PAS 3 Problems

I'm about to throw in the towel. I have a PAS 3 I'm rebuilding. Also, I bought a power supply from SDS and installed it the other night. Brought the PAS up with my Variac. But nothing happened. No juice. So I pulled the transformer leads from the board and measured the voltage. Sure enough, the transformer is fine. I then took a wild guess about the diodes and reversed the tranny leads. Everything came to life (that shouldn't have done it. Maybe it's coincidental). Voltages looked good, tubes all lit up, and then I quit for the night. This morning, I went out to put a signal through the thing and look at it on my scope. So I successfully turned everything on again and was going to flip the unit over and check pin voltages. But as I picked the unit up, the power went out. I tipped it back over and giggled the rectifier, because it’s a bit lose. The unit went back on momentarily then shut back down for good. Next I went back to the beginning and pulled the wires from the board again. The tranny built up 12 volts, just as it should. So the tranny is fine. Then I reconnected the leads (again trying leads both ways around) and the unit wouldn't pull any power. I don’t get it. It worked seemingly fine last night. This morning, no. I checked everything once again. And to my great embarrassment, I'd screwed up with two resistors. I had a 47K 1 watt where there should have been a 10K 1 watt, and vice versa. I then swapped them around, plus changed the two power diodes, just in case. The old ones tested fine, but I put two new ones in anyway. No change. It's just dead, won't even pull 1V. With the transformer leads removed from the board, it's still working fine, 12 volts. But resoldered in the board, something is blocking the voltage. I don't even know what to test for now. Obviously, something in the circuit is blocking the voltage, but I can’t figure out what it is. And it worked before. So I’m stumped. Did I cook something?

Looking at your board, the solder joints look suspect. From what you’re describing it could be an intermittent connection. I would try to inspect for loose joints. Resolder all joints with more heat. The solder should flow across the entire pad if it is heated correctly.

Also be sure there are no shorts due to unintentional solder bridges between pads.
Have you measured the voltage on the primaries of the transformers when the power supply is not working ? Is the transformer a dual primary transformer?

I agree - you may have some cold solder joints, based on the lack of flow and buildup in the photo. Please post a photo of the reverse side of the board.

Also; did you check all of the diodes? And the fuse?

Do you have a lab power supply you can sub for the PT? If not, did you use a bulb limiter to see if you have excessive current draw?

Yeah, some of the solder joints are messy, but they are secure. They look the way they do because I soldered the wires while they had excessive leads poking through, which I bent over to keep them secure. They look dodgy because of how I clipped them afterwards. But I love soldering, and I assure you, the solder was flowed onto both sides of the board. Also, I've checked everything with a magnifying glass and a DMM several times, looking for faults and mistakes. As far as I can tell, and I'm picky, all connections on or from the board are fine. I've come to believe that the problem must lie somewhere other than the board.

When I brought the unit up the first time, I went from a Variac to a DBT, to a monitor that reads voltage, amps, etc. that the unit is pulling. Everything went fine the first time, except the very odd situation that swapping the holes the transformer wires were placed in. I did that to check the diodes. But it shouldn't have made a difference. But the first time I actually got the unit powered up, I checked all the leads for proper voltages. Everything was great. I went to bed, thinking I'd go back out in the morning to do a further check out and run a signal through it. It turned on fine. But as a picked it up to turn over and check the pin voltages, the unit shut off. I jiggled the rectifier tube a little and the unit came back to life, for about 40 seconds, and then turned off again and has never turned back on. There is no voltage present at the insertion point of the transformer leads at the board. Off the board, I get my 12 volts. I'm stumped. My next move is to test the rectifier tube. But I don't have a tube tester and I live in a very rural area.

Another thing, just for reassurance, I replaced the diodes. I then tested the old ones, and they are fine. And the fuse is fine, as it sits before the transformer. With the leads out, the fuse holder pulls the expected voltage. With the leads in the board, there's no voltage.

I sent you a PM on the other forum yesterday. You may want to check it.

need pics of both sides of board as well as pics if the preamp boards

10-foe! I hope this clear enough to make everything out. I should have taken another picture last night. I took the board completely out, changed the diodes, put the two out of place resistors where they need to be, but I didn't take another picture, as none of the wires needed to be relocated. There were all in the right place. I changed out the diodes just to be damn sure one or both of them weren't reversed.

Damn it. The picture didn't attach.

This what I just got from the transformer:

Procedure: 1.) Variac. 2.) Dim bulb tester. 3.) Power monitor/meter. 4.) PAS with DMM measuring at the leads.

With one leg unhooked: 11.55 volts at leads with DMM; at the meter: @110 volts, 0.043 amps; 81 watts.

Both legs in board: .007 volts at leads with DMM; at the meter: @ 82 volts: Variac unable to produce 110 volts, only got as high as 82 volts, pegged; .374 amps. 24.7 watts.

OldFrisco wrote:...I assure you, the solder was flowed onto both sides of the board. Also, I've checked everything with a magnifying glass and a DMM several times, looking for faults and mistakes. As far as I can tell, and I'm picky, all connections on or from the board are fine.

