JCM900 SLX 2100

[shortfuse]

I have decided to punish myself again.  I picked up this head for a very reasonable price (so I thought) but once home and playing it, it just did not have the power I thought it should for a 100W amp.  Plugged in the bias probes to check that 1st and couldn't get the bias above 22ma, swapped those tubes out with the set in my 800 same result.  My amp is EL34 and matches the attached schematic from Doug's library. http://www.el34world.com/charts/Schematics/files/marshall/jcm900_slx_100w_2100.pdf
What components around in the bias circuit I did check so far seem to match the values on the schematic and the color codes on the resistors themselves.  I only checked back to the voltage divider at R26 & R27 and had -52 schematic says -48 so I figured that was close enough and to move on.  It was late last night when I did check so i may have overlooked something I will be more thorough tonight.  My 1st question is can the V2 47K trim pot just go bad or loose range?  I ask because I have never come across one that worked fine and just went bad on it own (but then again it's not like I have worked on hundreds).  But not knowing if the bias was ever set right to begin with leads me to wonder.  My first thought was to just change the range resistor which I believe is R28 from 47K but not sure what value I should start at.  I have read here in the past that said to double the value and work higher or lower from there.  I will check all voltages and double check more tonight before trying that.
This amp has also had B3 (bridge) on page 1 of the schematic modified to 4 1N4007's soldered together and modified into the board.  I have read on the Marshall forum this is a common problem with the SLX where B3 burns through the board with these amps but you should see the butchery on the mod that was done.  Looking at a pic of the mod on that forum I think i can fix mine up and bypass the traces straight to the components C8, 8K2 & RD2 if this is causing problems as well. I will post pic's later.

[PRR]

If it's not making full 100 Watts, and not red-plating, the problem is not bias.

(Bias is for the NO signal condition. When you beat the heck out of it, it finds its own bias condition.)

B+ voltages, idle and while playing LOUD.

Sick power tubes?

Driver stage voltages.

[PRR]

> B3 (bridge) on page 1 of the schematic modified to 4 1N4007's

That's a heater rectifier powering 0.9A of heaters (I think). The 1A diodes are NOT enough for this load. Probably survives a 90 day warranty. But they really ought to be 2A, and I would just get a 10A 100V packaged bridge.

[shortfuse]

Purchased new pre-amp tubes on the way home had 4 new EH-EL34's already at the house.  Checked out as much as I could all cleaned all the sockets real well.  Found several very poor solder joints on the rear board and re flowed everything that looked suspect.  Found R35 loose (would wobble to the touch) on the board re-soldered it.  Still could not get bias above 22ma changed R28 from 47K to 22K and was able to bring it up to 40ma.  Cleaned the pots really good and the amp works and sounds great.
As for the B3 mod that was done by someone else it looked like crap but it was working without issue so I decided to leave well enough alone because the potential to make things much worse out weighed the correct fix.  Don't know why they dug into the traces when all they had to do is what is in the attached pic 900br3.
Played it for a few hours and I really like it it is like my 800 with an extra gain stage not real thin and sounding like mush as my old 4100 did.  This one will be a keeper for the collection of an 800, 900, & 2000.

[shortfuse]

> B3 (bridge) on page 1 of the schematic modified to 4 1N4007's

That's a heater rectifier powering 0.9A of heaters (I think). The 1A diodes are NOT enough for this load. Probably survives a 90 day warranty. But they really ought to be 2A, and I would just get a 10A 100V packaged bridge.

PRR - I think I quoted wrong after looking at them again they appear to be 1N5408 (at least that size) but you seem to think I should upgrade then to 10a and yes I agree a package unit would be best.  What is the size of a 10a Bridge I see Dougs has 3a and they are 3/4 X 3/4.  Should I change out BR1 & 2 as well if I am going to be in there?  I guess a better way of asking would be would you change out 1 & 2 while in there if you had to do #3 anyway?
Thanks
Steve

[sluckey]

1N5408s are big enough for a 0.9A load. Can't you look at them and read the numbers?

I would only replace the other bridge rectifiers if there are signs of overheating or they don't work.

[PRR]

> you seem to think I should upgrade then to 10a

I'm a bigot. Especially at today's rectifier prices. If a hardly-big-enough $1 rectifier lets go on the 100th cold-start, that's $25 of my labor/salary plus listening to the irate user. If I have the beast open, I'm going to OVER-size it.

Whoever designed that PCB didn't leave enough room for a proper rectifier. It's too close to the caps and the board edge. There's not much copper underneath (small rects dump much of their heat out the legs to the PCB copper).

I'm a hay-wire guy. If the board is already munged-up, I'd take the PT leads out of their dandy stress-relief holes to get some space. Then take a 4A or 10A in-line FWB, work the + and - legs into the PCB, bend the ~ ~ leads up, solder PT leads to them with shrinktube. Bring that back down and wire-tie the PT leads to the FWB leads or to the old PT lead holes to damp vibration. (I'd have to be in there to get inspired.)

http://www.digikey.com/product-detail/en/KBL04-E4%2F51/KBL04-E4%2F51GI-ND/754841
http://www.digikey.com/product-detail/en/GBU1004-G/641-1367-5-ND/2074832

That is unless I got totally upset and moved the rectifier onto the chassis, ran wires over and back.

http://www.digikey.com/product-detail/en/MP1010G-G/641-1377-ND/2074842

They used to sell 10A with 0.250" lugs so you could use spade terminals. Everybody solders 10A now? have lugs moved up to the 25A class? No matter. The spades were poor connectors anyway. Might look "more professional" but a soldered 4A or 10A does the job and longer than many spade connectors.