Newbie troubleshooting JCM 2000

[dhahnke]

Hey all, I apologize for yet another troubleshooting thread, but I honestly wasn't finding the information I was looking for.

Basically, I just got a JCM 2000 DSL100 (1999 build), blew an output tube (tube 2 when looking from the left) and ordered a new set to replace it.
- I started by making sure the speaker was still plugged in, no input is plugged in, and pulled all the output tubes.
- I verified pin 5 was stable (grounded to chassis, positive probe in pin 5) and saw steady numbers in each tube's pin 5 on standby (.28-.030) and steady voltages when warmed up (on for about an hour) both on standby and off standby (.016-.017).
- I put the new tubes in and tried reading the bias pins and the left side is sitting at .9mv and the right side is sitting at .6mv.
- When adjusting the bias screws nothing changes no matter how far i turn them
- I tried this with standby on and standby off, with standby off (amp live) and I heard a LOT of noise through the speaker with no input (volume at .5/10)

What am I doing wrong here, or is the amp hosed?


Edit: I tested pin 5 wrong and used VAC not VDC, I updated the results in a later post but will put them here as well.

Voltage DC - Output tubes from left to right (when viewing the back of the amp)
1 2 3 4
-41.79 -40.70 -43.93 -44.55 Cold, Standby Off
-42.72 -42.73 -46.14 -46.13 Warm, Standby Off
-42.38 -42.39 -45.78 -45.77 Warm, Standby On

 

[Dogs of Doom]

to the forum...

doesn't sound like a lost cause, but, there's a lot of tests you probably should do before installing the new tubes & powering up.

I'm not a tech, but have had my share of issues w/ this amp.

stay tuned, I'll try & find something & hopefully someone more experienced will show up...

 

[Matthews Guitars]

What are those voltage readings again? .016 to .017? Volts? No way that's right. Bias VOLTAGE should be generally somewhere between 30 and 60 volts with no tubes installed. Current measurements require measuring through a 1 ohm resistor connected between pins 1+8 and ground. A normal current range would be generally in the 30 to 45 mA range per tube.

I had a DSL100 that had no audio output at all. That problem turned out to be blown 1 ohm (green body) flameproof safety resistors in the bias circuit. They were wide open. Probable cause: Shorted output tube drawing excessive bias current.

 

[Spanngitter]

- I verified pin 5 was stable (grounded to chassis, positive probe in pin 5) and saw steady numbers in each tube's pin 5 on standby (.28-.030) and steady voltages when warmed up (on for about an hour) both on standby and off standby (.016-.017).

What is that value? Cents, Dimes, Volts, Ampere, DC, AC?
To have the Power Tubes in a safe range you need to measure Volts DC when measuring directly at Pin5 and mV DC when using the Bias Port.
Later one only works when tubes are inserted as this Voltage is a direct result of current flowing thru the tube and a 1R Resistor to GND, hence 1mV = 1mA.
Also pls rmember that you measure for 2 tubes per side so Voltage needs to be divided by 2 to have a per tube reading.
If all this sounds like french or spanish villages to you, pls do you and your amp a favor and hand it over to a tech who is knowing what he's doing.

 

[Gunner64]

I would check the 1ohm cathode resistors associated with the bias test points. I also would change the bias pots, they are known to fail. They're cheap, like $5 a piece.

My dsl 100 amps are set at -72 a pair. Imo the -90 Marshall recommends make the amp sound, let's say not it's best.

And confirm your meter is working correctly, is set correctly, and has good batteries.

With Some meters if you happen to be in the ohms setting at hit it with hv an internal fuse will blow and you'll get craziness. I have seen guys do it. Make certain the meter isn't the problem FIRST.

If you can't chech or do any of the above I agree. A qualified tech is called for.

 

[dhahnke]

What are those voltage readings again? .016 to .017? Volts? No way that's right. Bias VOLTAGE should be generally somewhere between 30 and 60 volts with no tubes installed. Current measurements require measuring through a 1 ohm resistor connected between pins 1+8 and ground. A normal current range would be generally in the 30 to 45 mA range per tube.

