Paramount pops and sparks [resolved]

[notbent]

Hi there
I built my 240v [I'm in Australia and Bottlehead had just made these transformers available] Paramount 2A3's around 15 years ago as well as a Foreplay 3 and they've been better than great!

Recently I turned them on and the left Paramount made a loud snap and popping sounds with what looked like sparks at the bottom of the 12AT socket.
When I flipped it I saw that the black wire from the PS board to B9 at the 12AT socket had shorted against B1, burning off about an inch of insulation.

Also the 5W wire wound resistor had a long crack in it, and crumbled to pieces when I touched it to remove it.

I replaced the resistor [as well as the tubes} hoping for a quick fix, but it blew straight away glowing red as it did so.
I pulled the PS board and removed and checked all six rectifiers and the other four resistors. All were good.

Same result on restart... 5W glows red.

Where do I go from here? I'm no electronics expert, and if not for the incredible instruction manual, I'd have had no hope building them at all!

Cheers, Andy

Here are my resistance readings:

1  1.5K
5  3.99K
6  4.1K
7  3.4M
9  3.6M
10  3.4
11  17.9M
12  OL
14  OL
15  OL
16  1.2K
17  4.8
18  3.7
19  3.3M
20  68K


A1  4.4K
A2  1.5K
A3  3.3M
A4  4.5K


B1  3.4M
B2  68K
B3  OL
B4  4.2
B5  3.9
B6  3.5M
B7  218
B8  OL
B9  3.8

 

[Paul Birkeland]

You very, very likely need to replace all of the transistors on the driver PC board if B9 and B1 shorted.

 

[notbent]

Righto I'll get onto it and report back.

Thanks Paul

 

[notbent]

I can source the 2N2907A transistors

Can I substitute a MJE350G for the MJE350?

And a MJE5731AG for the MJE5731A?

 

[Paul Joppa]

The Zener string was probably taken out, too.

In 2009 we came out with the Paramount version 1.1, which incorporated an improved "soft-start" driver circuit. There was an upgrade kit for older Paramoiunts. It's been out of production for ten years, but it may be possible to resurrect that upgrade kit. If so, that would be the best solution.

 

[notbent]

Hi Paul

I should have mentioned I replaced the Zener string after the calamity with spares I bought when I built the amps.

The old ones checked ok.

And back then I installed the additional power switch that you suggested before the soft-start came out. It stopped the pop on startup and no problems since.

I'd sure be interested in an upgrade kit if I can't fix this with new transistors.

Cheers, Andy

 

[notbent]

I removed the shunt regulated board to replace the transistors but they all checked ok on my multimeter diode setting.

Then I turned the amp on, no tubes or board, and the 5W wire wound resistor yet again glowed red before I yanked the cord.

What else could cause this? Could the transformer be blown?



 

 

[Paul Birkeland]

Can you post pictures of what you have now? There isn't much left in there to upset the 270R/5W resistor.

 

[notbent]

I hope this works

 

[Paul Birkeland]

If you temporarily remove the black wire going to the PC-2 from the power supply board (just heat up the joint on the PC-2, pop the wire off, then leave it poking straight up), does that remedy the overheating of the 270R resistor?

 

[notbent]

The resistor blew again.

 

[Paul Birkeland]

OK, with the resistor removed, if you turn the amp on what are the DC voltages at each solder pad where the 270R resistor would go?

 

[notbent]

556V at the pad closest to the PC2 and 0V on the other.

 

[Paul Birkeland]

OK, this tells us a lot!  Now there's a red wire that leaves the power supply PC board and goes up to terminal 16.  I would go ahead and pop that wire out of the power supply PC board and leave it sticking up, then put a 270 in and fire it up again and check those same voltages (they will probably hold now).

 

[notbent]

It still blew loud too.

I left the wire off the PC2 and then hooked it back up again for the second test.

Same result firecrackers

 

[Paul Birkeland]

So with the red high voltage wire disconnected from the power supply PC board, you still blow the 270 ohm resistor?  If this is the case, that would indicate that something on the board is causing excessive current draw in the second power supply filter stage and blowing out the 270 ohm resistor. The next step would be removing the power supply board and removing that set of filter caps, putting the board back in, then firing it up.

This is a rather challenging amount of work if you don't have a powered solder vacuum, as getting the board out without one is likely to destroy the board.  You're going to want something like the Hakko FR-301 in order to continue debugging, or you'll have to send it to me for repairs.

 

[notbent]

Here's my rig.

I really want to save it!

 

[notbent]

Sorry text crossover.

I've already had the power supply board off twice to replace the diodes using solder sucker wire no probs.  I think.

I'll replace the filter caps tomorrow and get back.

Thank you for your help

Cheers, Andy

 

[Paul Birkeland]

Do consider the possibility that it may not be the filter caps.  An important step in diagnosing would be firing up the board without the second bank of filter caps.  I think with the solder sucked off that you can just smoosh the board back onto the pins on the power transformer to see if the 270 ohm resistor gets toasty.

 

[Karl5150]

Andy, that is a beautiful setup, hope you get it sorted soon - back to the music.
Karl