Valvestate 100 OD ch issues

[jp3zone]

Guys, this thing is driving me nuts! When the amp came in the only description I got was: "Theres no sound"...It came in with burned out regs., thanks to our old friend the tube retainer, blown IC5 logic switching chip and a million dry/cracked joints. I replaced/repaired all of the above and now its working, except for the od1 and 0d2 channels.

There is signal going to the poweramp when playing the guitar thru it, but not distortion tho, just lowish volume perfectly clean signal. od2 is bit louder than od1, but neither of the two's gain pot has any effect on the signal gain, only the volume pots have an effect.

As far as I can see with the scope, the signal drops way down right after R60, before IC5 (replaced it with new mc74hc4316n) when selecting the od chs. When the clean channel is selected, the signal after R60 is perfectly restored again... I checked the zeners connected to the switching array, all check out fine.


Any ideas?

[oc disorder]

I presume VR5 and VR7 are the gain controls switched alternately around IC7a.
As there may be a slight loss through the 4316 perhaps IC7a is faulty ?
Even though you replaced the 4136, as they are sensitive could your new one be stuffed?
What about socketing them, then with IC7 out measure the signal at pin 2 ???

[Enzo]

I'd focus on IC7 also. You may lose signal at R60, but is it still gone at IC7-1? If signal is there, than all that input selection stuff around R60 is working. Likewise if IC7-1 and IC7-7 have good signal then suspect IC6 looms up.


And you replaced IC5, but did you verify it has proper power supply at pins 9 and 16? See teh schematic, it has its own little +/-5.6VDC rails.


And for that matter, the two OD channels combine and run up to IC2 which selects clean or OD. A bad side to IC2 would kill the ODs.

The weak signal you do hear is probably crosstalk.

And working in from the back end, use your fingers. Grab something metal, like a small screwdriver or a resistor lead, and touch the input pins of each stage. It should inject some hum that you can hear. Work back until you cross a stage that no longer amplifies the hum signal. Classic signal injection.

[jp3zone]

OK, it's been a few days since I had chance to check out the amp again. VR5 and vr7 are indeed the gain controls. IC5 is fine, I tried a few new ones in there, same result. I'm gona replace IC2, IC6 and IC7 as soon as I get the M5201's, not gona bother just changing IC7 until then.

Enzo, the rail voltages on ic5 check out, so we can rule that out. I'll try out you sophisticated hum injection "system" .

Thanks for your time fellas, I'll report back soon

J.P