Polytone 104 power amp DC offset

[Delta362]

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:

I'll try the shorting jack and let you know what I find out.

[Delta362]

No change to the DC offset when using the shorting jack. I remembered I had an extra Eminence Man o War speaker stashed that I could try. Powering on and off, as well as signal passed thru fine. When going back to my test speaker setup (Eminence TX Heat), it had that sizzle sound for only about a second. Then it played fine, but also gave that sizzle sound when powering off.

Looks like the power amp is working properly. Both channels on the preamp seem to work fine, as does the vibrato. Now to figure out what's up with the reverb, but that'll be another thread if I get stuck on it. Thanks for your help!

[Dave H]

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 ?

Try changing C4. It's been running reverse polarity. Connect the new cap + to ground - to Q2 base. I think the polarity on the schematic is wrong for C1 and C4

[Delta362]

Interesting observation. When I replaced the caps, I followed the same orientation as the originals as well. I've attached the photo of the original way it came to me.

[Helmholtz]

A DC-voltmeter across the cap will show if the polarity is correct.

[J M Fahey]

80 something mV offset is fine, not worth trying to correct that.

I would replace C4 50uF NFB capacitor by a fresh one, it might be leaky.

Please explain how you are measuring and numbers you get, you can NOT get 110W RMS into 8 ohms with meager +/-40V rails.
Not without squarewave distortion that is, are you scoping the output?
Notice suggested load is 2 ohms for "220W" (which are still somewhat optimistic)

Old circuits like this often thump, and sometimes Op Amps squeal when turned off, when supply voltage decays they become unstable.

[Delta362]

Thanks for the comments!

As has been suggested, I'll be replacing C4.

Regarding the power numbers, I did scope it, but mistakenly interpreted my vertical scale. Not a chance it's well over 110W. My apologies.

[Delta362]

Try changing C4. It's been running reverse polarity. Connect the new cap + to ground - to Q2 base. I think the polarity on the schematic is wrong for C1 and C4

Dave, the schematic shows the - terminal for C1, not the positive like most times I've seen electrolytic caps drawn in a schematic. Are you suggesting the + side of C1 should be on the incoming signal or on the R1 side?

[Helmholtz]

Connect the preamp and see post #20.

[Delta362]

Connect the preamp and see post #20.

Will do. Thanks!

[Delta362]

I measured all the voltages on the electrolytics first. C1 and C4 polarities were opposite of the cap. The others were correct. I replaced both C1 and C4. In both cases, the magnitude for C1 and C4 were less than 100mVDC. The output voltage still stayed around 80mVDC.

[Dave H]

Dave, the schematic shows the - terminal for C1, not the positive like most times I've seen electrolytic caps drawn in a schematic. Are you suggesting the + side of C1 should be on the incoming signal or on the R1 side?

Yes, + side of C1 to the incoming signal - to R1. The base current of Q1 causes a voltage drop across R2 taking the base negative. It's the same for Q2. If the output is at 0V the voltage drop across R8 takes the base of Q2 negative.

[Helmholtz]

Acc. to literature Al Ecaps can stand a continuous reverse voltage of - 0.6V (@ elevated temperature) to -1.5V (@ room temperature).

[dmeek]

I believe the bases of Q1 and Q2 are at 0V, the emitters are negative.

[Helmholtz]

Both bases are referenced to 0V via 10k resistors.
As Dave H explained, the base currents will draw the bases slightly negative as measurements confirmed. Speaking of less than -100mV.
Emitters will be around 0.6V more negative.

If the transistors have different base currents (due to different Hfe), the base voltage difference will cause a DC offset at the output.

Higher Hfe means less base current and thus less negative base voltage.