Tracing noise problem, TwinRev.

[Sea Chief]

The fact that there was a change in the reading at all when the circuit was closed (both probes in contact with something) means that there was a readable result there. Voltages do not read as open circuits. There is only voltage or not. If your meter was set for DCV as you say then the 1........ reading indicates an over load circumstance for the range setting on the meter. If your meter was set for resistance in ohms (which it should NOT have been) then 1........... means an open circuit.

With me so far?

With the above information reported to be factual by YOU a reading of 1........ means you were using the meter incorrectly in one capacity or another. (<period)

Yes Im with you. But please remove the idea I put my DMM to ohms.

As I recall, testing a components' voltage is one probe on it, other probe on chassis. If this cap is being tested at it's HV end, to the screw which is tied ground, then I can say it is being tested, as any component would be " from it, to chassis ground".

As Im getting 1.... open circuit, then this implies there is -no- voltage where there - should- be voltage. That is all I can attribute to this DMM reading. This is a logical reasoning, albeit not a technically knowledgeable reasoning.

Can you comment on my reasoning here: does it have validity-?

Thanks SC

[Sea Chief]

Chuck H sorry just read your " overload " possibility reason, for my 1.... reading.

It could well be I set it to 200vdc ( expecting this diddy cap to be 120v or something max), & not 600vdc.

Muppet if so. Will do asap. SC

[Chuck H]

please remove the idea I put my DMM to ohms.

Ok. Done

As I recall, testing a components' voltage is one probe on it, other probe on chassis.

Not always, but in this case yes.

If this cap is being tested at it's HV end, to the screw which is tied ground, then I can say it is being tested,...

Yes

As Im getting 1.... open circuit, then this implies there is -no- voltage where there

This is where you're not understanding the meter. Set for VDC and with a closed circuit the meter will show a voltage between zero and it's range capacity. A reading of 1........ with the meter set to VDC does not mean an open circuit. You cannot read an open circuit with the meter set for VDC. With the meter set for VDC an open circuit would read zero and not 1..........

1........ is an "error" indication on your meter. For example, if you're trying to read ohms across an open circuit but there is no continuity at all then there is a missing element for the reading and that is an error. As indicated, but I will repeat it, an open circuit does not register 1.......... with the meter set for VDC. An open circuit should read zero. The only way 1........ would be indicated with the meter set for VDC is if 1........ is an error code (as I suspect) and the voltage being tested is above the range setting on the meter.

Can you comment on my reasoning here: does it have validity-?

It was a little off. I hope I explained why well enough.

[Sea Chief]

Chuck H My fault! I had indeed set it to the 200 vdc notch, not the 600vdc setting.

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SO (!). From HV north side of cap, to screw = 260vdc

And reading across the cap = 195vdc

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Apologies chaps, SC

[nosaj]

And south side of cap to screw is?
nosaj

[Sea Chief]

And south side of cap to screw is?
nosaj

Zilch. 0.00 vdc nosaj.

thx SC

[nosaj]

That was a basic way to test that cap. it is not passing any dc to ground. Cap is good.
nosaj

[Chuck H]

[USER="2808"]And reading across the cap = 195vdc

That's actually funny. I did indicate taking a reading from both ends of the cap didn't I. What I meant was a reading to ground from either end. Which we have now in your reply to Jason.

[Sea Chief]

That was a basic way to test that cap. it is not passing any dc to ground. Cap is good.
nosaj

Yup understood. Hmm.. would've been convenient if it tested bad.

The overall very bad popping & loud crunchy noises, have got better in general I must say. But the occasional very loud one tells me I haven't solved it.

Nor the Normal channel under-par volume thing.

Also I get now a very high pitched continuous whiney noise now, when tuning Reverb knob above 4. Gets louder as you go higher.
Incredibly high & piercing, painful. And a bad groundy hummmm as the same knob goes above 3, getting louder as you go higher.

I wonder if this signifies any next port of call area to inspect. Possibly something here causing the original pop/ crunch issue-?

Thanks, SC

[Sea Chief]

That's actually funny. I did indicate taking a reading from both ends of the cap didn't I. What I meant was a reading to ground from either end. Which we have now in your reply to Jason.

You did & I was going to next. Just thought reading across it was a good idea at this juncture.

Anyway cap good it seems.

[Chuck H]

Anyway cap good it seems.

Unfortunately? Let me recap (no pun intended) some things...

Trying to fix the absurd popping noise. (ignoring other matters for the moment)
Isolated to V1 circuit (eventually). Which also seems to suffer lower than normal gain.
Poked at and/or resoldered connections for that circuit.
Tried a different tube there.
Cleaned and re tensioned the socket.
Examined a potential layout issue with the screw.
Tested for DCvoltage at the tone stack output without finding any.

Did we ever get voltage readings for that tube? I can't remember now.

I'm still suspecting a bad socket at this time. Did you try gently rocking the tube in the socket with the amp on? I can't remember now.

And what happens if you smack the amp while it's on? With V1 in place of course since this is the circuit we're still evaluating.

[Helmholtz]

No voltage is the same as zero voltage.

The term "open circuit" only makes sense with resistance measurement.

[pdf64]

I think the normal channel low gain thing is probably a complete red herring. The channel volume controls may have very different tapers / characteristics.
Bear in mind a new CTS 10% audio pot has just been fitted to the normal channel. Whereas the fx channel may still be on an original 35% J taper or goodness knows what.
And we know that with modern CTS, the wiper doesn’t leave the metal end (ie starting) pad until it’s been rotated 15% (as per photo below).

How about refitting the master volume, at least as an ear saver whilst troubleshooting?

[Sea Chief]

I think the normal channel low gain thing is probably a complete red herring. The channel volume controls may have very different tapers / characteristics.
Bear in mind a new CTS 10% audio vol pot has just been fitted to the normal channel. Whereas the fx channel may still be on an original 35% J taper or goodness knows what.

Hi pdf, sorry shoulda said: actually the cts i fitted had a solid smooth shaft so not useable with orig knob.

So I put back the original pot, as no change occured with this test.

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When it gets to 5 some unpleasant distortion is introduced, like a crappy stomp box sound " with it's od knob dialled to 1/4 ".

So there is defo something amiss. But whether it's the source of the issue though..

thx SC

[Chuck H]

..

Did we ever get voltage readings for that tube? I can't remember now.

I'm still suspecting a bad socket at this time. Did you try gently rocking the tube in the socket with the amp on? I can't remember now.

And what happens if you smack the amp while it's on? With V1 in place of course since this is the circuit we're still evaluating.