That may be true, but hopefully you'll accept the comments two of us have made as constructive and consider that your soldering technique could be refined. Good joints fill the pad on the back side, though they don't need to fill the top side, and won't unless the pads are through-hole. Your iron may need to be a little hotter, and/or your contact time a little longer. Also check that you don't have any solder bridges. You might want to clean your board with a q-tip and ipa to get rid of some of the excess flux mess.

OldFrisco wrote:Both legs in board: .007 volts at leads with DMM; at the meter: @ 82 volts: Variac unable to produce 110 volts, only got as high as 82 volts, pegged; .374 amps. 24.7 watts.

Either you have a short or the transformer is failing under load, which I've had happen. I'd suspect a short on the board. Use your DMM to check for shorts on the board, with no power applied or connected.

Also, the second photo doesn't show D1 and D2 on the back side - are they there, because they aren't on the top of the board?

The two missing diodes are for using the board with SS rectification. "Two diodes are needed if using the 12AX4 regulator, four diodes are needed if you are replacing the 12AX4 with solid state rectification. The diodes at the top of the figure to the right are the optional ones used only in place of the 12AX4."

I do take and appreciate your points regarding soldering. I'll go back and reflow all the leads. That's easy. I do, however, feel embarrassed, in that the board is a messy and ugly job of soldering (no excuses). Perhaps I overestimate the quality of my work. So, only because I already feel so ridiculously inferior on this site, I'm sending a picture of another amp I built and have never had a problem with. I wish to either soothe my insecurity, or learn that I DO need to relearn — embarrassing as that might be.

I just feel like there is something obvious that, nonetheless, I'm overlooking. I just rechecked the V1 pin arrangement. I've looked for shorts and verified continuity where indicated multiple times, but nothing shows up. This is making me feel SO stupid.

So please don't feel that I'm dismissing anyone's remarks. I assure you, I am not.

I reflowed all the solder joints on the power supply board. I then confirmed all continuity to their respective sites on PC-5 and PC-6, plus ground and V1. Then I stared, real hard, at it for another hour. Everything has stayed the same. What I can't figure out is, why did it work apparently perfectly, then suddenly refuse to turn back on? In any mechanical situation, normally, if a mechanism is working, then stops working, something in the system failed. But I can't find anything wrong or different. All the voltages came up perfectly the other night. I was going to go ahead and run a signal through it, but it was already midnight. So I shut it off, and the rest is strange history. But I sincerely thank you guys for trying to help me.
I'm going to stare at it some more tomorrow. I'm also going to talk real sexy to it and offer it a drink — you know, invite it over to my place for a little Scotch and sofa. If that doesn't work, I'm going to tie it up and spank it. I'm out of ideas.

Frisco
I just started following this thread but is the B+ high voltage present in this preamp? I know you have been talking about the 11.5 or 12v supply but see if you have the higher voltage for the B+. Lets assume you do and the filament supply is the problem. Pull the 12Ax7s out of the Preamp and lets see if the 12v comes up on the transformer. If it does, you have a transformer breaking down under load. It worked once after the board replacement so it were wired wrong, it wouldn't have worked any time.
Pull the 12AX7s and leave the 12X4 in and lets see what we get.

There isn't any voltage at B+ either. You put your finger on the very odd issue: it worked for an hour, then turned back on the next morning. Then, after 2 minutes, it went dead again. That makes me think something failed, but I don't know what. I'm kind of out in the sticks up here in Northern California, so there really isn't any local source for parts, or I'd try another 12AX7. I know bad rectifier tubes can do some very weird things. I'm going to go ahead and order the cheapest one I can find on Amazon, just to eliminate everything likely to be the problem. After that, I'm going to be forced to buy an new transformer, just to eliminate that, as well.

Frisco
It is most likely the power transformer. You may have a loose wire on the primary going to the transformer or you may try jumping the power switch. See if the 120vac is reaching the primary wires. If it is, pull out the power transformer and you can pull the end caps off the power transformer and inspect the wiring inside under those caps. Did you check the 120vac going to the black wires? Do this before anything else. I'm assuming the power transformer is original, correct?
Mike

Mike, when the tranny is wired into the board, it screws everything up. Wired up, the unit wouldn't even pull any negligible voltage. With the wires unhooked, everything is normal. The problem is, the tranny is original, and the bells are not removable, that I can see. Otherwise, I'd pull them off and inspect the wires, as they are old and it's obvious and visible that the insulation on the wires has some serious wear on it where they exit the bell. I put some shrink tubing on them to try and keep them from shorting on the bell, but there is not short that I can see. Still, your suggestion is exactly what's been on my mind today. But I really don't want to spend $100 on a new transformer, if I don't have to. I don't know that there are even any available. Do you know how to get the bells off without tearing the entire transformer apart?

