JVM 205H red plating V3 blowing 500ma ht fuse and VERY low Bias voltage on v3

[Smitty02]

Hi guys I need some help with a Marshall JVM 205h. It come in for a re tube and bias as V3 red plated and is blowing ht fuse after about a minute and v3 bias voltage is next to nothing V2 is fine and biases ok bench tested all resistors in bias supply 1k 5 watters are good both R39 and R36 5k6's are ok v3 bias voltage is about -7v v2 I set to 35mv via bias pot, using test points V3 tp voltage is showing -137mv at last check I am confused as why its blowing ht fuse and the bias is screwy on v3 also the bias looks to be tied into the heater supply, I am not real good at reading schematics but getting better still not comfortable with this one wish they posted test voltages....I have not pulled the board yet I did reflow some joints from top as it has 2 sided traces. On bias 2 all resistors check ok.. Fuse wont blow if I pull both power tubes and Im stumped, though I have not flipped the board and tested the caps here is schematic. Any help I would be grateful Also why is it not blowing the bias supply fuse ? Shorted tube socket on V3 ?? I see no evidence of tracking on socket or tube no caps appear to be suspect to the eye....The string of resistors between the bias pots test good but the last 220k voltage goes from -45vdc to -7vdc onward to tube I did reflow some joints and get it up to -29vdc but still popped fuse and went back to 7vdc on V3 jvm205-65-02-iss3.pdf

[g1]

Not sure if you have checked the bias voltage at the socket with the power tubes removed?
C8 or C14 could cause your problem. You could disconnect them and see if the bias voltage comes back up (tubes removed).

[Smitty02]

Yeah i did and it stays the same with tubes out, planned on flipping the board tomorrow and measuring the caps C14 is a litic and appears to look good C8 is a small blue ceramic I traced it up to c12 and c14 off the PI and still got low reading off c13 i believe.. I dont have amp here its at my friends place....Pi tube tested good and also put in a new JJ... But I will flip the board tomorrow and check all of them in the bais supply. I assume that it blew a power tube Just wondering why and what it could have took out... to blow the mains fuse... Thanks for the input I will be checking it out tomorrow any other input is welcome Thanks...I will try to get a pic posted with readings.....

[Enzo]

Measuring a cap with a meter won't tell you much. cap might read 40uf like the label says and still leak like a screen door over 100v, which makes it useless.

Don't even put output tubes in it until you have reliable bias voltage at the tube sockets pins 5.

If the bias voltage is missing on V3 socket, turn off the standby switch. That removes B+ from the PI. Does the bias voltage return to that socket? If so, then C14 is suspect.

C14 is not likely an electrolytic, is it? Only 47nf and 630 volts.

The bias voltage comes through R39 to the grid, is it open? That from R49, is it open? And that through R51, lower left. Is it open? Since BIAS1 seems to be working, we know the basic supply is OK to C35. Follow the voltage through those resistors from supply to grid pin, does it disappear aanywhere? If so precisely where?

[Smitty02]

I may have got C14 wrong going from memory, and cant read schematic well without making it jumbo, my bad. I was thinking of the 2, 20 or 22 uf 160v caps sitting on top the row of bias resistors and pots R395k6 reads good R49 220k also but drops from -45v to like 7v I replaced R49 as the drop was so drastic. No bias does not return with stndby off BTW I also was rushing to post this and got he tube socket wrong I believe Its the V2 socket one farthest from PT checked all with meter and found no open resistors but need to follow it up stream I know I followed up to the big poly cap pair under PI socket C14 and C12 think dont have amp here not real good at schematics the 82k and 100k on PI are ok also.. I will be going to look at it again soon and will post my reading in awhile... Thank you Enzo I will check your suggestions sorry about the wrong socket it is V2 that wont bias but listed as Bias 2 I added a couple shots from the net first one is the biasstring of resistors the top most closest to the bias con is a 220k this is where i get -45v and drops to -7 on the left side yet I traced this low voltage back to the , I know its hard to see but the 2 poly caps in the upper right on top of PI socket top cap reads -7v coming out to bias supply.. Its a strange issue as I had reflowed the resistors in the bias row and the pots also from the top of the board and go bias on V2 up to -28v but with tubes in it it soon blew the 500ma ht fuse not the bias supply fuse ??

[Enzo]

Whichever side has lost bias, follow its bias voltage from the supply, through the resistors to the tube socket. It disappears somewhere.

[Smitty02]

Hi I just got back from looking at again C12 is where I traced it back to C14 is passing good bias voltage, I heated up the c12 joints from the top of the board tested it and had decent but not the same as V3 bias voltage but when I started to turn up the bias pot V2 I got to -28vdc and I lost it it drifted right back down to -7vdc.. At C12 the PI side of the cap (473 630v) read about 270vdc and went down to -7vdc .. I pulled this cap and am going to replace it and figure I will replace c14 while I am at it... I am not sure that this is the culprit and could a shorted V2 power tube cause this ? Is it possible the 47k bias pot is wonky ? as I turned it up it came up good till I got to about -28vdc and at c12 I had -36vdc at the time it drifted down....better but still weaker than c14... Sound like I am on the right track ? Thanks BTW Im not real good at tracing a power supply at sometimes there are other things running off of it, and this is a 2 sided board with traces on both sides, I have yet to pull flip the board I was able to heat up the joints from the top and carefully pull each leg of c12 up without damage both had straight leads, a plus.. Been looking over the schematic and it looks to me like C12would effect the good bias supply on V3 yet V2 is where the low bias is unless I am not looking at this correctly Swear bias 2 is the problem side, tube farthest from PT, C14 is got good -V aaarrrggh LOL

[Smitty02]

I replaced C7 C8 C12 C14 replaced resistors on bad side Re bias'd and all is well... Case closed Thanks Enzo and G1