left Paramount now blowing a fuse

[Doc B.]

You don't actually upload photos here, you link them. So the image needs to be hosted online somewhere. One thing that has me puzzled in your images is that it looks like the transformer pins that are poking through the solder pads for the 6.3V heater winding aren't soldered, or at least not very well. It actually kind of looks like they have been desoldered. If they have not been altered it may just be the camera angle, as the amp ran great here for many years.

[rbc3]

12 ohms resistance between the two terminals of the IEC socket.  Power switch is on.  Amp is unplugged.  IA on C4S board is disconnected.  Tested and working fuse is installed in the IEC socket.  No tubes are in the amp.

[rbc3]

Response to Doc...  The only soldering I've done is to switch the OT to 4 ohm (came tapped as 8 ohm).  Replaced RCA and speaker binding posts (desoldered leads from old and resoldered to new, not too difficult).  And now desoldered the red lead from IA on the C4S board.  I haven't touched the power board at all.  I did notice that the pins on the power board don't have much solder on them.  I could go through them with a touch more solder to ensure a good connection.  I suppose a solder connection could have become cracked during shipment or something.  <shrug>

[Paul Birkeland]

You can replace IA for the rest of the tests, and leave the 5670 plugged in. 

The next test is to pop out the red wire that goes to the PC-2 from the 5-lug strip by the 9 pin socket.  I would pop the wire out at the terminal strip.  Next, turn the amp on and see if the fuse holds (with 5670 in place).

-PB

[rbc3]

I resoldered the red lead from terminal 16 back to IA on the C4S board.  Double checked the good fuse (still good).  Desoldered the read lead from terminal 16 that goes to PC-2.  Plugged in the 5670 tube (300B tube still removed).  Plugged the amp in.  Switch on power.  Nothing... blown fuse.

[Paul Birkeland]

There's a red/black twisted pair of wires leaving the IEC power entry module and going towards the power supply board.  If there's room, side one of those wires out of the PC board, then see if the fuse pops when you throw the power switch.

[rbc3]

Just to be sure, I should desolder either the red or black lead going from the IEC power socket to the power board?

[rbc3]

I don't have easy access to the end of the wires that are soldered to the power board because the filament choke is in the way.  So I desoldered the black lead from the N terminal on the IEC power entry module.  I suppose that means I can't be sure if there's a short in the wiring between the power entry module and the power board though.

Installed a new fuse that I checked to confirm it was good.  The red lead from PC-2 is still disconnected from terminal 16.  And the 5670 is still plugged in to the amp.  Turned power on and it didn't power up (expected since I disconnected the black lead from IEC power entry module).  But at least the fuse didn't blow.

[rbc3]

I hope the silence means you guys are busy and not something worse.

[Paul Birkeland]

Yeah, that doesn't exactly test what I was looking for.  I thought that perhaps there might be a short in the wiring going from the IEC inlet to the power transformer. 

[Doc B.]

This amp ran without issue for 7 years and also ran even after the base was knocked in shipping. So I would SWAG that the problem stems from some change that occurred after that. I would suggest checking the rectifiers on the power supply board. It's a bit of a stretch but one could have been blown when a new tube was introduced, if the tube shorted.

[rbc3]

OK I'll reconnect the black lead to the N terminal on the IEC power entry module and try to disconnect either red or black lead on the power supply board to check for shorts between the IEC power entry module and the power board.  I'm not sure how successful I'll be in reconnecting later with such a tight space.

I'll also check all six Schottky rectifiers.  Do I need to desolder them from the power board to test or is it OK to test while mounted on the PCB?

[Doc B.]

I think you should be able to test them in place, with the tubes removed. 

[rbc3]

Red lead coming from PC-2 is desoldered from terminal 16.  The black lead to the IEC power entry module is disconnected, fuse and tubes are not installed.  Here's what I got for the Schotky rectifiers in circuit.

Left Amp (looking with front of amp with badge facing me) - the one blowing the fuse
1N5820 3A Schottky rectifier black lead on silver stripe (rear outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (rear outer) 205mV
1N5818 1A Schottky rectifier black lead on silver stripe (rear inner) 161mV
1N5818 1A Schottky rectifier red lead on silver stripe (rear inner) 196mV
1N5818 1A Schottky rectifier black lead on silver stripe (front inner) 161mV
1N5818 1A Schottky rectifier red lead on silver stripe (front inner) 195mV
1N5820 3A Schottky rectifier black lead on silver stripe (front outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (front outer) 206mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (left) 737mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (left) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (right) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (right) 734mV

Right Amp (looking with front of amp with badge facing me)
1N5820 3A Schottky rectifier black lead on silver stripe (rear outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (rear outer) 196mV
1N5818 1A Schottky rectifier black lead on silver stripe (rear inner) 151mV
1N5818 1A Schottky rectifier red lead on silver stripe (rear inner) 195mV
1N5818 1A Schottky rectifier black lead on silver stripe (front inner) 152mV
1N5818 1A Schottky rectifier red lead on silver stripe (front inner) 195mV
1N5820 3A Schottky rectifier black lead on silver stripe (front outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (front outer) 196mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (left) 737mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (left) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (right) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (right) 731mV

[Doc B.]

A diode test usually reads in ohms, not mV. Were you using the diode test setting on your meter? If that is the case I would say that the consistency from amp to amp is a sign that the rectifiers are OK.