Repairing Acoustic G60T Amp Head

[ogeecheeman]

A fellow brought one to me after he tried a recap.No electronic background at all.What a mess. I did however,get it repaired or so I thought-Squeal issue.Replacing the twin 40uf nailed it. Sounded great.A month later customer is back. Says volume slowly dies after it's been on a while. Haven't seen this happen yet but...Fire it up and with amp on Vol 1 setting I have crosstalk between both channels.In other words both channel pots (including Vol2 drive pot)are operating at the same time in Vol1 setting. Switching to Vol2 setting seems to work normally with no interaction from Vol pot 1.And still no gradual output decrease. I have no change in 1 khz sine wave at junction between (R38/R39)1meg resistor and 1meg pot at output of Vol2 regardless of what position (Vol1-Vol2) switch is in.If I understand this circuit correctly that signal should go to ground through the Vactrol and die when Vol1 is switched on.I have 3 VDC at switch.It does drop off at the switch when I switch it.LED's DO CHANGE correctly corresponding to the proper channel. Can you tell me if Vactrol on main board corresponds to Vol 2 circuit? Seems like Vol2 circuit not shutting off. When switching to Vol 2 I do see Vol1 drop off at Vactrol on front panel control circuit board. Can you tell me where I may find specs or schematic on the Vactrol?
Thanks
Gordo

[leydenjar]

First tube amp I owned. Got 'er in 1988!

I think I'm gong to have to find one of those just for the sentimental value.

Learned all my Sonic Youth and Pixies tunes on one of those!

[52 Bill]

Welcome to the place.

...Fire it up and with amp on Vol 1 setting I have crosstalk between both channels.In other words both channel pots (including Vol2 drive pot)are operating at the same time in Vol1 setting. Switching to Vol2 setting seems to work normally with no interaction from Vol pot 1.

The Vactrol that is supposed to ground the input and the output of the Vol2 circuit is not working, probably due to a burned out bulb. On the schematic this is marked V2.

Can you tell me where I may find specs or schematic on the Vactrol?

The original part was a Vactrol VTL9A10/2 which means it was designed to work with a 9 volt power source and it contains two cds cells. If you read through the thread, the burned out bulb can be replaced with a 10 or 12 volt hobby lamp.

I usually remove the unit from the pc board and then carefully pry open the spun aluminum cover, exposing the two photocells and the lamp. Then I snip out the dead bulb and solder in a new bulb. If you don't want to remove it from the board, you can carefully cut off the cover, solder in the new bulb and then glue the old cover back on or cover the assembly with some insulated tape to block out the ambient light.

[ogeecheeman]

here's the thing.On the control board the 20vdc goes through the 150ohm resistor then the zener.right there on the board it says 10v.I'm reading 3.5vdc. Something tells me that isn't correct.I pulled the Vactrol and still I get 3.5vdc. Bad zener?

[52 Bill]

Either a bad Zener or something is pulling down the voltage. I'd check the Zener first.