PAS 3 stopped working with no tale tail signs of why???

My friend recently acquired a completely original, factory wired PAS3 which was brought up slowly (over the course of a day) using a variac. This was done because there was no knowledge of how long it sat unused before now. He used it for a couple of days with his system (at full voltage) before the issue occurred.

While he was adjusting an RCA jack (with everything powered up) from his Maggie amp to the PAS he heard a pop in the speakers and the PAS stopped working. He quickly powered everything off, and reconnected the PAS to his ST-70, and the PAS would still not power up.
He removed the cover and there are no smells, dripping, and nothing appears burned.

I’ve tested the 12X4 using a Hickok 600A and it passed. I checked the incoming voltages using a DVM and get the correct voltage at the unswitched outlets and only about 15vac at the switched outlets. I also tested the power switch, and it passed. I don’t know what the resistance readings should be for the PA211 transformer, so I can’t check to see if that’s the problem.

Has anyone encountered this type of behavour and/or help me think through where the issue lies? I’d really hate to just start replacing parts (and ringing up a parts bill for him). I believe that a new power supply is in order if he wants to use it daily, but I’d like to get it up and running before that gets replaced (if possible).
I’ve attached a pic of it (post mortem) to show just how clean and original it is.

Thanks for any and all thoughts, suggestions, opinions, etc.

Anyone? Give me your best guess?

Look at the Troubleshooting procedure on PAS manual it can be helpful.

Measure the B+ at the quad capacitor to see if you have 330V(check the manual for the correct voltage, I forget) and if that is good, make sure the 12AX7 filaments light up. The selenium rectifier in the voltage doubler circuit is known to fail. You should have approx 22-23 volts DC going to the filaments on the phono board and line stage board. The 12AX7 filaments are wired in series. Also since you have a tube tester, check the 12AX7 tubes.

Let us know what you find.

Sal

cc43fan,

Time to put away the DVM and break out the test lamp. Use a 110 volt lamp with pigtails attached. A 4 watt nigh light type bulb is perfect. Test the primary side of your pas starting with line side if the transformer.

The 15 volts you measured from the switched outlets says that somthing is open-circuited and a test light should lead you right to it. If you don't have 110 to 120 volts on the primary of the transformer, work your way backwards to the plug wiring until you do.

Standing by...

Tony

Oh Geez... I didn't read about the 15 volts on the switched outlets... Disregard what I said and do what anbiteet66 wrote.

Sal

anbitet66 wrote:cc43fan,

Time to put away the DVM and break out the test lamp. Use a 110 volt lamp with pigtails attached. A 4 watt nigh light type bulb is perfect. Test the primary side of your pas starting with line side if the transformer.

The 15 volts you measured from the switched outlets says that somthing is open-circuited and a test light should lead you right to it. If you don't have 110 to 120 volts on the primary of the transformer, work your way backwards to the plug wiring until you do.

Standing by...

Tony

Hi Tony,
Thanks for getting my head right around this....Turns out it was one wire from the switch to the switched outlet/transformer mains got nicked by the cover screw. I didn't spend enought time checking for continuity the other night, and got stumped. Once I found the wire with no continuity, I jumpered it, and it worked. This nick was so small I needed my magnifier to find it!

Thanks again for helping me think this through!

Chuck

I'm glad you found the problem, and it was nothing serious. Adding a fuse would probably be a good idea.

If you want to give your friend a large performance improvement for very little money, head for the filament supply. Replace the selenium rectifier with a couple of UF4007 diodes, and add as much capacitance as you can fit in there.

The diodes will get the filament voltage up out of the 11V range, closer to the 12.6V ideal. Adding capacitance to the filament supply has s very dramatic effect on a stock PAS. I have tried using in excess of 100,000 µF (using an external cap bank) and the improvement was easy to hear. There's no need to get that crazy though. A couple of diodes and two Nichicon KW 22,000µF caps will give a big improvement for very little money. And it's very easy to do, which is always good!

1973shovel wrote:I'm glad you found the problem, and it was nothing serious. Adding a fuse would probably be a good idea.

If you want to give your friend a large performance improvement for very little money, head for the filament supply. Replace the selenium rectifier with a couple of UF4007 diodes, and add as much capacitance as you can fit in there.

The diodes will get the filament voltage up out of the 11V range, closer to the 12.6V ideal. Adding capacitance to the filament supply has s very dramatic effect on a stock PAS. I have tried using in excess of 100,000 µF (using an external cap bank) and the improvement was easy to hear. There's no need to get that crazy though. A couple of diodes and two Nichicon KW 22,000µF caps will give a big improvement for very little money. And it's very easy to do, which is always good!

I had suggested that he purchase the tubesforhifi power supply upgrade if it's going to be his daily driver. I hope in it's currect original condition it plays for a long time.