HELP?? Noise while turning/adjusting ANY instruments volume.

[Theors]

Just gonna through this out there.......
I've got an annoying problem while turning/adjusting the volume on ANY instrument!... or volume pedal... or acoustic pickup with a volume....etc,. Looks like a classic sign of a dirty/worn volume pot on the instrument etc,.OR a bad cord...but not so. It all started when I successfully cleaned the Master Volume pot and all others on my 1991 Tube Works RT-2100 MosValve+Real Tube OverDrive 100 watt Combo Amp. Concept and Design by B.K. Butler. I used a can of Deoxit Gold G5 spray. It worked GREAT! Noisy Crack POT SOLVED!! But not so fast grasshopper...Now it's developed this annoying glitch while adjusting ANY volume going in to the input ...either channel. I  have switched the Master Volume pot out of the amp...no diff. Cleaned guitar pots...changed cords...different power outlets...tried ANYTHING with a volume...to no avail. Still got the problem. What could be taking the input signal and making noise ONLY when it is being adjusted with a volume pot? Any suggestion? I have soldering skills and can work a multimeter. By the way...the amp works perfectly normal on both channels. All amp pots are clean and no noise.
Signed, Baffled

[J M Fahey]

You have DC at the preamp input jack.
To confirm it, set amp volume to 0 just not to hear hum or noise, plug a known good cable at the jack input and measure the other end of the cable, at the free/unplugged plug, between tip and body.
Use the muktimeter DC scale, start with the 20V one and then go down (2V , then 200mV)  to see which one gives you the clearest reading.

Post results here.

PS: I *think* you are using the guitar straight into the amp, no pedals involved, are you?

[Theors]

Correct. Straight into the amp. On my multimeter ( which I've included a pic of ) I have a reading of 13.70 DCV at startup and slowly climbs to 13.99 DCV in approx 5mins.

[Enzo]

We'd like a schematic of your amp.  That is beyond the "normal" up to a volt that some amps create.  That sounds more like a real issue.  If the input stage is solid state, many have a pair of clamping diodes to the +/-15v rails, and if one diode goes leaky, your symptom can result.  An open ground at the input could allow a floating condition, but then you'd have a ton of hum.  If the input stage is transistor, a leaky or shorted input cap could cause this.  Tube input?  Hard to imagine it making 13v there.

[Theors]

I know!!! I saw the volt reading and said WTF??? That's why I included a pic of my volt meter so there's no confusion on what setting I'm using. Gotta go to work for the day now....I REALLY APPRECIATE you guys helping me out. Keep throwing the ideas out there. I'll check back later today......

[Theors]

Oh boy would I like a schematic too!!! Can't locate anything on the circuit.

[DrGonz78]

Here is the schematic...

[Theors]

Wow! Thanks a million for the schematic!!!

[J M Fahey]

1) thanks drgonz for the schematic  :dbtu:
2) 13.7V DC???  OUCH !!!!!!!!!!
3) I see you are sleeping with the enemy 
Don't get the full picture yet, but the schematic shows what looks like an "input in use" sensor circuit, not sure if it's some kind of mute or it enables some function, but there you have 2 x  22k resistors connected straight to jack ring pin leaf contacts and on the other end to "CA" .
Said ring pins are grounded if nothing is plugged in, you'll have current passing through the 22k resistors.
If you plug into any of both inputs, one 22k resistor gets open and you halve passing current; if you plug in both, you fully stop current there.
That can be detected for some function I mentioned.
The only "CA" I find (you might find others) is on the emitter of "Y" 2N5401 , to the right bottom of the tubes area, which is also connected to the power supply node "C+" , which happens to be .... +13.7V  :trouble

Now those +13.7V shouldn't be a problem because if:
a) nothing is plugged in, 22k resistors get shorted to jack ring legs , so 0V there.
To boot, jack tip legs are also grounded, as an extra security layer.
b) you plug into 1 or 2 inputs:
the jack tip contact gets ungrounded, and signal gets into the preamp.
c) the jack ring contact also opens, current through 22k resistors stops, some function is triggered.
You have +13.7V 2 mm away from signal contacts, but it "SHOULD" be no problem, meager 13.7V can (should)  not be able to jump 2mm of air or PCB material .
I guess either the jack is damaged, has bent leads, whatever, or resoldering shorted nearby contacts or leaf switches are worn or bent or PCB is contaminated , and now those 13.7V cxan contact the *audio* pins.
They travel backwards to your guitar and make a scratchy mess.

