Strange LO Oscillator won't oscillate

B

Bill Bowden

Guest
It's a Autodyne converter in a small AM Radio. DC voltages look ok but
doesn't oscillate. Originally, the problem was a weak transistor with a hFE
of 4. I replaced the tranny with a 2N2222A and it worked fine for a couple
weeks and then died. Now the DC voltages look ok with about 1.5 volts on the
emitter. Usually, this type of oscillator has the coil tap connected to the
emitter through a cap with no bypass to ground. But this one has the tap
connected through a 100 ohm resistor in series with a 3.3nF cap and then
through the secondary of the antenna loopstick to the base of the tranny.
This radio worked pretty well with only one IF stage, so I'm wondering if
this configuration provides additional signal gain from the converter
section? I plan to play around with the tranny bias to see if it might
oscillate with a little more or less current.
.. .


+V +V
| |
| IF Stage winding
/ |
\ |
/ 150K Feedback winding
\ |
| |
| /c
Antenna winding-----b|
| \e
/ }
\ ___ |
/ 100 | /
\ | \
| --- / 2.2K
--- 3.3NF --- \
--- | |
| | |
| GND GND
Tap
|- Primary winding-|
| |
|-------||----------
|
|
GND




--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
Bill Bowden wrote:
It's a Autodyne converter in a small AM Radio. DC voltages look ok but
doesn't oscillate. Originally, the problem was a weak transistor with a hFE
of 4. I replaced the tranny with a 2N2222A and it worked fine for a couple
weeks and then died.
Click to expand...

Was the original transistor silicon or germanium?


Now the DC voltages look ok with about 1.5 volts on the
emitter. Usually, this type of oscillator has the coil tap connected to the
emitter through a cap with no bypass to ground. But this one has the tap
connected through a 100 ohm resistor in series with a 3.3nF cap and then
through the secondary of the antenna loopstick to the base of the tranny.
This radio worked pretty well with only one IF stage, so I'm wondering if
this configuration provides additional signal gain from the converter
section? I plan to play around with the tranny bias to see if it might
oscillate with a little more or less current.
. .

+V +V
| |
| IF Stage winding
/ |
\ |
/ 150K Feedback winding
\ |
| |
| /c
Antenna winding-----b|
| \e
/ }
\ ___ |
/ 100 | /
\ | \
| --- / 2.2K
--- 3.3NF --- \
--- | |
| | |
| GND GND
Tap
|- Primary winding-|
| |
|-------||----------
|
|
GND
Click to expand...
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com...
Bill Bowden wrote:

It's a Autodyne converter in a small AM Radio. DC voltages look ok but
doesn't oscillate. Originally, the problem was a weak transistor with a
hFE
of 4. I replaced the tranny with a 2N2222A and it worked fine for a
couple
weeks and then died.


Was the original transistor silicon or germanium?
Click to expand...

I believe it was silicon since the e/b voltage read about 0.5 as I remember.
But the new 2N2222A hasn't died (I don't think) since the emitter current is
about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or
germanium? It did work for 2 weeks.
..
Now the DC voltages look ok with about 1.5 volts on the
emitter. Usually, this type of oscillator has the coil tap connected to
the
emitter through a cap with no bypass to ground. But this one has the tap
connected through a 100 ohm resistor in series with a 3.3nF cap and then
through the secondary of the antenna loopstick to the base of the tranny.
This radio worked pretty well with only one IF stage, so I'm wondering if
this configuration provides additional signal gain from the converter
section? I plan to play around with the tranny bias to see if it might
oscillate with a little more or less current.
. .

+V +V
| |
| IF Stage winding
/ |
\ |
/ 150K Feedback winding
\ |
| |
| /c
Antenna winding-----b|
| \e
/ }
\ ___ |
/ 100 | /
\ | \
| --- / 2.2K
--- 3.3NF --- \
--- | |
| | |
| GND GND
Tap
|- Primary winding-|
| |
|-------||----------
|
|
GND
Click to expand...


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
Bill Bowden wrote:
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com...

Bill Bowden wrote:

It's a Autodyne converter in a small AM Radio. DC voltages look ok but
doesn't oscillate. Originally, the problem was a weak transistor with a
hFE
of 4. I replaced the tranny with a 2N2222A and it worked fine for a
couple
weeks and then died.


Was the original transistor silicon or germanium?


I believe it was silicon since the e/b voltage read about 0.5 as I remember.
But the new 2N2222A hasn't died (I don't think) since the emitter current is
about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or
germanium? It did work for 2 weeks.
Click to expand...

They require different biasing. .5 v is low for silicon, but high for
Germanium. A failing Germanium transistor often had an open base, which
would read higher. Some were so leaky that they were self biasing. GE's
early transistor radios had to have bias resistors added when you
replaced a bad transistor, because they depended on the leakage to make
it operate.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:BrqdnXocHooeN-zInZ2dnUU7-N-dnZ2d@earthlink.com...
Bill Bowden wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com...

Bill Bowden wrote:

It's a Autodyne converter in a small AM Radio. DC voltages look ok but
doesn't oscillate. Originally, the problem was a weak transistor with
a
hFE
of 4. I replaced the tranny with a 2N2222A and it worked fine for a
couple
weeks and then died.


Was the original transistor silicon or germanium?


I believe it was silicon since the e/b voltage read about 0.5 as I
remember.
But the new 2N2222A hasn't died (I don't think) since the emitter current
is
about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or
germanium? It did work for 2 weeks.


They require different biasing. .5 v is low for silicon, but high for
Germanium. A failing Germanium transistor often had an open base, which
would read higher. Some were so leaky that they were self biasing. GE's
early transistor radios had to have bias resistors added when you
replaced a bad transistor, because they depended on the leakage to make
it operate.
Click to expand...

There is only one bias resistor (150K) so it must be silicon. As I remember,
germanium required a low resistance (maybe 300 ohms) between emitter and
base to reduce leakage. My first transistor radio (Zenith 1957) cost $50 and
had germanium transistors in sockets. My dad's radio (GE table model) used 5
"D" cells. with a tap on one cell used for negative 1.5 volt bias. I think
they used the negative bias to reduce leakage. .

Anyway, I got it working. Looks like something is wrong with the oscillator
coil. I changed the emitter resistor to 1.1K to increase the DC current to
about 1mA and soldered a resistor to the tap on the coil and it worked for
about 5 minutes. I figured the heat from the iron had something to do with
it so I heated the coil connections again and the thing ran all day. If it
dies again, I'll rewind the coil.




--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
 
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