I just blew up my soft start upgrade. Help?

[Quietmouse]

Hey everybody! It's been awhile.

I purchased the soft start upgrade for my Paramounts about a year and a half ago but am just now getting it done.

The problem is, while checking my voltage on pinB6 of the 9 pin socket, I blew up the MJE5731AG transistor on the A side and the LED at D1 on the A side, the LED nearest the transistor.

It happened as I was pulling the probe away to check the other pin. I heard a pop and the visible damage is that these two parts are chipped. I have 1 of the tiny LEDs fro when I built my set up years ago so that part is here. Can I use transistor MJE5731A from the previous board to replace the one I blew?

Is that even the right thing to do or are there other problems that are not visible I should fix before I try it again. The voltage on pin B6 was 299 volts so I am guessing all that happened was I bumped something with the probe and shorted it. But that is just a gues from an amateur, any help is greatly appreciated!
Thanks!

[Paul Birkeland]

Yeah, a short across the MJE5731A could cause it to pop (and other assorted nastiness).

You can try replacing the damaged parts, but realistically you'd want to at least replace both transistors and both LED's on the toasted channel. 

When you have the board out, take your meter, set it to Ohms, then measure the impedance across each of the zener diodes.  Measure with your probes in both orientations on each diode.  You should see a reasonably high resistance in one direction, and a really high resistance in the other direction (maybe even "OL").  If you see low resistance in both directions, remove that zener diode and replace it. 

-PB

[Quietmouse]

Thanks Paul.

I will order the transistors and LEDs from Mouser I guess.
When you say to replace both LEDs which two would they be? Or did you mean to replace all 4 that are on the circuit board?


Another question. I see in a Sticky at the top of this section where someone had posted voltage checks to avoid frying amps checking these at the pins on the small socket. It looks like they have been erased though. Have they? Or am I missing something.

[Grainger49]

I'd make a suggestion PB and PJ (and probably every other paid Bottlehead) have made before.  Get some small heat shrink tube and insulate all but the tip of your meter leads.  I did that years ago.  Just to be safe as age makes my hands less steady.

[Quietmouse]

Great and useful tip Grainger. Does anyone know the other point I can check voltage at that are mentioned but not listed in the sticky?

[Paul Birkeland]

When you say to replace both LEDs which two would they be?

The one that exploded, and the one next to it.

[Quietmouse]

Thanks Paul. You guys are great.

[Quietmouse]

I replace all 4 LEDs and both of the MJE 5731 transistors. I checked the zener diodes as directed earlier in this thread and they responded fine. When performing the first voltage check in the manual, between the ground bus and socket terminal b6, the voltage quickly rises to 90 or so volts then drops to around 50 volts.

Both tubes light and the two LEDs on the A side of the PCB light but the two LEDs on the B side of the pcb never light.

Any suggestions on what to check next?

Thanks,
Rob

[Paul Birkeland]

What are the voltages at IA, OA, IB, OB, KregA, and KregB?

[Quietmouse]

IA= 542v
OA= 45v  (started at 56 v and drifted down till stabilizing at 45v)
IB= 45v
OB= 44v
KregA= 1.18
KregB= 2.47

Thanks for the help.

[Paul Birkeland]

It's still possible that you have a shorted zener.  Also, is this a 2A3 or 300B Paramount? 

OA/IB should be regulated to ~300V or so.  It should be higher than 300V until the 5670 warms up, then KregB will rise to over 2.5V and the regulating triode of the 5670 will draw the rail down.

Your regulated rail never gets there, and this can happen if you have a bunch of shorted zener diodes. To test this, you can simply heat up and remove one leg of any zener in the string to remove the entire string, then recheck your voltages.

If this isn't it, then you'll want to remeasure the IA, OA, IB, and OB voltages with no 5670 installed.  If this happens to be a 300B Paramount, it's better to do these checks with a 300B in the socket.

-PB

[Quietmouse]

It is a 2A3 paramount.

Is there a way for me to check the other transistors on the board without removing them?

I will try the tips you gave me.

Since it IS 2A3 should I have the 2A3 in but the 5670 out when doing the second voltage checks you listed?

Thanks!

[Quietmouse]

With one leg of Zener Diode removed:
IA= 542v
OA= 45v
IB= 45.5v
OB= 44v
KregA= 1.19v
KregB= 2.5 v

With Zener leg removed + 5670 removed
IA= 543v
OA= 44.3v
IB= 44.3v
OB= 43.6v
KregA= 0v
KregB= .186v

With Zener reconnected and 5670 removed:
IA= 542v
OA= 42.8v
IB= 44v
OB= 43v
KregA= 0v
KregB= .184v

All tests were done with the 2A3 in place.

[Paul Birkeland]

Now place the Zener back in place, then check those board measurements with no 5670 installed.

[Quietmouse]

With Zener reconnected and 5670 removed:
IA= 542v
OA= 42.8v
IB= 44v
OB= 43v
KregA= 0v
KregB= .184v

The voltages seem very similar every way except the change with the tube out. I am too ignorant to know what this may be telling me. Except, maybe it is showing I have not found the problem yet?