Jcm800 2204 blow main fuse

[Nicolas S]

Hello, today i played my jcm 800 at moderate volume and i smell like a hot tube . I checked the tube and one or both (i can't remember were redplating ) . I shut the amp and removed the chassis after letting it cooled (it was really hot ) .I checked the bias and one tube was at 34 mv and the other at 29 mv . I reduced the bias to 30 for the hotter of the two and played the amp without apparent problem at low volume for 5 mins .

I rechecked the bias it was ok and i checked the voltages and when doing that i accidentaly touch two pins and it arced and the main fuse blows imediately ...

I think it was just a matter of replacing the fuse so i didn't worried


. I then decided that it was time to replace the filters caps as i have two in hand and i thought it could be the problem . So i did that and i also replaced the power tube .

I put a new fuse of 2 amp (i live in France so we have 220 volts ) but the new fuse is rated for 500 volts and is made of ceramic .

I flipped the main switch, the light and the heaters came on, and then i flipped the stand by and the main fuse blowed imediately ...

I removed the power valve and put a new fuse witch blowed when i flipped the stand by ....

Does i have to worry for my power transformer and how to be sure he's still ok or not ???

help !!!

 

[Tatzmann]

Wiring, sockets, stby switch, fuseholders, selectors, rectifier diodes, filtercaps, caps, need checking. That will get
you started.

Pull the preampvalves also.

Bad powertransformer problem would usually show up while flipping the mainsswitch.

You could disconnect all the secondaries, for checking, but its probably just a diode.

 

[Resident 217]

Thats annoying I bet, the service manual might help

 

[Chris-in-LA]

Take the power tubes out. Turn the amp on. Does the fuse blow?

 

[Matthews Guitars]

Also check the grid resistors. One or more may have burned open.

 

[Marcomel79]

Take the power tubes out. Turn the amp on. Does the fuse blow?

I removed the power valve and put a new fuse witch blowed when i flipped the stand by ....

 

[Nicolas S]

Thanks for the replies, i will check the grid resistor and disconnect the secondaries of the PT to test it . I let you know

 

[Nicolas S]

I did some test and here the results :

There is continuty between the two secondaries and the center tap, same with the two 3.25 v and the center tap

If i mesure the resistance between the two secondaries, i read like 79 ohms

do you think it's ok ?

There is something else i can check ?

Thanks for your help

 

[Kuga]

I did some test and here the results :

There is continuty between the two secondaries and the center tap, same with the two 3.25 v and the center tap

If i mesure the resistance between the two secondaries, i read like 79 ohms

do you think it's ok ?

There is something else i can check ?

Thanks for your help

Turn on power transformer with secondary wires disconected but isolated them. Check voltage.

 

[Nicolas S]

ok i'll do that

 

[Nicolas S]

So i checked secondaries voltage desoldered from the stand by and the CT desoldered also and i got 333 v on each side between the HT and the CT . Looks like a good news for my PT no ?

 

[dtier]

Yes, trans looks good. I would junction test the diodes next as Tatzmann suggested. One or more likely shorted during your incident. You should have a diode junction test on your meter if it is digital.

 

[dtier]

Individually they should read around .6 volts forward and OL reversed. If you get .00 both ways and or a continuous tone they are shorted. There should be 4 for the HT and 1 for bias. 5 total.

 

[Nicolas S]

Bingo !! That's exactely what i was doing at the moment and two are shorted, .00004 something in both directions !!! i hope it just a matter of replacing two diodes . Thanks a lot dtier

 

[dtier]

Should be but I would check out everything else mentioned above when you get past blowing the fuse.

 

[Nicolas S]

so i replaced the diodes, (not easy i had to disconnect some wires to lift the board ), i checked the grid resistors, both are at 1.0k, checked all the connections, replaced the fuse, the power tube and ....
Yes ! It makes noise and a good one !

i biased at 30ma with the plates at 430 volts.

i will keep an eye on the tubes because the former were redplating ..(Russian Mullard el 34)

 

[Ivan H]

Glad you got it sorted @Calvinr. A tip that will help avoid this type thing in future, cover about 95% of your meter probe's metal tip with heatshrink, leaving just the very end exposed. This will prevent the possibility of accidental shorts when probing inside the amp. Cheers

 

[Nicolas S]

That's a really good tip . Thank Ivan