Marshall JCM 2000 repair issues

[bnwitt]

dcline,
The capacitor you mention is not in the DSL50 but only the DSL100. It is C46 in that amp shown on the power section schematic coming off of V8's G2 terminal.
Barry

[AmpFix]

Or any other suggestions. I've worked on quite a few of these myself and have seen many of the problems mentioned in the forum but this disc cap thing is a new one for me.
Thanks in advance

As mentioned earlier, that cap is not on the DSL50.

try plugging into the effect loop in to see whether the problem (hum) persists. If it does, the problem is in the power amp, if not, its in the preamp.

[dcline]

aaaaaa it cleaned right up and got quite a bit louder. No hum... So I have preamp problems...
I've already switched out tubes... Any other suggestions?
Thank you!!!

[AmpFix]

aaaaaa it cleaned right up and got quite a bit louder. No hum... So I have preamp problems...
I've already switched out tubes... Any other suggestions?
Thank you!!!

Next step, which is easy, is to remove each preamp tube one at a time with the red channel selected (goes through all the tubes). After that, it really depends whether you are qualified to work in a live electronic device like a tube amp.

[The Dude]

1) Put a cable between effects send and return and try the amp. You could simply have a bad switching jack in the loop.
2) Have you checked the solder on the input jack itself?
3) Check voltages on preamp tube plates and/or check all of your plate resistors. You could have a plate resistor that has started to go towards open.

[dcline]

I'm self taught, but have actually been repairing amps for a few years. Still learning. I've even built a couple.
What should I expect during this procedure?

[Audiotexan]

<self EDIT>
Reading from the start to maintain context/progression: recommended!

Failing to notice the date on the thread before commenting: priceless.

/facepalm.


(BTW, this is not a comment about anyone but myself.) >.<

[dcline]

V3 is acting very strange... hums really bad when touched... tried replacing and it got very distorted.
Wish I had a voltage chart for this amp.

[dcline]

Tried the cable in the FX loop. No difference.
I've spent all my time today on the output section checking voltages, resistors, solder joints... Time to check everything in the preamp section. Gonna pull the board with the input jack tomorrow. It had some bad solder joints that I fixed. I think the bad connections caused the problems I'm having. When the owner called me and told me this issues he was having, I told him to hit the top of the amp and let me know what happens. Well, he hit it and there has been very low output ever since along with a slight hum. Gonna get back to this tomorrow. Thanks again everyone for your help.

[dcline]

Sure enough, it was an open plate resistor on V3. R1 to be exact.
Thanks everyone and especially thank you jcmjmp for the tip on tracing it to the preamp section!

[clintronics]

Just had one of these with low volume, but sometimes normal, intermittently. turns out BR1 did not have enough solder on it

[bnwitt]

I wonder whatever happened to wardcband the fellow who started this thread.

[fatcharlie]

Hi All,

It may be that this thread is dead but I thought I'd give it a go before starting a new one.

I have the same issue on my JCM 2000 - DSL 401.... the green channel light is flickering and the power is cutting in and out. I have read this thread and tried disconnecting and reconnecting all the leads (with the amp switched off, diconnected and left for several hours in case of capacitors still holding charge), I've pulled and pushed the speaker jack in and out several times to try to dislodge and accumulation of dirt. I also wiggled (gently) all the valves in their sockets, pulled them slightly then pushed back home without actually taking them out. All of the previous is just to try to make sure the connections are all good.

I'm still getting the same problem. Thought I'd solved it once - got all the way through Mary Had a Little Lamb with no probs. I tried a Clapton number and it started again - maybe it just doesn't like EC?

I can hear a clicking noise when the amp is cutting in and out but I'm blowed if I can locate exactly where it's coming from.

Bottom line is I can't afford to take it to a repairer - no cash.

Can anyone help by leading me through a diagnostic/repair method?

many thanks,

Roger.

[fatcharlie]

Is this thread still active?

Hi All,

It may be that this thread is dead but I thought I'd give it a go before starting a new one.

I have the same issue on my JCM 2000 - DSL 401.... the green channel light is flickering and the power is cutting in and out.

I've had the amp for 19 years and it's been a gem - it's had a hard life, being stored on my boat when I was a travelling musician in the Mediterranean, then stored in the garage for the last couple of years. I'm kinda thinking it's simple bad connections but i thought I'd have a look online first.

I have read this thread and tried disconnecting and reconnecting all the leads (with the amp switched off, diconnected and left for several hours in case of capacitors still holding charge), I've pulled and pushed the speaker jack in and out several times to try to dislodge any accumulation of dirt. I also wiggled (gently) all the valves in their sockets, pulled them slightly then pushed back home without actually taking them out. All of the previous is just to try to make sure the connections are all good.

I'm still getting the same problem. Thought I'd solved it once - got all the way through Mary Had a Little Lamb with no probs. I tried a Clapton number and it started again - maybe it just doesn't like EC?

I can hear a clicking noise when the amp is cutting in and out but I'm blowed if I can locate exactly where it's coming from.

Bottom line is I can't afford to take it to a repairer - no cash.

Can anyone help by leading me through a diagnostic/repair method?

many thanks,

Roger.

[oc disorder]

Hi fatcharlie, Easy for me to answer from the other side of the world ! I think in your case the quote from bnwitt all those years ago applies !
" I've got to tell you that these amps are notorius for cracking solder joints at varying locations."
Enzo , the Guru on this forum usually suggests to make a ball of your fist and thump the top of the cabinet !
With failing solder joins (sometimes referred to as dry joints) they get aggravated with vibration so Mary Had A Little Lamb is a little too gentle whereas a Clapton number perhaps one where he's "fanging into it" may aggravate the possible faulty solder joins !
The fact that your light is flickering but the amp still works leads me to that .. it's got nothing to do with the burnt/faulty jack socket .
Ideally you need to be able to dismantle the amp and preferably have a temperature controlled soldering iron and skills to suit !
Anyway one step at a time .. thump the amp and get back here with the results ! BTW Welcome to the forum.. Usually its a good idea to start a new thread and perhaps provide a link to a thread (like this one) to refresh our memories... some of the people in this thread haven't posted for ages.
The fact that you have had the amp for 19 years without failure is remarkable! Even more so when you admit it's had a hard life !
Surely you can rake together 150 bucks to support a struggling good tech? Don't eat for 4 days and drink water , I'm sure you wont die !
That's why Jimmy Pages amps never failed ! Well you did choose fat in your user name and I'm an Auzzie !