Trouble shooting push-pull amp

[Terry Pinnell]

This circuit is part of a very old project.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Until a few hours ago it was working after a fashion. But while
attempting to improve it I've clearly screwed something up. The
voltage at the +ve terminal of the output cap is now at Vcc (24V),
instead of around 12V. Adjusting the R2 100k preset makes no
difference. Staring and fiddling has got me no closer to discovering
why.

Can anyone help isolate the likely cause please?

(Also posted with attachment in alt.binaries.schematics.electronic)

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[j.b. miller]

Obvious items.... are C2 ,Q3,Q5 shorted ?
Hey at least it's a start....

 

[Stefan Heinzmann]

Terry Pinnell wrote:

This circuit is part of a very old project.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Until a few hours ago it was working after a fashion. But while
attempting to improve it I've clearly screwed something up. The
voltage at the +ve terminal of the output cap is now at Vcc (24V),
instead of around 12V. Adjusting the R2 100k preset makes no
difference. Staring and fiddling has got me no closer to discovering
why.

Can anyone help isolate the likely cause please?

Seems you have no trouble shooting it, as it has already happened ;-)

Have you got a scope or at least a multimeter? What about measuring a
few more voltages in the circuit? You're not exactly giving away much
information. Could Q5 have melted down?

--
Cheers
Stefan

 

[Tim Wescott]

Terry Pinnell wrote:

This circuit is part of a very old project.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Until a few hours ago it was working after a fashion. But while
attempting to improve it I've clearly screwed something up. The
voltage at the +ve terminal of the output cap is now at Vcc (24V),
instead of around 12V. Adjusting the R2 100k preset makes no
difference. Staring and fiddling has got me no closer to discovering
why.

Can anyone help isolate the likely cause please?

(Also posted with attachment in alt.binaries.schematics.electronic)

Measure the Vbe and Vce of each transistor, from the speaker backwards,
and look for readings that make sense (i.e. 0 < Vbe < 0.7V, and Vce of
around 12V). Also measure across your diodes. If that doesn't get you
there, start measuring across resistors (oh joy).

Chances are you've fried a transistor, you've probably made one fail
short on the supply-rail end but you may have made one fail open on the
ground. Failing that it could be a short or an open nearly anywhere in
the circuit.

Find the bad part and replace.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[legg]

On Wed, 12 May 2004 20:26:22 +0100, Terry Pinnell
<terrypin@dial.pipex.com> wrote:

This circuit is part of a very old project.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Until a few hours ago it was working after a fashion. But while
attempting to improve it I've clearly screwed something up. The
voltage at the +ve terminal of the output cap is now at Vcc (24V),
instead of around 12V. Adjusting the R2 100k preset makes no
difference. Staring and fiddling has got me no closer to discovering
why.

Can anyone help isolate the likely cause please?

(Also posted with attachment in alt.binaries.schematics.electronic)

When you've found the dead semicoductor, relocate R8's connection to
the emitter of Q6, not to the kelvin signal ground.

RL

 

[Terry Pinnell]

"Ban" <bansuri@web.de> wrote:

Terry Pinnell wrote:
This circuit is part of a very old project.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Until a few hours ago it was working after a fashion. But while
attempting to improve it I've clearly screwed something up. The
voltage at the +ve terminal of the output cap is now at Vcc (24V),
instead of around 12V. Adjusting the R2 100k preset makes no
difference. Staring and fiddling has got me no closer to discovering
why.

Can anyone help isolate the likely cause please?

Terry, even a very minute short on the output will blow one of the 3055, in
your case the upper one. But it is possible to add a current limit. Put two
resistors into the emitterQ5 collector Q6 lines, so the voltage is 0.7V with
max.current. For example 0.1R will limit at 7A. then use a small 2N3904
with the C to BaseQ3 and a 2N3902 to BaseQ4 to shorten the drive current.
with the base to the new resistors. You can add some R/C to allow higher
current for a limited time and even some foldback action with an additional
resistor to the respective supply rails.

Many thanks for all the helpful replies, here and in
alt.binaries.schematics.electronic. First thing I'll check this
morning is whether Q3 and Q5 are shorted. If so, I'll fix and then add
some minimal protection along the lines Ban suggests.

