C4S/Shunt Regulator PC board - 2 LEDs blown up when turned on

[Anton]

Hi
I just finished assembling first of Paramount monoblocks. Resistance check was OK.
When I plugged in and turned it on - only two LEDs (on B side ) lighted up, LEDs on A side didn't. 15 seconds later B side LEDs blown up and I turned everything off.

Any ideas why this happened and where to start to fix the problem?


Thank you

[Doc B.]

When the LEDs blow violently like that it means all the semiconductors on that half of the board are probably blown, and the board is basically hosed. The best bet is to build a new one. Also the zener diode string may be blown and need to be replaced. Typically the problem is due to a miswire or bad solder joint somewhere around the driver stage. Double check all of the connections on the nine pin socket and the terminal strips on either side of it, and also check the PC board solder pads. We can sell a replacement board at our cost.

[Anton]

Doc,
Thanks for the reply.
I replaced the board - I have two of those in the kit.
I also checked diodes and replaced one on them.  My resistance check is not ideal - resistances on some terminal strips are higher than supposed to be.
So I replaced the board and rechecked on connections and joints. Put in tubes and tried to turn it on again. As before - I had only 2 LEDs lighten on and lamps were not glowing. I turned it off fast to avoid blowing up LEDs once again. So - I still have the same problem. I am going to go and check all connections from the beginning again. Just was wondering if somebody probably see what is my most likely problem - based on symptoms: only 2 B-side LEDs lighten on and tubes do not work....

Thank you

[Paul Joppa]

Just to be sure, this is a stock Paramount, not the soft-start board just released, right?

The Zener diodes protect the Q2 transistors from damage by excess voltages during the initial turn-on transient, and problems there can cause cascading failures of other components on the boards. So the first thing is to check their orientation; then check each diode to be sure they are working - read the resistance each direction; it should be low one way and bvery high the other.

Assuming the Zeners are OK, there are only two paths for excess current through the LEDs on the B side. One is the 300K resistor R2 - I'm not sure what part is being shipped currently, I think a higher-rated part is standard now. Check the resistance, anyhow. The other path is through the big transistor on the B side, and i think the current kits are shipping with the MJE5731A rather than the original MJE350. Orientation is important, and the metal side faces the opposite way between those two parts - so it's easy to get it wrong.

Finally, check the 90.9-ohm resistor R1 on the B side for correct value.

[Grainger49]

Anton,

Look on page 13 of the manual.  Verify that the diodes here on the power supply PCB (printed circuit board) are oriented as in the picture, be certain that the diodes are on the left side of the board as you look.  On page 15, verify that the diodes (rectangular black plastic and metal) are back to back, meaning the metal sides are facing each other.  Verify that the capacitors all have a white stripe toward the right when the diodes are on the right.  These things get reversed very often.  But may not have anything to do with your problem.  This is the voltage that gets delivered to the shunt regulator/active load circuit board.  It can be checked by taking the wires from this PCB to the shunt regulator/active load PCB and measuring the voltage produced by the power supply PCB.

On the bottom of page 25 the picture shows the shunt regulator/active load PCB.  Look at the 4 LEDs and two metal cased transistors.  These LEDs drive me nuts.  I mark the "silver ends" with a sharpie so I can see it well during assembly, I'm old.  Verify that the LEDs and the transistors are oriented as shown (check the transistor tabs). 

And finally on page 28 in the picture on the bottom of the page verify that the two new transistors are oriented properly.  The one at the bottom has its metal side toward the outside of the PCB.  The one on the upper side is probably different than shown in the picture.  Bottlehead's supplier only had a substitute.  Doc posted this explanation under Crack:

http://www.bottlehead.com/smf/index.php/topic,1288.0.html

Good luck, post back!
 

[Anton]

Grainger,
Thank you for the response.
1) I checked rectifiers orientation from page 13 - everything seems to be right. See attached pictures for reference.
2) From page 15 diodes orientation is right - see photos
3) All capacitors are oriented in right direction - see photos
4) LEDs are oriented correctly - but you can't see that on photo
5) Transistors are oriented as they should - see photos
6) I believe that transistors on my shunt board (as on page 28) are oriented correctly - see photo. I have MJE350 transistor.

I still can't figure out what is wrong.
Will keep checking everything else - probably will find something...

[Anton]

Paul,
Thank you for your reply.
Yes you are right - I am building stock Paramount.
1) I checked orientation of diodes - seems to be right - please see photo
2) I checked diode's resistance in each direction: in one direction it reads around 2.6 - 2.7 megaOhms for each of them, and in opposite direction it reads 40 megaOhms for some and infinity for others
3) R2 resistor (B side) checked - 300 Kohm
4) On B side I have MJE350 transistor installed - see the picture in my earlier post above. Orientation seems to be correct
5) R1 resistor (B side)  checked - reads 91 ohm

[Grainger49]

Anton,

My above post, reply #4, covered checking everything on the board that has an "Orientation."  Sounds like it passed.

