Silvertone Solid State 100 (similar 1423)

[Mars Amp Repair]

Hello folks,
This amp is haunting me! It motorboats. Here is what I've done:
1) Unit is motorboating. NOTE: It does not motorboat on load.
a. Unaffected by any of the control settings.
b. Bridged all electrolytic caps to no avail.
c. Added the Zobel network that is on the schemo but not in unit as it didn't motorboat on load, to no avail.

2) Disconnected signal path by lifting C9 0.047uf coupling cap to no avail which tends to indicate that the issue is strictly in the audio power stage.
3) The output transistors are the original 3155 variety and the others are also original.

I can't seem to get the schemo to upload today, but it is available at Electrotanya.com under Silvertone 1423 which is not an exact version, but very close. https://elektrotanya.com/showresult?...a=All&kat2=All
I can't get the URL to upload either. Maybe the one I pasted will work.

I'm kinda stymied. Any ideas where to go from here would be greatly appreciated!
Thanx, Glen

[TomCarlos]

I saved the schematic in JPEG and PDF. Here you go.

Silvertone_1423.pdf

[g1]

Bridged all filter caps at once, or one at a time?

[Mars Amp Repair]

Bridged all filter caps at once, or one at a time?

I initially did them one at a time, then decided as old as the caps are, to tack-solder them all in. As I mentioned, to no avail. I'm going to have to put more attention to the audio power stage as disconnecting C9 brought no difference in the motor-boating. I actually have a 2nd one of these amps the customer supplied that is a parts donor. So far no need for it other than physical comparison, as it is inoperative.
glen

[Mars Amp Repair]

I saved the schematic in JPEG and PDF. Here you go.

[ATTACH]n999229[/ATTACH]

Thanx, Tom.
Glen

[Mars Amp Repair]

Well, here it is.
I had isolated the issue being from the power amp stage by disconnecting C12 in the signal path to the power stage. Upon focusing on the power stage, it turns out the cathode bypass caps C15 and C15A 4000uf @ 2.5V each were completely missing from the amp with no evidence that they were ever there (I'm sure they were there initially). Admittedly, I completely missed them on the schemo. So that resolved the thumping issue.
I will be suggesting this customer just have me replace all the electrolytic caps in this amp, which is what I should have done to begin with rather than try to cater to his budget (;- ).
Thanx all....Glen

[Helmholtz]

Well, here it is.
I had isolated the issue being from the power amp stage by disconnecting C12 in the signal path to the power stage. Upon focusing on the power stage, it turns out the cathode bypass caps C15 and C15A 4000uf @ 2.5V each were completely missing from the amp...

Confused.
Cathode caps?
What amp/schematic are you talking about?