ACOUSTIC 260 Bass Amp is noisy

[saint gillis]

Hi,
I'm repairing an Acoustic 260 Bass amp.
The power transformer has been changed for 230V operation, the supposed 70VDC are only 63VDC, which I suppose is not a big issue.

The power amp part is quite noisy (kind of filtered white noise), and the preamps also bring a very audible white noise.
Does this sound normal to you or not? If not what could I check?
(the amp has been recapped, and the output power transistors have been changed, and the calibration procedure has been done)

Here's the schematic

 

[Khron]

I'd follow the signal starting at the input, either with an oscilloscope, or an audio probe.

Not entirely impossible that some of the small transistors might've grown leaky / noisy over the years.

 

[saint gillis]

Thank you, I"ll try this

 

[saint gillis]

There was something dodgy around Q23, these old carbon comp resistors... I've changed R89, R90, R87, R88. Now the power amp is clean !
Still a lot of noise from the preamps.. I'm continuing

 

[CJ]

Looks like that model does not have a driver transformer which usually means a bit more noise,

 

[saint gillis]

The power Amp noise level is not awesome, but really more than acceptable
Now I've managed to bring the 1st channel preamp to an acceptable snr, I didn't change transistors but carbon comp resistors and these capacitors (tantalum I think, correct me if I'm wrong) :



Now let's work on channel 2..

 

[saint gillis]

This was a tough repair. Some transistors to replace, some resistors and of course the polarized capacitors, but it ok now

There's one last thing, when I switch the amp on and off several times (the amp being loaded), the fuse blows after 2-3 or 4 times, is this normal or related to the design?

 

[saint gillis]

Sometimes when the cab gets unplugged / plugged back, the main fuse blows.. I don't understand, maybe it's related to the output capacitor before the speaker?

 

[dmp]

I repaired my Acoustic 140 awhile back and those little axial black caps through me off too - they are in fact polarized and you want to replace all of them. Not sure if they are electrolytic or tantalum, but I put in electrolytic.
Blowing fuses is not normal. Replace any remaining polarized caps.
The 140 has a little inductor in the tone area that picks up a little hum. You can't get rid of it all.

 

[emrr]

I have a 370 that developed a wild oscillation that I need to get back to.

 

[saint gillis]

I repaired my Acoustic 140 awhile back and those little axial black caps through me off too - they are in fact polarized and you want to replace all of them. Not sure if they are electrolytic or tantalum, but I put in electrolytic.
Blowing fuses is not normal. Replace any remaining polarized caps.
The 140 has a little inductor in the tone area that picks up a little hum. You can't get rid of it all.

It happens only with the amp loaded...

 

[dmp]

I suggest replace all the polarized caps, including C64 1000 uF
The fuse blowing is protecting other parts. If a cap has failed to a short, it will pull too much current.

 

[saint gillis]

I have to check again, but I think they were all changed, C64 for sure

 

[saint gillis]

I think my problem was that the guy who worked on this amp before me put a 230V power transformer a bit over-sized, I've added a 10 Ohms thermistor and it's now alright !