JCM2000 DSL100 Repair inquiry

Handel85

Member
Joined
Aug 3, 2021
Messages
65
Reaction score
81
I have tried about everything I know how to do to fix my amp. I don’t think it’s the dreaded bias drift issue…yet. I just can’t get the volume issue I’m dealing with corrected (volume is weak and even cranked it’s not as loud as it should be) and I believe I need someone who is willing to let me ship my amp or if you live close by, to work on it.
I did apply the speaker output ground fix for the 16ohm speaker issue, which at least allowed me to hear my amp, albeit at a reduced volume. Prior to that mod, the amp would eventually produce no sound at all after only a few minutes upon being powered on.

I’m in the Lexington, KY area. I can ship as well but wanted to start by seeing if anyone interested in working on it is within driving distance. PM if interested.

And if someone thinks they can walk me through fixing the amp on here, be my guest. I’m at my wits end with it. I spoke about this in another thread so sorry for the similar content in another new thread.
 

Handel85

Member
Joined
Aug 3, 2021
Messages
65
Reaction score
81
Man, I get the impression this amp is not a fan favorite in this forum. I’ve not talked much on here but at least for this topic Ive not had many bites. I get it, this amp came with a faulty board and was not the shining star of the Marshall family. But when it works it sounds so good to my ears. I’d just really prefer to take it to a reputable tech at this point. I know everyone is busy but if anyone knows a good tech in the KY area I’d greatly appreciate it
 

