JCM800 2203 No bias voltage. Help!

[retrobob]

I just bought what was said to be a working amp. But was found to have a blown fuse, and one bad power tube.
Replaced the 4amp fuse. Have a new set of EL84s, but I have no bias voltage measuring pin 5 on the OP tubes to ground. ?
Can someone help me find the problem? :bkw

[Blue Strat]

I think you mean EL34s.

Check for voltage on the other side of the "grid stopper" resistors (the ones connected to pin 5). If the voltage is there but not on pin 5, the resistors are open. It's very unlikely for both to be open, so the problem is likely further back. Check the 220K (or whatever value) bias feed resistors for voltage, then the pot, then the resistor before the pot. Sounds like an open connection somewhere.

Note that running an amp with no bias voltage will continually cause the power tubes to run wide open, possibly short out, possibly take out the screen grid resistors (on pin 4), etc.

With the checkered history of this amp, I'd be replacing all components surrounding the power tubes and bias circuit. This is about $2 worth of parts.

[Swarty]

I would expect an amp with no bias voltage to take out the 1A HT fuse, not the 4A mains fuse. Make sure you didn't stick a 4A fuse in the fuse holder labeled "HT". In my experience the most common failure in the bias circuit is one of the 10uF filter caps shorted.

[phsyconoodler]

You need to rebuild the bias supply first and do your testing with no tubes in the amp.When you get the correct negative voltage then you can test with tubes in.I would suggest replacing the diodes and caps related to the bias supply.Don't get tempted to not change the diode.

[John Phillips]

I would expect an amp with no bias voltage to take out the 1A HT fuse, not the 4A mains fuse. Make sure you didn't stick a 4A fuse in the fuse holder labeled "HT". In my experience the most common failure in the bias circuit is one of the 10uF filter caps shorted.
My experience too - although usually, the bias voltage doesn't drop right to zero.

BTW, if this is a model with the pots mounted on the chassis, you can still get at the underside of the board to do the job properly by undoing the pots and jacks, and lifting the whole thing up. Do NOT be tempted to do it from above the board by snipping the old components and soldering the new ones to the ends of the wires, the joints on the underside are easy to overheat and you'll only have to lift the board anyway to do it right after it's failed again.

[Blue Strat]

BTW, if this is a model with the pots mounted on the chassis, you can still get at the underside of the board to do the job properly by undoing the pots and jacks, and lifting the whole thing up. Do NOT be tempted to do it from above the board by snipping the old components and soldering the new ones to the ends of the wires, the joints on the underside are easy to overheat and you'll only have to lift the board anyway to do it right after it's failed again.

+1000

I suspect a lot of problems like this (no bias voltage) are the result of people replacing bias supply components, like the bias range resistor, from the top of the board. It works temporarilly but is a very shabby way of doing things and can lead to results like these.

[retrobob]

Thanks everyone. Yes I did mean EL34s. The 4 amp was the blown fuse, not the 1a HT. I did replace the diode that leads to the 10uf caps.(still nothing) Seems I'm not getting any voltage reading from the neg. side that diode, the 10 uf caps, bias pot and the resistors in that area, including the two 220k resistors that connect the grids. The resistors values measure ok. I'll try replacing the 10uf caps and see what happens.
BTW the tubes have been out and I have been doing the work from the underside of the board, no clipping!

[TheAmpNerd]

Check the AC side of the diode using AC scale.
If no AC, then caps are not your problem
Check somewhere before diode in bias circuit.

If AC, and diode conducts DC..then
problem is down stream of diode.

Check each node until problem is found.

Else, problem is diode.

Good luck keep us informed.

[retrobob]

Turns out, its a bad power transformer. :BITCH No voltage from the white wire that goes to the board. Took it out and removed the endbells. Looks fine but smells burnt. Time for a new tranny.

[John Phillips]

That burnt smell is quite distinctive and definite, unfortunately... :(

[Swarty]

If it is just the bias winding that toasted, you don't need a new PT. Look at the schematic for a 50w. They get their bias voltage from the secondary. You can do the same with a 100w. Just look at the schemo for a 50w... basically move the white wire to one of the PT legs on the standby switch. But before you do that, examine the circuit carefully. I suspect something shorted that caused the bias winding to toast/open.

[John Phillips]

If it is just the bias winding that toasted, you don't need a new PT. Look at the schematic for a 50w. They get their bias voltage from the secondary. You can do the same with a 100w. Just look at the schemo for a 50w... basically move the white wire to one of the PT legs on the standby switch.And increase the value of the limiting resistor by a factor of ten! Otherwise you'll get something like a couple of hundred volts on the bias circuit (I don't know exactly, I've never done it) and blow up the caps.

I did think of this too, but given that the transformer is already out and dismantled, it really isn't worth putting it back now. Burning smells from the winding are a sign of major trouble, and I wouldn't count on it not shorting somewhere else too. If it didn't smell bad, I might take the chance that it was just a connection on the bias winding that had come apart inside.

I would also strongly second checking the bias circuit for shorts, especially the caps... although I recently repaired a JMP 2203 that had a blown PT, for no other detectable reason - it had just been turned on after years in storage, and the HT winding smoked - I'd guess the transformer was damp.

[Swarty]

Yeah, the 50w shows a 220K resistor and 27K on the 100w. There is a good possibility that only the bias winding would be affected by a short in the bias circuit. If filament and B+ voltages are OK I'd give it a shot.

[John Phillips]

Actually, will it work at all? The reason it does on the 50s is because the HT winding is center-tapped to ground so can be used to generate a negative voltage, but on the 100s it's floating and bridge-rectified, with the center tap providing the half-voltage reference for the first filter stack... at +245V :).

Maybe that's why the 100W transformer has a separate bias winding!

[Swarty]

I'm thinking it will work. It is AC until it gets to the diode. You will probably have to play with the voltage divider to fine tune the voltage.

[TheAmpNerd]

I agree with JP.

You got the tranny out. Stick in a new one and save the
original to go along with the amp if you ever sell it.

The process of the power tranny burning has already started,
the bias winding is already toast.

I'd get a MM power tranny in there and be happy knowing that
it will last for 100 + years. That is a lot of peace of mind when
you start playing that amp cranked up.

[John Phillips]

I'm thinking it will work. It is AC until it gets to the diode.Yes, but there's no ground connection to the winding, so you can't develop a voltage relative to ground - not a negative one anyway, since the diodes at the ground end of the bridge rectifier will block that.

(*I think! No coffee yet this morning... *)