Grundig AM/FM portable "Transistor 305" troubleshooting

[klem kedidelhopper]

I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base and emitter voltages in the last IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.

I considered trying to inject a signal and/or signal tracing, however
with the circuit voltages so far out of whack when it fails I think
that approach would be futile. The base and emitter voltages in these
three stages increase substantially when the radio quits. Some
voltages are jumping from for example 1.2V up to 4.0 V. With
conditions like this you would think that the cause would be obvious,
and perhaps I've been staring at this this too long and it is. However
the remedy has stubbornly eluded me thus far. The intermittent nature
of the problem is what makes it so difficult.

This is typical of what is going on: If the radio has been off for a
long period of time it will typically work for several hours and then
quit. In the beginning of this Quitting sequence, just after it first
quits it will try to come back on intermittently for a short period of
time, crackling etc, (as though something was intermittently breaking
down). Eventually it will just remain silent. There was an
electrolytic in the audio driver section, C57 a 100uf/3.0 volt with
slightly high ESR. Replacing it improved the low frequency audio
response, however the AF amplifier is not the problem though. You can
still get a good audio signal from the volume control out when it
fails. So the AF section seems to be unaffected. The voltages to that
part of the circuit substantiate that as well. And the RF and
oscillator circuit voltages during dead time seem to be fine as well.
The problem when it occurs affects both AM and FM.

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

I know that there are technicians out there better at this than I am,
and I would really appreciate it if someone could please take a look
at the schematic along with the voltage readings I obtained during Go
and No Go conditions and give me your opinions. With three stages
affected I suspect that I'm looking for a common denominator but I'm
just not sure. The initial voltage readings (on turn on with the unit
working normally) will be listed in the left hand column. The voltage
readings during a failed condition are listed in the right hand
column.
The major differences are marked with an asterisk*.

I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readings
Test point Radio working: Radio not working
------------------- --------------------
-------------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

Thanks once again for looking. Lenny

 

[spamtrap1888]

On Mar 4, 7:47 pm, klem kedidelhopper <captainvideo462...@gmail.com>
wrote:

I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base  and emitter voltages in the last  IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.



I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

"A drawing error occurred."

Try cutepdf.

 

[Winston]

klem kedidelhopper wrote:

I would like to ask for help once again with the repair of my own
personal old radio.

Hey Lenny,

It looks like a thermal intermittent.
Transistor AF126 is being told to 'cut off'
when the area around it heats sufficiently.
I suspect trim pot R22 (Just below AF126)
has a fractured solder joint which pops
open under thermal expansion.

Use your plastic spudger tool to gently
wiggle R22 and see if you can reproduce the
problem at will.

Next available moment, suggest you replace
the solder on R22 using plenty of liquid
RMA flux, then clean the area with naphtha
and an acid brush. Use lots of ventilation
and 'exam gloves' on your hands. Naphtha
is an excellent cleaner and will suck the
oil right out of your hands. DAMHIKT.

--Winston

 

[Franc Zabkar]

On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper
<captainvideo462009@gmail.com> put finger to keyboard and composed:

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readings
Test point Radio working: Radio not working
------------------- --------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

I suspect that C45 (near pin #8 of 7209-301) may be shorting.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Winston]

klem kedidelhopper wrote:

R22 seems to set a bias point. I subjected R22 to
some stress. It's definitely not a cold solder joint on that pot.

When you re-soldered the connections to R22, did
the intermittent go away?

--Winston

 

[klem kedidelhopper]

On Mar 5, 6:07 am, Franc Zabkar <fzab...@iinternode.on.net> wrote:

On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper
captainvideo462...@gmail.com> put finger to keyboard and composed:









http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

                                    Voltage readings
  Test point           Radio working:      Radio not working
-------------------         --------------------
  E. AF178                -1.10V                    -1.20V
  B. AF178                -1.50V                    -1.50V
  E. AF124                -1.10V                    -1.20V
  B. AF124                -1.32V                    -1.44V
  E. AF121                -0.85V                    -0.90V
  B. AF121                -1.20V                    -1.30V
  E. AF126 (1)           -1.00V                    -3.70V *
  B. AF126 (1)           -1.20V                    -4.00V *
  E. AF126 (II)           -0.80V                    -3.70V *
  B. AF126 (II)           -1.10V                    -3.60V *
  E. AC151 (I)           - 0.90V                   -0.80V
  C. AC151 (I)           --3.05V                   -2.95V
  E. AC151 (II)          --0.66V                   -0.62V
  C. AC151 (II)          --3.20V                   -3.10V

