damage to bass amp head?

[Rob Strand]

I'm puzzling over how you could have +/65V on the collectors AND have them short to ground!

I suspect they are false measurements through the power supply - I wouldn't be acting on that just yet.

The measured voltages are not characteristic of  a blown output stage.
Many things have to be working for the voltage to stabilize even to 0.65V.

The base voltages of TR1, TR2, TR4 are missing, you should check those first.  A stuffed front-end can cause weird problems like this.

However, the part that doesn't make sense is the voltage across the base-emitter of TR8.
Is the emitter voltage of TR8 positive or negative? 
If the TR8 emitter voltage is in fact negative, I'd suspect it might just the be front end.

Faulty caps C1, C2, C10 might do some evil also.

[Dimitree]

sorry, maybe I was not clear enough

TR3 and TR11 have their own heatsink, let's call hA, and their collector is shorted with hA. The collectors are at +65V, and indeed the voltage on hA is +65V (it is isolated from ground, there's no mechanical connection with ground, resistance to ground looks infinite).
The same with TR16 and TR17, they have another heatsink hB, and the voltage on it is -65V, same as their collectors.

Is that normal? Isn't that dangerous (if I touch the heatsink with bare hands?)

I couldn't measure the base voltage at TR1, but I probed on C2/C3/R4 and found that I can clearly hear my bass guitar there.
TR1 emitter/TR4 base is fluctuating,
TR2 base/TR5 emitter is fluctuating too.

Is the emitter voltage of TR8 positive or negative?
If the TR8 emitter voltage is in fact negative, I'd suspect it might just the be front end.

from a new measurements I did now, it is negative and not 1V/0.5V anymore 

I'm posting the schematic again since it's on the previous page and it is uncomfortable to turn pages

[Rob Strand]

TR3 and TR11 have their own heatsink, let's call hA, and their collector is shorted with hA.
...  The same with TR16 and TR17, they have another heatsink hB, and the voltage on it is -65V, same as their collectors

OK I understand what they are doing.  They do this to simplify mounting and to allow them to use slightly smaller heatsink and they don't need to mount the transistors using insulators - it's all done to reduce cost.    It also explains the unusual connection of the VBE multiplier (TR15).

Is that normal? Isn't that dangerous (if I touch the heatsink with bare hands?)

It's not a good idea to go touching stuff over 50V.  You could accidentally touch the other heatsink
then you are dealing with 130V DC.

I won't say it's not normal but it isn't common.   The use of separate heatsinks is very unusual for a high-power
amplifier.  In the 1970's quite a few kits//modules did this.  When you design amplifiers you need to prevent thermal runaway.  Usually that means all output transistors and the VBE multiplier all on the one heatsink.

from a new measurements I did now, it is negative and not 1V/0.5V anymore

Hmmm, it's hard for us to determine the fault without the correct voltages.

Eb7+9's suggestion that the TR15 is dead is a good place to start. 
Maybe pull it out and check it.
*** do not power the amp up if you remove anything ***
***  you will destroy other parts for sure                   ***

However, it does not explain the lack of sound.  With TR15 shorted it should only sound bad (fizzy).

[Dimitree]

I replaced TR15, and I since I found an old speaker, I took the opportunity to actually "hear" the issue.
after replacing TR15 that issue is still there, and now that I can hear, I can describe it:
I thought it was sounding like a 50/60Hz distorted hum (probably appearing 1 octave higher due to harmonics), but comparing with a 60Hz tone, I found it was different, and seemed more like 90Hz or 180Hz..but I need to measure to be more precise.
I immediately removed the speaker, I was afraid of blowing it, and furthermore you can't control the volume (as we already know, since it's from the power amp) so it is really annoying after some seconds, it would blow your mind if not the speaker.

so swapping TR15 didn't help,
I will check the voltages again tomorrow and let you know

[Vitrolin]

maybe the problem is in your power supply
if you have a oscilloscope you should check supply rails to see the ripple.

[wavley]

Is that really -.65V on your speaker out?  650 mV is a lot of DC offset for a speaker out and isn't really good for a speaker.  You should really be seeing something well below 50mV there. 

I would look at your +/- supply caps and see if there's something hinky there.

[Eb7+9]

I had another look at the schematic and R8 (300r) needs to see some voltage in order to turn on Tr7 and the rest of that branch (TR15 and TR12) ... so obviously there's some not happy with the input differential circuit

what's the voltage at R3 ...?

[Dimitree]

I don't have a capable oscilloscope to check the ripple (my cheap oscilloscope have a maximum input voltage of 50Vpp), but, checking the voltage with the oscilloscope over the zener (15.8V) seems totally stable (maybe it's obvious..I'm not that expert).
Also my multimeter doesn't show any major fluctuation on the +/- rails (maybe obvious that too).
Last, 24V powering the relay looks fine, with no evident ripple

the voltage at R3 is 0V when the bass is not connected (and as I said, I probed just there and I got nice and clean bass sound)
the voltage at both sides of R8 is the rail voltage, -65V.

There seems to be some 50Hz noise (1.7V amplitude!, about 0V to -1.7V) on TR1 emitter / TR4 base. Is that normal?
that does not happen on TR2 base / TR5 emmiter, where the signal is a stable -1.7V

[Eb7+9]

the voltage at R3 is 0V when the bass is not connected (and as I said, I probed just there and I got nice and clean bass sound)

can you explain this better ...

you land the probe at R3 and the amp starts working ??
I'm assuming you mean scope or meter probe

cold solder there ?!

if the first transistor TR4 in the diff-pair is not getting its base current
it would explain lack of current in R8, ... and dead Vbe multiplier ... etc etc

try re-flowing solder joints at top of R3 and base connection of TR4 input device

[Dimitree]

sorry for not having been clear about that.
I mean that I connected an audio probe to R3, and I could hear the bass nice and clear through another amplifier connected to the audio probe.