The very last actions I took seemed innocent enough, which is why I
was at a loss about these symptoms. I had been getting a distorted
output, and C1 +ve end was well above 12V. I'd tested all four small
BJTs and the two power types. Therefore I began to mistrust my
recollection and scruffy handwritten notes that it *used* to perform
properly. (I built it 23 years ago.) So I speculated that R2 wasn't a
large enough range to give me optimum adjustment, and replaced it with
breadboard connections to 220k. It was at that stage that the C1
voltage became fixed at about 24V.

Thinking about it overnight, I suspect that I may heve been careless.
I have that particular stripboard circuit unscrewed from a large case
containing other sections. But it's still hard wired to the 2N3055s,
which are fixed with usual insulation to rear of case. So it's
temporarily on a piece of cardboard suspended on top of other wiring,
with power temporarily from my bench supply. I'm careful to switch off
that supply each time I make any tests. But a hefty charge must remain
on C1, and one possibility is that some of it got into the wrong place
while I considered the circuit 'unpowered'.

Digressing, I often get into this loop. Temporarily testing or
modifying a finished project, with some parts still in the case and
others on breadboard, with power supply and probe wires etc across the
bench - a scenario which risks adding *new* faults as well as those
under investigation. (Oh, did I mention my natural impatience too?
<g&gt

Finally, the simulation
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp2.gif )
prompts me to wonder whether another possibility might be that I
simply haven't got R2 and R4 correctly adjusted. They do seem
critical, and several I simulated end up placing C1 at the sort of
high level I reported. Assuming that's not the cause, when I do have a
'working circuit' again, what is the correct sequence for adjusting
these two presets? Is it just a matter of trial and error, or is there
an optimum quiescent current I should first set with R4 before
adjusting R2 to get 12V at C1?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

legg <legg@nospam.magma.ca> wrote:

When you've found the dead semicoductor, relocate R8's connection to
the emitter of Q6, not to the kelvin signal ground.

RL

Can you amplify (sorry!) on that please? Why would that be an
improvement? The original 30-year old circuit on which I based mine
presumably connected all low power signals to 'Signal earth', and only
the final output return to 'Power earth'.

BTW, that term 'kelvin signal ground' is unfamiliar to me. (A google
news search on it turns up only your post here.) Could you explain its
meaning please?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Paul Burke]

Terry Pinnell wrote:

BTW, that term 'kelvin signal ground' is unfamiliar to me. (A google
news search on it turns up only your post here.) Could you explain its
meaning please?

A Kelvin connection is a separate trace to the high- impedance sensing
circuit, added to avoid voltage errors caused by a (relatively) high
current through the track resistance. It's often used in sensor bridge
circuits, particularly where the distance from the bridge to the
conditioning circuit is large.

Paul Burke

 

[Terry Pinnell]

Paul Burke <paul@scazon.com> wrote:

Terry Pinnell wrote:

BTW, that term 'kelvin signal ground' is unfamiliar to me. (A google
news search on it turns up only your post here.) Could you explain its
meaning please?


A Kelvin connection is a separate trace to the high- impedance sensing
circuit, added to avoid voltage errors caused by a (relatively) high
current through the track resistance. It's often used in sensor bridge
circuits, particularly where the distance from the bridge to the
conditioning circuit is large.

Thanks, Paul. In that case, I don't see its relevance? Unless RL is
using it to mean 'signal ground'.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Stefan Heinzmann]

Terry Pinnell wrote:
[...]

Many thanks for all the helpful replies, here and in
alt.binaries.schematics.electronic. First thing I'll check this
morning is whether Q3 and Q5 are shorted. If so, I'll fix and then add
some minimal protection along the lines Ban suggests.

The very last actions I took seemed innocent enough, which is why I
was at a loss about these symptoms. I had been getting a distorted
output, and C1 +ve end was well above 12V. I'd tested all four small
BJTs and the two power types. Therefore I began to mistrust my
recollection and scruffy handwritten notes that it *used* to perform
properly. (I built it 23 years ago.) So I speculated that R2 wasn't a
large enough range to give me optimum adjustment, and replaced it with
breadboard connections to 220k. It was at that stage that the C1
voltage became fixed at about 24V.

