question on dc voltage & volume pots

[blindsirens]

Hey there,I'm having a grand time troubleshooting a Mesa Dual Rectifier.

Scratchy volume controls on BOTH channels. With controls all the way down I read no DC then as you turn control up you get more and more DC voltage with the center tab reading more then the outside tab. At halfway up it reads about 8-10 volts. The other tab is connected to ground. This noise is completely gone when I pull the efx return 12ax out(and yes I've sub'd good ones just to be sure)and the dc is no longer on the pot.

Anyways I started pulling up one leg on all capacitors in the vicinity and some not so close(I don't have a schematic that matches this,its an older dual)and found an electrolytic leaking around 7volts dc. Well after continuing this and damn near lifting everything but the filter caps the dc voltage is still there. Are there circuits or situations where DC can be okay on volume pots? I've heard something about how presence controls sometimes have this.
Are there other explanations as to why DC can escape to a volume pot besides leaking caps? Could bad grounding do this? There is a relay in this efx loop circuit,could dc voltage make its way from these?
I was sure I was going to find a leaking cap and see the dc gone after pulling its leg up. I can't trace the signal visually or with my meter due to a stack of Vactrols,part congestion and this being a dual sided PCB that you can't take out.
Should I have the volume dial on halfway in order to see this voltage in circuit and which tab should I try and follow back the outer or middle? I feel like if I have the volume on then I'm merely following the dc voltage after the problem and not going towards the source. As you can see I am just not clicking with how to go about this without shotgunning on a wild goose chase. Thanks for any advice out there.

[stokes]

I would say to trace the dc voltage from the outer leg of the pot.The cap that is connected to this leg has to be the leaky culprit or the dc would not get to the pot itself.When you turn the control down and see the dc disappear it is because at this setting the dc is being grounded,along with the signal.Since it disappears when you pull the effects return out,it must be coming from the plate on this tube.Check the caps in this circuit,in case you have more than one leaky cap.But as I said earlier if the dc is hitting the pot the cap connected to the pot is leaking.

[Rick Erickson]

This sounds like an oscillation problem. Do you have an oscilloscope? It would really help here. The other possibility I can think of is a grounded cathode on the tube following the Vol. Pot.. This can force DC on the pot via the grid. The original (Marshall) Plexi Re-issue amps had a problem with this on the first stage cathode. This is why there is a mounting screw missing on the PC board of these amps - they ran the cathode trace too close to the screw hole.

RE

[Amp Kat]

Don't think for a second that a bad preamp tube can cause this symptom making you think it's a pot. Of course it is also leaking DC putting it on the grid. Of course a bad pot could easily be the cause.

[Rick Erickson]

Don't think for a second that a bad preamp tube can cause this symptom

You really mean ["can't" cause this symptom}, right?
He said he had swapped tubes and that the DC went away when he pulled the FX Return tube. This sounds like RF oscillation to me.

RE

[stokes]

The only thing in the circuit that blocks dc is a cap,a pot doesnt block dc voltage so I dont see how a bad pot would cause dc volts to be where they shouldnt.How does an RF oscillation create dc voltage?In any event any dc current reaching the pot has to be leaking thru the cap connected to the pot.If the cathode of the following tube were grounded instead of going thru the resistor it would throw the bias of the tube off but any dc flowing thru the cathode would still go to the plate,it aint likely to detour thru the grid on its way to the plate.Looking at the schem for the Dual Rect,I am assuming you are refering to the master volume controls,since you say the dc goes away when you disconnect the loop effects return,if this is the case check the loop active master control,if you see dc volts here as well then I would suspect the .047 cap from V4A,pin 1.

[blindsirens]

With the Master volume off it reads zero on ALL pots. As you turn the Master up the DC voltage peaks halfway at about 9.7vdc and then begins to trail back down to 1.1 volt when maxed. This is the center tab of the pot. The outer tab reads around 1 volt when maxed out. If you leave the Master volume up so its reading around 5vdc the Treble pot will read anywhere from 144mv to 1.2mv depending on where its set (the middle tab). With the Master volume still up you can read 140-2.7vdc on the bass pot and around 140mv on the Mid pot. There is no dc on the presence or gain pots. The other channel reads IDENTICALLY. The scratching sound is only heard on the Master volumes.
The dc ONLY appears on whichever ch is selected. When a ch is not selected there is no dc on it.
Regardless of where the efx loop selector switch is set you still get these readings. The only way to remove it is to pull the efx 12ax. The amp will actually run fine and the master volumes are silent with no dc. Like I said I pulled a leg on every cap in the send/return socket and MOST small caps throughout the amp and its STILL there. I did find a 2200uf cap (Which I don't even see on the schem I found,unless they mistakenly wrote 220uf) leaking 7volts but the dc voltage is still there with the leg pulled. I don't think its possible to shotgun this Mesa as much I‘d like to start swapping caps.