Frisco,
This site gives excellent advice as there are people on here that have a passion for these tube amps and have many years of experience. For that reason I always prefer to listen to their advice before I speak up to share mine, but your problems sound to me to be a ground problem. If this were mine, I would focus on checking all the voltages at critical measuring points while giggling the wires. This will also help you to verify the solder joints are solid. First compare all the voltages as you giggle the wires at each ground terminal. Yes it will take some time to check/measure all of them. Definitely do this before you start looking for a replacement transformer. These transformers are usually solid and last for several decades before they have any issues. The more likely problem is with the wiring and most likely with a ground IMO.

Good luck, Mike

First of all, I apologize for the ridiculously long post. I’m offering this in case it might be of help to someone else at a later time.
So, I believe I solved the mystery. Much like everyone who weighed in on this problem, very early on, I strongly doubted the problem was to be found on the board. However, beyond that, nothing made sense. My first doubts came up about my installation of the diodes, which weren’t the problem (I replaced them anyway).
Also, just to be thorough, a minimum of a dozen times, I checked and re-checked all connections and continuity. It was for this reason only that one of my posts was taken with offense. I sincerely apologize if I did offend. That wasn’t the intention. My point was simply that I had already inspected the solder joints, even the original joints, with a magnifying glass many times, such that it was easy to verify that they were not the source of the difficulty. The problem is, if the roles were reversed, I would have said exactly the same thing. It would be the obvious. And after the suggestions, I went back and re-flowed everything, just to finally eliminate that potential cause. The challenge was, without the opportunity to actually inspect the board themselves, in person, and given that in the photos the joints didn’t exactly appear to meet typical standards, it was next to impossible for anyone to see what I was able to see. My intention was simply to insure everyone who was so generously attempting to help me that solder joints, as the potential cause, had been inspected, tested, and eliminated. I simply wanted to move forward and look for other causes. I was only trying to assure everyone that I had made certain the joints were sound. However, as I’m sure I would have responded, if the roles were reversed, the skepticism persisted, and I didn’t know how else to get everyone to move to the next possibility. All I wanted to do was create some confidence that I can solder well, and that I’m not a novice who might think they know how to solder but actually lacked some basic skill.
Again, I’m sorry if I offended anyone. That was definitely not the intention.
Anyway, one thing that did not make any sense from the beginning was, when I first tried to power up the unit, nothing happened. I studied the situation, looked for bad joints, improper placement, and all the possible knucklehead mistakes that are common.
As a Hail Mary, I reversed the transformer leads on the board. I simply desoldered them and swapped their respective holes. I thought this might reveal something about the diodes, which I actually knew it couldn’t. But I was getting desperate.
And as soon as I reversed the leads, everything was perfect. The problem is that it shouldn’t matter which of the two holes either lead goes into. That made no sense.
All of that produced another two days of unsuccessful diagnostics.
Yet, as I indicated in my initial posts, the transformer leads were suspect from the beginning. For whatever reason, the leads, as they exited the transformer, had rubbed excessively on the holes in the bells. The cloth had been rubbed away, but with a magnifying glass, I could see that the other insulation was intact. So I slid some shrink tubing over all the wires and hoped for the best. However, the transformer isn’t one that allows for the easy removal of the bells. I would have preferred to have been able to push the shrink tubing on deeper than the bells made possible. But I did the best I could. And it was the B+ group that were most worn, not the 12v side.
Okay, you already figured it out. Switching the placement of the wires moved the point at which the leads exited the bell and eliminated a short. But after working on the unit a bit more, it seems the wire once again shorted against the bell, on the inside. It wasn’t visible from the outside. And when I pulled the wires from the board, it moved them enough that they read appropriately. Put them back in the board, and they once again contacted the bell.
I had to unstrap the transformer in order to get the bells off, but once I did, the problem was pretty obvious.
Anyway, I treated all the wires with shrink tubing, where the leads articulated the bell on the inside, and started putting everything back together.
But just as I was beginning to re-solder everything, when I twisted the transformer back into position, one of the B+ wires broke off. And it broke at the last point before it goes into the winding. So there’s now nothing to solder a new lead back onto. I may have to buy a new transformer after all. But I need to go out today and take a closer look.
Problem is, the wires are now so brittle, after I don’t know how many decades, maybe I should just try to find another one. However, I don’t even know if there are any presently available.
Sorry for the over-long post, but I thought it might prove useful for someone else down the road.
Thanks for all your help.

Frisco
Merry Xmas. Anyway, why not upgrade the PA-211 power trafo to a torroid? They are 28usd and they are heftier than the original and I've used 4 of them and they work fantastic.
I don't believe you offended anyone on the solder joint issue. It was a suggestion by some and I would have suggested it as well because they did look suspect in the photos but photos can be deceiving.
It's good that you went over them. I figured it was a Power trans issue because the transformer is rather weak in those. There is also the fact that it worked at times so that eliminates incorrect wiring. Put this in.
http://www.analogmetric.com/goods.php?id=1026

I may be able to repair my unit. I'm going to check it out today. But the torroid is something to keep in mind, just in case. Thanks for the suggestion.

This is bonkers! Look how small these leads are. And this picture was taken through a magnifying glass.

Frisco
I have fixed transformers as well but only the heftier ones. Just be sure to goop down the wires after you resolder them.