If you were getting a distorted output all of a sudden, it is possible
that the amp was oscillating at a high frequency, which frequently
results in overheated output transistors. This may therefore have caused
the death of Q5. As Ban already noted, your amp has absolutely no
protection against any sort of mishap built in. I can't even see
anything that would limit the bandwidth, which presumably makes it
rather difficult to control a tendency to oscillate. I'm rather amazed
it worked for 23 years

Thinking about it overnight, I suspect that I may heve been careless.
I have that particular stripboard circuit unscrewed from a large case
containing other sections. But it's still hard wired to the 2N3055s,
which are fixed with usual insulation to rear of case. So it's
temporarily on a piece of cardboard suspended on top of other wiring,
with power temporarily from my bench supply. I'm careful to switch off
that supply each time I make any tests. But a hefty charge must remain
on C1, and one possibility is that some of it got into the wrong place
while I considered the circuit 'unpowered'.

By removing the circuit board from its case you changed the layout of
the wiring, which may well have started the oscillations. No short or
capacitor discharge into the wrong place is necessary to explain the
outcome. It could have happened without any carelessness of your side.

[...]

Finally, the simulation
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp2.gif )
prompts me to wonder whether another possibility might be that I
simply haven't got R2 and R4 correctly adjusted. They do seem
critical, and several I simulated end up placing C1 at the sort of
high level I reported. Assuming that's not the cause, when I do have a
'working circuit' again, what is the correct sequence for adjusting
these two presets? Is it just a matter of trial and error, or is there
an optimum quiescent current I should first set with R4 before
adjusting R2 to get 12V at C1?

Note that your simulation shows a tendency of the circuit to oscillate,
which supports my argument.

R4 controls the quiescent current in the output stage, hence it is very
critical. If adjusted wrongly the output stage simply overheats,
possibly in a very short time.

If you simulate the circuit, have a look at the *currents* in the output
stage, too. Looking at the voltages may not give you the real picture.
Well, actually, the only thing to give you the real picture is the real
circuit ;-)

--
Cheers
Stefan

 

[Terry Pinnell]

Still unresolved.

The cause appears to be none of the suggestions made so far. All 6
transistors are OK. Fresh from the workshop, attached are some actual
voltage measurements which I propose studying this evening. Maybe with
a glass of dry white...
(Also posted in sed, and same illustration is at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp3.gif )

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Keith Wootten]

In message <f557a057pr3qkgje86rn1vsb3rk8h8ln4a@4ax.com>, Terry Pinnell
<terrypin@dial.pipex.com> writes

Still unresolved.

The cause appears to be none of the suggestions made so far. All 6
transistors are OK. Fresh from the workshop, attached are some actual
voltage measurements which I propose studying this evening. Maybe with
a glass of dry white...
(Also posted in sed, and same illustration is at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp3.gif )

Do you really have 24.7V and 80mV on the same track? If so, you

probably have an ex-track.

Cheers
--
Keith Wootten

 

[legg]

On Thu, 13 May 2004 09:21:04 +0100, Terry Pinnell
<terrypin@dial.pipex.com> wrote:

legg <legg@nospam.magma.ca> wrote:

When you've found the dead semicoductor, relocate R8's connection to
the emitter of Q6, not to the kelvin signal ground.

RL

Can you amplify (sorry!) on that please? Why would that be an
improvement? The original 30-year old circuit on which I based mine
presumably connected all low power signals to 'Signal earth', and only
the final output return to 'Power earth'.

BTW, that term 'kelvin signal ground' is unfamiliar to me. (A google
news search on it turns up only your post here.) Could you explain its
meaning please?

The connection of R8 to signal ground, rather
than to the emitter of Q6 introduces some
discontinuity in the function of Q6. This
acts to add to Q6 off-bias when Q5 is conducting
through the load or through Q6, but produces a
sudden increase in Q6 on bias when Q5 turns off.
This does not come from the control circuit, so there
will be a tendency to oscillate, unless R8 is connected
to the emitter.

R8 is not part of the 'low power signals'.

Setup

-R2 at midpoint
-R4 at minimum value.
-Apply power. VC1 should read ~6volts
-Decrease R2 to set VC1 to 1/2VCC
-Increase R4 until supply current starts
to increase, then back it off.

This design requires ClassB biasing.
It cannot be safely biased as classAB.

The quiescent voltage on C1 will also tend to
drift down with increase in temperature of Q1.

Q1, Q2 and Q3 form a signifigant silicon short
if the control circuit tries to handle ANY quiescent
current in Q5.

Adding an emitter resistor to Q4, Q5 and Q6
might allow the circuit to run in classAB.

