JCM 2000 TSL 100 Volume on clean even when pots are turned down all the way...???

[DrGonz78]

So, I had to clean some scratchy volume/gain pots on all the independent channels of my TSL100 and now have a problem. The problem is with the clean channel in that when I have both the volume and gain knobs off there is still a good signal present. I have read a few instances of this happening and in one case the guy sent it back to have it worked on. In that case is was a faulty tube and setting bias to get it fixed. Another couple of people think this supposed to be this way, and I know that is just not the case. So I have changed the 200k Gain pot and later the A1M pot just to make sure those were not the culprits.

The symptoms are simple: when you have the the volume & gain all the way off = some signal. When you turn up the volume pot going up to 10 there is a part around 6-7 that gets quieter then signal comes back to normal. When you have Volume all the way down and raise the gain pot to 10 there is no signal and a bit of a hum is then present. (There is a guy that described the exact behavior on the Marshall forums website but no solutions or any follow ups at all there...) When you switch to the crunch channel and turn up the volume (with out gain) it's loud. If you turn up just the gain on the crunch channel there is some signal that will come through and there is no hum. Same thing on the Lead channel too. Before I did all this it was fine.

I may have confused the order of my power tubes but I always mark 1-4 on some masking tape and keep them in the right order. If I moved the tubes around in the original order>>> could that cause the bias to not be set right now? I have not ever biased an amp and need to learn how. I have all the instructions on how to do this but just gonna take it slow as I go there.

I have inspected the clean channel circuit board and there is nothing wrong that makes any sense for it to behave this way. None of the wiring is hooked up incorrectly and nothing clear in the regards to a damaged trace or something on those lines. I will next take the whole thing apart again to inspect the lead board to make sure there is no solder problems on any CON type ribbon connection going to that board. Will make sure that no traces or anything was damaged on the lead board as well. Will do that tomorrow night so I will have to keep ya'll posted as to anything I find there...

But does anyone know or has anyone come across these symptoms on this amp before? Any help or thoughts are appreciated, thanks.

[DrGonz78]

Just wanted to mention about the above post that once I turn up the volume pot and raise the level of gain there is nothing wrong w/ the amp. It performs as good as it ever did before these other symptoms described. The hum only happens when the volume is all the way down and the gain is turned up independently.

[Enzo]

You are experiencing "crosstalk." One section of the amp is picking up signal from another. Like when you can hear people talking in the apartment next door.

The reason the signal is LESS when you turn the volume up some is that the amplified crosstalk is in reverse polarity or phase compared to the signal path, so the crosstalk is cancelled by the initial small amount of signal. COntinue to turn up past that minimum point and the amp overwhelms the crosstalk and you don;t hear it.


Is it causing any operational issues? Or is it just something you notice.

[DrGonz78]

It was supposed to be so easy to clean a pot and then solder back on the board. In this case all but the clean channel pots were better and not scratchy. It is not causing any operational issues, with the exception of the gain pot making hum as described. The hum only occurs in a way that you would never set the amp settings to play. As I add even the smallest amount on the volume pot it is then like a normal amp again. Very strange.

I will have to check all the wires and all the boards. I even tried to solder the effects loop jacks and other various grounded jacks on the output board to make sure and that was not the problem either.

The thing is about this amp is that I have always had a lot more trouble w/ the clean channel than anything at all on the amp. I use to service the amp at field services here in phoenix, back when I was using it hours and hours each week. The last time I brought it there the clean channel was making this light gurgling noise or something. As if it was a ground issue or some component that was making an almost phased in and out gurgling type light hum. The noise was really noticeable when it was on clean and no signal from the guitar. Once you started playing it was not easily detectable at all. None of the other channels had anything like this going at all and it was very unusual. I took it to Field Services and they did something to it that quieted it down a bit for a while but yet that demon of a noise reappeared again. So, I gave up for a while on fixing that noise. I then opened the amp up around August 2011 and scraped away at the chassis where the mains ground jumper was located. They actually had sprayed that part of the chassis w/ an anti-rust protection. After scraping that off it made that noise go bye bye. That lil bit of noise is still there but not like before and even field services could never find that lil demon in the clean channel.

So anyhow sorry to get so side tracked but thought that might be some useful history to this amp to chew on.

I do have a couple direct questions in relation to the schematic and clean channel circuit board. What is Bead L2? What is OPTO1 & OPTO2? Those are a few components in amps where I have no idea what they do or how to test them etc etc etc. I wonder especially in this case since I have pretty much ruled out most or all the other components on this little clean channel board.

[Enzo]

For small inductances - like suppressing RF sensitivity at an input or something - instead of a small coil of wire, they use what is called a "ferrite bead." Look that term up. L is the symbol for inductor, so L2 bead means the ferrite bead at L2.

