Express build dc on pots

[francospence]

Hi there, my express has scratchy pots and I'm putting this down to the dc I'm measuring in the tone stack. I get readings varying from 120 Ma to 380 Ma on the treble pot leg which connects to C2. I've tried replacing C2, C3 and C4 but it only has marginally changed the dc readings. The highest readings seem to happen when the bass pot is on 10 and the treble pot is on 0 if that helps. Any ideas on where to look next would be appreciated, the pots are all cts and pec. Thanks in advance,
Best regards Lee

[tubeswell]

You could just have a dodgy pot. Sometimes pots can develop leakage when they are soldered in as the result of flux or solder running down into the pot, which causes all sorts of scratches. On second thoughts you could have grid current from the second triode coming back through the vol pot (if it is leaky).

[francospence]

Thanks for the reply, all the pots are scratchy(some more so than others) and considering the considerable dc showing on them I'd tend to think it is a bad cap leaking dc through, I was actually quite careful not to heat the pots up too much when soldering the ground buss to them, I'm just not sure which caps to check next,
Cheers Lee

[tubeswell]

Did you actually lift the tail end of each cap from the circuit and fire up the amp with your DCVmeter between the loose end and ground to see if each cap really was leaking?

[Richie]

i'd suspect the mica treble cap if the controls have dc on them

[francospence]

Thanks for your repies, I actually just lifted the treble mica cap off at the tail end and measured at the same tag on the treble pot and the dc reading was roughly the same, which was around 360 Ma, as for the bass and mid .022 caps, I disconnected the tail and temporarily soldered in another .022 orange drop and the dc readings were still the same both times, I should have measured the dc from the tail to earth as mentioned but I assumed the dc would drop at the pots if I either lifted the tail or changed out the cap, any advice on where to check next, or should I go back and lift the tail ends of C2 C3 and C4 and measure them to ground? Thanks for your help once again, Lee

[VacuumVoodoo]

Are you trying to measure DC potential [volts] with the meter set to current [mA] range? Or is "Ma" it just a typo?

[francospence]

My apologies I meant mA ( 2nd bottle of wine in after 2 hrs of dealing with myspace code

[ChrisM]

My apologies I meant mA, the multimeter was set to dc voltage in the mA range,
Best regards Lee

Well that made no sense.

Hello Monday morning

[francospence]

I should add it's Monday night where I am
Cheers Lee

[VacuumVoodoo]

Perhaps you should get some sleep and set the meter to V or mV range after you wake up.

[Structo]

Perhaps you should get some sleep and set the meter to V or mV range after you wake up.

Yeppers

[francospence]

Sorry, Seems I had mA on the brain( must have been a combination of wine, and 2 hrs of myspace programing :/ And replying on a tiny I phone screen) so the voltages I'm measuring at the treble pot are in the 150- 300 mv range, on measuring all the pots the treble pot is where the highest Voltages are, I've lifted the tail of the 500pf mica cap (C2) and the reading didn't change much, and dropped in a replacement .022 in C3 and C4 and once again no real change to the voltage measured, any ideas on what I should check next?

[Bob-I]

Before you go too crazy here, check out your voltmeter. I got burned awhile ago when my meter was reading non-existant voltage. Another meter measured it correctly and all was well

[francospence]

Well it seems both the C2 and C4 were leaking dc, that's why lifting one at a time showed no improvement. Replaced both and we're down to under 2 mv on the pots, all scratchy noises have disappeared as a result. Glad to get that fixed, now where's that wine
Cheers Lee