That dreaded Bridge Rectifier in my JCM900 SL-X

metareal · Post by **metareal** » Tue Jul 15, 2008 5:25 am

Hi All,

I was having a look inside my JCM900 SL-X (2100) yesterday, as I am planning to do some work on it, and I spotted a burned pcb section and some extension leads added under the infamous bridge rectifier (BR3 on the schematics).
This is what the pcb looks like from underneath the bridge rectifier:

Also here is a shot of the BR itself with a not so healty cap (C8) next to it.

The amp works fine, except for that fact that it is not as loud as you would expect a 100Watt amp to be (but this could also be caused by many other things, ex: 13 years old filter caps, old power valves).
To me this looks like a tech job, where the person that had the amp before me had the BR fail and took it to a tech, who might have replaced the BR and added the extensions to bypass the burnt pdc and give the br legs a bit more beef considering the why it fail in the first place: i.e. overheating.

The BR that is in there now is a W02G, which is rated at 2A and 200V. I do not know what the stock value should be for this (I will phone up marshall today and ask them). I am very tempted to take this out and replace it with something much beefier! What bridge rectifier would you guys reccomend here, so that I dont incur into the same overheating problem again? What sort of specs?

Thanks
Ed.

Post by **Brian Wallace** » Tue Jul 15, 2008 6:36 am

Go bigger with this rectifier. I have done this to a few with lasting results.

Sparky · Post by **Sparky** » Tue Jul 15, 2008 7:24 am

Is that for the low-voltage supply? If it is, current draw should be minimal and I'd be surprised if it was overheating, but I'd defer to Brian if he's seen similar problems with these as I've never worked on one.

That 10000 uF cap looks like it ready to 'splode. I'd change that first.

metareal · Post by **metareal** » Tue Jul 15, 2008 8:44 am

Sparky wrote:Is that for the low-voltage supply? If it is, current draw should be minimal and I'd be surprised if it was overheating, but I'd defer to Brian if he's seen similar problems with these as I've never worked on one.

That 10000 uF cap looks like it ready to 'splode. I'd change that first.

Yes it is the supply for the preamp valve's heaters. I have read of the BR3 rectifier overheating in various places and that is the reason I went to check, I think it is a common SL-X problem. I have a feeling that the stock rectifier was rated at 100V, but I still have to get through to Marshall to ask about it. Here is the circuit for it:

I had a look at bridge rectifiers on Farnell and in this casing (WOG) you can only seem to get up to 2A and 200V which is what I already have...Should I get something with a higher rating even if it is in a different casing?

I will definately change that cap! It looks evil!
Ed.

metareal · Post by **metareal** » Tue Jul 15, 2008 11:26 am

Hi,

Right, I have gathered quite a bit more info on this.
The ratings for the stock BR are: 1.5A 100V. So the one in my amp has definately been upgraded to the largest size available in a BR with that type of casing. I would probably install something even bigger even if that means using a different casing style.

The main problem however, is not heat. The overheating is caused by the BR operating outside of its range, due to the C8 capacitor next to it being too large. This apparently causes the voltage supplied to the heaters to slowly ramp down until eventually the amp sounds quiter and quiter...

I was advised to swith to a capacitor half the size of the stock one (5000uf). If I understand this correctly the C8 cap is a smoothing capacitor for the rectifier, correct? If this is the case, would halving the farads supply less current to the BR making it work properly and supplying the right voltage to the heaters?

Thanks
Ed.

the_haunted · Post by **the_haunted** » Tue Jul 15, 2008 10:05 pm

If you change the cap to a smaller size the hum will go up; you need a lot of filtering to make the DC quiet.
Now, IIRC the SL-X has 3 12ax7's on DC, so the current draw would be approx 1A. 1.5 amp is not enough margin for it to last forever with no heatsink as it will run hot.
I would keep the cap the same.
Just out of interest push in the top of that cap(or cut the plastic off) to see if it's actually puffed up the aluminium, or if just the plastic has puffed up.
I see PSU caps like that all the time, and it's mostly just the plastic cover on top.

metareal · Post by **metareal** » Wed Jul 16, 2008 5:26 am

Hi The_Haunted (awesome band!!)

What do you think, if I replace the rectifier with this:

CP1004

Post by **Brian Wallace** » Wed Jul 16, 2008 7:20 am

I used a 20A square rectifier and mounted it to the chassis, it was all I could get at the time of the repair. It was overkill, but the amp has never had a problem since. I would go with the something double the current draw, or more, of the 12ax7's and put a heat sink on it if you are going to mount it to the board again.