Marshall Bass 60 needs help

[mirko]

Hi there I'm new to the community, hello!

I found a Marshall Bass 60 which is playing fine until a certain point (after 1hour or so) when it drops in volume (power?).
It never goes off completely, but stays down for a while, pumping up and down.

- Is it the preamp?
- or power amp?

I have changed the potis, well I checked them and re soldered them. At first I thought it fixed it but now the same thing again.
It's not distorting or anything, just volume.

If it would be the power-amp losing power, wouldn't it distort?
So I lean more towards checking the preamp stage...

I got the schematics here: Marshall 5506 STD, it uses TL072 ICs, for the power section 2x MJ3001, 2x MJ2501.

I just found out that sometimes also the 10kHz EQ Knob puts it into distortion - alot, but that's a new thing to me right now.

Help appreciated.

Mirko

[Jazz P Bass]

The very first item on my troubleshooting check list is the Power Supply.
The main + & - rails should be steady & have a minimum of Vac ripple.
The opamp + & - power rails should also be rock solid.
The Vac ripple on them (ideally) should be zero.
When the amp misbehaves, what is happening to the rails?
If nothing noticable shows up on the power supply, then you can divide the issue down to power stage or preamp.

[mirko]

AC +-27 V ready
DC +- 36.5 V steady
DC Rails Op-Amp on 15.0 one 15.4 V all the way through pass the point when the volume drops to very quiet.

then, fiddling around on the EQ or Volume, or tapping around on the PCB all of a sudden comes up again. This is very pleasant when Volume is all the way up.
but then, I leave it alone, walk away 5 to 10 meters, and i t sets out again.

I, somehow think, it's not the knobs but then again, there is current flowing through them, maybe they are screwed beyond fixable.
I noticed at the volume know, that one I had taken out, when measuring a real raise in ohm when turning.

Now the amp is playing very quiet for the last 20 minutes.

The Voltages are all cool, Op-Amp has gone up to 15.2, 15.6 V

Mirko

[g1]

Sounds like it could be dirty pots or jacks, or possibly a bad solder connection on the circuit board. Try cleaning the pots and jacks with appropriate type cleaner. Especially FX loop jacks or power amp in/out jacks if it has them.
See the sticky post about switching jacks (at the top of the maintenance page) for more info. about the loop jacks.