Marshall JCM800 BASS

Szybas · #1 11-13-2010, 05:03 AM

I've got Marshall JCM800 100W Bass head. It's definately a copy. Outer look suggest cheap and kind of lousy construction, however when I look inside I see everything being handmande and FULL tube constructuction.
-one tube rectifier
-three tube preamp
-four tube poweramp

Anyway one problem occured recently: I can't get any sound from it beside strange buzzing coming from inside of the amp. My father suggested that it is transformator problem or damaged solder joint.

Would You help me how and what to look for to repair it?

phatbass · #2 11-13-2010, 05:11 AM

Tube amps have internal MAJOR/LETHAL voltages; even with the power off the capacitors remain charged.

They are one item you definitely DON'T want to "have a go" at fixing yourself unless you know what you're doing.

MAMMOTHvolume · #3 11-13-2010, 06:27 AM

+1 tech time

T-Bird · #4 11-13-2010, 07:36 AM

Hi.

Another vote for tech time.

Do post pics of it though, perhaps someone can identify what it actually is.

Regards
Sam

delta7fred · #5 11-13-2010, 08:15 AM

Quote:

Originally Posted by phatbass

Tube amps have internal MAJOR/LETHAL voltages; even with the power off the capacitors remain charged.

They are one item you definitely DON'T want to "have a go" at fixing yourself unless you know what you're doing.

+1000

I only got a 400v jolt once, that was enough (and I am an EE and am supposed to know what I am doing.

Here are a few 'rules' when dealing with tube amps, there are probably a lot more but these are a good start.

1. Don't open it up unless you understand the risks and know what you are doing.

2. If you do go in then always put your left hand behind your back or better still in a pocket. Never ever hold the chassis with it.

3. Always discharge the smoothing capacitors after you switch off and before you do anything else.

If you don't understand 2 and 3 then take it to a tech.

chadds · #6 11-13-2010, 08:23 AM

Number 1!!!!!!!

delta7fred · #7 11-13-2010, 08:37 AM

Quote:

Originally Posted by chadds

Number 1!!!!!!!

Yea, I could probably have worded it better

1.2.3.4.5.6.7.8.9 Take it to a tech

Szybas · #8 11-13-2010, 08:47 AM

Ok, thx guys, although I understand what I am doing, I'm not qaulified person but I know basics of electronics, however I'm gonna take it to the tech cause I don't want to mess up my amp.

Mr. Foxen · #9 11-13-2010, 08:54 AM

Sounds like he already has it open. No tech you take it to won't have been there once, looking blankly at a bunch of amp gubbins. As long as its not plugged in, you can't connect yourself up to the mains, just get a nasty belt from it, not ideal if you have a weak heart, but probably educational rather than fatal. Have you verified you cables and cabs are all fine? Do that first, they are safe and passive, and have solder joints same as in the amp, first thing to learn to recognise, good and bad solder joints. Other stuff will require a basic grasp of electronics, such as the difference between a resistor and a capacitor, impedance vs. resistance, and why voltage isn't the bit that kills you. If you have that sorted, add an understanding of how valves work, and find a schematic, and you should be good to go, if not, learn it before trying to work on amps. I want to encourage people to learn the inside of amps, cause there aren't enough good amp techs around.

If dude is determined to try and fix it himself, helping him do it right is better than telling him he shouldn't.

T-Bird · #10 11-13-2010, 09:10 AM

Hi.

Quote:

Originally Posted by Mr. Foxen

I want to encourage people to learn the inside of amps, cause there aren't enough good amp techs around.

An old, non-working tube amp with a questionable past is not the safest way to learn. At some point, which comes pretty fast, one has to work in there with the mains voltage on.
For an inexperienced DIYer: A VERY BAD IDEA. A 500V jolt through ones finger will restrict ones playing for a long time. I know that for a fact.

Quote:

Originally Posted by Mr. Foxen

If dude is determined to try and fix it himself, helping him do it right is better than telling him he shouldn't.

A bit of an analogy that I read from some old MI magazine way back in the stone ages, modified to the present:

I know my way around vehicles. I wouldn't dream of helping someone over the 'net if he/she wants to fix bad brakes on their car. That just puts their well being as well as others at risk. A deadly risk.

There's no telling how lethal or potentially dangerous any el. equipment is unless a qualified tech goes through the wiring and tests the insulation etc..

I'm glad to see that Szybas plans to take it to a tech. A wise move if anyone asks me.

Regards
Sam

Szybas · #11 11-13-2010, 09:19 AM

@Mr.Foxen
Back of the amp is already opened. It was like this since I'd bought it. I can see all the tubes, wires, some solder joints and transformators.

alembicguy · #12 11-13-2010, 09:27 AM

Take it to a qualified tech. DYI over the internet is a really bad idea when dealing with electricity!

Mr. Foxen · #13 11-13-2010, 10:08 AM

Try and find a pro recommended by someone who has used them and knows what they are up to, just because someone charges a lot of money to do a job doesn't mean they'll put the same care and time in that you could, see here: http://forums.vintageamps.com/viewtopic.php?f=9&t=94526

Warning, DIY over the internet content. I got the bill with the amp, and a dreadful job was done very expensively, and it still didn't work right.

delta7fred · #14 11-13-2010, 11:08 AM

I am the last person to discourage people from learning how to do things for themselves but I have to agree 100% with T-Bird, an unknown, non working tube amp is definitely not the right place to start learning electronics.

If you really want to learn start on the basics, simple one transistor stuff and work up, once you get a reasonable grasp of transistors you can move onto tubes.

It could be a while though before you get competent enough to repair the tube amp.