Peavey Classic 30 Help

[stacked]

Hi,
I have just bought a Peavey Classic 30 that was not working. It had blown fuses both the 3a and 1a.
I replaced the fuses and it made a loud hum then blew the 1a fuse but the tubes were still lit.
I powered it down - took it apart and replaced all the main filter caps. The boards look very clean - no burnt signs any where - spelling trouble.
After replacing the caps put it back together and same symptoms.
Can anyone help with where to try and look to fix this amp?
All help is very much appreciated.
Thanks

[Enzo]

Well I would have replaced the four power tubes before replacing caps. Your blown high voltage fuse wants to agree with me. Pull those tubes, does it still blow the fuse?

Alternately possible shorted flyback diode on the output transformer primary. Other thing much less likely.

[stacked]

Well I would have replaced the four power tubes before replacing caps. Your blown high voltage fuse wants to agree with me. Pull those tubes, does it still blow the fuse?

Alternately possible shorted flyback diode on the output transformer primary. Other thing much less likely.

thanks for the reply- pulled all four el84 tubes but left in the 3 12ax7 tubes - same thing - hum then fuse is blown.
Where can i find the flyback diode - Is it in the case on the output transformer? which place would be best to order it from?
Thanks again

[DrGonz78]

The bottom side of the board that has tube sockets is where you'll find those flyback diodes. CR4 & CR6 is what I see on the schematic. They run from plate to ground. Test them with a meter to see if they are bad. You can get those parts straight from Peavey.

[Pedro Vecino]

They are in bad place. It forces the assembly to be removed again.

[Enzo]

But you can test them for shorts without pulling the boards, just probe via the tube socket pins.

[stacked]

But you can test them for shorts without pulling the boards, just probe via the tube socket pins.

Asking a stupid noob questions
1. If I power up without the fuse in - any chance of something else getting fried?
2. what do i set the voltage meter at for testing these socket pins and which ones? And the diodes?
3. if I pull the boards again - Anything else while they are out?
4. should i have the speaker hooked up to have a load on the OT?

Thanks again everyone. Looking forward to getting this fine amp in service - I have a gig next Sat and might use it.

[stacked]

Asking a stupid noob questions
1. If I power up without the fuse in - any chance of something else getting fried?
2. what do i set the voltage meter at for testing these socket pins and which ones? And the diodes?
3. if I pull the boards again - Anything else while they are out?
4. should i have the speaker hooked up to have a load on the OT?

Thanks again everyone. Looking forward to getting this fine amp in service - I have a gig next Sat and might use it.

Looking at this again - Am I just testing for Continuity? Not a reading - In testing for shorts OK but what about those diodes?

[Enzo]

Eye on the prize, we are looking for what is blowing the fuse. If pulling the power tubes didn't help, we want to check flyback diodes, which are CR4 and CR6 on the schematic. Those are wired from power tube plates to ground. SO do this, even easier than at the tube sockets. Unplug the amp and power off. Now look at the circuit boards, ther are three. The center one is narrowest. Look down at the end near the fuses. There should be three Molex connectors near that end. Those are the white plastic connectors with wires. Two of them are crosswise to the long board, and the third is lengthwise. the lengthwise one is also farthest from the fuse end. I mean farthest of the three connectors, not at the far end. NOTE wwhich way the wires are on it. It can be installed backwards. Should be brown blue and red wires. Pull the connector off the board. Now measure resistance to ground from the pins on the board where the blue and brown wires were. Should be the end pins (red in the center). It should measure open or high resistance. A short to ground means a bad diode.

And while you are at it. Hold the loose molex connector in your hand, and measure resistance to ground from any of the three wires. Should read open. (Checking for short to frame.

If it still blows fuses with that Molex pulled off, there is a problem in the circuitry, like a shorted filter cap or some such.

You should always have a speaker or load connected to a tube amp when operating.

[stacked]

thanks for the info _ I think we are getting somewhere. I pulled the molex connector the reading for the brown wire was 326 the reading for the blue wire was - nothing - open. I put a fuse in with the molex off and the 1a fuse did not blow.
So what should i do now?
Thanks again

[stacked]

Also just getting a reading on the molex connector itself they all were open - no reading

[Enzo]

The loose wires - the wires ending in the molex connector - SHOULD all measure open to ground. Otherwise it would mean a short to frame. SO if you measured 326 OHMS (was that it? always include units) to ground from the pin where the brown wire USED to be, then that is a sign the flyback diode is bad. Unsolder the diode and see if the resistance reading turns into open. The other side - where the blue wire USED to be - measures open as it should.

I recommend replacing bad diodes, but for testing the amp will run without them. In the sense your car will operate without the seatbelts, but I recommend we use them anyway.

[stacked]

The loose wires - the wires ending in the molex connector - SHOULD all measure open to ground. Otherwise it would mean a short to frame. SO if you measured 326 OHMS (was that it? always include units) to ground from the pin where the brown wire USED to be, then that is a sign the flyback diode is bad. Unsolder the diode and see if the resistance reading turns into open. The other side - where the blue wire USED to be - measures open as it should.

I recommend replacing bad diodes, but for testing the amp will run without them. In the sense your car will operate without the seatbelts, but I recommend we use them anyway.

Thanks so much yes that reading 326 Ohms was from the connector on the board. On the connector they were all open.
I'll pull the diode and see what happens then report back here

[Enzo]

If the one diode is bad, please go ahead and replace both of them.

[stacked]

I've pulled the diodes but only have an MDL type 1a glass fuse - Is this ok or should i wait until I can get Agc fuse for further testing.
This is without the diodes in - I've pulled both.
One other note - When I turn the amp one there was a loud hum before the fuse blew. This was with and without the tubes in .
Thanks in advance