Paraglow II - "Tucker" Board Problem?

kohked · 6589

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Offline kohked

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on: April 21, 2014, 06:11:54 PM
Greetings....

I've had a problem with one of my (beloved) Paraglow II's....a very low, distorted volume output.   I recently re-capped this amp and it made no difference.    After searching this forum, I see that the symptom is manifest by one of the LEDs on the "Tucker" board to be very dim and the other is not lit at all.    The OTHER amp has two brightly glowing LEDs.

Can anyone help me debug this board? (or is the problem "upstream" of the board?)


TIA,


Best,


Dennis



Offline Doc B.

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Reply #1 on: April 21, 2014, 07:14:14 PM
It's most likely that the Tucker board needs to be rebuilt. If the resistors look OK you could try replacing the transistors and the 431 chip. That was a board that John designed and sold, so unfortunately we don't have any documentation to share or PC boards. I don't know if John still has any of those boards or not as those were sold quite some time ago.

The only other alternative we could offer is for us to repair it with our own parts. We would have to make up some of our own boards with custom values and it would be a little different looking underneath, but function the same. You would need to have us modify both amps to match.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Online Paul Joppa

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Reply #2 on: April 21, 2014, 07:41:37 PM
Just a note - that board used an LT-1431, not a 431.

Paul Joppa

Offline Paul Birkeland

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Reply #3 on: April 22, 2014, 05:10:05 AM
Just an FYI,

I have repaired a fair number of Paraglows over the last couple of years, and generally I go for a pretty bulky rebuild of the power supply, and I put in a shunt regulated driver stage that is similar to the Paramount 1.0, but tweaked to give the correct plate voltage.  I also replace the cathode resistors with 4 of the same part in a series/parallel combination to reduce the stress in that position.

If you were patient and willing to go with a different driver tube, the Paramount Soft Start kit would work well, but is somewhat of a tight squeeze.


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline kohked

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Reply #4 on: April 22, 2014, 05:50:59 AM
Thanks guys....let me noodle on this for a bit.    I'll take a closer look at the board today and see if there is anything obvious.

BTW, any idea what transistors are used?   There are data sheets out there for the chip so maybe I'd get lucky and be able to source one.


best,





Offline Paul Birkeland

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Reply #5 on: April 22, 2014, 06:59:25 AM
IIRC, there's just the regulator chip that PJ mentioned, then a 2N2907 and MJE-350.

It wouldn't hurt to replace the MJE-350 with the MJE-5731A, being careful to flip it around to face the opposite way. 

Also, there's a dropping resistor that the regulator works against, and you'd want to check to be sure that it's not charred and still measuring properly.  (In our current driver configuration, that resistor is replaced by a C4S)

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline kohked

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Reply #6 on: April 22, 2014, 01:36:43 PM
Guys, thanks for the prompt replies....

In the daylight, it seems that there is some "browning" on the board at the base of the MJE-350 power transistor.   I'd bet it has cooked itself.   I noted your recommendation for the higher power 5731a part as well.    I'm just an amateur at this so didn't understand the remark about "flipping around" the 5731a?    Are the pins in a different order or location?   Thanks again.


Offline Paul Birkeland

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Reply #7 on: April 22, 2014, 02:04:22 PM
The middle pin is the same, the outer two are flipped, so the metal sides of each transistor face in opposite directions.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline kohked

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Reply #8 on: May 11, 2014, 12:14:10 PM
Greetings again.....

So........

I've replaced the two transistors and the chip on the C4S/shunt reg board.   The amp is sitting here on my workbench, still with no LED's lit, but all the "check out" and setup parameters in the original documentation to spec (can that be?).   A few minutes ago one of the LEDs lit, but the B6 voltage climbed up to the 280V range (should be 225V)  and the trim pot doesn't seem to be able to change it.

I'm probably at the limit of my knowledge/skill here as nothing else seems obvious (and it was working well for > 8 years).

Whaddya' think?   Should I send it back to one of you guys for a look see?  I realize these boards are no longer available, so is there a substitute/functional equivalent solution that performs as well?

best

Dennis



Offline Paul Birkeland

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Reply #9 on: May 11, 2014, 06:10:31 PM
Hello Dennis,

When I get Paraglows in for repair, I essentially install the Paramount V1.0 driver PC board with adjustments to keep the plate voltage in spec.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline kohked

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Reply #10 on: May 12, 2014, 05:01:33 AM
Paul,

Thanks....

Ping me off the forum (kohked(at)yah00.com) and we can work out the details of getting this to you and working again.

best,


Dennis
« Last Edit: May 12, 2014, 08:18:14 AM by Caucasian Blackplate »


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