I am looking at the manual picture of the finished board and your picture side by side now.  Just about everything looks exactly the same.  The jumpers are right, diodes, capacitors and metal transistors are right.  The wires exiting the board look right, but I can't see the colors or where they end up.  

The MJE350 has changed, I can't verify that.  The resistor in the "R1" position on the A side is the resistor between the two transistors at the bottom left of the board.  Yours looks smaller than in the manual.  The color codes are too small for me to read.  It should be a 237 ohm 1/8 watt.  (this very could be a parts supply problem where a different version was used, measure this resistor)

You will want to clip the leads on the new Zener diode, they could short to the chassis.

I guess I would go back to removing the wires (red and black wire labeled 300B page 23) from the power supply board to the regulator board and measuring the voltage.  The idea is to see if everything back at the source is good and move forward.  After that maybe run the regulator board without the output to the tubes?

Keep an eye on the wire colors they have to go back in the same spots.

Hopefully PJ or Doc will chime in and tell us what the unregulated output voltage of the power supply should be.

[Anton]

Hello,
just wanted to share a progress on my problem and ask for advise.
I decided to go ahead and build another Paramount to check if I have the same problem. So I finished it and check resistances. Everything reads fine except of B6. It should read 3.7 mega Ohms and my reads 0. When I disconnected the lead from PC board - it reads infinity, when connected it back - again zero.
Does it mean I need to replace C4S/Shunt regulator PC board? That is the one I blown LEDs on. I replaced the LEDs but probably damage was bigger than just LEDs?

[Doc B.]

Yes that would indicate that something is shorted to ground. So either something is miswired, or it's possible that a component on the PC board failed in such a way that it shorted to ground. I think that I have posted the procedure for testing the transistors before (actually I think I have posted it about a zillion times before). Does anyone have a link to a post about testing the transistors? Basically you measure the resistance across each possible combination of two of the three legs. If any reading shows really low, like 100 ohms or less it is blown. They should read around 1Kohm or higher.

[Anton]

OK,
Here is another update.
I took C4S/Shunt Regulator PC board from my first assembled Paramount and soldered it to my second Paramount (which had problem with 0 resistance on B6 when C4S/Shunt Regulator PC board connected). When I replaced C4S/Shunt Regulator PC board - B6 resistance became around 3.5 Mega Ohms. So - that part is OK. But when I plugged it in - same problem as with my first Paramount - only 2 LEDs lighten up. So - either this C4S/Shunt Regulator PC board is damaged as well, or both power supply boards are damaged. I don't know what else to think. I checked all my wiring already twice, I assembled 2 amplifiers hoping that if I made some mistake in first one - I will unlikely make the same mistake again. But I still have the same problem. Is there any way to somehow test PC boards (resistance check?) to confirm if they are functional???

I sent e-mail to Eileen asking for replacement C4S/Shunt Regulator PC board and power supply board. I want to replace both of them simultaneously. If that will not solve the problem - I don't really know what to do next.

Will appreciate your feedback. Thank you

[Paul Joppa]

I am a little confused by the multiple boards and amps, but it's probably not worth the effort to figure out exactly what the sequence was. As I said we have seen cascading failures. In addition we've seen cases where a semiconductor is damaged but not completely blown - simple resistance checks aren't always enough to detect a part that will not work at circuit voltages.

The bottom line is, if one of these boards has blown, you should replace ALL of the zeners as well as the board with ALL new parts. A damaged board can blow the zeners, and a damaged zener can blow the board.

The power supply is probably fine, since the B side LEDs glow properly.

When you replace the board and zeners, and check the B6 resistance (should be high), do this check as well: measure the resistance to ground of both ends of the two 174K resistors. You should find one point with 350K ohms, two points at 175K, and one point at 2.5K. The bias current for the A side LEDs goes through these resistors, so this is to ensure that they should light.

We have lately found that a good test is to connect the voltmeter to B6 and monitor it at startup - standing by to pull the plug quickly if necessary. It should show around 380v initially as the zeners clamp the voltage; in some 10-20 seconds it should drop to 350v as the regulator tube kicks in. If it rises above 385 initially, pull the plug IMMEDIATELY - either the zeners are not clamping the voltage, or the MJE350 is blown and driving too much current through the zeners. If the initial voltage is less than 350v then at least one of the zeners has shorted itself.

(By the way, these voltages apply to the Paramount 1.1; the soft-start upgrade and the Paramount 1.1 are different.)

I am sorry you're having such a frustrating time with this. But we'll get it working, one way or another!

[Grainger49]

Anton should be able to lift the wires from the power supply to the regulator board and verify that open circuit the power supply produces the right voltage.  I just don't know what it should be.

[Anton]

Paul,
Thanks for the advise.
I am going to order a replacement for zeners and for shunt regulator board. I was wondering if I have to order power supply board too, considering you said it is probably fine? Can I possibly again damage new shunt regulator board and zeners if power PC board is not good?

[Paul Joppa]

I don't see a way for PSU board problems to damage the SR-C4S boards.