OPTO for optoisolator - same thing as we often call and LDR - light dependent resistor. And LED shining on a photcell. MAking the light glow changes the resistance of the cell.

[DrGonz78]

So I got home tonight and went to work taking the amp apart as to inspect anything that may be causing troubles. I found W4 on the output PCB (which comes from Main PCB W2) that was totally a cracked solder joint. It is a yellow wire jumper that says it is for "Feedback" and not sure what that does exactly but it seems to have been the culprit for the cross talk. Soldering that made the cross talk go away on the clean volume pot. So now as you turn up the volume it does not drop out.

However, there is still a nasty hum coming from the (EDIT: Not Gain as I was saying) "Volume" pot and turning it up makes the hum more intense. As described before, this hum is really loud when "Volume" pot is turned up all the way with the Gain pot all the way down. Once I turn up the "Gain" it has a nice sound but still a tiny bit of hum present. It's as if the Volume pot is not doing it's job in many respect and is the main problem now. When I turn down both the gain and the volume there is bleed signal of the guitar still present. When I turn up the presence pot all the way up (clean channel) it intensifies the hum.

At this point I am not really sure where to look or what to trouble shoot next. I am about ready to send it field services so they can really give a good test. My questions here are: could it be a bad tube? Could it be the bias is acting funny? or both?

The replacement pots were both the right ones and appear not to be the source of this problem since this occurred in the same way w/ the old A1M(gain) & 200k(volume) pots that were on it originally. I want to also think that the clean board has a cracked trace or something wrong there but I have inspected it over and over w/ nothing leading to the conclusion.

Edit: I was mistaking in my post about which pot referenced. The Volume pot is the one that is in question and the Gain is working fine.

[DrGonz78]

Well there is definitely something going to ground making this hum. I tried out my Ibanez rg440 tonight on the amp to try out another guitar and got some funny results. When I play in clean w/ the guitar set to humbucker setting on the pickup it is quiet and sounds really nice. When I click the tone pot that then sets it to single coil mode the hum from hell is apparent. So there is definitely something on the ground side somewhere in the circuit that is causing the pot to basically not work. I tested the J174 transistors and put new ones on just to make sure those were not bad. Those were not the culprits.

Could I run ground jumper to the 200k volume pot to see if that helps? If I were to do this, where on this pot would find the ground bases? There are 4 legs and 2 that hold it to the board for better support. The 2 legs that hold it on the board for stability are not in circuit as a ground. I would guess that those metal casings are still a ground source to the pot. Can I place a ground one of those legs to ground out the pot?

Only other thought is that one of CON type connectors to the Lead channel has issues or cracked solder joints. I did look at that board and tested continuity in the wiring and the board too. So, that seems to be further down the line but still in my head for troubleshooting. That Lead channel board is the toughest to get out of the chassis but I don't think it flexed really.

[J M Fahey]

I tried out my Ibanez rg440 tonight on the amp to try out another guitar and got some funny results. When I play in clean w/ the guitar set to humbucker setting on the pickup it is quiet and sounds really nice. When I click the tone pot that then sets it to single coil mode the hum from hell is apparent.

Then it's clearly coming from your guitar.
Can't blame the amp.

My questions here are: could it be a bad tube? Could it be the bias is acting funny? or both?

Tube? unlikely. Bias? impossible, not related.

What is Bead L2? What is OPTO1 & OPTO2? Those are a few components in amps where I have no idea what they do or how to test them etc etc etc. I wonder especially in this case since I have pretty much ruled out most or all the other components on this little clean channel board.

I tested the J174 transistors and put new ones on just to make sure those were not bad. Those were not the culprits.

It looks like you are pulling *all* parts at random and checking them outside, to find "the bad part which is giving you trouble".
Well, troubleshooting does not work that way, sorry.
And although you find that:

It was supposed to be so easy to clean a pot and then solder back on the board.

in fact you are doing much much more than that: you open the amp, pull the chassis, pull all jack and pot nuts plus maybe a couple screws, pull the board out, either pulling some connectors or twisting wires , unsolder and desolder, and then do everything backwards to re-mount "as original".
Much fumbling and messing and moving things, way beyond "simply soldering and desoldering"
You may be creating problems that weren't there before.
JM2C.

[g1]

I'm unable to keep track of what the problem is here so could you please answer the following questions.

I tried out my Ibanez rg440 tonight on the amp to try out another guitar and got some funny results. When I play in clean w/ the guitar set to humbucker setting on the pickup it is quiet and sounds really nice.

So with the rg440 on humbucker setting what problems are you having with the amp?

something on the ground side somewhere in the circuit that is causing the pot to basically not work.

Once again, with rg440 on humbucker setting. Which pot is not working? It has no effect at all?

