Polytone 104 power amp DC offset

[Delta362]

I'm working on a Polytone 104. I've uploaded the schematic, plus my measurements as a second pdf.

I'm having an issue with the power amp that I should know how to address, but am having a mental block on it. The version of the power amp I have is slightly different than the schematics I've found, though it's close. Notable differences are +/-41V power supplies, 2N5880/2N5882 output transistors, R4 and R5 are 6.8k, R10 is 2.7k (it's original), and D1 anode connects to Q4 base directly - there is no 100 ohm in that path. It also came in with two resistors that were burned - R18 (470ohm) and R11 (couldn't read the color bands). Q3 is in a finned heatsink, while Q4 and Q5 are in some special black housing allowing for the close coupled diodes on top for heating compensation.

D1 and D2 are also MZ2360 (I missed marking that on the schematic measurements pdf).

I recapped the power amp and guessed on R11 (I used a 604 ohm resistor) based on the higher stock value for R10 in this particular amp. I am getting about 0.230VDC on the output that I can't seem to dial out. I'm running with a speaker for now..

With no signal applied, there isn't much voltage drop across R14 (0.03VDC) so I'm interpreting that as Q5 isn't conducting enough. For reference, R13 for Q4 on the positive leg is dropping about 0.6V. I've tried a decade box across R24, then R10 and then R11. The only thing that made much of a difference to Q5 current is R10, but adjusting that also seems to drive the positive transistors more as well. I still am left with a small DC offset on the speaker output.

I put an 8 ohm dummy load on it and can drive the signal thru it. It amplifies the sinewave ok as my input signal increases, though that DC offset still exists. I'm not connecting a speaker to it until I can get it lower. Looking for some help on how to address this. Thanks!

[Delta362]

A couple other differences: It also has two 2N5880 and two 2N5882 output transistors. There are three 0.15 ohm resistors in parallel on the board where R22 and R23 would be. At the two output transistors of each side, there are two more 0.15ohm resistors connected in a Y. The ends of the Y go to each collector (Q6 and calling it Q8), while the junction part of the Y goes to emitter of Q4. Same concept happens with the negative pair (Q7 and calling it Q9) and Q5.

[dmeek]

A small DC offset can be caused by a mismatch of the input pair Q1-Q2. you could try swapping them and see if the offset changes polarity.
If you can, please post photos of the circuit board, both sides.

[Delta362]

I'll try swapping those input transistors. In the meantime, here are the front and back photos of the power amp board. The long blue wire across the front was in there when it came in, as were the two 10W 0.30Ω resistors to make up R21.

[Delta362]

And a couple photos of the output transistor wiring

[Delta362]

I pulled Q1 and Q2. Q2 has an hFE of 78 and 645mV Uf, whereas Q1 has an hFE of 172 and 630mV Uf. The way the board pads are and with how short the leads are, I'm not gong to be able to swap positions on them. Q2 pads are <| whereas Q1 pads are shaped like |>. The leads on them are too short and fragile to bend them in the other direction. Is there a suggested replacement transistor I could put in place of them? I have a fairly decent stash of a variety of transistors, but want to make sure I substitute with an acceptable one.

[Delta362]

I do have a pair of C1815O that have 120hFE and 713mV Uf. I realize the pinout is not quite the same, but would those be a viable substitute as long as I line up the EBC arrangement?

[g1]

Is there a suggested replacement transistor I could put in place of them? I have a fairly decent stash of a variety of transistors, but want to make sure I substitute with an acceptable one.

2N5066 maybe? It's hard to find a datasheet for the MPSA09 but it is included in the attached.

[Delta362]

I substituted the C1815 for Q1 and Q2. Now the output is down to 0.080VDC. I still have not connected a speaker to it. Is there anything else I should be looking at (check matching of the original output transistors with each other and with the other pair)? Do I play around with R24, R10 or R11?

[Delta362]

Well just realized when looking at the output waveform across an 8Ω dummy load that I have much more than 110W! Guess I actually have the 220W power amp with this one. I was using the 110W version by mistake.

I'll look over my notes and see where else I find any differences as this one looks pretty darn close to my values. I'll put the 680 in for R11 instead of my 604 that's there now.

[dmeek]

For the record, here is the MPS-A09 datasheet from my absurdly large databook collection.This from the Motorola 1969 Semiconductor Databook. MPS-A09.pdf

You could zero the output by adding a small voltage to the input like this, though I don't know how stable it would be. Matched transitors is the best.

[Delta362]

I checked the signal coming thru the amp and am getting a nice sine wave on the dummy load. DC offset was 80mVDC. I then turned it off and connected a speaker and powered on. Well that wasn't a very good experience. It seems there is a couple hundred mVDC that comes on initially and then settles down to the 80mVDC. The speaker let me know right away. When powering off, the offset then pulled towards 0 and continued into a few hundred negative mVDC before coming back to 0.

Would that imply the output transistors are not balanced? Would I need to ensure Q6 and Q8 are close to each other (as well as Q7 and Q9)?

Do I need the Q6/Q8 to be matched to Q7/Q9?

[dmeek]

Matched output transistors are needed to balance the current at high volume but shouldn't affect the DC offset. DC balance is done with the two input transistors. A leaky transistor could cause strange problems though.
80mV is not too bad. If it was below 50mV I wouldn't worry about it. Turn-on and turn-off thumps are normal.

[Delta362]

Thanks for the explanation on the matching requirements!

It's not giving the normal thump that I hear sometimes on amps. This is more of a sizzle sound that gets increasing in loudness for a couple seconds then quiets down. When turning off, it takes several seconds (much more than the turn on version) before it gets back to 0. That's what I'm worried about now. I'm hoping my speaker wasn't on the edge and this took it closer to its limit.

Is there something I should check that might allow for a faster decay?

[glebert]

Thanks for the explanation on the matching requirements!

It's not giving the normal thump that I hear sometimes on amps. This is more of a sizzle sound that gets increasing in loudness for a couple seconds then quiets down. When turning off, it takes several seconds (much more than the turn on version) before it gets back to 0. That's what I'm worried about now. I'm hoping my speaker wasn't on the edge and this took it closer to its limit.

Is there something I should check that might allow for a faster decay?

Since this is a separate preamp and power amp, you can plug a shorted RCA connector onto the power amp to eliminate the preamp from the diagnostic. (This assumes you are using the preamp during your testing).

Both of my polytones do weird things when powering off. Usually nothing for like 15 seconds and then a thump. :shrug: