Marshall JCM900 SL-X 2100 (5881 version) problem

cycopeter@pandora.be · Post by **cycopeter@pandora.be** » Mon Aug 16, 2004 11:53 am

My JCM900 has following problem: capacitor C8 10000µF,16V and diodebridge BR3 (between V4 and V3 heaters) have been replaced due to malfunction. (the C8 had a bulb and BR3 was broken) The replaced BR3 immediately blew again after the first time back in action. When replaced by a bridge with higher voltage/current capacity, the amp started working again but each time after 20some minutes, and when the tubes really heaten up I witness short output errors. The output disappears for a short time (between 1 and 5 seconds) only to return directly afterwards and then again followed by the same error shortly later. The time between errors is not strictly periodic but happens each time unexpected. It seems like the BR3 bridge gets really hot, hotter than normal in my opinion. When VR1 and VR2 (pre-amp volume and gain sensitivity) are turned completely open, the error happens quicker. I was adviced to replace all pre-amp tubes (ECC83, 4 in total) but after having done this (all new ECC83's) I see no improvement. I'm pretty sure the error is in the pre-amp circuit but I can't find it. 3 other technicians, with tube amp repair experience have looked to it but neither of them could find the error.
Is there somebody here who has experienced the same problem, or can help me give some advice what to do next, any help is very welcome.
The three technicians that have looked to it were contacted by a local store and had all original Marshall schematics available. The schematics I have were taken from www.drtube.com . (I don't think there are major differences though. The pcb in my 900 is "SLXa-60-00".
The serial no. is 957554140.
If someone could help me but needs more info, please don't hesitate to contact me. Thanx.

owen291 · Post by **owen291** » Mon Aug 16, 2004 2:38 pm

This might sound farfetched but I read once long ago that when replacing cap, the new ones need to be "formed." I'm not very sure about the details on this since i haven't done this myself. But I know that it requires hooking up the amp w/ new caps to a Variac and SLOWLY bringing up the voltage from 0 to 117 VAC (should take 10 min.) I hope someone can shed more light on this topic.

Post by **VelvetGeorge** » Mon Aug 16, 2004 6:20 pm

These types of problems can really drive you (and a tech) crazy.
There's a good chance that some other part was fatigued when the orginal parts failed, but they didn't fail completely.

Someone will have to put your amp on the bench and let it cook for a while until it starts acting up. Only then can you start to diagnose the parts that are failing.

Where are you located?

George

cycopeter · Post by **cycopeter** » Tue Aug 17, 2004 11:08 am

Hey, I'm from Belgium. I bought this amp 2nd hand but tested it long enough to hear for any problems. Then we practiced a few times without any problem. But being a electronics smuk as well, I couldn't resiste opening it up. There I immediately saw the back cap (see pic) had a big "bulb" , usualy a sign of a bad capacitor. The bridge rectifier (just left from the cap, also had 1 diode maling short circuit. After having replaced bouth , my problem started to occur each time when my tubes really heaten up after 10 to 15 minutes. And then it re-occurs each time like I explained in my first posting.
Anyway, like you said , this problem can really drive me crazy!
Cheers, Pete

jhadhar65 · Post by **jhadhar65** » Sun Apr 17, 2005 9:50 pm

I know this is an old post, so I have to ask if you've solved the problem with your SL-X yet?

cycopeter · Post by **cycopeter** » Mon Apr 18, 2005 12:21 pm

I could fix my SLX but fear I searched in the complete wrong direction. Here's what I did and the problem was over and gone; never had it again.
Look at the picture attached:
I re-soldered all the points on this scheme (didn't change any part), took extra attention to VR3 and VR4, soldered the points very carefully and sprayed a fair amount of contact spray inside the pots.
While doing this, I noticed C16 was kinda loosened from the board and soldered it again.
Then I let the spray dry up and tested the amp. Because the problem always occured after some 20 minutes when the tubes got really heaten up, I waited long enough but didn't find any new errors. Then I took the amp to some practices and it never happened again. Played a show with it , without any problem and used it for demo recordings later. It sounded back like if there never was any problem in the first place. I really was satisfied with it.
Then I found a JCM2000 DSL100 head for a really nice price 2ndhand so the SLX2100 became my backup amp. I did use it a few times later but when I found a Mode Four for again a great price, I decided to sell the SLX. The person I sold it to has the amp for a while now and never got back to me saying there was something wrong.
So I found the problem, fixed it and it's working good again.

Hope this explanation helps ppl with kinda the same sympthons on their SLX2100. I have read quite a few reviews where pppl mention bad solder points on the PCB's. I experienced one myself now and hope I'll have better luck with my DSL100 and Mode Four.

Greetz, Peter