Marshall JCM 2000 DSL FIXED!!

[j45]

Hello everyone,

Enzo was kind enough to explain to me how to Post,

I apologize for interfering with another thread,

Here is the marshall story,

Picked up this amp, when connected to a load(speaker) was making a loud hum and cycling as it warmed up. I suspected tubes,but checked the board first.

Sure enough, R9 was open and burned. No problem, changed R8 (1ohm).Also changed the burned cap 22n to 22n@1kv.
Returned amp to music store and told owner it needed tubes,change them all.
Customer installed 4 brand new EL34s.

Got the call that something smelled and tube overheating.When I got the amp back, the new tubes were barely in their sockets!! I assumed this was the problem and drove them home. Saga begins,tube glow would start and hum also, Bias drifting upwards (one side). Proceded to use schematic,downloaded the wrong one. Enzo started digging in at that point.He was leaning towards tube malfuntion.How could that be? they are brand new?

Downloaded the correct schem. removed tube sockets, and checked. measured all component in bias supply, ALL GOOD!!! What the #%#.

Enzo suggested pin 5 of All the tubes. Lo and behold, since both tubes in each channnel are tied together (clue). something was bringing down that negative voltage.Would start at -40v then drop as amp warmed up.Sounded like a filter bad to me. Finally started swapping out the tubes.Hello!!!!the voltage was correct,after I removed the "bad tube". I put in an old 6L6 just to test,voltages on pins 5 became stable.

To confirm, put 6v into the bad tube and got weird voltage readings, about 1.5 v this is not good, Brand New Tube had failed!!!!!!But would only read bad as it warmed up!!!

Wow, I am a TV tech and little surprises me ,seen lots of weird, but usually new parts are good, well.... 98% anyways!!!!

So there you have this repair story, I dont work on amps much, but this will not deter me in the future, learned alot in just a couple of days,

This is a very high quality site, and appreciate the info I have gotten here!!

Thanks,
Marv

[Alex R]

I get about a 5% failure rate on new tubes I buy in bulk, I think that's about normal. Not like other electronic components, and probably not like how it used to be with tubes - but there are so few plants making them now and in relatively small numbers for a small but hungry market that I guess quality control isn't something they've had to think much about. However there is Chinese competition for the current two (or is it more) Eastern European plants, and I'm sure they will be working on quality (something they had a problem with in the past), so maybe that will improve things. Till then, 'bad new tube' is a definite possibility for diagnosis.

Glad you fixed it! Not too hard compared to TVs I guess.

[j45]

Hi Alex,

Well... Tvs and amps, I guess everything boils down to troubleshooting and the odd "dog".which we all have had.
Yes I did hear the chinese were looking at tube manufacturing. Alot of the non OEM parts I get for tv repair come from there. Thing is, I noticed higher failure rates (over time) on some of them ,Caps and transistors.

The real evil for me is, surface mount, which is the way of electronics,difficult to troubleshoot and repair.

What is still nice is that some manufacturers are looking back to the past and re-engineering old designs, which is great for us from the repair end.Unfortunately this wont happen for tvs. For instance Technics Amps have a stereo module that blows up. The Ic costs $100.00. By the time you are done, the customer is looking at purchasing a new unit.

Hope my next amp repair goes a little easier.

Marv

[Alex R]

When the word gets out with the local musos you'll get plenty of amps, many of them have will never have had an amp repaired, I guess they just sell them on... Usually it's pretty straightforward work, and this forum is a great resource - good luck with it.