Acoustic 470 Repair Issue

[tpalmer]

Hey, I'm new to this site. I have an Acoustic Control 470 head I've been trying to fix up. Got it about 90% there but still one problem I haven't solved. There is a "ghost distortion" in the signal, not real prominent but definitely there. Not present when not playing, audible when listening to a note decay, and seems to have a hint of tremolo in it even with the Intensity knob all the way off. Distortion knob is off as well, so it's not the culprit.

Can anyone help? I've already replaced all output transistors with matched sets and replaced most/all the 40909 and 40910 as well. Thanks!

[gbono]

Is it crossover distortion? What do you see on a scope when looking at the output?

On another note - what volatge to read at the collector of Q401 - 3.1V????

[tpalmer]

Thanks for the quick reply.

Q401 collector is reading 2.75V. I checked the collector at Q402 and got practically nothing. Significant?

The scope shows strong oscillation in the signal, and definitely coming from the tremolo circuit. Oscillation rate tracks with the tremolo Speed setting. Again, this is with the Intensity off. When playing through the amp the oscillation is barely audible.

With the signal oscillating it's hard to tell if any crossover distortion.

Really appreciate any help.

[Enzo]

Please connect the amp to a different speaker and report on the sound.

[tpalmer]

Connecting to a different speaker yields the same results as before - no change in the audio. Some signal distortion with ghost tremolo most audible as the note decays.

[Enzo]

Look at the schematic, the collector of Q402 ia ground. SO I wouldn;t expect much voltage there.

If your trem won;t turn off, plug your test signal into the accessory return jack so as to bypass the preamp.

Where are you grounding your scope?

Disconnect the speaker. How much, if any, DC voltage is there on the speaker jack?

How much ripple is on the +92v supply?

How healthy are C403, C404?

[tpalmer]

Sorry for the late reply, had to go out of town.

I disabled the tremolo with a makeshift switch, no longer in signal.

Attaching a pic of my scope at the output, grounded to the negative terminal of output jack. Putting about 50mv to the input. Definitely seeing some crossover distortion, but appears to be offset somewhat.

Disconnected speaker and measured DC voltage at output - reads about 42V and diminishes over several minutes until less than 1V.

Checked C403 & C404 with an ESR meter, both appear to be healthy.

Haven't yet checked ripple at 92v supply - will do that next.

Thanks again, any clues based on scope view?

[Enzo]

That is what the bias control is for. Adjust it until the crossover notch just disappears.

[tpalmer]

AAAAAHHHHHH! That would make sense. It's always something stupidly simple that trips me up.

Thanks so much for all your help.