PAS 3 Power Supply

So I just finished building my ST70 kit and bought a PAS3 to use with it. When it arrived I powered it up and the rectifier tube and pilot lit but the 4 12x4 tube filaments didn't. I figured there was a power supply problem and knew the cap can and selenium rectifier should be replaced so I just went ahead and put in an Erhard Audio power board. That turned out to be a mistake because the symptoms didn't change. I should have done some troubleshooting first but now it's water under the bridge.

So what we have now is measuring the rectifier tube we have 12.5 DC across pins 3 &4. Only 250AC on 1 and 6 to ground and only 50DC on 7 to ground. I removed the rectifier tube and have 375ac on pins 1 & 6. I put the tube back in and disconnected the Erhard board and pin 7 goes up from 50DC to 341DC but still not up to 405 as it should. I have 2 rectifier tubes and the symptoms are the same with each.

As for the filaments on the 12x4 there still dead. I removed the tubes and checked for continuity for their filaments and they look good.

By the way the transformer gets very hot but the outside does not look burnt but is nice and shiny. Anyone have any idea whats wrong here.

thanks
Joe

harris wrote:So I just finished building my ST70 kit and bought a PAS3 to use with it. When it arrived I powered it up and the rectifier tube and pilot lit but the 4 12x4 tube filaments didn't. I figured there was a power supply problem and knew the cap can and selenium rectifier should be replaced so I just went ahead and put in an Erhard Audio power board. That turned out to be a mistake because the symptoms didn't change. I should have done some troubleshooting first but now it's water under the bridge.

So what we have now is measuring the rectifier tube we have 12.5 DC across pins 3 &4. Only 250AC on 1 and 6 to ground and only 50DC on 7 to ground. I removed the rectifier tube and have 375ac on pins 1 & 6. I put the tube back in and disconnected the Erhard board and pin 7 goes up from 50DC to 341DC but still not up to 405 as it should. I have 2 rectifier tubes and the symptoms are the same with each.

As for the filaments on the 12x4 there still dead. I removed the tubes and checked for continuity for their filaments and they look good.

By the way the transformer gets very hot but the outside does not look burnt but is nice and shiny. Anyone have any idea whats wrong here.

thanks
Joe

I assume it's a z-psu you have installed. Was the board installed or did
you have to install all components yourself ?
Can you double-check the wiring ?

Yes it's a ZPSU and I populated the board and installed it myself and double and triple checked the wiring.

harris wrote:Yes it's a ZPSU and I populated the board and installed it myself and double and triple checked the wiring.

Is the capacitors soldered the right way ?

tube out 375vac, tube in 250vac
seems the 12X4 may be bad

Correction to my first post the 4 12ax7's have no filament voltage and I don't know why.. The 12x4 does have a lit filament and per the last post suggesting a bad 12x4  I have 2 12x4's both showing the exact same readings so should I buy another, what are the chances of my two being bad with the same readings? I did install the caps with the correct polarity.

harris wrote:Correction to my first post the 4 12ax7's have no filament voltage and I don't know why.. The 12x4 does have a lit filament and per the last post suggesting a bad 12x4  I have 2 12x4's both showing the exact same readings so should I buy another, what are the chances of my two being bad with the same readings? I did install the caps with the correct polarity.

All filemants of the 12ax7 are fed from a 24Volt DC created by voltage-doubling
the 12V AC ( that also lits the 12X4 )
Thus you have made a mistake in the board.
Re-read the manual.

First, was the PAS claimed to be working when you bought it? If so, did you try it before assuming a power supply problem based only on no visible glow from the 12AX7's? Note that, depending upon the tubes used, the 12AX7's may not exhibit a readily visible glow. Besides the more obvious observation of "do they get warm" the voltage to the filaments can be confirmed by measuring the DC voltage at pin 5. It should be about +12VDC or -12VDC depending up which tube is measured. However, even if the PAS was functional before you upgraded the power supply, it appears that, currently, there is a problem with the high voltage side, at a minimum.

50VDC at pin 7 of the rectifier tube, plus a hot transformer, indicates an usually heavy load, and you should refrain from any prolonged testing in that condition to avoid ruining the transformer. The fact that the voltage rises to 341VDC after disconnecting the power supply board is a good sign and exactly as expected. The 405VDC you expect will appear when pin 7 sees the first filter capacitor on the power supply board, once the fault is corrected.

Before tracking that down, I would first suggest confirming if filament voltage is actually present for the 12AX7's. In this case, the Erhard board will be fully connected, EXCEPT the connection to pin 7 of the rectifier to ease the burden on the transformer. You have already confirmed that the rectifier filament glows, and has proper voltage. Since the voltage doubler circuit that powers the 12AX7 filaments uses the same supply, the 12AX7 filaments should also have power which can be confirmed by measuring the voltage at pin 5 for each tube, relative to chassis ground, a stated above. I would do this first to confirm that you don't have a double problem to track down.

If the the 12AX7 filament supply is confirmed, then focus on the high voltage supply. Since there appears to be a loading issue, you'll have to find where the excessive load originates. First, and most obvious, is the power supply board, and in particular the electrolytic capacitors. I know you've said they're installed in the correct polarity, but check again. If you're absolutely certain, then disconnect the power supply board from eyelets 16 on both the PC-5 and PC-6 boards. Connect pin 7 from the rectifier to the power supply board and measure the voltage at pin 7 again. If the voltage is still low, the problem is most likely on the power supply board. If the voltage is no longer being pulled down, then then reconnect the PC-5 and PC-6 individually to see if doing so loads the supply. Note that with nothing connected to eyelet 16 of PC-5/PC-6 there should be an infinite resistance reading from these eyelets to ground.

Try this and see how far you get.

George, thanks for the excellent advice.
I just want to add that removing the 12X4 will disconnect the B+ for the process of
diagnosing the filament power supply.

Thanks to everyone who suggested troubleshooting ideas. I still haven't figured out what the problem is but I think I'll try another avenue and so have a few questions. I'm thinking of just scrapping my power supply and circuit boards and putting in the Erhard Audio regulated power supply, PH10 Phono Preamp and the TCLA with tone controls board. I would also swap out the selector switch and volume control as well as the connector block. This will basically give me a new unit mirroring their PAS NG. I successfully assembled the Bob Latino Stereo 70 kit so I think I'm up to doing this work.

So my questions:

What do you think of this Erhard upgrade sound wise?
Should I keep the tube rectifier or go solid state?

thanks