Polytone Mini Teeny Brute Distorted Output

[misterc57]

This Mini Brute has distorted output when playing a guitar.

With distortion set off, sounds are cracky, static, distorted and jumpy output, starting at low volume and on up. Person I bought it from thought it had a bad speaker.

I found...

no DC on output
this amp has same issue when connected to an external speaker
this amp's speaker is fine connected to a different amp
this amp's main output (preamp out) sounds fine connected to a different amp

I see a clean sine wave at this amp output (at all volume levels), when injecting a 1K signal at the input. See scope shot

So it seems the preamp section and speaker are fine in this amp. But why the clean 1K output signal on the scope? Is it because it is a constant signal unlike a guitar signal?

The scope image and testing was done with the volume around 3, tone controls at mid point and distortion set to OFF.

Any suggestions and does anyone have the schematic for the power amp section of this amp?

Thank you, Mark

[glebert]

Does the 1kHz sine sound OK when going to the speaker? I would also try a triangle or square wave to see how it handles transients. Also, the scope shot appears to be a pretty small signal, only a couple volts peak-peak. I would go into a load with something closer to full range to see if there are voltage points where things go haywire.

[misterc57]

Yes 1Khz signal sounds steady going to the speaker.

I connected a guitar signal from a Peavey preamp to the Poly RCA amp input at the chassis. Getting the same bad sound.

This has pretty much isolated the problem to the Poly amp and the Poly preamp and speaker are fine.

[Delta362]

See if this schematic matches your power amp

[Jon Snell]

Signal voltage measurements would help.
Sig generators are stable guitars are very peaky.
Unplug the amplifier input J1. Plug that into your test amplifier and note the results.
Plug a signal into the input socket S1 and note at what level the distortion start.
What sort of distortion ... you have no indication.

The amplifier is generic from most of the other Brute series. Plenty of punch and presence.

I had one the other day, in my workshop, that distorted. The cause was possitive feedback from a badly earthed reverb tank.

[misterc57]

V measurements coming.

Just got the chassis out (boy is that tight to contort out of the cab).

Not finding an exact schematic match yet. Attached one that may be close.

This one has a LM391N-80 and MJ4502 / MJ802 finals.

[misterc57]

So yes earlier I had a low 1KHz input signal (maybe 50 mV), which I bumped up to 175-200 mV range.

I now see distortion on the scope with the amplifier volume set to "1", see image.

Also I am seeing the following V on the board, supply V look good.

LM3901N-80 pin 16, -33V
LM3901N-80 pin 15, +33V

MJ802 -33.6V, -33.5V , 29mV

MJ4502 33.6V, 33.5V, 2.2mV

Still looking for the proper schematic.

The output of the amp sounds fizzy, faint, crackling.

Thank you all.

[The Dude]

Any DC on the output?

Edit: Never mind. I see you already checked that. Sorry.

[The Dude]

You have over a volt from what I presume is B-E on the outputs. You didn't specify which pins of the transistors are B-C-E, so not sure. Either the transistors are reverse biased or possibly open. You should only have around .7V B-E. I think this might be the problem and your driver transistors are driving the speaker instead of the outputs. I would check the output transistors.

[g1]

The IC in your picture matches the schematic. It's possible someone used subs for the finals in a previous repair. The drivers look like they are probably U07/U57 style. Are there other discrepancies to the schematic aside from the finals?

[g1]

Is this schematic a better match?

[misterc57]

Is this schematic a better match?

Yes that looks to be the correct schematic. Thank you

[misterc57]

Proper schematic attached. Q4 seems backwards from schematic indications.

Q3 MJ4502 E: 33.6V
Q3 MJ4502 B: 33.37V
Q3 MJ4502 C: 2mV

Q4 MJ802 E: 30mV
Q4 MJ802 B: -33.5V
Q4 MJ802 C: -33.6V

There is a repair on the Q3, see image, small component screwed onto the Q3 can. Black and a red wire are taped on the inside of chassis. Has number 19C on it, perhaps a transistor?

How do I pull the board up to see the bottom? Unscrew the can caps?

Thank you

Polytone 378 Power Amp Schematic.pdf

[g1]

Yes, you would unscrew the can caps.
The part mounted on the transistor is for thermal tracking. I would think it is the diode pack CR2 ?
Are you positive about the pinout for Q4? Is R19 ok?
What are the voltages for Q2?

[misterc57]

Yes, you would unscrew the can caps.
The part mounted on the transistor is for thermal tracking. I would think it is the diode pack CR2 ?
Are you positive about the pinout for Q4? Is R19 ok?
What are the voltages for Q2?

R19 is showing 4K in circuit, compare to R18 showing 0.28 R

Removed R19 and out of circuit it reads 1R