Marshall JCM 900 fried! Need help :)

[Audiotexan]

JCM 900 series was 1990-1999
<link snip>
That's why I don't suspect bad caps. Not saying they couldn't be, but this amp isn't that old and all those caps are in the low voltage section, so again I don't suspect them.

Thanks for clearing the foggy memory. Yeah I was thinking 800's
Still, at roughly 20 years old...and under the stress of a tube guitar amp's conditions (especially what folks tend to put Marshall's through "If it's not on 10, you're not playing a Marshall" mentality), if you're in there for any reason, I'd yank em! Again, they're cheap! And it's good insurance. But I'd completely agree if they were 5-10 years old (eevn though I'd still test them on principle of label shrink).

Respectfully,
AT

PS: phenomenal catch on R8 and R9! Didn't even see those until you mentioned them!

[audiopete]

Thanks AT. I agree, better to have new caps then wonder. The only thing is if you don't have a good desoldering gun or experience, you can make a big mess of it. Those boards can be tough to access with all the wires attached, although with this one, pulling the pots and jacks from front/rear panels should make access easy.

[sibo]

Just to confirm, R32 did have a little burn mark on there (great eyes man) as did R8 and R9. I measured and they are within the given 10% values when off but who knows what happens when its on.

Am I right assuming that one board (the one where power goes in) is called power section and the other preamp section? If so, looks like C14 on power section and C41 on preamp section has a little brownish dot on them as well. I scraped it off and couldn't find any hole under but maybe I should change them anyways?

There's a capacitor missing in C5 power section, is that alright?

Can I harm anything by trying to change these components and not the tubes at first? After all the amp isn't really mine so I wouldn't wanna spend more dough than necessary

[audiopete]

Check and make sure the fuses are the right value - slo blo 500mA (the "T" stands for time delay blo). The LEDs are probably fried too.

[audiopete]

Can I harm anything by trying to change these components and not the tubes at first? After all the amp isn't really mine so I wouldn't wanna spend more dough than necessary

Pull the power tubes while doing any testing or work. I can almost guarantee one or more are bad and if you fire it up again you will do the same damage potentially. Get a new quad of fresh EL34s....its necessary.

[J M Fahey]

(had to reassemble cuz of my daughter running around x)

This is also part of a repair: get someplace where you can work unmolested (kids/wife/pets) so you can concentrate.
And where you may leave that amp open for days, until it´s repaired.
If anything, cover it with a tarpaulin or at least some old bed cover.

Besides the plain time loss, assembling/reassembling unfocuses you, the contrary of what you need when doing deductive, analytical work such as troubleshooting.

That does not mean that every hour or two you leave the workbench and go cuddle and play with your beautiful daughter

Which by the way flushes your mind, when you come back you see things that you didn´t before.

[sibo]

Haha word J M Fahey, ain't easy having kids running around.. I usually have my workbench but at the moment stuff are stuffed in the way so this will have to do for now I'm afraid.

Buying tubes in Sweden is fucking expensive and if I would import I might aswell turn it in to an qualified tech and pay about the same so I decided to place an order of four matched EL34L JJ Electronics since they were bloody cheap. I did some reading and seems they have more headroom which probably is less desirable in a Marshall for most people but that'll have to do!

Next step is to order all components. Given that I posted pictures of the board, is there any schematic that I dare to trust for reading values now that I know I will be replacing R8, R9, R31-34, C13 and C14 in power section - C41 in preamp section? Did I miss anything?

[sibo]

To clarify,
for example I don't know what to replacement resistors I should order for R8 and R9. Can I check some schematic for their values knowing they're called R8 and R9 in the power section?

I also ask this because I want to know no one modified this in a bad way before I got my hands on it. If so I'll get right values of what I'm about to change.

When I browse these components there are many choices available. I tried to sum it all up so it should be easy to read.. Please do correct me if I'm wrong:

Power section
R8 and R9: Wirewound resistors with silicon coating unknown values
R31-34: Wirewound resistors with ceramic case. 2.2 kOhm, 4W
C13, C14: Aluminum electrolytic capacitors with axial connections. unknown capacitance, 100V

Pre section:
C22:Aluminum electrolytic capacitor with radial connection. Unknown values
C29, C30: Aluminum electrolytic capacitors with axial connections. Unknown values

Thanks!

[sibo]

Bump.

Also, I noticed that the cable from the head to the speakers seems a bit on the thin side and I can't remember if that was always there or if it's something he put there.. It looks like one of those cheap angled "guitar starter cables" that are thin and cheap. In my book that might at least add up to the problem..?

[g1]

What is the exact model number of this amp (ex. 4501, 4102, etc.)
Yes, they used a very flimsy looking speaker cable, I don't recall ever having a problem with one unless it was damaged/abused.

[sibo]

4102, Hi gain dual reverb 100W combo.

I'll change it just to make sure it isn't a problem.

Any thoughts on my previous posts regarding those components?

[g1]

R8,R9 These are the resistors in parallel with the OP valve fuses? 100K 1/2W
C13,C14 10uF 100V
Pre section C22 - parallel with D4 zener? 1000uF 16V (should be same as C21)
C29 - parallel with R48? 10uF 16V
C30 - connects to TR1? 2.2uF 16V

[sibo]

For R8 and R9, can I use carbon film resistors instead?
Also, for R31-34 can I safely choose a R with higher rated effect? For example 11W.

[g1]

Yes and yes. For R31-R34, check that the physical size will not interfere with the mounting.

[sibo]

Now I've got all components and tubes. About to start working but before I do, how do I unload the capacitors safely? People always talk so much about lethal voltages being stored for days in those.. Can I just use a copper wire from anode to cathode on that big cap or do you have some other suggestion(s)?

Tyvm!