No reading when biasing JCM2000 DSL100

[Slinkymcvelvet]

Hi all,

I'm new to the forum and I'm looking for some info. I play rhythm guitar in a Scottish Status Quo tribute band, hobbies also include guitar building and repair so I'm fairly competent with a DMM and soldering iron.

I'm willing to try my hand at most repairs within my knowledge, being a jack of all trades but master of none, but I will back off when something's beyond my capability or knowledge, hence my question here.

I've had a 2nd hand Marshall JCM2000 DSL100 head for over a year now, driving a 4x12 with Celestion GT12-75's at 8ohms. On purchasing, I asked the seller how long he'd had it and when the valves were last changed, and going by his reply I reckoned I'd get another year out of it before having to re-valve it.

However, almost as the year was up, just last week the amp blew the HT fuse. So I took the opportunity to change the EL34's and and purchased new ones.

I changed the HT fuse and changed the valves at the same time, following the prescribed method of cleaning the valve seats with the old valves and contact cleaner. The amp was then allowed to sit on power for around 15mins before turning on the standby, all under an 8ohm load.

When attempting to bias the new valves, I hooked up the DMM with the black "COM" lead to the centre pin and touched the right hand side pin. The reading was 85mv, which I was happy with and so moved over to the left pin. There was absolutely NO reading coming from the left hand side. On touching the left pin, I did however notice a very slight electronic "contact" crackle coming from the speaker I had hooked up (if this is any help) but on touching the right, no sound.

I had no time to investigate this further as I needed the amp that night for a rehearsal, but it seemed to work fine for the 3 hrs I was playing through it. Could I have done further damage?
More to the point, what could be allowiing a good reading on the right side but none on the left? A Faulty bias pot?
What do you all think?

Thanks, in advance.

 

[anitoli]

You need to check the cabinets speakers ohm rating. A 4x12 with GT12-75 would normally be a 16 not an 8 ohm cabinet. If you were running an impedence mismatch you could have some real problems.

Use an ohm meter and find out for sure what the cabinet is.

 

[Slinkymcvelvet]

Hi Anitoli,

I know all about correct impedances re matching, forgive me but I should also have stated that the cabinet is rated at 8ohms and the amp is switched to 8ohms. There is no mismatch at all, so that is not the problem.
What caused the HT fuse to blow in the first place and why is there no reading from the left hand side pin when biasing?

PS, I built and wired the cabinet myself, and although I can't remember the impedance of each speaker, they were all matched and the total impedance doesn' exceed 8ohms.

Thanks anyway.

 

[anitoli]

Ok with the ohms. You could have shorted a power tube. It does happen. This could cause repeated blowing of the HT fuse and could have also damaged some components. Have you looked inside to see if anything looks burned or if a power tube has a funny color?

Meant to say when you lost the HT fuse you probably shorted a power tube.

 

[chadjwil]

It seems to me that if the amp sounds fine, most likely the 1ohm resistor for that pair of tubes has failed. That would be the best case scenario for you since you can check and replace those parts without even removing the board.

If that's the case, you may as well replace them both while you're in there with high quality parts. If you're a little OCD about your amp like I am, go ahead and buy 6-10 of them and use the two that check closest to 1ohm exactly.

 

[Slinkymcvelvet]

Thanks for the input guys, keep 'em coming!
I'm going into the amp this afternoon, (GMT!) and I'll look for burned parts first. As for the tubes, (or valves as I call 'em) I couldn't tell if any glowed brighter or looked out of order when the HT fuse blew. The band was in full swing and then suddenly no sound from me. I didn't have the foresight to put a new fuse in and check the old tubes first, instead I changed everything all at the same time. That's when I discovered there was no bias reading from the left pair.
On inspection of the old tubes, out of the amp, there isn't any obvious discolouration of them.

Keep the suggestions coming and I'll keep you posted.

