PAS 3X power transformer

by Plumberboy1 Tue Jan 20, 2009 2:25 am

Plumberboy1: Posts : 2
Join date : 2009-01-20

Post n°1

PAS 3X power transformer

by Plumberboy1 Tue Jan 20, 2009 2:25 am

How can I check the power transformer for proper operation? I took apart my preamp a few years ago to try to rebuild from scratch mostly. I started to put it together and then put it away. I got it out the other day to resume the project. I got new boards from Vintage Electron and installed them and removed the original selector switch to install the Curcio Audio Selector switch. I custom made a new rear panel out of stainless steel that is perforated with 1/16 inch holes. and left the row of outlets out and installed gold plated RCA jacks. I powered it up today on my variac. The tubes on the PC 6 board do not light up. The new switch does not hook to the board the way the old switch does. There are many empty spots. I'm wondering if I am having Tranny problems, If it is bad can I get a toroidal?
I built two of the Sound Values/Sound Valves Amp kits and a Van Alstine PAS 4SI preamp kit. The preamp kit was a blast to build. I was supposed to send it back to Frank for a free bench test. I didn't do it, I crossed my fingers and turned it on. It has worked flawlessly since new. The Soundvalves is a different story. Not as much fun to build. One started acting up, I tried to call the company, but they went belly up. I talked to Frank personally and he warned me they were an unstable circuit. I still use one today but the bias won't adjust to equal out the lights. I suppose it is a simple fix. I built this stuff in 1997.

Greg

by **Bob Latino** Thu Jan 22, 2009 6:05 pm

Bob Latino

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: PAS 3X power transformer

by Bob Latino Thu Jan 22, 2009 6:05 pm

The best way to test the power transformer on a PAS-3 is to check it at the 12X4 rectifier tube. Set your meter for AC voltage.

Where the two RED wires from the power transformer come in to the 12X4 rectifier on pins 1 and 6 see if you can measure about 330 to 340 volts AC from each pin to the chassis ground.

Next measure ACROSS the two heater wires that come in on pins 3 and 4 of the 12X4 and look for 10 to 12 volts AC, again, ACROSS these two pins.

The RED/YELLOW wire is the center tap for the two RED wire windings and goes to ground. There shouldn't be any voltage there.

If these voltages check out then your power transformer is OK.

Bob

Last edited by Bob Latino on Mon Jun 17, 2013 10:38 am; edited 1 time in total

by Roy Mottram Sun Feb 15, 2009 3:35 am

Roy Mottram

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Join date : 2008-11-30

Post n°3

PAS3 upgrades

by Roy Mottram Sun Feb 15, 2009 3:35 am

hopefully when you put in the new PCBs you also updated the original power supply, either with an SDS cap board or something equivilent.
Bob gave you the correct directions for checking the transformer. You can also check for 400vdc on the quad cap or power supply, about 320v on pin 16 of the PC5 (line PCB) and about 220v on pin 16 of the PC6 (phono PCB). You should also have 12vdc on each tube filament (pin 4 to pin 5).
Note that it's not always obvious the filaments are wired in series unless you look at the schematic, so the power supply for the filaments is actually 24vdc.
New PA211 transformers are available, also I have a specially designed toroid transformer that fits into a PAS3, but it's set up for 12v filaments, which is a simple 10 minute rewire.
Roy www.tubes4hifi.com roymottram@yahoo.com

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PAS 3X power transformer