Help with DC on volume pot

[mkusan]

Originally Posted by Mooly

Thanks for the readings... they seem OK and don't show much all that much drift. Also I'm sure the resistors will be OK. Unless you just desolder one end with braid and at the sametime use a croc clip as a heat shunt on the leg you can end up making things worse.

The resistors are the very last suspects. They all look normal tbh, the "hole" is just the ceramic or whatever coating... not a problem.

Look at the circuit and the "top" channel.
Resistor R531 can either bypass the filter consisting of C519 and C521 when the switch is "off" or place those components in circuit when on. So measure the DC voltage across those two series caps from end to end. That is from the junction of C517/R531 and C521/R539. If that is not zero volts DC then you will get a thump as the switch is operated. All those points are referenced to ground via R529 and R539 so they should be truly at 0.000 vdc. Make sure there is no signal applied.

Mooly please don't thank me, you are helping me I am very grateful for that. I am enclosing picture which will reveal the magnitude of my ignorance in electronics and in art but please don't laugh at me.

red line 10.82V
yellow line 2.15V
green line 0.0V
blue 13.08V

[MikeBettinger]

Originally Posted by mkusan

@amptech
I think certain time line of my (wrongdoings) is needed here. I recapped amplifier in February this year, since then it worked flawlessly until two weeks ago when old problem of popping while cold while pressing low filter turned up again, cold weather maybe

Then I opened the amp again and changed transistors (since I presumed they are most thermally sensitive of all components). Then those tantalums and then ceramic disk. I may be wrong but changing C501,C502 ceramic 300pF to film caps made the popping less intensive.

thank you all for helping me.

I agree that the resistors are the last to suspect. But the ground to the board being slightly bad, cold solder joint could possibly add a small potential and would account for the multiple symptoms as it is common to both the filtering and the bass control, also to both channels.
The image you posted was good but it is still hard to make out what is what and I assume by zooming out it will get harder to read. Measuring on the milivolt range of your meter using the main supply ground as your negative and the positive to the ground side of the resistors I mentioned see if you read anything. As Amptech pointed out it should be zero volts. Possibly during your first work you disturbed the connection and it started working, much like rocking a control back and forth to quiet it down. With time bad connections come back. Sometimes a can of freeze stray can help locate the culprit!

Mike

[Mooly]

Very difficult working from pdf's on screen.

Measure on DC volts between these two points with the filter switch ON which places the caps in circuit.

Also measure DC voltage from ground to these two points which should also be zero.

[Mooly]

I'll post another diagram which might make more sense in a few minutes.

[mkusan]

I have some new info that might be of relevance, I left the amplifier turned on over night and in the morning the right vu-meter needle was closer to zero, imagine my vu-meters have a scale from 0 to 10, immediately after powering on my left channel is on 1 and right on 3. In the morning left was on 0 and right on 1, and this time needles were jumping up with volume up or down with volume down (without any input). Looks like what ever DC is there it was lower after the amplifier was on for eight hours.

Originally Posted by MikeBettinger

I agree that the resistors are the last to suspect. But the ground to the board being slightly bad, cold solder joint could possibly add a small potential and would account for the multiple symptoms as it is common to both the filtering and the bass control, also to both channels.
The image you posted was good but it is still hard to make out what is what and I assume by zooming out it will get harder to read. Measuring on the milivolt range of your meter using the main supply ground as your negative and the positive to the ground side of the resistors I mentioned see if you read anything. As Amptech pointed out it should be zero volts. Possibly during your first work you disturbed the connection and it started working, much like rocking a control back and forth to quiet it down. With time bad connections come back. Sometimes a can of freeze stray can help locate the culprit!

Mike

What is not very visible in the picture is that yellow, violet, red,gold 4.7K resistor has a slightly wrinkled, shrinked surface while other resistors in that row have almost a glossy surface, I would say the difference between them is like a skin from a 20-year old girl and from an old lady. It is also the only resistor that measured its nominal value in circuit, all other measured slightly lower value. Perhaps I should measure it from the underside, pcb side to exclude cold solder joint. Your suggestion is noninvasive, if I understand correctly one side of every resistor should read 0.0V relative to ground, main ground that would be E14?

