Woogie SS power amp troubles

[Swampy]

Hey Everyone,

I'm currently troubleshooting a 'Woogie' hybrid guitar amp (made in South Australia) and have hit a brick wall in my thought patterns.

The power amp is comprised of two pairs of npn transitors, the bases of the pair with collectors connected to v+ (lets call them Q1 and Q2) are connected to the emitter of a pnp transistor (Q3) and the bases of the pair with emitters connected to v- (Q4 + Q5) are connected to the collector of an npn transistor (lets call it Q6).

Symptoms are:

Low voltage - 20v from v- to v+, should be about 60 (I'm using a lightbulb limiter)

Reading of .023 volts on the base of Q6 relative to V-, even though it's tied to 'theoretical ground' (the point that the r22 ballast resistors all star at, and is coupled to the output jack via a big fat cap) via a 150 ohm resistor.

Also, 'theoretical ground' measures at only 3.3 volts relative to v-, shouldn't this be more like about 10volts (halfway between v- and v+).


So, if I pull Q6 my [ v- to v+ ] reading drops even lower to 3.1 volts and current draw is epic, says the brightness of the lamp. Voltage proportions seem about right though (all be them super low), theoretical ground is at 1.5 volts, base of Q4 and Q5 are at .7 volts.

Then, if I lift the connection to the bases of Q4 and Q5 [ v- to v+ ] rises to about 55 volts.



All power transistors test good and Q5 has been replaced already (as the old one measured slightly funny), Q3 and Q4 are also new.

I've been over this one with a fine toothed comb and am going a little crazy here, can anyone lend some insight?

I can scan in a schematic if need be, you probably won't find one on the net.

Chur,
Swamp

[Jazz P Bass]

Please post the Woogie schematic.
My head hurts reading what you posted.

[Steve Conner]

It doesn't make sense unless Q6 is actually PNP and Q3 is NPN. Then it's just a standard quasi-complementary output stage. Double check what the transistors actually are and what they should be - has some newbie put the wrong ones in by mistake?

Also post the schematic. I've only seen one Woogie in my life, in the hands of a touring band from New Zealand.

[J M Fahey]

has some newbie put the wrong ones in by mistake?

I guess that would be the OP
Sorry.
I also think both output transistors are NPN, (the output capacitor says it's an *old* design), the "upper" driver is NPN and the "lower" one PNP.
What the OP calls "theoretical ground" must be the speaker output rail.
Of course we still need the schematic.

[teemuk]

Schematics are always nice, especially rare ones that are otherwise hard to find... but honestly, even without one, based on description, we can follow at least this far:

Basic quasi complementary output with two parallel pairs of NPN outputs, ordinary NPN/PNP driver scheme, the branches marked "x" likely hold at least a resistor but with a good chance also some other components like caps and diodes. Since output is reputedly capacitively coupled the V- rail is likely common but - to be honest - the description on this part didn't make so much sense... the mentioned 150R resistor, could be a ground reference in case speaker is disconnected... but it also could be misinterpreted part of Zobel, feedback, bootstrap, ...or anything. The description was a bit vague at best.

But we should still know the basic topology for the output and from that we should be able to assume few reasonable readings based on the circuit.

The node before the coupling cap to speaker should indeed read half of the voltage measured from V+ to V-. Base-emitter junctions should be naturally about 0.6V above or below the value, the bias string in between driver bases should have some reasonable voltage difference, sufficient enough to bias the output stage.

[J M Fahey]

I'd love to have the original scanned (or hand traced) Woogie schematic, not for us but for Swampy himself, so we all can talk about the same part, suggested voltage readings, etc.
Otherwise the explanations can be *very* confusing (see original post)
As of the partial schematic posted above, is not a good example because it has ballast resistors on the lower transistors collectors; to do their "ballast" job they must be in the emitters.

[Jazz P Bass]

I can scan in a schematic if need be, you probably won't find one on the net.

Chur,
Swamp

Post the scheme please.
As we patiently wait.........

[Swampy]

Hey Everyone,

Thanks to you all for your advice so far - I gotta apologize for taking my time to reply, life has been happening.




Here's my scrawlings of the part of schem we are interested in as it was when bought to me - I'm a hundred percent my cartology is accurate, whether or not the circuit is functional is another matter (and in this case it obviously isn't).



It seems a little retarded that the base of Q1 is connected to V+ through a 100R, and as some have already mentioned Q3 and Q6 should be reversed - if someone can explain how that would be applicable in this situation twould be grand.

And yes, I'm relatively knew at transistor circuitry but have been working with tubes for a while now.


Also, Steve, do you remember the bands name? I'm a kiwi, not many of our fine local talents make there way together offshore, it seems to cost a lot to travel from here to there (but somehow much less to go the other way??).

Swamp

[teemuk]

Google: compound / Sziklai pair and Darlington pair.

Learn how each pair can be used to make up a circuit behaving either like a NPN or a PNP transistor. Darlington is simple: You have emitter of the driver connected to the base of the second transistor. In Sziklai you have collector of the driver connected to the base of the second transistor. The driver determines the overall "polarity" and the phase inversion makes the following transistor "play along".