I had a DSL100 that had no audio output at all. That problem turned out to be blown 1 ohm (green body) flameproof safety resistors in the bias circuit. They were wide open. Probable cause: Shorted output tube drawing excessive bias current.

Yepp, that was volts. I can take pics or a short video if you think that'd be helpful to see. My concern is all the static that comes through when the standby is set to ON as that's what I heard when a tube blew (air leak from one of the pins).

 

[dhahnke]

What is that value? Cents, Dimes, Volts, Ampere, DC, AC?
To have the Power Tubes in a safe range you need to measure Volts DC when measuring directly at Pin5 and mV DC when using the Bias Port.
Later one only works when tubes are inserted as this Voltage is a direct result of current flowing thru the tube and a 1R Resistor to GND, hence 1mV = 1mA.
Also pls rmember that you measure for 2 tubes per side so Voltage needs to be divided by 2 to have a per tube reading.
If all this sounds like french or spanish villages to you, pls do you and your amp a favor and hand it over to a tech who is knowing what he's doing.

Sorry about that, should have specified. All measurements taken (unless stated otherwise) are in volts.
I re-measured the bias with the standby turned on (waited 30-40 seconds before turning standby on) and could the tubes warming up through the speaker, but there was significantly more static with standby on vs off. Anyways, the right side was sitting around 92mv. The guide I was reading did not state whether bias needed to be taken with standby on or off so that's definitely my bad. My concern though is I'm going to blow something..haha
I have a large set of resistors in my garage, I'll try to find that 1 ohm resistor today.
Thankfully I have a lot of experience working with electronics so what you're saying makes sense and I'm comfortable with, but working with tube amps is new to me so I want to be sure I'm doing things correctly as to not damage anything.

 

[dhahnke]

To add, my biggest concern is the hefty amount of static I hear when turning the standby on. I also noticed as the tubes were warming up with the standby on I could hear the tube pinging through the speaker.

 

[dhahnke]

I would check the 1ohm cathode resistors associated with the bias test points. I also would change the bias pots, they are known to fail. They're cheap, like $5 a piece.

My dsl 100 amps are set at -72 a pair. Imo the -90 Marshall recommends make the amp sound, let's say not it's best.

And confirm your meter is working correctly, is set correctly, and has good batteries.

With Some meters if you happen to be in the ohms setting at hit it with hv an internal fuse will blow and you'll get craziness. I have seen guys do it. Make certain the meter isn't the problem FIRST.

If you can't chech or do any of the above I agree. A qualified tech is called for.

Good to know, I'll look into grabbing a pair.
I mentioned in a post above, I believe I was attempting to set the bias with the amp in standby OFF, the guide I was using didn't specify so I thought that would work fine as I wanted to not turn standby on without verifying no problems. I was obviously wrong
Thanks for the info on the bias voltage, I'll give that a shot. What's the safe max/min voltage that you'd want to see on the bias pins?

 

[dhahnke]

Finally measured the left side and was seeing 278mv with the dial adjusted all the way down (clockwise) I confirmed it was down by turning it counter clockwise and seeing it rise to 310mv. So I guess maybe that's why one tube went dead?
I'm unsure what my next steps should be. I keep seeing the Dr.tube bias drift mod and heard of people getting new boards, but I'm not sure if that's the right direction.

 

[iamme]

I put a new board in and it had parts stuffing issues (the parts populated were wrong, especially in the bias circuit), so new ones need to be checked for correctness...that said if you are comfortable doing the bias stability mods (dremeling around pins and such), then it really does clean up the issue and it cannot fail....(I have heard even the new boards have issue...mine does not, after I fixed the issues with it it has been stable for years...other issues with the early ones (TSL/DSL) are the Dagnall transformers are shit (mine caught fire on the 5v rail of the power transformer...I now have Classic Tone...too bad they went out of business...they were great AND cheap/

 

[dhahnke]

I put a new board in and it had parts stuffing issues (the parts populated were wrong, especially in the bias circuit), so new ones need to be checked for correctness...that said if you are comfortable doing the bias stability mods (dremeling around pins and such), then it really does clean up the issue and it cannot fail....(I have heard even the new boards have issue...mine does not, after I fixed the issues with it it has been stable for years...other issues with the early ones (TSL/DSL) are the Dagnall transformers are shit (mine caught fire on the 5v rail of the power transformer...I now have Classic Tone...too bad they went out of business...they were great AND cheap/

So you think the bias mod would completely fix this issue?
Also, if I'm already going to be digging into the amp, is there anything I should do to it while I'm in there (fix/replace/upgrade)?