I suspect that C45 (near pin #8 of 7209-301) may be shorting.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

The "whisker theory was also posed to me last time I took a crack at
this repair. I disconnected all the cans at that time but the problem
still persisted. I'm trying to understand though how C45 could affect
this? It seems like a somewhat removed part of the circuit but perhaps
I'm not seeing the whole picture. Could these two affected transistors
be turning on, and if so wouldn't the collectors be very close to the
same potential as the emitters? I never looked at that. Collector
voltages are not listed on the schematic for those stages. Still if
that isn't the case I have to try to determine how this voltage is so
drastically rising. R22 seems to set a bias point. I subjected R22 to
some stress. It's definitely not a cold solder joint on that pot.
Unless it's breaking down internally but I would have thought that my
poking and prodding would have helped that along. In fact I had hoped
that once things became "thermal" they might also become
"mechanically" intermittent as well, but that was not the case. Now
hypothetically speaking in the case of one of the 820 ohm emitter
resistors opening, would that affect both stages? So many
possibilities here. I also thought of putting the soldering iron onto
the leads of several components upon turn on to attempt to bring on
the problem ahead of the time it usually takes but I am leaving that
as a last resort. Here is another link to the schematic just in case
the first one doesn't work or fails:
http://db.tt/bbVeazAe
Lenny

 

[William Sommerwerck]

This is a good example of the dilemma anyone fixing an electronic product
faces -- do you want to find out exactly what's wrong, or do you just want
to get it working again?

My own bias is towards the former, but eventually one has to stop
experimenting and "fix the damn thing, already".

Given that it's battery-operated, a thermal problem seems unlikely. A bad
solder joint, or a cracked trace or solder pad seem likely. It might also be
a bad transistor.

I would unsolder all the components around the suspected-bad area, and
troubleshoot for a cracked trace/pad. If you can't find any, replace all the
unsoldered components with new ones.

 

[amdx]

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

I have found a lot of intermittent problems with a can of freeze mist,
hair dryer and the tip of a soldering iron.
The first thing I would try is heating it with a hair dryer to see if
I could get it to go intermittent. If it does then go to your suspect
area and just barely dribble the freeze mist on, one part at a time.
It usually takes a fine touch on the freeze mist button to not
put out a big gush.
Another thing to try; touch transistors with the tip of your soldering
iron to heat it up. Sometimes you can find an intermittent transistor
this way.
Radio Shack has freeze mist.
Mikek

 

[spamtrap1888]

On Mar 5, 7:02 am, klem kedidelhopper <captainvideo462...@gmail.com>
wrote:

On Mar 5, 6:07 am, Franc Zabkar <fzab...@iinternode.on.net> wrote:



On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper
captainvideo462...@gmail.com> put finger to keyboard and composed:

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

                                    Voltage readings
  Test point           Radio working:      Radio not working
-------------------         --------------------
  E. AF178                -1.10V                    -1.20V
  B. AF178                -1.50V                    -1.50V
  E. AF124                -1.10V                    -1.20V
  B. AF124                -1.32V                    -1.44V
  E. AF121                -0.85V                    -0.90V
  B. AF121                -1.20V                    -1.30V
  E. AF126 (1)           -1.00V                    -3.70V *
  B. AF126 (1)           -1.20V                    -4.00V *
  E. AF126 (II)           -0.80V                    -3.70V *
  B. AF126 (II)           -1.10V                    -3.60V *
  E. AC151 (I)           - 0.90V                   -0.80V
  C. AC151 (I)           --3.05V                   -2.95V
  E. AC151 (II)          --0.66V                   -0.62V
  C. AC151 (II)          --3.20V                   -3.10V

I suspect that C45 (near pin #8 of 7209-301) may be shorting.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

