Link ch1 to MV? [ Crate Bv-omfg]. Plan B!

[Sea Chief]

G'day gents.

Well Ive not sorted my ('96) Crate BV bad noise prob. Traced to V2 a & b but cannot be found. Tried: 3 other tubes, shorts xyz, replaced every V2 thing, inc socket/ & noise remains.
So its a total loss.. unless I can make it useable as it is.

It has one ch2 MV (ch1 has Vol only). When I turn ch2 MV down the noise diminishes to 0.
As its a full 50w I can just use ch2 @ max MV 1.5.. quiet but just about useable for home/ noise annoying but minimal.

Is it possible therefore to add an MV pot to ch1 to also dial down/ out the noise? (set underside & glued on 1.5 for all I care). Or even simpler in theory, wire up ch1 Vol to ch2's MV?

[Schematic is on post #19 of http://music-electronics-forum.com/t34224/ . Warning: reading that thread will induce mania or even blindness. defo some baldness anyway ]. thx S.Chief.

[Steve Conner]

Sounds like you eliminated everything except carbonised spots in the circuit board, so time to look for those!

[Sea Chief]

Sounds like you eliminated everything except carbonised spots in the circuit board, so time to look for those!

Is that a serious suggestion Steve, or..?

[Steve Conner]

Yes. If the PCB designer didn't leave enough clearance between tracks that have a large voltage difference, the board material can break down. It looks like a little black spot in the PCB. Old Mesa Boogies were notorious for it.

[Sea Chief]

Yes. If the PCB designer didn't leave enough clearance between tracks that have a large voltage difference, the board material can break down. It looks like a little black spot in the PCB. Old Mesa Boogies were notorious for it.

I cant see anything suspicious around V2 at all. Id have though if a general pcb deterioration, then the prob wouldn't be so localised to a specific area as has been established. Ive scraped and finely scratched between the pcb tracks all around V2 in case of any track contamination. All seem fine as other preamp areas. Nothing measuring 'across' at all.

Is there anything at all, absolutely any component whatsoever in the whole amp circuit that could 'leak' back or fwd to affect V2 that I havent crossed off the list of all possibilities, however unlikely/ even the tiniest slimmest chance of affecting it?

[Steve Conner]

The channel switching system could well be the culprit. A few people already suggested checking for dirty contacts on the footswitch jack. It wasn't clear whether you checked this. Connecting a footswitch and verifying proper operation might be a good place to start.

Sometimes on a double-sided board it can track right through the thickness of the material, between tracks on opposite sides. Examine with the lights out and the power on to see if you can see any sparking.

[Sea Chief]

Well, re FS socket I sprayed contact cleaner in & rummaged well with a rough tiddler s'driver so contacts good/ checked afaict. I dont have a FS itself; I did try my PV Bandit65 ch/rev one but didnt work.. not right type I guess.

Its a single-sided board, & apart from V2 are which looks knackered after many solders/ test/ replacements (and prob there b4 I even started to touch of course) so much so I wont replace anything else now or tracks will disintegrate entirely.. it all looks minty. I hold board up to light and check each congested track area for any obvious cross-referencing bits/ blobs/ crud. I really cant do anything else surely now (but as s'one said 'it has to be something' so Im at a brick wall, & 6ft thick).

[Jazz P Bass]

The footswitch is a plane jane TRS plug.

Have you replaced R13 V1 plate resistor?

Next, have you explored disabling the switching matrix?
Schematic page 2, top center.
Three resistors marked with a F: R49, R53 & R55.
Lift one end of each.

[Sea Chief]

The footswitch is a plane jane TRS plug.

Have you replaced R13 V1 plate resistor?

Next, have you explored disabling the switching matrix?
Schematic page 2, top center.
Three resistors marked with a F: R49, R53 & R55.
Lift one end of each.

Hi JP.

Replaced both V1 plate Rs 1st up way back.

No I havent tried disabling the switching bit- didnt know how or it had any bearing to the prob tbh/ glad there's another avenue to try tho. I'll knock up a basic TRS switch, try that. Then assuming n/g I'll lift those R legs.

(Once Im restocked with solder that is.. Im all over the shop with V2 socket out and ruined tracks at the mo.. so need to do a major salvage job 1st). thanks SChief.

[Sea Chief]

The footswitch is a plane jane TRS plug.

Have you replaced R13 V1 plate resistor?

Next, have you explored disabling the switching matrix?
Schematic page 2, top center.
Three resistors marked with a F: R49, R53 & R55.
Lift one end of each.

TRS (Peavey) F switch used: switches with a bump (prob my backwards Q4), but noise remains.
FS socket thoughrally s'paper scoured/ cleaned.

R49, R53 & R55 one end of each lifted: noise remains (I think/ fairly sure) under a fairly prominent hum.

[Sea Chief]

Test repeated: noise defo there still under the hum.

[Sea Chief]

JazzP/ anyone..

any thoughts on plan B/ to add an MV pot? (or link ch1 vol to ch2's MV) it seems a lost cause trying to find the bad noise, I guess some amps just have these.

[Jazz P Bass]

If you removed Jfet Q6 gate resistor, R39, wouldn't the MV work for both channels?
Q6 is on the tail leg of the MV pot.
Being a Jfet, it is a normally low resistance between the drain & the gate.
It needs a gate voltage to make it high resistance (off)
It either gives it a ground reference or it doesn't.
With a ground the MV pot works.
Without it, it does nothing.

[Sea Chief]

I shall investigate JP..

Lifted a leg of R39: bad buzzy hum even with MV on 0.

[Jazz P Bass]

Duh. The P channel J176 requires a voltage at the gate.
Sorry about the confusion.
How about you dispense with the Jfet all together.
Ground the bottom of the MV pot.
As long as the MV pot has a ground, it should work.