All in all, the drivers and the output transistors in combination need to form a NPN/PNP transistor pair to the output - that's pretty much how a push-pull buffer can ever work.

That is, unless you have an interstage coupling transformer to phase-invert the signal coming to the other half of the buffer. Sort of like a Sziklai pair but with phase inversion handled with a transformer instead of a transistor. Of course your schematic doesn't show the voltage amp configuration, nor the biasing scheme. Didn't you mention you can scan the factory schematics?

Now what you have in that sketch looks like some strange attempts to form Darlington or Sziklai pairs but then doing it all wrong and mixing them all together in some bizarre manner, which I doubt could ever work.

How much dubious work has been done to that thing already in its history..? Is it a safer bet to start completely from scratch?

[J M Fahey]

Agree with Teemu.
Rebuild that otput stage as:

and play it safe.
Try to draw the rest of the power amp, the input and biasing part, so we have no surprises.
Good luck.

[Swampy]

Thanks for your advice folk. I've rebuilt as per the attatched schematic, but there's still an issue to iron out - the speaker output node is being pulled up to v+, and I suspect
Q9 has something to do with it. Originally the collector and emitter were reversed, but that would mean the c - e junction would be reversed biased and cease to operate in the correct manner wouldn't it?

Also, speaker output node was being pulled up before I connected Q1 - 6.

Oh, and the unvalued block above the 330R I've not come across before - it say L3K5 on the component, but I don't think it's a resistor.


Q10

b 0.587 v
c 15.19 v
e 0.00 v

Q9

b 54.8 v
c 54.6 v
e 54.5 v

Q8

b 54.6 v
c 55.9 v
e 54.9 v

Q7

b 54.1 v
c 54.9 v
e 53.5 v

[J M Fahey]

Q9 must be NPN (BC547/546), emitter "down" and collector "up"; Q8 is PNP (TIP32C), Emitter "up", collector "down".
The lower pair of .22 ohms ballast resistors must be on Q4/5 emitters.
They might even have been as you draw them in the original amp, it's a common error.
The feedback resistor *must* be connected to the "speaker node" on the leftof the output capacitor, not on the right.
The DC discharge resistor at the output must always be connected to the right side of the output capacitor, but to the left of the relay, or it will pop badly when the relay works.
The 100 ohm resistor + 100 (unreadable) capacitor in series should be 10 ohm in series with 100nF (0.1uF) capacitor.
The 330 ohm in series with an unknown element are not needed, can be pulled out.
With these mods the amp should work.
Please label all parts, it's easier to talk about "R12" than "the one on the left of ... and below ..."

[Swampy]

Right. Mother of all fuckups in that case- V+ and ground connections to the board were reversed (the two come from a rectifier bolted to the chassis, via the power transistors and solder to spots on a etched board).

So I've rebuilt (again), I got the power transistor section and drivers down now, but there's still a problem earlier in the circuit - now it's pulling the speaker node down to about 10 V.

Also, I can solidly vouch for the fact that I've probably put a transistor in the wrong way, no matter how many times I check and double check there's something fundamental missing in my knowledge that keeps me from stabbing in the light.

Anyway, can you explain the functions of Q8 and Q9 - I've gathered that Q10 raises the DC to bias to the base of Q9, and that Q9's role is negative feedback orientated. Does that make Q8 a straight up buffer?

Also, I'll scan in a new schem in the morning.

Swamp

[Jazz P Bass]

Also, I'll scan in a new schem in the morning.

Swamp

Yeah!

[teemuk]

The 330 ohm in series with an unknown element are not needed, can be pulled out.

No. It's most likely a HF compensation (resistor+cap). There is no "miller cap" around the voltage amp transistor, so instead there's plenty of HF compensation implemented to global feedback loop, base of the voltage amp, and on the base of the NPN driver. All in all, the thing seems like a design that might be very suspect to oscillation so I wouldn't start removing any of the compensation stuff. If it works, don't fix it...

Also, the feedback loop coupling can no way be that low in capacitance. Similarly the HF shunt cap at voltage amp's base seems excessively high value in capacitance. These likely should be reversed and I think this is once again another error in tracing the circuit.

I'm quite sure Q8 and Q9 are also drawn to have incorrect polarities, which should be reversed. The circuit very much looks like the typical topology with one input transistor driving one voltage amp transistor. The common circuit that was used before using full differential input became almost a standard. In such, NPN input stage with global feedback to emitter, would drive a PNP voltage amp transistor that has a bootstrap load on it's collector. Collector signal would then be buffered by the output transistors. I could be wrong but all circuit elements at least hint very strongly to such scenario.

The odd transistor thingie biasing the input doesn't seem to make much sense either.

In all honesty, I think we're throwing around too many suggestions without being too familiar with the actual circuit. So far the sketches have seemed to be full of errors.

I'll further on retain myself from this until I see a factory schematic. Otherwise this seems to take a route of completely "shotgunning" the amp and rebuilding it along the way.