The "whisker theory was also posed to me last time I took a crack at
this repair. I disconnected all the cans at that time but the problem
still persisted. I'm trying to understand though how C45 could affect
this? It seems like a somewhat removed part of the circuit but perhaps
I'm not seeing the whole picture. Could these two affected transistors
be turning on, and if so wouldn't the collectors be very close to the
same potential as the emitters? I never looked at that. Collector
voltages are not listed on the schematic for those stages. Still if
that isn't  the case I have to try to determine how this voltage is so
drastically rising. R22 seems to set a bias point. I  subjected R22 to
some stress. It's definitely not a cold solder joint on that pot.
Unless it's breaking down internally but I would have thought that my
poking and prodding would have helped that along. In fact I had hoped
that once things became "thermal" they might also become
"mechanically" intermittent as well, but that was not the case. Now
hypothetically speaking in the case of one of the 820 ohm emitter
resistors opening, would that affect both stages? So many
possibilities here. I also thought of putting the soldering iron onto
the leads of several components upon turn on to attempt to bring on
the problem ahead of the time it usually takes but I am leaving that
as a last resort. Here is another link to the schematic just in case
the first one doesn't work or fails:http://db.tt/bbVeazAe
Lenny

I was able to open the pdf after downloading it.

While I haven't done the math, I would suspect C44. First, it is
suspect merely by virtue of being a 40 year old electrolytic capacitor
that is connected to both failing stages. Further: if, when you turned
the radio on, C44's leakage current was initially low, but with
operation it increased substantially, then a bad C44 would affect the
bias point R22 was trying to set, as current began to flow through
R21. Moreover, a short through C44 to positive should have an effect
similar to the short through C45 to ground that Franc suspects. Thus,
I would try replacing C44.

 

[Franc Zabkar]

On Mon, 5 Mar 2012 07:02:27 -0800 (PST), klem kedidelhopper
<captainvideo462009@gmail.com> put finger to keyboard and composed:

The "whisker theory was also posed to me last time I took a crack at
this repair. I disconnected all the cans at that time but the problem
still persisted. I'm trying to understand though how C45 could affect
this?

Sorry, I'm not convinced that C45 is the culprit. I was confused by
the negative voltages and the upside down circuit diagram. :-(

I still think that some component is reducing the bias on the base of
AF126 (I). I was looking for a path to ground via a likely suspect,
and C45 and C46 looked like possible candidates.

As for R22, I don't understand how an open circuit could reduce the
voltage on the base of the transistor.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[hrhofmann@att.net]

On Mar 5, 11:18 am, amdx <a...@knology.net> wrote:

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

   I have found a lot of intermittent problems with a can of freeze mist,
hair dryer and the tip of a soldering iron.
  The first thing I would try is heating it with a hair dryer to see if
I could get it to go intermittent. If it does then go to your suspect
area and just barely dribble the freeze mist on, one part at a time.
It usually takes a fine touch on the freeze mist button to not
put out a big gush.
  Another thing to try; touch transistors with the tip of your soldering
iron to heat it up. Sometimes you can find an intermittent transistor
this way.
Radio Shack has freeze mist.
              Mikek

What he says is what I would do also>>>>>

 

[Winston]

Franc Zabkar wrote:

(...)

As for R22, I don't understand how an open circuit could reduce the
voltage on the base of the transistor.

Emitter current through R24 is adjusted by the setting of
R22, per the note to the right of R24. As R22 is adjusted
higher in resistance, we can expect less current to flow
from the base to emitter, causing AF126 to tend towards
cutoff. (The base becomes less negative *in relation to
the emitter*).

What if we broke the connection to R22 by lifting it's rotor
or fracturing either of it's solder connections?
The base of AF126 will be biased more positive in relation
to it's emitter (towards cutoff) because R22 isn't there to
provide a parallel current path to ground.

See how the AGC rectifier (AA130) biases the base of AF126
more positive (via R21) as IF voltage increases? This
also tends to push AF126 towards cutoff. It's a negative
feedback loop that keeps IF voltage constant.


--Winston

 

[Dave M]

Franc Zabkar wrote:

On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper
captainvideo462009@gmail.com> put finger to keyboard and composed:

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readings
Test point Radio working: Radio not working
------------------- --------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

I suspect that C45 (near pin #8 of 7209-301) may be shorting.