So I would replace those jacks, while out clean the PCB area very well, I'd use Isopropyl alcohol because it dissolves both grease and salts (Big Brother does not let you USA kids to buy 94% ethyl alcohol ) and recheck that DC voltage dissapears.

This is what tip and ring means, applies both to plugs and to matching contacts in jacks:


By the way the amp schematic has tip and ring jack contacts drawn the wrong way, tip (wherev 68k resistor go)  should be drawn the furthest away from the sleeve/ground contact, and ring between them ... as it physically appears in a plug.

And the input sensing circuit is also wrong: as drawn, whether 22k resistors are grounded or not , voltage at 2N5401 transistor does not change, since it comes straight from the nominal "+15V" OpAmp supply (actually +13.7V) called "+C".
Those errors make me lose trust in that schematic.
It may be close enough as a general idea but each part should be checked/matched with what's actually on the PCB.
Of course, the amp works or they wouldn't have sold so many but the schematic is iffy.

[Theors]

To: J M Fahey....wow I think you are nailing this. Funny you said this,...

"I guess either the jack is damaged, has bent leads, whatever, or re-soldering shorted nearby contacts or leaf switches are worn or bent or PCB is contaminated , and now those 13.7V cxan contact the *audio* pins. They travel backwards to your guitar and make a scratchy mess."

Because when I cleaned the amp pots with Deoxit Gold G5, I sprayed all pots on the PCB in their place. I had quite a mess and continued to spray other components like input jacks and tube sockets. Even tried to tighten up the tension on the two input jacks so the cord would fit more snug. Then I proceeded to blow all the excess spray off with an air compressor. Reassembled the amp and that's when the problem started. So...then I thought the spray might be contaminating the PCB. So I sprayed a product 70% Isopropyl Rubbing Alcohol mostly on the underside of the board and lightly brushed with a tooth brush. Also sprayed it on the pots and input jacks. I wasn't to sure about spraying the alcohol on the component side of the PCB so I didn't. Then lightly blew off and dried the whole thing with air compressor. Could I have damaged something? Or do I just clean it better?

[Enzo]

You probably either bent a jack contact, or in spraying cleaner, some dirt wound up lodged in the contacts. No, your alcohol didn't hurt anything.

[Theors]

I have other input jacks I can try out on the input board by just soldering the replacement jack to the input board in a way that it bypasses original jack and still uses the resistors ( which I have checked out and are fine ) but like Fahey says the schematic seems to be wrong? Here's what I have, 3 wires,... Red (I) Black (G) Purple (CA). 1st what does Purple (CA) represent?

[Theors]

Hurray!!! I removed the channel 1 input jack from the input PCB. Installed a two wire input jack on the input PCB to the Tip/Red (I) and Sleeve/Black (G). All voltage is gone at instrument cord and PROBLEM SOLVED!!! So.....what did I just eliminate? I can't for the life of me see anything wrong with the 5 point input jack that I just removed. Here is the Amplifiers input jack features posted in the users manual:

INPUTS — Input 1: 0 dB. - Most sensitive input.   Input 2: -8 dB. - Less sensitive input for Steel guitars and ex- tra-high output pickups. NOTE: Inputs have true active summing, therefore two guitars of any impedance may be summed together by using both inputs. It is also possible to use a "Y" cord to give even more gain. (Single guitar Y-ed into the two inputs).

Anybody? Will this setup that I just did work from here on out? Safely? Effectively? What features did I just lose? (if any).

[J M Fahey]

Well, those powered 22k resistors were sent to the input for *something* .
I guess they might trigger some kind of electronic mute although that's not clear from the not_too_accurate schematic.

Suspect same as Enzo: when tightening jacks you bent some contact.

You may use the amp as is, with the simpler jacks, and if it works for you, fine, but the proper repair is to hunt down exact same model jacks and replace them.

[Theors]

Thank you all very, very, much! At least I know the problem was at the input jacks PCB.

One last question,... do you know what the (CA) Purple wire on the input PCB which goes to the main PCB is for? Basically 3 wires. Red(I) Black(G) and Purple(CA)