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Workin' with a PCB (newbie alert:-O)

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9/10/1999 4:09 AM

Workin' with a PCB (newbie alert:-O)

I've been lurking around a little while. I'm trying to learn more as I read. Now I need to ask for some tips and/or advice.

I have a '73 Marshall 1987 that I'm looking at doing some maintenance to, and possibly tweak it in the future. It has the PCB. Right now I'm looking at replacing the bias filter caps.

I'm curious how you guys go about pulling the board up and clear to get the access needed to do component changes on the board. Do you pull everything off the front of the chassis, choke leads, or what?

I have the filter caps drained and it's in a safe condition. since I think some of you guys have seen these amps a few times I though I'd ask for some pointers on technique. Any help is appreciated.

TIA,

-Dave

9/10/1999 4:12 AM
Dave M.	Sorry about the spelling goofs. -Dave

9/10/1999 12:16 PM

Hi, Dave. The last time I did this to a Marshall I just removed all the pots and jacks from the front panel, remove the nuts holding the board, and then swung the board over for access to the back. Lotsa parts to keep track of, but it doesn't take too long.

By the way, these Marshall PCB's are crap (from a PCB materials standpoint) and can easily be wrecked from too much heat. Here's a tip: IF YOU ARE GONNA REPLACE A PART, WRECK THE PART TO SAVE THE BOARD! In other words, clip the cap out leaving about 18" of lead to the PCB, then heat the lead and remove it with pliers. No need to keep the part intact if you are just going to toss it! This way, the board gets less heat for less time and less stress also.

Good luck!

9/11/1999 3:50 AM

Marshall bias voltage (?)

Thanks for the tips. I got the new parts soldered in, but I haven't solved my "problem".

Marshall guys, I am seeing a variance of -27 to -37.5 vdc of bias voltage. I've heard that I should probably be seeing -25 to -55, or so. I replaced the bias supply filter caps, but no change.

What first alerted me to this was that I could get *no less* than 40 ma per tube when I went to bias the amp. The amp (1987) is running 420 volts at the plates.

So, I'm wondering what else I should check?

-Dave

9/12/1999 11:46 AM

Dave,

What tubes are you using? There are two versions of the Marshall Bias circuit, one for US and another for Europe(EL34). The US version has much higher bias supply available. It's pretty straight forward to swap between the two circuits. Pittman's tube amp book actually has a little section telling you how to convert. You do need to check this out, as 40 mA is a fair bit higher than I'd run most modern tubes in a 1987.

Liam

9/13/1999 12:08 AM

Whoops! goofed up my first reply..sorry.

The tubes are Svetlana EL34. AFAIK the amp is a U.S. version. My first inclination in saying that is the polarity switch on the front panel.

I'd like to see an accurate schematic for this amp. The only one's I have are from Doyle's book, or a few on-line. I haven't checked all the components in this amp, but I have found a couple of weird values so far; i.e. 82k bias splitters, 1k catode resistor from V2A.

-Dave

9/13/1999 10:07 AM

Dave,

I'm pretty sure there's a schematic in the Doyle book for your amp. If you're running EL34's you're going to need the UK spec bias circuit, I would think. I can't get hold of any of my amp books, as I'm moving house at the moment, but if you get stuck, try to get hold of Trace (his email will be on another post somewhere), or Speed. These guys are pretty hot on Marshall problems, and have saved me a hell of a lot of time and trouble. The main thing you need to check is the resistor values in the bias circuit match the schematic, and the bias pot is functioning correctly.

Post again if you can't get anywhere with this. I'll have my books out of storage in a few days.

Liam

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