- Franc Zabkar

My $0.02 worth...
Since two stages are being upset, look for what's common to both circuits.
I don't know how the PCB traces are laid out, nut since the layout diagram
indicates that both stages are enclosed by (metal?) shields, it would be
possible that the PCB trace(s) that connect the shields together might be
intermittently broken fron the rest of the circuit. That would cause the
symptom of the measured B and E voltages being high negative, indicating
that the collector circuits of both stages are open. Look for a broken trace
or solder joint somewhere in the collector circuits.

I've seen broken shielding enclosure connections cause all sorts of weird
symptoms.
--
Dave M
A woman has the last word in any argument. Anything a man says after that is
the beginning of a new argument.

 

[Franc Zabkar]

On Mon, 05 Mar 2012 15:39:48 -0800, Winston <Winston@Bigbrother.net>
put finger to keyboard and composed:

Franc Zabkar wrote:

(...)

As for R22, I don't understand how an open circuit could reduce the
voltage on the base of the transistor.


Emitter current through R24 is adjusted by the setting of
R22, per the note to the right of R24. As R22 is adjusted
higher in resistance, we can expect less current to flow
from the base to emitter, causing AF126 to tend towards
cutoff. (The base becomes less negative *in relation to
the emitter*).

AISI, the voltage readings on the circuit diagram are using the
positive terminal of the 9V battery as the 0V reference. Therefore all
the voltage measurements are negative numbers, which means that the
transistor is being turned on harder during the fault condition. The
voltage on circuit ground would be -9V.

Or am I having a brain fart? (Quite likely)

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Sylvia Else]

On 5/03/2012 2:47 PM, klem kedidelhopper wrote:

I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base and emitter voltages in the last IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.

I considered trying to inject a signal and/or signal tracing, however
with the circuit voltages so far out of whack when it fails I think
that approach would be futile. The base and emitter voltages in these
three stages increase substantially when the radio quits. Some
voltages are jumping from for example 1.2V up to 4.0 V. With
conditions like this you would think that the cause would be obvious,
and perhaps I've been staring at this this too long and it is. However
the remedy has stubbornly eluded me thus far. The intermittent nature
of the problem is what makes it so difficult.

This is typical of what is going on: If the radio has been off for a
long period of time it will typically work for several hours and then
quit. In the beginning of this Quitting sequence, just after it first
quits it will try to come back on intermittently for a short period of
time, crackling etc, (as though something was intermittently breaking
down). Eventually it will just remain silent. There was an
electrolytic in the audio driver section, C57 a 100uf/3.0 volt with
slightly high ESR. Replacing it improved the low frequency audio
response, however the AF amplifier is not the problem though. You can
still get a good audio signal from the volume control out when it
fails. So the AF section seems to be unaffected. The voltages to that
part of the circuit substantiate that as well. And the RF and
oscillator circuit voltages during dead time seem to be fine as well.
The problem when it occurs affects both AM and FM.

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

I know that there are technicians out there better at this than I am,
and I would really appreciate it if someone could please take a look
at the schematic along with the voltage readings I obtained during Go
and No Go conditions and give me your opinions. With three stages
affected I suspect that I'm looking for a common denominator but I'm
just not sure. The initial voltage readings (on turn on with the unit
working normally) will be listed in the left hand column. The voltage
readings during a failed condition are listed in the right hand
column.
The major differences are marked with an asterisk*.

I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readingso
Test point Radio working: Radio not working
------------------- --------------------
-------------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

Thanks once again for looking. Lenny

The bias of AF126 II is derived from the emitter of AF126 I (hereinafter
'the transistor'), so if the latter's bias arangements go awry, so will
the former's, which is thus of no interest.

If pin 10 of F IV became open circuit, the base of the transistor would
be pulled towards -9V through R5, R4, R26 and R21 in series, being a
total of 47.2K. For that to put the base at -4V requires a base current
of 0.1mA, there being no other source for the current.

For the emitter of the transistor to be at -3.7V requires a current of
4.5mA through R24 (the 820 ohm resistor). Given the calculated base
current, this would be the case if the beta of the transistor were 42,
which seems entirely plausible.

So if the transistor has a beta of around 40, the behaviour can be
explained by an intermittent open circuit at, or in the vicinity of, pin
10 of F IV.

Sylvia.

 

[Winston]

Franc Zabkar wrote:

(...)

AISI, the voltage readings on the circuit diagram are using the
positive terminal of the 9V battery as the 0V reference. Therefore all
the voltage measurements are negative numbers, which means that the
transistor is being turned on harder during the fault condition. The
voltage on circuit ground would be -9V.

