Marshall JCM 2000 repair issues

[wardcband]

Hi Guys

First of all, Im very naive when it comes to technology and the associated terminology so bear with me

Ive had my amp for 9 years from pretty much new and never had it serviced or had any problems with it. then recently it lost a lot of power, no crackels hisses etc, just a loss of volume overall.

I was told about a guy who did repairs locally, so took it to him, who diagnosed the fault, and said i needed a new set of valves. i expected as much, and although a little disappointed at being quoted £120 for 4 pre an 2 outputs (its a 50 watt amp) as well as fitting, didnt have much in the way of options so went with it. I mentioned that Id heard theyd need biasing, and asked if he could do that, and he said that as he was getting a matched pair of mullards, he wouldnt need to bias them. this worried me, but hey, hes the expert.

I went today to check it worked with my cab and guitar and to pay and bring it home, but on plugging in there was just crackling and we couldnt get a sound from the guitar. he ummed and ahhhd, took off the back and felt the output transformer which was hot, and he said it looks like your output transformer has gone. He was however getting a signal with his signal generator, but it was very weak, and he said it had been much louder before Id arrived.

He mentioned the cause might have been that he had been using an 8 ohm load, and me plugging into the 16 ohm cab must have pushed the transformer over the edge.

Now, I dont have the knowledge to know whats going on, and feel a bit trapped into the situation, and worried about how much Im going to be charged now. Does all this sound ok or should I be concerned? As far as i can tell this guy knows his stuff electronically, and he comes recommended from a friend who i trust very much, but he is not a musician at all and doesnt work solely with amps.

I have now left it with him to test the output transformer and see where we go from there.

If anyone could chime in about the type of issue Im having and confirm if the output transformer should be the first suspect, and whether the rest of the work so far sounds reasonable itd really put my mind at rest.

Thanks in advance,

Kris.

[Mars Amp Repair]

Well,
First of all, which JCM 2000 is that amp. JCM number are just like a series designation. The other numbers might be a chassis number like 2202 or TSL-100.

That is a bit weird to have the OT go bad like that, espically if you weren't playing all that loud at the time. I think the jury will be out on that one for a while.

As far as replacing tubes with matched set not needing biasing, he's got a problem there. The Matched set only means tjhat all four power tubes will be drawing the same current at a specific bias voltage setting. That could be 100ma for one set (which would be low for 4 tubes) or 220ma for another set (which would be too high for 4).

Either way, the bias being off doesn't ususally take the OT out. If it is of the TSL or DSL variety, there is a common anti-oscillation disc capacitor that shorts & could mimmic a bad OT to an unexperienced tech...so I'd be careful at accepting this 1st diagnosis. get a second opinion from a tech that at least understands how biasing works!

glen

[wardcband]

hi thanks for your reply,

its the dsl variety, 50 watt version, two power valves not four. i assume it still needs to be biased?

I kinda wish I could just get it back and go somewhere else, but hes already got the new valves and stuck em in, how does the £120 charge sound for new pre and power valves/service/fitting?

He says the powers are mullards (which my friend doubts as they are hard to get apparantly) and i dont know what the pre amps are, they are still ecc83s tho.

thanks again

matt

[bnwitt]

If your tech said a matched pair of power tubes in a fixed biased amp doesn't need to be biased, I would be suspicious of his technical skills period.

[lowell]

You should get your amp back from this guy and take it elsewhere. IMHO. To touch the OT and say it looks as though it is bad... that's an awful assumption to make. A bad OT is last in line when diagnosing such symptoms. It is not to say it CAN'T be the OT... but a bad assumption.

[wardcband]

thanks guys, wnet and got the amp back, and found a good guy elsewhere. However, before I went to get the amp, i told him to take out the new valves, send them back and put mine back in. Shock horror, it works. Like it did before I even had a problem, like its fixed.

Could this be to do with the fact that hes probably changed (by accident) the positions of my preamp valves? Im sure Ive heard that one of the four preamp valves deals with the drive or something, I could be completly off with this, but if that one was dying this would make sense. Im still going to take it for a service with the real tech, just pondering thats all.

Thanks so much for your input

[bnwitt]

thanks guys, wnet and got the amp back, and found a good guy elsewhere. However, before I went to get the amp, i told him to take out the new valves, send them back and put mine back in. Shock horror, it works. Like it did before I even had a problem, like its fixed.

Could this be to do with the fact that hes probably changed (by accident) the positions of my preamp valves? Im sure Ive heard that one of the four preamp valves deals with the drive or something, I could be completly off with this, but if that one was dying this would make sense. Im still going to take it for a service with the real tech, just pondering thats all.

