The dreaded Acoustic G100/60T channel switching problem

[Zouto]

Hi, the channel switching on an Acoustic G100T suddenly stopped working: it changes between Vol.1 and 2 but doesn't switch off/short Vol.2 when Vol.1 is activated. Crosstalk between Vol.1 and 2 pots creates strong squealing when Vol.1 is activated. Switching to Vol.2 works fine. The LEDs are working properly, the footswitch jack is isolated from chassis. I suspect one of the Vactrols to be blown. So what to do, butcher the Vactrol(s) and replace the blown bulb(s) with a LED? I'm not quite sure, which Vactrol switches Vol.2 off.
So help is urgently needed.
Thanks a lot,
Zouto

[soundguruman]

you can get the vact spec sheet
you can make sure the "on" voltage for the vacts is correct and good.
you can power the vact with external power supply, and test the "on" resistance with an ohm meter.
after that, any bad ones become obvious.
you can replace any bad vacts which is advisable.

no I would not modify the vacts.
Just buy some new ones. Try Mammoth Electronics for deal on Vacts.

you can swap out the preamp tubes one at a time and try to eliminate noise or microphonics.

[Zouto]

Well, thanks, the old VTL9A10/2s are not produced anymore, the 'bay stuff is expensive, mostly used and unreliable. So I thought it would be the best to replace the obviously blown bulb inside the Vactrol thimble. I've swapped caps and tubes recently, switching worked fine without any squealing up to now.

[52 Bill]

Well, thanks, the old VTL9A10/2s are not produced anymore, the 'bay stuff is expensive, mostly used and unreliable. So I thought it would be the best to replace the obviously blown bulb inside the Vactrol thimble. I've swapped caps and tubes recently, switching worked fine without any squealing up to now.

You can rebuild the Vactrol if the bulb is burned out. It originally was rated at 9 volts, but any tiny 10-12 volt bulb should work. Take a resistance reading across the lamp terminals. It should give you a low ohm reading. If it is open circuit, the bulb filament is open. Years ago Radio Shack used to carry a grain of wheat lamp that worked for this. I've seen similar lamps at Mouser, Jameco, etc.

I've seen guys hack open the case and just solder in the new bulb onto the old wires. I prefer to remove the Vactrol from the pc board and disassemble it. The top cover is crimped onto the base and can be reused if you are careful.

[Zouto]

You can rebuild the Vactrol if the bulb is burned out. It originally was rated at 9 volts, but any tiny 10-12 volt bulb should work. Take a resistance reading across the lamp terminals. It should give you a low ohm reading. If it is open circuit, the bulb filament is open. Years ago Radio Shack used to carry a grain of wheat lamp that worked for this. I've seen similar lamps at Mouser, Jameco, etc.

I've seen guys hack open the case and just solder in the new bulb onto the old wires. I prefer to remove the Vactrol from the pc board and disassemble it. The top cover is crimped onto the base and can be reused if you are careful.

Bill, thanks. I get 100 Ohms and 160 Ohms at the lamp terminals, 9,21 Volts supply and 130K and 321K Ohms at the Vol.2/Drive switching Vactrol, when Vol.1 activated . I think that's way too high for shorting Drive and Vol.2. The other Vac has 670 and 710 Ohms when Vol.1 activated.

[52 Bill]

I agree those resistances are too high to kill the signal. Do the ohm readings across the two photocells change at all when you switch the channels? If the lamp is working at all the resistances should change some.

By the way you can take these readings with the amp in standby mode. Your lamp reading may be low only because of parallel resistances.

[Zouto]

Yes, the resistances change a bit when I switch the channels, 1M5 or so. Maybe the lamp is too weak or the LDRs got out of specs. It's a pity that there's no direct replacement for the VTL9A10/2, otherwise I'll build some optocoupler with a LED and two LDRs.

[52 Bill]

I dug through my old parts and found a working VTL9A10/2. With the lamp off, both photocells read more than 30M. With the lamp on 9 volts dc, both cells read less than 1K.

Hope that this helps when you build a replacement.

[Zouto]

Thanks Bill, I've got to build a new opto with two cells since one of the cells seems to be dead. For reliability I tend to use a small LED with series resistor that will fit under the thimble hood, I think this should work.

[Zouto]

It seems that one small switching transistor on the control pcb/ 10V rail is blown, for I don't get 9V on the vacs anymore, and the Vol.2 LED is always on. Any idea what tranny it is? It's labeled "PN 4248", some Acoustic Pts.No.obviously. I get 10V on the 10V rail, Volume FS tip connection at the pcb, zeners etc are ok, but no/ ultralow Voltage after the transistor.

[52 Bill]

PN4248- Data sheet sez its a 40 volt PNP small signal transistor. In that circuit I would think that anything would work.

Edit: I just looked up the schematic, the original transistor is marked as a 2N6747. The only thing that the transistor does is turn on the distortion channel led.

If there is no voltage getting to the Optos then test the switch and the normally closed contact of the foot switch jack. Also be sure that the jack is not grounded to the chassis.

[Zouto]

The standard G100T schematic shows a 2N6474 (TO-220), a LM383 and an unknown Q1 for reverb, but no PN4248 in the channel switching circuit. I couldn't find a correct schematic for the actual circuit.
http://techpreservation.dyndns.org/b...Schematics.pdf

[Zouto]

After fixing the broken 10V cable, I checked all voltages and everything was fine. After retubing and checking Voltages with light bulb tester, the bulb got rather bright and all Voltages dropped, B+from 410V (without power tubes) to 283V(306V in 50W mode), bias tap from 58V to 36V, only switching circuit Voltages remained 10V as before. I checked all fresh filter caps, but could not find any shorting to ground. Could it be some PT issue? Troubleshooting and fixing this G100T turns out to be absolutely cumbersome......

[52 Bill]

And what happens with no tubes and no limiter?

[Zouto]

And what happens with no tubes and no limiter?

Without tubes & limiter all Voltages are OK: B+ 471 Volts, bias -68Volts, heaters 6.3V/AC, 10V/20V supply. Swapping tubes brought the same Voltage drop again, I tried different sets, swapped rectifier bridge with 4x UF5408 for no bettering. With tubes I use the limiter for saving main fuses.