Or am I having a brain fart? (Quite likely)

I had the fart.

You and Sylvia are much closer to the truth.
I now see that R5, R4, R28 and R21 would tend to turn on
the transistor (AF126 I) if the path from the positive
terminal of the battery, (through the diode "1.4 St1",
through pin 10 of F IV) were opened, because pin 10
of F IV is only one diode drop less than
the positive terminal of the battery normally.

So, I retract my diagnosis and now think that pin 10
of F IV and both pins of diode "1.4 Stl" should be
cleaned and re-soldered.

--Winston

 

[Winston]

Sylvia Else wrote:

On 5/03/2012 2:47 PM, klem kedidelhopper wrote:
I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base and emitter voltages in the last IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.

I considered trying to inject a signal and/or signal tracing, however
with the circuit voltages so far out of whack when it fails I think
that approach would be futile. The base and emitter voltages in these
three stages increase substantially when the radio quits. Some
voltages are jumping from for example 1.2V up to 4.0 V. With
conditions like this you would think that the cause would be obvious,
and perhaps I've been staring at this this too long and it is. However
the remedy has stubbornly eluded me thus far. The intermittent nature
of the problem is what makes it so difficult.

This is typical of what is going on: If the radio has been off for a
long period of time it will typically work for several hours and then
quit. In the beginning of this Quitting sequence, just after it first
quits it will try to come back on intermittently for a short period of
time, crackling etc, (as though something was intermittently breaking
down). Eventually it will just remain silent. There was an
electrolytic in the audio driver section, C57 a 100uf/3.0 volt with
slightly high ESR. Replacing it improved the low frequency audio
response, however the AF amplifier is not the problem though. You can
still get a good audio signal from the volume control out when it
fails. So the AF section seems to be unaffected. The voltages to that
part of the circuit substantiate that as well. And the RF and
oscillator circuit voltages during dead time seem to be fine as well.
The problem when it occurs affects both AM and FM.

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

I know that there are technicians out there better at this than I am,
and I would really appreciate it if someone could please take a look
at the schematic along with the voltage readings I obtained during Go
and No Go conditions and give me your opinions. With three stages
affected I suspect that I'm looking for a common denominator but I'm
just not sure. The initial voltage readings (on turn on with the unit
working normally) will be listed in the left hand column. The voltage
readings during a failed condition are listed in the right hand
column.
The major differences are marked with an asterisk*.

I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readingso
Test point Radio working: Radio not working
------------------- --------------------
-------------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

Thanks once again for looking. Lenny

The bias of AF126 II is derived from the emitter of AF126 I (hereinafter 'the transistor'), so if the latter's bias
arangements go awry, so will the former's, which is thus of no interest.

If pin 10 of F IV became open circuit, the base of the transistor would be pulled towards -9V through R5, R4, R26 and
R21 in series,

Yes, I now see that R5, R4, R28 and R21 would tend to turn on the transistor if the
path to pin 10 of F IV were interrupted.

being a total of 47.2K. For that to put the base at -4V requires a base current of 0.1mA, there being no
other source for the current.

For the emitter of the transistor to be at -3.7V requires a current of 4.5mA through R24 (the 820 ohm resistor). Given
the calculated base current, this would be the case if the beta of the transistor were 42, which seems entirely plausible.

So if the transistor has a beta of around 40, the behaviour can be explained by an intermittent open circuit at, or in
the vicinity of, pin 10 of F IV.

I agree. Cleaning and re-soldering pin 10 of F IV as well as both pins of
the diode "1.4 St1" are good next steps.

--Winston

 

[Franc Zabkar]

On Tue, 06 Mar 2012 06:40:59 -0800, Winston <Winston@Bigbrother.net>
put finger to keyboard and composed:

So, I retract my diagnosis and now think that pin 10
of F IV and both pins of diode "1.4 Stl" should be
cleaned and re-soldered.

AISI, if the diode were open, then that would disturb the bias on the
preceding stages (AF121 and AF124/125).

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Jon Elson]

klem kedidelhopper wrote:

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off.
OK, this HAS to be a bad electrolytic capacitor. That is the only component

that has that sort of memory. Replace electrolytics in the area where
the problem is suspected. They are often used in bias networks.

