left Paramount now blowing a fuse

[rbc3]

I'm not sure why the digital display of my multimeter switches from the ohm symbol to an mV symbol when I switch it to diode test mode. The diode test setting on the dial is in the ohm section of the settings...

I was concerned that the four round rectifiers weren't working because they didn't read OT in one direction which is why I tested the right amp too.

Is it starting to sound like there is a short somewhere?  What happens if one of the loose pins on the cracked four pin socket were to touch the chassis plate?  Could that be the problem?

[Doc B.]

OK, your meter is simply reading mV. I think your rectifiers are OK based on your measurements. Rectifiers in a bridge circuit will always read below OL in either direction, because the meter is in effect also reading the "backwards" rectifier that is connected to the rectifier you are measuring. If they blow they can create pretty close to a dead short so you would probably see a measurement much lower on the bad rectifier than on the good ones.

Actually it has sounded like a short is the problem from the beginning, but what is shorted is still the mystery. If one of the pins of the four pin socket is touching the chassis, that's a serious shorting problem that could certainly take out a fuse. If that is something that needs to be addressed I would make it the next priority.

[rbc3]

Reading about diode test mode of my multimeter and it's measuring the voltage drop across the diode in diode test mode and not resistance.  I can also switch the multimeter to resistance mode and measure the resistance in forward and reversed bias.

[Doc B.]

Yup, that would be what it is doing if it reads in mV. Same thing applies as measuring resistance, it will be confused by the bridge arrangement. If a four pin socket pin is possibly contacting the chassis, that would be the best place to next focus your examination.

[rbc3]

OK.  I wired up a new 4 pin socket.  Here's the photo.

https://www.dropbox.com/s/7x4q8p7xg3xgml9/20150430_171447.jpg?dl=0

I resoldered the black lead to the N terminal on the IEC power entry module.  I resoldered the red lead from PC-2 to terminal 16.  The 5670 is not plugged in.  I checked to confirm the fuse was good.  Powered up the amp.  Powered off, checked fuse... blown.   

On the power supply board, I desoldered the red line that comes from the IEC power entry module (next to FC-1).  It was a tight squeeze, but I managed to get in there with some needle nose pliers to pull off the wire.  I replaced the fuse with a tested good one.  Turned the power on.  Turned power off, checked fuse... not blown.  So there's no short in the lines going to the power supply board.

I suppose we've narrowed it down to the power supply board itself.  If the Schottky rectifiers on the PS board are OK, then maybe it's something else on the board?  I could also do the same for the leads coming off the PC board, which we haven't determined aren't causing a short.

[rbc3]

OK, now I feel I'm on to something!  I resoldered the red lead going from the IEC power entry module to the power board.  I desoldered the red lead from the power board that's going to FC-1.  I desoldered the red lead at the power board that's going to 16U.  And I desoldered the red lead from the power board that's going to the 9 pin socket B1.  I rechecked the fuse to ensure it's good.  Plugged in the amp. Powered it on.  Powered it off, and check the fuse... it's good!

I also checked the voltage at each of those three points that I desoldered the red leads.  The point on the power board at which the red lead is going to FC-1 is 6.8 volts.  The point at which the red lead is going to 16U is 580 volts (seems high since the manual states 457 volts at terminal 16).  The point at which the red lead is going to socket B1 is 0 volts, which seems odd.

I believe that means the power board is OK and there's a short somewhere coming off the power board.  I hope I'm on to something.  I think my next step is to reconnect each red lead, one at a time and then I can isolate which path the short or problem might be down.

[Doc B.]

It's possible that the Zener string next to the nine pin socket has a blown diode or diodes. Try a diode test on each individual Zener to see if any are shorted.

[rbc3]

All five diodes in the Zener string show 580 mV drop in forward bias and OL in reversed bias.

[rbc3]

I'm astounded! I resoldered the red lead lead from FC-1 to the power board.  Checked the fuse, good.  Powered up. powered down.  Fuse is still good!  I resoldered the red lead from 16U.  Powered up.  Powered down.  Fuse still good!  I thought, well it must be a short in the heater line for the 9 pin socket.  I resoldered the red lead from B1.  Powered up.  Powered down.  Fuse still good!  I plugged the original tubes in and powered up and they lit up!  No blown fuse.  I cant check for audio because it's too late tonight and I have kids.  But tomorrow morning I might be back in business!  Woohoo!

PS I swapped out the stock 4 pin socket in the right amp too so now they both have gold plated copper 4-pin bakelite sockets.  I know it's way overkill but they're looking pretty good with everything but the 9-pin sockets as gold plated copper, Cardas binding posts and Vampire RCA sockets.

[Doc B.]

Well, I wish I could say I knew what exactly happened, but I can't really find a pattern when I go back over the posts. Could have been some intermittent short, or maybe it was the socket issue and the start up inrush of the caps after being discharged for a while was just enough to pop that final fuse after you did the replacement. Anyway I am glad you got it running! Good job of staying at it, that's what separates a good tech from a mediocre one.

[rbc3]

Yep, they work great!  I've been running the stock supplied tubes all day.  Just a little bit ago, I dropped in some NOS 90's Sovtek tubes that I recently picked up.  Everything is running as expected.  SO HAPPY TO HAVE MY PARAMOUNTS BACK!  I'm spoiled and gave up listening on my vintage solid state Yamaha M-4.  It's just not the same as these Paramounts!

Thanks for all the help and putting up with my noob questions.  It only cost me around $7 for blown fuses and I learned my way around the amp a lot more.  If the short shows up again, I know how to proceed through the checks to try and figure it out.