RL

 

[Terry Pinnell]

Keith Wootten <keithw@nononono.co.uk> wrote:

Do you really have 24.7V and 80mV on the same track? If so, you
probably have an ex-track.

Yes - that was it! A couple of others over in
alt.binaries.schematics.electronic spotted it too. Proved to be a hair
line crack on a stripboard track.

I hadn't really looked at those figures before I posted them. Like to
think I'd have spotted that inconsistency after dinner <g>. Just glad
I measured that node.

Thanks to all here for the help with the detective work. With that
mystery solved, I can return to getting a decent output now! I'll come
back soon to seek further help in minimising distortion.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Given]

"Terry Pinnell" <terrypin@dial.pipex.com> wrote in message
news:22k7a0952nmtucvvnvhsnh47829mt2s22k@4ax.com...

Keith Wootten <keithw@nononono.co.uk> wrote:

Do you really have 24.7V and 80mV on the same track? If so, you
probably have an ex-track.

Yes - that was it! A couple of others over in
alt.binaries.schematics.electronic spotted it too. Proved to be a hair
line crack on a stripboard track.

I hadn't really looked at those figures before I posted them. Like to
think I'd have spotted that inconsistency after dinner <g>. Just glad
I measured that node.

Thanks to all here for the help with the detective work. With that
mystery solved, I can return to getting a decent output now! I'll come
back soon to seek further help in minimising distortion.

--
Terry Pinnell

If its 23 years old, your electrolytic capacitors are in all likelihood past
their best. Electrolyte eventually "dries up" and the hotter the cap gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only candidate
for this failure mode. Other things happen when the cap is not charged for
long periods of time (google electrolytic capacitor re-forming if you want
to find out what, how and why). The actual capacitance can decrease hugely,
leakage current increases - I have replaced caps that measured 10s of ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

Terry

 

[Terry Pinnell]

"Terry Given" <the_domes@xtra.co.nz> wrote:

If its 23 years old, your electrolytic capacitors are in all likelihood past
their best. Electrolyte eventually "dries up" and the hotter the cap gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only candidate
for this failure mode. Other things happen when the cap is not charged for
long periods of time (google electrolytic capacitor re-forming if you want
to find out what, how and why). The actual capacitance can decrease hugely,
leakage current increases - I have replaced caps that measured 10s of ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

Thanks. They *look* OK, but from waht you say I suppose it could be
that in the earlier stages of my testing they were reforming. Latest
results seem to be OK as you probably saw.

Interesting that my '1000uF' is actually about 700uF!

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Ken]

On Sun, 23 May 2004 19:13:59 -0700, "Terry Given"
<the_domes@xtra.co.nz> wrote:

If its 23 years old, your electrolytic capacitors are in all likelihood past
their best. Electrolyte eventually "dries up" and the hotter the cap gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only candidate
for this failure mode. Other things happen when the cap is not charged for
long periods of time (google electrolytic capacitor re-forming if you want
to find out what, how and why). The actual capacitance can decrease hugely,
leakage current increases - I have replaced caps that measured 10s of ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

My Philips TV is made in early 1976 and still going strong.
I use it every day and all capacitors is original!
World record?

 

[Terry Given]

"Terry Pinnell" <terrypinDELETE@THESEdial.pipex.com> wrote in message
news:7em0b09sdjrstjrglkp0p14b79lk8aqs43@4ax.com...

"Terry Given" <the_domes@xtra.co.nz> wrote:

If its 23 years old, your electrolytic capacitors are in all likelihood
past
their best. Electrolyte eventually "dries up" and the hotter the cap gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only
candidate
for this failure mode. Other things happen when the cap is not charged
for
long periods of time (google electrolytic capacitor re-forming if you
want
to find out what, how and why). The actual capacitance can decrease
hugely,
leakage current increases - I have replaced caps that measured 10s of
ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

Thanks. They *look* OK, but from waht you say I suppose it could be
that in the earlier stages of my testing they were reforming. Latest
results seem to be OK as you probably saw.

Interesting that my '1000uF' is actually about 700uF!

--
Terry Pinnell
Hobbyist, West Sussex, UK

I shot myself in the foot with a lousy electrolytic once (metaphorically
speaking - I did however destroy the caps in $3,000,000 worth of ac drives
once but thats another story).