Jon

 

[Sylvia Else]

On 7/03/2012 1:46 AM, Winston wrote:

Sylvia Else wrote:
On 5/03/2012 2:47 PM, klem kedidelhopper wrote:
I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base and emitter voltages in the last IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.

I considered trying to inject a signal and/or signal tracing, however
with the circuit voltages so far out of whack when it fails I think
that approach would be futile. The base and emitter voltages in these
three stages increase substantially when the radio quits. Some
voltages are jumping from for example 1.2V up to 4.0 V. With
conditions like this you would think that the cause would be obvious,
and perhaps I've been staring at this this too long and it is. However
the remedy has stubbornly eluded me thus far. The intermittent nature
of the problem is what makes it so difficult.

This is typical of what is going on: If the radio has been off for a
long period of time it will typically work for several hours and then
quit. In the beginning of this Quitting sequence, just after it first
quits it will try to come back on intermittently for a short period of
time, crackling etc, (as though something was intermittently breaking
down). Eventually it will just remain silent. There was an
electrolytic in the audio driver section, C57 a 100uf/3.0 volt with
slightly high ESR. Replacing it improved the low frequency audio
response, however the AF amplifier is not the problem though. You can
still get a good audio signal from the volume control out when it
fails. So the AF section seems to be unaffected. The voltages to that
part of the circuit substantiate that as well. And the RF and
oscillator circuit voltages during dead time seem to be fine as well.
The problem when it occurs affects both AM and FM.

The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.

I know that there are technicians out there better at this than I am,
and I would really appreciate it if someone could please take a look
at the schematic along with the voltage readings I obtained during Go
and No Go conditions and give me your opinions. With three stages
affected I suspect that I'm looking for a common denominator but I'm
just not sure. The initial voltage readings (on turn on with the unit
working normally) will be listed in the left hand column. The voltage
readings during a failed condition are listed in the right hand
column.
The major differences are marked with an asterisk*.

I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny

http://dl.dropbox.com/u/65394789/Grundig%20large%20schematic.pdf

Voltage readingso
Test point Radio working: Radio not working
------------------- --------------------
-------------------------
E. AF178 -1.10V -1.20V
B. AF178 -1.50V -1.50V
E. AF124 -1.10V -1.20V
B. AF124 -1.32V -1.44V
E. AF121 -0.85V -0.90V
B. AF121 -1.20V -1.30V
E. AF126 (1) -1.00V -3.70V *
B. AF126 (1) -1.20V -4.00V *
E. AF126 (II) -0.80V -3.70V *
B. AF126 (II) -1.10V -3.60V *
E. AC151 (I) - 0.90V -0.80V
C. AC151 (I) --3.05V -2.95V
E. AC151 (II) --0.66V -0.62V
C. AC151 (II) --3.20V -3.10V

Thanks once again for looking. Lenny

The bias of AF126 II is derived from the emitter of AF126 I
(hereinafter 'the transistor'), so if the latter's bias
arangements go awry, so will the former's, which is thus of no interest.

If pin 10 of F IV became open circuit, the base of the transistor
would be pulled towards -9V through R5, R4, R26 and
R21 in series,

Yes, I now see that R5, R4, R28 and R21 would tend to turn on the
transistor if the
path to pin 10 of F IV were interrupted.

being a total of 47.2K. For that to put the base at -4V requires a
base current of 0.1mA, there being no
other source for the current.

For the emitter of the transistor to be at -3.7V requires a current of
4.5mA through R24 (the 820 ohm resistor). Given
the calculated base current, this would be the case if the beta of the
transistor were 42, which seems entirely plausible.

So if the transistor has a beta of around 40, the behaviour can be
explained by an intermittent open circuit at, or in
the vicinity of, pin 10 of F IV.

I agree. Cleaning and re-soldering pin 10 of F IV as well as both pins of
the diode "1.4 St1" are good next steps.

--Winston

As Frank observes, if the 1.4 St1 connections are compromised, then the
bias for the AF124 would be affected, and on the OP's readings, it is not.

Now, we don't know the nature of the link between that diode and pin ten
of F IV, and I've seen Grundig do some rather questionable things, but
if it's just a circuit board track, the fault pretty much has to lie
either in the pin 10 soldering, or in the track itself.

Sylvia.