[PRR]

> cheap oscilloscope have a maximum input voltage of 50Vpp

For power supply ripple (and many other not-small sources): connect 100K and 1K as a voltage-divider from rail to ground, measure across the 1K. This is a 100:1 divider. (101:1 but we are not doing precision, just gross guesses.) A 50V power rail comes up as 0.5V. 10V of ripple on the rail comes as 0.1V. "Everything" is in range of 100:1 to 50Vpp. (Above 150V DC/RMS you need a >1/4W 100K resistor, do math.)

[Rob Strand]

Measuring the missing voltages around the TR1, TR2, TR4, TR5 will help a lot.
It's a 1 min job and will remove a lot of uncertainty.  It might even solve for problem!

[Dimitree]

so I took some time to measure the voltages using my cheap oscilloscope this time, instead of the multimeter.
this is what I found:




those are the signals marked as A,B,C,D



the noise on C and D is not from the oscilloscope, the oscilloscope can be a lot clearer (it shows an almost flat line when probing 0V), so I'd say that noise is actually there


and this is the output on the speaker jack



I removed all the other voltages since they all look like the output on the speaker jack (not stable at all, modulated by 50hz), so the measuring with the multimeter weren't really helpfull I think.
I still haven't got time to do the ripple measurement, will do tomorrow!

[Rob Strand]

I wasn't expecting to see waveforms like that!  Crazy stuff.

The waveforms at A and B look weird.  The voltage swings are high and A doesn't look like B, perhaps a result of the large voltages.

At this point I would be:
- putting a 100R or 1k load on the amp to see if the output changes.
- probing through the amp to see how at what point in the amp that exponential waveform starts and stops being present.

Then maybe,
- pulling out TR1 and TR4 and testing them.  If they looked OK then check TR2 and TR5.
In fact it might be easier to just replace them.

Another thing to keep in minds is some crap might be coming from the preamp.  Lift R1 and grounding it might help keep weird results to a minimum.

[R.G.]

Let's have another run at this. A quick look at the string of posts shows that you have probably, maybe tested transistors for shorts. OK, go to step 4.

It is quite difficult to selectively debug a power amp that is in a frenzy of some kind with high current output transistors throwing current around. So you need to get back to some stable, but low-power situation.

To recap what I said earlier, simplify the problem you're looking at. Take PL4 loose and temporarily short across it. This forces the input signal to be 0.00000V. Remove any speaker load. Now remove the output transistors, and temporarily short and temporarily tack solder a wire from pin 1 to pin 3 on TR16's empty position, and TR11's empty position. This lets the drivers (TR13 and TR8) act as a low power output stage, and the amplifier ought to work correctly into no load if the bias does not kill the drivers. To prevent that, temporarily short pins 1 and 3 on TR15. This forces the Class AB bias to zero, and makes the amp run Class BC. It will have some crossover distortion, but that's fine for the moment. Now you can check the amp for whether it puts out zero volts DC and passes audio signal without the output transistors detonating. You'll probably also want to disconnect that silly muting relay, or disable it by removing TR9 or R24.

With this set of things done, the amp should still come up and produce an output that is within 100mV of 0V at the "output", and pass audio, perhaps with some crossover distortion. If it does not, you have to start tracking back through the input and predriver stages to find out why.

You'll recognize that most of this is a quote from my original post.   

[Dimitree]

did a quick test today before desoldering the output transistors as suggested by R.G.

I connected a speaker, and the 50Hz buzz was there obviously.
then removed PL1 (the connector than joins the power amp to the preamp) and the buzz disappeared, and checking the output voltage with an oscilloscope, I noticed that it is a stable 200mV.

so the problem could be in the preamp? if so, that is weird, since the output from the "tuner out" is fine, and audio-probing on the PL1 connector I got a nice and clean bass sound.

[Dimitree]

found the schematic for the preamp:

‎bmamps.com-Schematics-Ashdown-Ashdown%20APC060%20preamp%20EB%20&%20MAG.pdf

so to recap:

1) if I connect my guitar to the input of the preamplifier, then connect LINE OUT to another amplifier, the sound on this other amplifier is perfect (this should tell us that the circuit up to LINE OUT is fine)
2) if I connect an audio probe to PL1/R1 on the power amp, and the output of the audio probes go to another amplifier, the sound is perfect on this other amplifier (that should tell us that the circuit till R1 on the power amp is fine)
3) if I disconnect the preamp from the power amp, the buzzing on the speaker is gone and the output is within 200mV
4) if I reconnect the preamp from the power amp, I immediately got back the huge buzz, and the output volume pot (RV9) cannot attenuate it.

to me 2) and 3) looks a bit of contraddiction 

[PRR]

found the schematic for the preamp:

I don't know what happened to your URL, but it won't work. Try this:
http://bmamps.com/Schematics/Ashdown/Ashdown%20APC060%20preamp%20EB%20&%20MAG.pdf

[PRR]

> if I disconnect the preamp from the power amp, the buzzing on the speaker is gone and the output is within 200mV

The buzz is in the preamp. Start with a Basic: are the power rails right and clean?

200mV DC is quite a lot for a speaker. It suggests you may *also* have a blown output stage.

[Dimitree]

the rails on the preamp are fine, no ripple at all.
the rails on the power amp may or may not be fine, I cannot determine it really well, I tried with the 100K:10K voltage divider, the scope shows a flat line, there is a bit of random noise (probably from the scope) and any 50hz noise

what I don't understand at all, if the buzz in in the preamp, why audio probing on R1 I don't get any noise at all?