Dogs of Doom

~~~ Moderator ~~~
Staff Member
Joined
Aug 2, 2012
Messages
27,845
Reaction score
39,306
Location
Los Angeles
the problem w/ the JCM2000 series, there was a lot that could go wrong w/ them...

could be cold soldering joints, but it could be anything.

Good luck!
 

Tatzmann

Well-Known Member
Joined
Nov 8, 2021
Messages
1,097
Reaction score
1,581
Had very low volume on mine,
maybe 0.5 watt output.

Bad solder joint on a phase inverter
resistor.

Good Luck.
 

Handel85

Member
Joined
Aug 3, 2021
Messages
65
Reaction score
81
Had very low volume on mine,
maybe 0.5 watt output.

Bad solder joint on a phase inverter
resistor.

Good Luck.
i apologize but I'm very new to amp repair basics. i can take this amp apart and put it back together, do basic soldering, install tubes, and I know how to bias it. that's about it.
do you have a picture of the phase inverter resister by chance or can point me to what it is on the schematic? thanks @Tatzmann !
 

Tatzmann

Well-Known Member
Joined
Nov 8, 2021
Messages
1,097
Reaction score
1,581
Ofcourse, i'll check my notes when im on
the computer.
 

Handel85

Member
Joined
Aug 3, 2021
Messages
65
Reaction score
81
thanks man. interesting backstory with this amp...the volume issue was preceded by me getting shocked from the grounding issue with the 16 ohm speaker jack. i raise chickens and use electric poultry netting, and the degree of shock was close to the amount i get if i touch that electric fence. wasn't too painful, but certainly uncomfortable. after I fixed this, i noticed the volume was like you said, sounded around .5 watts, dimed. I bet when it had the grounding issues, it caused a short in a component, perhaps the resistor you mentioned.

I'd love to be able to fix this without sending it in for repair. repair ETA is 2-10 weeks and i need my amp :)
 

Handel85

Member
Joined
Aug 3, 2021
Messages
65
Reaction score
81
Prolonged use of an amp with an dodgy secondary circuit on its OT can damage the OT, with a likely symptom being that the power output is crippled.
Thanks Pete. Any guidance you can offer on how I can safely test the output transformer with my multimeter? Does the amp have to be turned on?
 

Tatzmann

Well-Known Member
Joined
Nov 8, 2021
Messages
1,097
Reaction score
1,581
jcm2000 dsl100 r54.jpg

It was r54. Poke around there with a chopstick.
Check voltages at the phaseinverter cathodes.
On mine there was only 45v, should be way higher.
 

Tatzmann

Well-Known Member
Joined
Nov 8, 2021
Messages
1,097
Reaction score
1,581
I was reading the other thread you posted in,
your amp seems pretty bad. Redplating powertubes...the heaters on your preamps
are dc-rectified and the rectifier seems bad.

All known JCM2000 issues.

You should get the Dr.Tube Stable Bias Kit.

After youve done installing it we would need
to see some voltages, otherwise its hard
to further diagnose this amp over internet.
 

mrrstrat

Active Member
Joined
Sep 27, 2011
Messages
306
Reaction score
71
Location
Rockford, IL
The JCM2000 DSL 100 head is not a bad amp - I have had a number of them. They do have lots of things that can go wrong in them. Standardly when I would get one I replace all of the bias resistors and rebuild the power section, and change the 220K grid stoppers to 5.6K.

I have found in the past that a low volume output is indication of a single point of failure that is happening. Tracing the stage output from input to tube-to-tube would show where it is dropping out. Start with the tubes - put in a set known to be good or have a tester to test them (and not some 1950s uncalibrated $100 tester off of eBay). I have had a couple JCM 2Ks that ate the cathode follower tubes up that required a nice strong tube in that position. Thats my only real complaint with the JCM 2K but that circuit is part of the Marshall sound. Start with tubes and make sure the amp has a chance to even work properly. Rotate the preamp tubes a few times to be sure it is not a tube problem. Measuring voltages is the next step but if you don't know what you are looking for you can accidentally fry something bumbling around.

Unfortunately the JCM 2K is getting old enough that the ones you get will need some TLC to get and keep going. Many shops I am aware of in the Chicago area are "parts swappers until problem goes away" shops. The good techs are far in between and few these days. This forum is awesome for great advise if you know our way around electronics, but the JCM2K in particular I would not advise to be someones starter learning project.

The advice you are being given is spot-on where to check. Just be careful - If I read it correct the amp has already zapped you. A safety concern right off. A "modern amp" like the DSL should not be shocking you at all. I agree with the red-plating comments in this as well. Bad OTs can be easily checked for but I found that a bad OT usually has other fixes needed and starts to be a repair I turn down as the cost to repair (and stay repaired) creeps toward the tipping point of not being worth it to the person getting the repair. Amps like yours are what I snag to fix and use myself.

I am a E.E. and run a repair shop in the west Chicago area and am Leary of some of the used DSLs out there, as I know of a guy in the Peoria area that for years would gather repairs from the Chicagoland area and work on them in his shop. The guy was an infamous drunk and a butcher/hacker that would jack up amps - he worked on hundreds. Make sure your amp was not one of these bubba-fix amps and does not have signs of someones creative mods that have failed to work. About 1/3 of the repair work I did in 25 years is to fix the hack and chop shop repairs these yo-yos would do. I work on tube radios mostly these days and stopped working on amps as there is too many jacked up ones out there and no real money in repairing them :) . Also since the quality of the standard model offerings from the companies dropped to the point they are disposable - lots of cheap Far East junk. The DSL is hardly an example of a robust build but its not too bad.
 
Last edited:

thetragichero

Well-Known Member
Joined
Jun 6, 2010
Messages
3,446
Reaction score
1,406
Location
upstate ny
for what it's worth 50s/60s tube testers off ebay (or i've found flea markets) are great. likely need some drifted components replaced and many have a selenium rectifier that should be replaced with silicon, but bog simple machines and lots of documentation out there to calibrate yourself if you have a multimeter. just gotta make sure the meter on the tester works unless you have a source for replacement (like my dad's retired EE friend whose house looks like an electronics surplus store). bringing an old tester back to life is a good project for a sunday afternoon
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,456
Reaction score
1,612
Location
Staffordshire UK
Thanks Pete. Any guidance you can offer on how I can safely test the output transformer with my multimeter? Does the amp have to be turned on?
Oops, apologies that I overlooked your query.
A useful check is to measure the OT primary resistances; end to end, then each end to the CT. So 3 resistance readings.
If the end to end doesn’t equal the sum of each end to CT readings, then the OT primary insulation is screwed, OT needs replacing.
There’s a fair chance your old OT failed the above test.

Obviously the amp needs to be totally disconnected from the mains, and deenergised.
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,456
Reaction score
1,612
Location
Staffordshire UK
I have had a couple JCM 2Ks that ate the cathode follower tubes up
Unless used with an indirectly heated rectifier (eg GZ34) to provide a slow ramp up of HT voltage, the DCCF circuit puts its valve under enormous stress when HT is applied.
In the case of an amp with silicon rectified HT / standby, and a DCCF, elevating the heater circuit to about 80VDC, and protecting control grid - cathode from the extremely high momentary forward bias (eg by the of a silicon diode) seem to be good practice to protect that valve.

See ‘A useful mod’ http://www.valvewizard.co.uk/dccf.html
 


Top