 

[anitoli]

Look at the drawing:

http://www.drtube.com/schematics/marshall/JCM2-60-02 (1998) iss5.pdf

http://www.drtube.com/schematics/marshall/JCM2-64-00 (1997) iss1.pdf

If R-9 or R-6 were open there would be no sound at all as this would shut off the cathode connection to ground for that particular pair. With those 2 tubes out of the way there wouldnt be any push-pull operation.

You are getting sound.

Maybe there is an issue with the connector for the bias reading pins?
If you were to pull the chassis and measure the voltage drop across R-6-9 then convert to ma you can see whats going on for both sides.

This amp wont work with one side ( both tubes ) not operating.

 

[diesect20022000]

that and running it at 8ohms without the ground mod is asking for trouble man.
look up the joey mods thread on here. but otherwise i'm not a big help. i'm not a tech. just throwing that one your way because it does need done if you plan on using the 8 or 4 ohms without eventualy having OT issues.

 

[chadjwil]

Resistors can fail without showing any visible signs whatsoever. Solder joints can crack or go cold as well. If you had a tube failure it could have overheated one of the solder joints to the point that it no longer conducts...so don't just check the part, check the entire path from the socket to the bias board.

HT fuses just give it up sometimes for no good reason, as in without any circuit failure. A wall outlet voltage spike or anything like that can take it out. It's a fuse...it's there to protect the circuit. It's designed to blow before anything else fries. If you replace it and it keeps blowing then you have a real problem.

 

[chadjwil]

Anotoli, if you look at the bias board schematic the cathode still finds ground through pin 7 and the 33k resistor. Not ideal, but it's still there...I would imagine that would have a negative impact on the sound though. However, if that resistor was shorted somehow you wouldn't measure any voltage drop over it also...gotta do the checks!

Not to disregard your take at all, it's a very good point and sound advice.

 

[chadjwil]

OOPS! My fault Anitoli, I read that wrong. You are absolutely right about the ground on those two tubes. Nice catch

 

[anitoli]

Anotoli, if you look at the bias board schematic the cathode still finds ground through pin 7 and the 33k resistor. Not ideal, but it's still there...I would imagine that would have a negative impact on the sound though. However, if that resistor was shorted somehow you wouldn't measure any voltage drop over it also...gotta do the checks!

Not to disregard your take at all, it's a very good point and sound advice.

You sure about that? The cathodes connects 3-5 on CON1, 1,2,6 and 7 are for the bias regulation. Bias voltage feeds in at R77 then loops to both trim pots. 4 is a ground connection but not in the bias loop.

 

[Slinkymcvelvet]

Ok... found something... R9 has completely burned through.

http://i282.photobucket.com/albums/kk268/Slinkymcvelvet/IMAG0042_zps7d162893.jpg

http://i282.photobucket.com/albums/kk268/Slinkymcvelvet/IMAG0040_zps364fc747.jpg

http://i282.photobucket.com/albums/kk268/Slinkymcvelvet/IMAG0041_zps52dd6f9a.jpg

So... what next apart from obviously replacing that resistor?
What would have caused this?
Strange that you mention that there would be no sound with this fried as it was going the other night after replacing the tubes, unless it burned out as we finished playing and before I turned it off.
Any more thoughts guys?

 

[chadjwil]

I think you need to get an ohmeter in there and figure out exactly what's going on with that part of the circuit before you do anything. I'm afraid that just replacing that resistor might only result in turning it into another piece of charcoal.

BTW, just because it's all burnt to hell doesn't necessarily mean it's not a dead short now. It's unlikely, but not impossible. That would explain the sound being OK but no bias reading. Just saying it worth checking out. If it is, I would feel better about replacing it and firing it up

 

[chadjwil]

Just out of curiosity, have you ever checked the bias before now? Has it ever had readings on both sides?

 

[Slinkymcvelvet]

Just out of curiosity, have you ever checked the bias before now? Has it ever had readings on both sides?

No chadjwil, when I bought it off the guy I took his word that everything was fine with the amp and I played it before parting with my cash.
It seemed to be working fine for nearly a year and it is gigged/rehearsed with at volume for around 3hrs every week.
Throughout this time, the only thing I noticed was a slight electronic/ozone kinda smell coming from it but I thought that may be just the way these amps are. In hindsight, that could have been that resistor burning through.
I never ever thought to check if there was a bias reading until now.

