Problem with Valvestate 8280

[Johnrcurry]

This amp was working but crackling and distorting on both channels. It sounded like a bad solder joint. Poking around, I got a reaction/noise at the ribbon cable from the preamp board to the reverb board. Examining the reverb board, I found both loose solder joints at both regulator 1 and regulator 2. I reflowed/those joints and checked the remaining joints on the board. All looked good and I reassembled to try it out.

The amp played fine for about five minutes with no more crackling/distortion on either channel. Then there was a very harsh noise for about 5 seconds and all sound stopped.

Now, when power is turned on, there is a thump in the speakers, but no guitar sound through the amp. The channel switching, chorus on/off leds no longer light. I would appreciate it if someone could direct me where/how to start looking for the problem. I have precious little experience working on solid state amps.

The schematic is posted below. Any help will be most welcome.

stereochorusrev_2x80w_8280[1].pdfThanks

[Jazz P Bass]

I would suggest going back & rechecking your work.
At the very least, check the output voltages on the regulators.
It sounds to me like one of them went belly up.

[Johnrcurry]

Thanks for your reply, Jazz. Checking those regulators gave me these results. . . .

The 7812 (positive regulator) showed about +19v on the input and +12v on the output.

The 7912 (negative regulator) showed +4.9 on the input and +6.1 on the output.

That reading on the negative regulator doesn't make sense to me. It seems to me that even if the regulator had gone belly up, I would get a proper reading on the input. Am I wrong about that?

Thanks

[Enzo]

How about if the input to the negative regulator is missing? Then it can float up to whatever the circuit wants.

And remember the pins are not in the same order for the V- regulator.

Go see where the -20v is and where it disappears on the way to the Vreg.

[Johnrcurry]

Thanks Enzo. Looking for my missing -20 volts, I found 30vac coming into the Reverb board at the points marked "RD" on page 5 of the schematic, and up to FS1 and FS2. I found 0v on the other side of FS1 and FS2. Also, measuring across FS1 I get infinite resistance (open). Likewise across FS2. It looks like I could have blown those fuses?

[Enzo]

Well, yes.

[Johnrcurry]

Can someone suggest a source for these parts? I've attached a picture.

Thanks

[MarkusBass]

You can try mouser: 37402500000 Littelfuse Fuses
I've seen such fuses in computer equipments (modems) - they could be Chinesse.

Mark

[Johnrcurry]

Thanks, Mark.

[Enzo]

I buy them at Mouser. Either riffle through the paper catalog, or online look for radial leads or maybe throughhole in the mounting style filter.

[Johnrcurry]

Thanks, Enzo.

[Johnrcurry]

Just to close the loop on this thread. . . . I replaced the two blown fuses and the amp works fine. The original problem must have been the loose solder joints on the voltage regulators, and I must have shorted something (thereby blowing the fuses) when I was testing the fix.

Thanks for your help.