[DrGonz78]

JM: Yes I can't agree more that trying to even remove the boards from the chassis is causing more trouble than helping. I was very careful and even then it leading me into trouble. The guitar is not the issue here it definitely the Volume 200k pot on the amp. This hum occurs if I use PRS(humbuckers), Ibanez RG440, or especially my Tele. Now what I was trying to say about the RG440 is that it has 3 pickups on it: 2 single coil and 1 humbucker. There is tone pot that will split the humbucker into a single coil and that would make it hum even more. The PRS has 2 humbuckers so it is not allowing the the amp to hum as loudly as the single coil type settings on the other two guitars.

Anyway I did not remove the opto or bead L2 components... I was trying to look at all the components on the clean channel board to just test them all and to realize what function each component plays. I realize the part of shotgunning parts is not trouble shooting, oh well. I was getting a headache and scratching my head as to where the problem might be and those trasistors are right in the path of the schematic that leads to the VR5 200k volume pot. I guess with those I was wondering about electrostatic discharge or something that might have occurred. Still learning...Yeah the bias and tubes are fine, I was just letting my thoughts go the wrong way there... Anyway I appreciate all that you have posted and cannot agree more, but still I will troubleshoot further so lets see where this takes us.

G-One: Yeah the guitar is not the important detail it is the fact that there is a loud ground hum apparent on the Volume(Clean 200k) when I turn it all the way up. It is kind of working to effect the volume but only when the gain knob is engaged. I have totally looked over all the clean board and can't find a problem there in regards to bad pot, bad ground on the W1 jumper to the board, or the soldering.

So, I am now moving to the main component where the problem is occurring >>> VR5 200k Volume pot. It has a two wire CON4 that connects to the Lead channel board via CON10. CON10(lead board) goes to ground there and links part of signal to CON11. If that is the case: then CON11(lead) goes over to CON9(Main circuit diagram). Then CON9 is in a direct ground to W14 on the main board. As I was posting earlier there was some strange stuff w/ the ground on the clean channel of this amp for a while now. Hopefully, I will stumble onto the ground issue that might be causing this now failing Volume pot problem.

I will attach the schematic and wiring diagram if anyone is interested. The schematic that I am referring to is almost exactly like the board that I am using and the connection sheet comes from a gentleman that had the exact same amp as this one w/ the exact board versions.

Sorry if I ramble at times or just seem silly w/ my logic or the ways in which I work on amps. I really am just a novice at this and this amp is like a bag of tricks. Thanks for all your help, you guys have already shown me a lot. Just want ya'll to know I appreciate that the most, Thanks.

Clean channel board
http://www.drtube.com/schematics/mar...tl10-65-02.pdf

Lead channel board
http://www.drtube.com/schematics/mar...tl10-61-02.pdf

Main Circuit board
http://www.drtube.com/schematics/mar...tl10-60-02.pdf

[DrGonz78]

Just another thing to mention is that all three channels are getting the hum on the Volume pots and are all behaving just the same as the clean channel. Not sure this was the case all along but it is now very apparent. But only the clean has signal bleeding through w/ the volume and gain turned all the way down.

Edit: #1 reason I know it is not any guitars causing the hum... The hum is there w/ out any guitar cable or anything connected to the amp.

[DrGonz78]

Well I took the whole thing apart again and really went to town on all the boards. Since the amp is getting older there were lots of solder joints to look over and fix. I basically re-flowed the entire Main Board and looked over all the volume/gain pots that were removed for cleaning. Any solder joint in question I touched up and checked every ground to make sure it was good to go. All joints were bright and shiny again!

Anyhow I fired it up tonight to test to see what might have gone right... or more wrong... Well now it seems to be quieter and better grounded in a general sense. Now when I have gain knobs off on the Crunch & Lead channel and turn Volume knob up all the way the hum is gone. So, that part is good, but still on the clean the volume pot still triggers the hum. So, I am basically just done w/ it and will send it off to have someone else look at it too. That is if I don't look in there just one more time.

Edit: Oh and there is actually less signal bleed on the the clean channel when volume and gain are turned all the way down. Some where along the way the same thing started to occur on the lead and crunch channels, but now then turn all the way off w/ no signal bleed.

[g1]

Just wondering if you have the chassis in the cab or not. The extra shielding when the chassis is back in the cab may help the hum issue. Try it again once it is back together. Try the clean channel at normal settings with guitar input and see if the hum is still objectionable.
If it is still bad I would agree, time to send out, best bet to someone familiar with this model.

[Pross]

Do you have the connection diagram? Can I get it?

[DrGonz78]

It is a bit hard to read this diagram but it is accurate, just read it slowly to make sure. I forgot that this file did get knocked out by the server crash that occurred, so here it is again... I would spend time color coding wires from one board to the other for readability but that is too much work for me right now.