Thanks so much for your input

Sounds more like a loose connection or something, not a dying preamp tube. Could even be loose socket pins. I've got to tell you that these amps are notorius for cracking solder joints at varying locations. This can cause intermittent problems that come and go. I would take the amp to a good tech and have him go through it and beat on it, chop stick it (drop it down some stairs)etc, etc. I just had a DSL401 in here today that had several very poor original solder joints and fatique cracked solder joints on it's board and I see this alot with the JCM2000 series amps. After dissasembling and reassembling the amp for 3 different problems, I just chopsticked the heck out of the board and found two more bad joints at coupling caps. That kept me from having to take it apart two more times or having it come back after the customer pickup. You need a good tech with Marshall DSL experience or you'll be going back and forth for each new problem. I've attached my photo service record on just three of the problems with todays JCM2000 for an example. There were several more solder joint issues and this is usually the case with the JCM2000 series amps. I'm almost ready to buy a stereo microscope to inspect these boards to save some time.

[Mark Black]

My money would be on the "Mullard" replacement power tubes having been bad, especially if they were new "Mullards" (which are nothing at all like real Mullards apart from also being vacuum tubes).

Aside from that the JCM2000 series suffers from a long list of known maladies, most of which can be found by doing a search on this forum based on "JCM2000", "DSL", and "TSL". Cracked solder joints (rampant & widespread), a shorted (and under-specified) ceramic cap near the power tubes, and bad 16-ohm jack switching contacts probably top the list but are not the end of the possible issues. Something along these lines could have been the cause of the initial problem, as most usually cause intermittent symptoms which may at this time be temporarily resolved.

As bnwitt said a tech with a lot of Marshall DSL/TSL/JCM2000 series experience would be a big plus here.

That being said I think the JCM2000 amps are pretty good-sounding and versatile when working correctly.

Best of luck!

[bnwitt]

...That being said I think the JCM2000 amps are pretty good-sounding and versatile when working correctly.

I'd agree with that. The DSL401 I just repaired sounded great clean and had pretty good overdrive tones too. All available via footswitch.

[Mars Amp Repair]

Regarding the 22pf under rated voltage cap...I have now had 2 come back with the replacement 1KV cap shorted.
I am now replacing that little bugger with a 2kv cap...glen

[bnwitt]

Regarding the 22pf under rated voltage cap...I have now had 2 come back with the replacement 1KV cap shorted.
I am now replacing that little bugger with a 2kv cap...glen

Wow! What the heck is going on there in that part of the circuit that would blow a 1KV cap?

Wardcband this is academic to the discussion of your amp as only the 100 watt version has the cap in question.

[Amp Kat]

That amp is pretty succeptable to bad tubes and usually takes out resistors when it doesn't like them and I've seen a few plate resistors go on that amp also. There are two versions of these,one with bias pots on the outside and newer ones without. What I found amaazing is that he blamed the mismatch in loads as the culprit and that isn't the cause for blowing OT's. Now if the speaker came disconnected or open while the amp was cranked for all it's worth then yes that is a cause if the OT is even blown at all. The 1st thing he needs to do is test the transformer and these types are very simple to test either with ohms or voltage from the secondary to primary. I also agree that the Mullard reissues are most likely the whole culprit as I've had problems with almost every pair I've seen. I'd switch techs too.

[bnwitt]

I also agree that the Mullard reissues are most likely the whole culprit as I've had problems with almost every pair I've seen. I'd switch techs too.

Well the original problem was a loss of power with the original tubes in place so I'm still betting on a bad solder joint, dirty effects jack, or some other intermittent problem that has yet to be discovered. If a good tech doesn't give the amp a once over, I would expect the problem to return.

Barry

[Amp Kat]

Well the original problem was a loss of power with the original tubes in place so I'm still betting on a bad solder joint, dirty effects jack, or some other intermittent problem that has yet to be discovered. If a good tech doesn't give the amp a once over, I would expect the problem to return.

Barry

You may be right and I shouldn't have said whole problem because it definitely has other problems. Obviously when it gets moved a certain way is when it acts up so yes a connection,cold solder or even tube sockets and loop jacks could cause it. Those tubes are also problems but I doubt they would cause power loss unless bad out of box which many are.

[bnwitt]

Well Amp Kat, I don't know about you, but I have a problem buying tubes with misleading brand names on them. I mean really, who would buy a Rolls Royce knowing it was really a Yugo under the hood ornament. I find this recent practice of purchasing old (abandoned or trademark expired) brand names and then slapping them on modern tubes that have nothing in common with the originals quite deceptive and a bit unethical.