Can you tell me what the exact spec is of R9 so that I can get one ordered immediately from eBay or somewhere?

 

[chadjwil]

1 ohm 1 watt. Like I said earlier, if I were you I would order a handful of precision (1% tolerance) resistors, I used Vishay when I did mine, and replace them both (R9 and R6) with the two that check closest to 1 ohm.

In the meantime though, for your own good, please check out what's going on in that circuit.

The fact that it sounded fine has me bugged out a little, unless R9 is indeed a short now...or you just never knew how bad it actually was. lol! I'm suspicious that something else might be going on there, like the previous owner monkeyed around with it or something like that. Better to be safe than sorry

 

[Slinkymcvelvet]

1 ohm 1 watt. Like I said earlier, if I were you I would order a handful of precision (1% tolerance) resistors, I used Vishay when I did mine, and replace them both (R9 and R6) with the two that check closest to 1 ohm.

In the meantime though, for your own good, please check out what's going on in that circuit.

The fact that it sounded fine has me bugged out a little, unless R9 is indeed a short now...or you just never knew how bad it actually was. lol! I'm suspicious that something else might be going on there, like the previous owner monkeyed around with it or something like that. Better to be safe than sorry

That's fantastic info' chadjwil, and I'll get on to it right away.
Thanks to all the others too for contributing and I'll take it all onboard.

 

[RickyLee]

These guys have given you some good advice. A good thing to do is to take an Ohm's reading on all of the power amp resistors (with the amp OFF and unplugged from the wall of course).

The last set of valves could possibly still be good, or one or two of them that were on that problem side could have went bad taking that 1 Ohm resistor out.

Is this problem side on V7,V8 or the last two valves? Take a look at that small ceramic cap C46 that is across V8 pins 3 & 4. Does it look chicharone'ed? That cap is only rated for 500V and gets cooked easily. You can measure across that and if you see a small Ohm's reading it is shorted. That cap is actually in parallel with the 270 Ohm choke resistor. I would just cut it out entirely, which is what I did on my TSL100.

Also take a reading on the big 5W 1K screen resistors R60 R65 R74 R76. I would also suggest upgrading those small 1/4W resistors in the bias circuit with 1/2W metal or flameproof, while you have the amp opened up and fixing R9. But also, you will need to check some other things after replacing R9 and before powering ON, as chadjwil pointed out. Basically, you will need to put the amp in standby mode and check the bias supply voltage on pins 5 of the power valve sockets.

It sounds as if you were running the amp only with half of the valves operating.

 

[Slinkymcvelvet]

that and running it at 8ohms without the ground mod is asking for trouble man.
look up the joey mods thread on here. but otherwise i'm not a big help. i'm not a tech. just throwing that one your way because it does need done if you plan on using the 8 or 4 ohms without eventualy having OT issues.

Hi Diesect,
Are you referring to this on the Joey Mods thread?...

" One last preventive measure, On the rear of the chassis the speaker output jacks should have separate grounds installed. the 4&8 ohm jacks are grounded through the 16 ohm jack, (which Bypasses via an open/close contact), the 4&8 Ohm jacks when the 16 Ohm is used. Sometimes this contact doesn't close because of Dirt etc. so when You are plugged into the 4, 8, or Both, the Amp is actually seeing No load at all and: Bye-Bye OT and Maybe more. If You use only the 16 Ohm jack this is not a concern. This can be accomplished by soldering a short piece of wire to the shoulder of W6 Ground connection and to one of the metal bands on either the 4 or 8 ohm jack that is closest to the inside rear surface of the Chassis. When this is done however there will be no auto disconnect. ALL 3 jacks will be active ALL the time. Please DO NOT try and run 3 Cabs at the same time!!! "

Once again, thanks for the info, the new resistors to replace R6 & R9 are in transit as I type!