 

[Dogs of Doom]

sounds like the 1w 1R resister is dust, the one on the side reading 300mV is the blown one...

I'd suggest doing all the tests w/ the power tubes pulled, make sure it's working properly, then plop in the tubes. No need to blow a brand new set of tubes, while you're figuring things out...

If the bias pots are dust, the bias mod kit does not fix that. It also does not fix the 1w 1R bias resistor...

 

[Dogs of Doom]

my guess is to check these resistors:

https://www.drtube.com/schematics/marshall/JCM2-60-00%20(1997)%20iss1.gif

I believe that's the ones that are for the bias that pop. I'd recommend replacing both, if one is bad.

 

[rocker68]

DOD = words of wisdom.
Gunner knows these amps well.
All great advice.
I always check battery and fuses in the meter first. ( Cos I'm always setting the meter range wrong ). ( Doofus syndrome) .😲
Popular good sounding amp when healthy.
Good luck.

 

[Spanngitter]

Finally measured the left side and was seeing 278mv with the dial adjusted all the way down (clockwise) I confirmed it was down by turning it counter clockwise and seeing it rise to 310mv. So I guess maybe that's why one tube went dead?

Everything which exceeds 100mA per side squeezes an unhealthy amount of current out of the tubes so this side needs to be verified.
If the Bias Resistors R54/R68 have been confirmed healthy then you have another issue like a leaking coupling cap (C22/23) or resistors drifting due to heat.
At all i cannot really recommend usage of the Bias Port without verification by an Bias Adapter which measures directly at the tube as I had too many fails of these 1R seen (and repaired) in the past and also the aftermath they cause when an unexperienced relies on these values w/o counterchecking (either via Bias Adaptor or directly measuring on OT side (either voltage drop or direct current)

 

[Gunner64]

Check the 1ohm resistors dogs has circled, and the bias pots! If the elcheapo pots are bad you will have unstable bias, and that will Lead you on a wild goose chase searching for another cause when it was the pots all along.

Check the easiest causes first. Before talk of board changing, or Dr. Tube kit. ( which is the way I went. The kit solves the drift problem. Period, if needed.)

The pots are so cheap to buy I change them just to rule them out, and piece of mind knowing they are new.

In the 2 JCM2000 dsls I have, the pots in both amps (4) had no stops and had wonky, jumpy readings at the test points.

It absolutely could be the dsl drift situation, or other things guys have mentioned here, but if the pots are bad and are ignored any other diagnostics are futile.

Any repair of the situation you are describing will involve disassembly and soldering, and you being a self described "newby" I can't recommend a qualified tech enough.

There has to be 50 threads on this subject here on the forum. Do a search. Lots of info to graze on.

 

[rocker68]

Solid gold Intel. I'm bookmarking these replies to compile a flo- chart. Very helpful. It seems when attacking a problem amp for simple routine service, a deeper cause/effect rabbit hole appears. Handy to be able to solve surprise issues. Good stuff guys !!!!!!

 

[dhahnke]

sounds like the 1w 1R resister is dust, the one on the side reading 300mV is the blown one...

I'd suggest doing all the tests w/ the power tubes pulled, make sure it's working properly, then plop in the tubes. No need to blow a brand new set of tubes, while you're figuring things out...

I ordered a few resistors just in case they did blow (and because they're cheap). I'll pull the tubes and continue testing.

If the bias pots are dust, the bias mod kit does not fix that. It also does not fix the 1w 1R bias resistor...

The pots are so cheap to buy I change them just to rule them out, and piece of mind knowing they are new.

I was searching for these last night but could not find them except from one dealer who is out for the next few days due to personal issues. Would you happen to know the part number or any other sources?

 

[Gunner64]

Amprepairparts.com has direct replacements.