Disconnecting the zero voltage side of caps should remove the DC if that said cap is leaking right?

Theres no way a dc powered relay could contribute?

And again I am struggling tracing this as I run straight into a stack of Vactrols and the traces disappear underneath the board frequently. I'll check the loop active control and everything again tomorrow. There are two ceramics off the tone circuit I wanted to check but neither side read voltage so I didn't know what leg to lift. I do have a scope but have never looked for RF oscillation before. I'm willing to learn though.

[stokes]

The fact that the dc disappears when you pull the 12AX7 tells me the leaking cap is the one from the plate of that tube,as I said before.that cap feeds both master volumes by way of the effects loop master volume control.The only dc coming out of that tube is from the plate of V4A by way of the .047 cap from pin 1 of V4A.Look at the schem for the effects loop,you will see what I am talking about.That plate feeds your master vols on each channel.This is the most likely place for dc to reach the pot.RF oscillations are radio frequency and are introduced into an amp from external radio signals,they are not dc,they appear along with your ac signal in the amp,usually get in at the input.The dc on your pot is most likely coming from the cap I described.

[Enzo]

You have the two masters, and one or the other is selected to continue on out to the loop. The way it is selected is through a couple Vactrols. Any potential that a Vactrol is somehow shorted, miswired, etc?

The voltage max's at mid point, ground is ground, so we expect low voltage. At the top, there are parallel paths for current, so the reading drops. You get 1.1v there. Does chaning the EQ control setting affect that voltage? In any case, the voltage has to be coming into the pot via the wiper.

If that 100nf at the grid of the loop tube is not leaky - and simply removing it would answer that question - then nothing else IN THE SCHEMATIC makes sense. Especially when pulling the loop tube makes a difference.

Pulling the loop tube removes current through the tube, so voltage dropped acros its cathode resistors drops to zero. How then does the presence of cathode voltage translate into voltage on the master wiper? I'd be looking for a little bead of solder or other solder bridge, or maybe wires touching - perhaps in one of those Vactrol condo communities.

[stokes]

The loop tube plate has an.047 connected to the loop master control which feeds the master volume that has the dc voltage problem.Since pulling this tube stops the dc from reaching the master vol control the obvious source of the dc has to be that .047 cap leaking,no?I think you guys are overcomplicating this thing.

[blindsirens]

Disconnecting the zero voltage side of caps should remove the DC if that said cap is leaking right?

I'll be looking at it again today,but I've already lifted a fair amount of caps with no change. I'll see what else is there. How would I test a Vactrol to be sure its operating normal? Simple continuity test? I'll also look for bridged connections.

[stokes]

If a cap is leaking there is no "zero" side of that cap.The idea of pulling what should be the zero side is to ascertain there isnt voltage coming in to that point from somewhere else.You can check your coupling caps "in circuit"but if you see dc on both sides then you should lift the grid side of that cap and check it as there are some caps in some circuits that do have a dc source on both sides of the cap.If all your switching works properly then I think checking the relays is pointless.When you pulled that tube and the dc on the pot went away,it pretty much is obvious as to where the dc is coming from.

[Mark Black]

The loop tube plate has an.047 connected to the loop master control which feeds the master volume that has the dc voltage problem.Since pulling this tube stops the dc from reaching the master vol control the obvious source of the dc has to be that .047 cap leaking,no?Actually wouldn't that cap still be hooked to B+, regardless of having a tube present? If anything I would expect the voltage to go up...

Sorry - not trying to butt in & make trouble...:)

[stokes]

Yeah,it is still hooked up to the B+ but the ground reference is thru the tubes cathode.

[blindsirens]

All cap legs are lifted in the efx loop so I'm not sure how they could still contribute. I checked where the 1meg resistor meets 100n cap at RY2 and there was 0vdc. With/without the cap lifted.

There is no DC on the loop active control.

When I lifted the 20n cap between RY2 and v5grid and checked the point where it was connected to,there was the exact amount of DC there as on the middle tab of the Master volume where LDR14 connects. Also that point is 10ohms to ground. And the LDR14 point is 18ohms to ground. Is that through the master pot? Anyways its only there with efx loop switch set to bypass. With all other settings its gone so I guess the relay is working and it should be somehwere in the effects loop,but if at the above mentioned point I got zero...where is it?
Although I get some funny negative ohm readings with a 2.2meg resistor or two,the rest in the loop measure fine. Also I checked the send level/return pots and there was no dc voltage there either.
Damn actually I can't remember If I checked them in all positions of the efx loop switch. I'll double check this tomorrow.

