Need transformer help, please.

[viridian]

First post here after much research. I have rebuilt a few Bottlehead products with great success and am now redoing a FPIII extended with 6SN7s. I suspect that the power transformer may be a gonner, and it no longer has the paper label on top giving me the output voltages. At top left, I am getting 120V in, next to that the 6V, all good so far. On the opposite sides I get 180V AC and 1.8V AC. The last one has me scratching my head. Do I need a new transformer or is my problem down stream? Thanks so much for any, and all, gracious advice.

[Grainger49]

The best way to test the transformer winding giving 1.8V AC is to open one lead coming off of it.  That relieves the load.  But it sounds like you might have a shorted winding.

Try that and post back.

[viridian]

Wow, thanks so much for the super quick guidance; it is greatly appreciated. I should clarify, the voltage readings are with the diodes connected to the heater taps on top but nothing connected to the 1.8V and 180V taps on bottom. What should I be getting off of that 1.8V tap; should it be 180V as well?

[Paul Joppa]

Yes, the two high-voltage windings are identical. Another test for a possible short is to measure the resistance of the two windings.

The PT-3 was used in Foreplay, SEX, and Crack. If they are not wired exactly as in the manuals, the insulation is not safe, which is why we don't sell the transformer by itself. We have since replaced it with the PT-7 which has a single HV secondary, primarily for this reason. If you know the full history of the transformer, you may be able to guess whether and/or when it has been subject to misuse - which is my first guess of what may have happened. In any case, it appears to be unusable.

Contact Eileen for information about getting a replacement; she'll want to know the original purchaser's name and may want the damaged unit back for study. We do take component issues like this seriously.

[viridian]

Paul, thanks so much for taking the time to help, especially on the weekend. I love your kits! Well I'm at 86 ohms on the happy side and my meter is jumping around on the unhappy side which appears to be over range. I think the patient has expired, no?

I just want to be clear, I purchased the unit and it was not working and was the subject of some dubious mods, which I reversed when I disassembled the unit and put it back to right. There was questionable soldering work as well.

Since I have your attention, should I be putting back as the basic FPIII first and then adding the extended mod? I know that it's hard to give advice without having a look. I have restored three FP2s (I'm listening to one right now, with C4S and sweet whispers) as well as the Seduction and this is the first time that I have had issues. Heck, I haven't even gotten past the power supply yet!

Thanks again,
Marty Kohn
just South of you in sunny Portland, OR

[Paul Birkeland]

Marty - I think you can go back to a stage that's intermediate between the FP-III and extended FP-III.

My advice would be to start with a fresh power transformer, then verify that each of the power supplies flanking it are functioning.  This means lifting the red wire that leaves each power supply so you can look at the voltage output with no load.

Next, you'd want to test the functionality of the shunt regulator.  You can do that by lifting the red wire going to the I pad on the A side of each of the outside green C4S boards.  On the inner PC board, you should see ~150V DC on the + pads, and something like 3-12V DC Kreg, along with the four LED's lighting.  This is sufficient to OK the PC board.

Next, solder one red wire back onto one of the outside board, then take measurements and report back.

-PB

[viridian]

PB, thanks for the guidance. Everyone here is so generous, I can't thank the three of you enough! So, we have a plan, I will order the new transformer and follow your suggestions for sequentially testing the circut and report back. Might take a week or more.

All the best,
Marty

[Paul Birkeland]

Might take a week or more.

Take your time, plan on spending tons of time on something like this, as rushing can compound the problems.

[viridian]

Thanks PB, good to know that you're here. I'm a pro at compounding problems.......... I will be circumspect and take my time.

[viridian]

OK, I'm back at it. The new transformer is installed and I have sorted the power supplies. There is around 227V DC going to each side of the SR active load PC board. I am only getting the LEDs to light up on the B side of the board. I have checked the soldering on the board and can't really see anything that is out of place. My guess is that one of the transistors or regulators is the issue. Is there a short cut to test them, or should I desolder them from the board and start over? Thanks again for any, and all, guidance.

[Paul Birkeland]

Start with all the voltages on that center board.  There is an "I", "O", and "Kreg" pad on each side, it should help a lot to know what all those are.

-PB

[viridian]

Thanks Paul. The preamp is set-up for 6SN7s. The voltages are as follows:

Side A

K    0
I     229
O     0

Side B

K    0
I    218
0    155

Mea culpa, in taking the measurements, when I bridged the two wires connected to O on the A side I got a little flicker out of the LEDs, unfortunately, I also touched one of the leads from the MJE 350 and got a spark. Bridging them doesn't help now.

[Paul Birkeland]

Disconnect the two wires on the "O" pad on the side with 0V and see if it comes up.

-PB

[viridian]

PB, thanks and happy holiday. I disconnected the two wires from the O pad, but did not get the two LEDs on that side to illuminate.

Thanks,
Marty

[Paul Birkeland]

The LED's won't illuminate, but there may still be something other than 0V there.