The circuit was a 5V voltage reference using the humble TL431, running from
12V. I had a 5k resistor, so about 1.4mA, and 2 x 10k resistors for the
voltage divider. As you may well know, TL431's will oscillate with a certain
load capacitance - between 1nF and 100nF is the danger area (TI have some
nice pictures in their data sheets). So I used a 10uF smt electrolytic cap.
And the damn thing sang like a canary. Why? well, the 10uF 25V electrolytic
cap I used had 27 ohms (yep, no decimal point). I measured it to be so,
using a step-voltage from a 50 ohm source, edge showed step to 1/3 of Vstep,
then RC charge to Vstep (thanks Howard Johnson). I got an OSCON cap (0.02
Ohms) and a series resistor, and found >= 10R was where oscillation started
(froze it and baked it too, with freeze spray & heat gun). Actual solution
was to buy a better cap, around 1R ESR. and forever more, look at the ESR of
every electrolytic I ever use. In general, pick 105C caps - the sort of caps
you get from hobbyist stores tend to be really crap, and if the data sheet
doesnt mention ESR (or ripple current), you can be sure its very high (very
low).

I used to repair monitors and smps for a living before I went to uni and
became an engineer. After about 6 months, I learned to replace all
suspect-looking electro's - swollen tops (those grooves are so the case
ruptures there), goo leaking out the bottom, etc. If i had a hard-to-find
fault in a particular area, I would replace all the electros there, THEN
look for the fault. between that and fixing all the broken tracks/solder
joints, probably 75% of all faults had gone by the time I looked for them.
caps are cheap. time is not.


Cheers
Terry

 

[Terry Given]

"Ken" <___ken3@telia.com> wrote in message
news:j141b0habq6hngeeubv353ffdumteu2n3k@4ax.com...

On Sun, 23 May 2004 19:13:59 -0700, "Terry Given"
the_domes@xtra.co.nz> wrote:

If its 23 years old, your electrolytic capacitors are in all likelihood
past
their best. Electrolyte eventually "dries up" and the hotter the cap
gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only
candidate
for this failure mode. Other things happen when the cap is not charged
for
long periods of time (google electrolytic capacitor re-forming if you
want
to find out what, how and why). The actual capacitance can decrease
hugely,
leakage current increases - I have replaced caps that measured 10s of
ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

My Philips TV is made in early 1976 and still going strong.
I use it every day and all capacitors is original!
World record?

good design, good components. When I used to design AC drives we used Vmin,
Pmax (gives Iripplemax) for 3 years continuous operation at Tmax. Never in
30yrs had a cap "worn out"

I once reverse-engineered a tiny (1kW) toshiba drive. the bus cap was a
little electrolytic, 10mm diameter, 45mm long. It clearly wouldnt last long,
and usre enough, in the service section at the back of the operating manual,
it recommended replacing the bus cap every year That particular drive
failed catastrophically - we put a 2kW overload and left it, and it happily
blew itself to pieces (you can apply a dead short to the output of our
drives, and they will operate continuously). This was when our GM was asking
us how come they were so cheap. The answer was simple: mass manufacture,
high purchasing volume and low quality.

Terry

 

[Jan Panteltje]

On a sunny day (Sun, 23 May 2004 13:52:59 +0200) it happened Ken
<___ken3@telia.com> wrote in <j141b0habq6hngeeubv353ffdumteu2n3k@4ax.com>:

On Sun, 23 May 2004 19:13:59 -0700, "Terry Given"
the_domes@xtra.co.nz> wrote:

If its 23 years old, your electrolytic capacitors are in all likelihood past
their best. Electrolyte eventually "dries up" and the hotter the cap gets
the faster it does so - this is why smps caps crap out all the time, the
ripple current is high and they cook - in your amp C1 is the only candidate
for this failure mode. Other things happen when the cap is not charged for
long periods of time (google electrolytic capacitor re-forming if you want
to find out what, how and why). The actual capacitance can decrease hugely,
leakage current increases - I have replaced caps that measured 10s of ohms.
From your perspective, just replace them as a matter of course (most TV
techs do this before looking for faults).

My Philips TV is made in early 1976 and still going strong.
I use it every day and all capacitors is original!
World record?
Dunno if world record, I have some old Philips electrolitics from that time

in the box, and they are fine.
Electrolitics did often go bad also in Philips TVs.
I have an old Crown transistor BW portable, from the seventies I think, it
works, but needs an other component transplant now, vertical is jumpy.
JP

JP