[Mooly]

Look at this. The filter switch is now in the "on" position.
The two voltages shown must be zero with respect to ground and zero volts between them.

The two caps marked "Check" if fitted back to front or faulty could cause a DC fault.

[WayneK]

A mate of mine had a preamp that drifted dc. The cause was solder flux gave a leakage across a capacitor. Something to check.

[Mooly]

The VU meters are driven from the main speaker output via an opamp and rectifier. It's all AC coupled so the only way the meters can deflect is either because of a problem around the opamp stage or because the amp really is actually oscillating and is unstable and there really is an unwanted AC signal present.

You mention in post #1 that the amp has had quite a bit of work done to it. All these problems perhaps suggest something that has gone amiss during all the mods rather than a genuine "one off" fault.

You are going to have to work on this one stage at a time and it would be useful to also check the stability with a scope, however if you remove C635 and C636 feeding the VU meter stage then the meters should show zero at all times. If they do not and still drift and show various readings then that confirms a problem with the VU stage.

If removing the caps does cause the meters to read zero again correctly at all times then that points to instability in the amp somewhere.

[mkusan]

Originally Posted by Mooly

The VU meters are driven from the main speaker output via an opamp and rectifier. It's all AC coupled so the only way the meters can deflect is either because of a problem around the opamp stage or because the amp really is actually oscillating and is unstable and there really is an unwanted AC signal present.

You mention in post #1 that the amp has had quite a bit of work done to it. All these problems perhaps suggest something that has gone amiss during all the mods rather than a genuine "one off" fault.

You are going to have to work on this one stage at a time and it would be useful to also check the stability with a scope, however if you remove C635 and C636 feeding the VU meter stage then the meters should show zero at all times. If they do not and still drift and show various readings then that confirms a problem with the VU stage.

If removing the caps does cause the meters to read zero again correctly at all times then that points to instability in the amp somewhere.

Hi Mooly and thank you,
you gave me a lot of work, I will try to do some of it if not all during this evening.
Now that you mention opamp I remembered while googling dc on volume pot I stumbled upon this page:
The Repair Bench: Carvin SX200 Guitar Amplifier | GuitarKitBuilder.com

"With our digital multimeter set for DC voltage we began to measure, beginning with the volume pot, and as suspected found about 13 volts DC on the pot terminals that connect with resistor R19. This voltage is pretty close to the power supply rail voltage, and in our experience should be no higher than a few tenths of a volt. Measuring further we found similar voltages on the output and inverting input (-) pins of op amp A1A. We found -14.2 volts on R17 at input of op amp A1A. Likewise the DC voltages on these pins would normally be no higher than a few tenths of a volt.

At this point we couldn't be sure whether the high voltage was coming from the op amp A1A, or from transistor Q2. To isolate the source of the voltage we de-soldered and lifted one lead of resistor R19. We found that the high DC voltage was now gone from the volume pot and transistor Q2, but still present on the op amp terminals. Our conclusion was that the op amp contained an internal short circuit, that was connecting the power supply voltage directly to the output and inverting input pins.

One verification of our diagnosis was that with resistor R19 lifted, we could now use the B-channel through input jack A/B without any hum or scratchiness.

We used our de-soldering tool to remove the A1 op amp, an NJM4558D chip, and ordered a replacement from Digi-Key. When the new part arrived it was soldered in place and the B-channel was found to work perfectly. Amplifier fixed, case closed.."

I have the same opamp NJM4558DD, any resemblance to my case?

[Mooly]

Yours is a totally different design and application. Opamps are incredibly reliable providing that they are run withing their limits.

You must try it with those two caps disconnected to confirm the meters read zero at all times. If they don't then there is a problem with the VU circuit.