[blindsirens]

I was reading my post and its striking me as strange that the DC is on the master volume pot even when the Loop switch is set to bypass supposedly changing RY2 to be closed? And also at the 20n cap. Meaning say you set the master half way and read 5vdc on the middle tab thats what you would exactly see at the 20n cap.

[stokes]

Those two caps you mention the 100n and the 20n are connected to your PI cathode that has 48v.The 100n doesnt look like it could have any affect but check that 20n on both sides there is no need to lift it.If it has dc on the side connecting to the switch,that is your culprit.Didnt see that connection to the PI before, the notation on that page of the schem says to grid V5.But I really dont see how pulling V4 is stopping the dc from showing on that pot.That 20n and the .047 from V4a look like the only possible source for the dc to reach that pot

[Enzo]

And we should never rule out conductive crap on the circuit board.

[blindsirens]

I pulled the middle tab lead on volume pot connecting to LDR and measured 80vdc in loop bypass and 30vdc in all the other loop settings. It slowly rises over time,but starts at these amounts whenever you power it on. After finally giving in to the insanity I desoldered the LEDs and undid the pot harnesses in order to get a look at underneath(even then you can't fully flip the board). I am seeing a questionable area where one of the traces leaves the LDR heading for the relay and runs right by the high voltage end eyelet of a .68 cap (I believe its .1 on the schematic posted) this connects to pin8 and a few resistors. Anyways there is a plastic spacer hole here that cuts in extremely close to the trace running from the LDR and cap eyelet. There is 1meg continuity between the two points so I don't think its too bad,but I'll try and brainstorm a way to isolate them better. There is no green just a woody surface on the board around these points.
Then again I'm not sure which leg to follow from the four coming off that vactrol and I'm reaching for straws at this point.
I wish boogie would realize people need to get under their boards no matter how well you make an amp.

[blindsirens]

I moved the leg of that cap over to the top of a 100k resistor which the trace led to (It has different resistors on the schematic). And now it reads 110vdc at the lead going to the vol pot? I screwed up though as I did two things at once. I also reconnected all the caps I had lifted so I'm not sure what caused this. Anyways that eyelet is still there getting voltage and looks like it may be burned through to the trace from LDR,but its so slight I can't tell.
These are through plated boards so I'm not 100% sure enough to take it out or hard wire the LDR to the relay.

[Ptron]

Did you say that the voltage is present even if the loop is bypassed? When bypassed, V4 and the whole loop circuit should be completely out of the picture. If pulling and replacing V4 causes the voltage to come and go even with the loop bypassed, I would think there is some sort of unwanted connection between one of cathodes of V4 (or the junction of 22k and 82k resistor on the cathode of V4b) and the trace/wire that connects LDR14, LDR15, and RY2.

[blindsirens]

I was right about that hairy area on the board. It is arcing. You guys are awesome,thanks for helping out.
Right at the edge of the hole that holds a plastic PCB lift is a woody,rough type area. The eyelets for a 100ohm resistor(two resistors on schem) and the .68cap(.1 on schem) getting 210vdc are right at the holes edge. The trace leading from the LDRs to the relay runs right to the bare edge meeting damn near at the cap and then makes its way around to RY2. It didn't arc over the top of the board leaving a burn mark it actually arc'd THROUGH the board to the trace where the green finish on the board is just ending and the hole is punched out for the spacers. I couldn't see any obvious burn marks. I took the 100ohm(purpose?) resistor straight from pin 8 of the loop tube and jumped straight over the eyelets where the cap and one end was previously and put them all together at the 100k resistor which is where those ends lead anyways. I then scratched around the caps eyelet isolating it(I hate to do it but it was still arcing with those ends out of their holes). There is no DC whatsoever now and the amp has been running okay for about a half hour.

The only other thing I notice is that if I crank it right when I take it out of stdby I can hear a little crackling for about five seconds then it goes away. So either its still trying to arc over or its a noisey tube? Maybe I should think of a more permanent fix that won't look like hell. Also,what caused this nightmare I have no idea. I'm not sure how much I learned,but I swear I will not read too much into the symptons of amps even if it does sound like the obvious. I wonder if the newer Dual rectifiers punch those holes in different areas now. Dual sided boards are so time consuming. You have no idea where anything runs or goes. Thanks for the help fellas.