Marshall 8020 motorboating?

[Xico]

Hi, new here from Portugal.
I'm trying to recover a 8020 with a lowd thumping noise without anything plugged.
Turning the pots don't change the noise and using the headphones give the same output as the speaker.
I cleaned the pots and inputs, resoldered all the points and replaced all the electrolytic capacitors without changes in the noise.
Please, someone can help me repair this little amp?

[blindboybenton]

Hi and welcome
You say motorboating, ? Is this a pulsing tone or constant
Are the Voltages stable ?
My guess is on the Power chip , Try and scope the input to the chip, If you see no of noise on input most likely the power chip
Its a common issue with all the valve states
Marshall-Valvestate20-20W-8020-Schematic.pdf

BBB

[Jazz P Bass]

Here you can find the IC: https://www.digikey.gr/products/en/i...o/742?k=LM1875

[Xico]

Thanks!!
The noise is a pulsing tone, I will check the voltage and scope the input.

[J M Fahey]

Doubt replacing the 1875 will change anything, they are all the same.

If anything, I would suspect poor grounding (poor soldering) or an open Zobel net (R42 - C35).

I can imagine no reason why original LM1875 would "degrade" , and if unstable/ oscillating when new that would have been noticed long ago.

In any case I can suspect original degraded electrolytics and perhaps poor quality replacements (being "new" is not guarantee of anything, and they might be NOS) ... or poor soldering when doing so.

But .... an LM1875 becoming unstable? .... can´t even imagine how.

[Xico]

Thanks all!!
I don't have a oscilloscope, I could try to make a soundcard one. I think the soldering is ok, but I will replace again the electrolytics to be sure. I will check R42-C35 too. Until tomorrow I can't get a LM1875.

[Xico]

I'm going insane, only now I'm seen a lot of caps spaces (Cxx) populated with resistors .......
But after I replaced all ... the problem persist

[nevetslab]

I'm going insane, only now I'm seen a lot of caps spaces (Cxx) populated with resistors .......

Can you elaborate on this? This is Cxx part locations on the PCB, now occupied with resistors? Which ones, and with what values? I can see how someone might have wanted to DC-connect the stages, rather than having coupling caps between stages. Such as C19 replaced with, say, 100 ohms, or C28 replaced with 100 ohms, things like that. Now, DC-connecting IC1 & IC3 might be a problem, if this is being done. You might measure all of the IC Op Amp output terminals of the TL072's (pins 1 & 7), 1458's (pins 1 & 7).

[g1]

What colour are the 'resistors' in the capacitor positions? There are capacitors that look exactly like resistors except the body colour is different. The ones I have seen are usually green. If you measure them with a capacitance tester, they check out as capacitors, not resistors.

[Xico]

g1 was right, they are caps, green, yellow and pink with the same format and size of resistors.
Now I replaced twice the electrolytics, and the LM1875 and nothing change.

[J M Fahey]

1) I posted at The Gear Page how to use software scopes (no special card needed, just the built in soundcard) ; specially 2 attenuators, a fixed one and an adjustable one, so you can safely connect Tube and SS amp voltage levels to the weak and feeble soundcard Mic input which must be protected at all costs.
Both are clamping diode protected so no more than 700mV peak can pass through.
https://www.google.com/url?sa=i&rct=...41249428506440
I can not open that thread myself, my IP is blocked there, but have a look yourself.

The adjustable attenuator is:

The main point in these attenuators is the pair back to back diodes across soundcard input, burning it is not an option.

2) scope chipamp output and check for oscillations, which are seen as trace fuzzyness.
You will NOT be able to see a , say, 50 or 100kHz oscillation by itself, because soundcards are limited to 20kHz tops by definition, but scope trace will look weird, because the oscillation still interacts with the "visible" 1kHz audio signal.

3) short chipamp input to ground with a ceramic .1uF or .047uF .... any change?
We want to know whether presumed oscillation comes from unstable chipamp itself or is injected into it.

4) now that you have a scope, inject 100mV 1kHz sinewave at the input and follow it along the path, from input jack to speaker out, to find where it gets funny.