Marshall JCM 900 Power Board Burn

[Zentech]

Howdy all . New member here . So here is the scoop : A 1990 100 watt Marshall JCM900 DR original EL34 version . The 2K2 4W screen resistors heated up quite a bit , as well R30 was found open cracked and blown , R36 is so well cooked ( see pics ) So , I arrive at seller , ( he knows nothing , but says "it lights-up" as in the power tubes and power switch all come on ) . He has no cab , so I bring my 1x12 for testing . Amp turns on , I see the 4- EL34 power tubes normal heater filaments glow , got no sound with guitar pluged in , and in about 15 seconds I smell that sad electronic component burning smell . No blown fuses . All 3 rear fuses are correct value and not blown . Shut it down immediately . I buy it as is for a good price . Take it home , replaced R30 with 22K 4 watt , replaced R36 , checked all 4 2K2 4W screens which still all tested ok . Powered it up without any tubes installed , all voltages read ok , nothing heating , no burning smells . Installed fresh set of power and pre tubes , Biased it at min travel on bias pot for 455V plate with -37.5 ma bias ... looks good and stable , runs cool , and amp sounds superb with plenty power and volume . Later Replaced the 4 2K2 4W with 4 1K 5w screen resistors .
Ok , so question is : Anyone here might know what could have caused those components to burn ? The 4 Sovtech EL34's that were in it , all tested super strong on my Heathkit tube checker , but I did not chance putting those back in . R30 is supposed to be the HV feed resistor for the preamp section ? R36 , I'm not clear on but supposedly some kind of path to ground ? Strangely no blown fuses ? Stable bias . My deduction would be shorted EL34 tubes ? , or no load ? Anyone here have an idea ? Thanks so much for any feedback . Much appreciated . Could not upload pics here , but included a link to my Youtube with all the pics there . Dirty rock guitar track included .. lol https://www.youtube.com/watch?v=kFgc...ature=youtu.behttps://www.youtube.com/watch?v=kFgc...ature=youtu.be

[alexradium]

Burnt resistore on supply string can easily mean bad filter caps,if they are original from 1990 I would replace them all.
Testing tubes on an emission tester means nothing,its a diode test that doesn't reveal much of the actual working at nominal voltage.

[Mick Bailey]

Welcome to the forum. I'm with alexradium - the cap section immediately after R30 is probably shorting and this would also explain R36 burning. This resistor, in conjunction with the parallel cap (and on some models anti-parallel diodes) is an elevated-ground circuit that raises the amp's ground above the chassis ground noise floor to reduce hum. When you get a burnt resistor you should always look further and not just replace it - resistors don't burn up on their own and this is likely to happen again unless the root cause is eliminated.

That board is pretty burnt. All of that carbon needs to be removed as it will provide a conductive path that will only cause further problems. Sometimes it means careful Dremel work and maybe some track/pad replacement.

Another tip is not to modify an amp that's got a problem - even if it's just replacing screen resistors. Fix the amp first, then consider changes.

Tube testers (to quote Enzo) will tell you if a tube is bad, but won't tell you if it's good. Especially the case if yours is an emission tester

[Zentech]

Alex and Mick , I thank you both kindly for your valuable information . I am with you guys on this and know there had to be a fault / cause for resistors to heat up and burn . For example , I am not one to just replace a fuse and call it a fix . I know there is a reason for a fuse to blow , and want to investigate further as to the actual cause . The pics show how the amp was before with the damage . I have since cleaned the carbon , checked the board and traces , etc and put about 10 hrs of playing and on- time on this amp . The amp has been running very stable and without any issues . It runs extremely quiet at idle ( no hum whatsoever ) and nothing getting hot anywhere . Power Xfo and output Xformers both running cool . I am still wanting to diagnose / investigate the real actual fault / cause . Maybe someone else has come across this same issue on a 900 DR . For now I am just guessing it may have been due to bad power tubes . Thanks again .

[g1]

Also note that they went from 1K screen R's in the design to 2K2 for reliability reasons. So that is the price for that mod.

[Zentech]

G1 , Thanks . Yes , went to 2K2 . I replaced with more common 1K for EL34 . I did not consider that I was per say "modding" anything with that . Those 2K2's took the heat and did not give up . They all still measured 2K2 . I just thought best to replace with brand new screen R's . I'm still not 100% sure what the cause was . I'm attributing it to the EL34's that I took out ...... but still find it strange as they peg the needle as good on my tube tester . I've put a quad of JJEL34 in there and the amp has been running great since . Actually its one of the best sounding 900's I've had , go figure ! I've had about 6 of the different versions . SLX , MK3 , DR's with 5881 and EL34's . This one is killer .