Dyna Pas

[leadtower]

Does anyone have a PAS and if so do you use the SDS Power supply board? I just installed one in my PAS and I can't get any power to the boards. I get correct voltages on the 6x4 and good filament voltages from the transformer but I get nothing out to the boards. I would suspect the diodes except they measure 0 resistance one way and infinite resistance the other way so that should be good; no?

[mesherm]

I have an SDS board in one of my PAS 3xs and it works ok. The PAS uses a 12X4 though. The PT supplies 12 vac to the 12X4 and a voltage doubler to provide 24 vdc to the PCBs as well as B+. Are you not getting BOTH outputs?

[leadtower]

Sorry; yes it's a 12x4. I get 390v to one channel on the cap board but nothing to the tubes. It's definately not a solder issue because I re-soldered every point and double-checked them. Maybe you know this; on PC 5; PC board pin 16 is the power take-off. I have one lead going from the PC 5 to the Line-output of the SDS board and I have a lead from pin 16 on PC 6 to the Phono-output of the SDS board. I have the SDS board ground wire going to the original can cap ground pin along with the Red-Yellow transformer wire. The B+ I take from Pin 7 of the 12x4 to the B+ point on the SDS board.
I never hooked up the line-fuse on the SDS board. I wanted to make sure it worked first.
I know this is a lot to wade through and I appreciate it. I never had a problem with my SDS board in the ST 70.

[TomMcNally]

The "line" connection and fuse on the SDS board is just an isolated fuse, to add to the preamp for safety. It's not in the circuit otherwise. The B+ for the line level board comes from the hole next to the fuse by the resistors, and the phono power comes from the hole near the .1 capacitors, near the fuse.

[leadtower]

Yes; That part I got. Any hints on troubleshooting the filament circuits? I know they are series wired.

[TomMcNally]

First, check the filament loop on the PAS with an ohm meter ... you should get a low reading ... if you don't - you may have an open filament or loose tube ... meter each tube to find it. If you DO have a reading, double check the board, diode polarity, etc and see if you have 24 volts DC where you should.

Are you measuring B+ on the LINE and PHONO points ?

[leadtower]

Here's what I have on the SDS board. The rec filament/transformer inputs; each channel measuring 13.8 v.
The diodes I get 13.8v on the pos side and nothing on the neg side for both channels.
The tube filaments outputs; one channel I get 27.9v and the other channel I get nothing. I know that accounts for the problem of one channel on the SDS board having no output but I don't see why!
I measured all of the caps and the filament filter caps measure good. The Filter caps for the B+; one side has 312v and the other side has nothing. The B+ is measuring 390v off the SDS board and the ground has continuity with the chassis. All of the pinouts on the rec tube measure good. I have no tube on the PC boards that has any filaments lit up.
Baffling. Hope this helps for troubleshooting.

[TomMcNally]

I don't have a schematic for the SDS board, I'm just looking at the board layout and the PAS schematic. It looks like the diodes should have the banded end pointing at each other. You would want to measure the voltage across the two output points (not to ground) and see what you get. If you have voltage there, you must have the wires from the circuit boards connected wrong. The B+ would be referenced to ground.

I would suggest looking at the pictures and diagram and making sure everything is OK .... http://www.quadesl.com/pdf/pas_capboard.pdf

It's not easy since the board isn't silk screened.

[leadtower]

Thanks Tom; I have the banded ends of the diodes facing in the same direction; maybe thats it. In the instructions for the SDS board they are silk-screend that way but let me switch them and see what happens. I have a spare pair. But let me ask; why would they measure o'kay if they were installed backwards? And how would that affect only one channel?
Aagin thanks and I'll post the result.

[TomMcNally]

I'm only going by pictures which are hard to see ... if they have it silk screened then they are probably OK. The diodes are in a voltage doubler circuit, measuring the diodes isn't important, the question is, do you get 24 volts (27 unloaded maybe) on the output side ? When you say "one channel" do you mean stereo channel, ie: left and right ?

Attack this logically ...

1> is there voltage at the filament output terminals
2> are the correct wires from the PAS board filament strings
hooked up correctly
3> do the tubes light up

All 3 condition must be YES before it will work,
so attack them one at a time. You won't measure
the filament voltage to ground - but across the terminals

If you can't get it going ... let me see some hi-res closeup pix !

[leadtower]

Yes; I get 13.7v at the outputs of the filament wires and I triple checked them to their locations on the PC boards. None of the tubes light up. I still need to determine the proper voltages on each 12ax7 tube yet but in the meantime I checked each filter cap on the SDS board and I get 314v on two of the caps; 380v on one of the caps and nothing on one of the caps. Could that cap be bad by any chance? It's new but I know that doesn't always mean anything; could the cap be bad?
There is a site called Triage Audio with pics of the SDS boards completed and mine seem to match up correctly. That leaves a mis-wire on the PC boards but there is only one B+ wire to each board and the two filament wires and I checked them thoroughly. I have one other question; if one of the tubes is bad would that screw up the filament voltages?

[TomMcNally]

The two tubes on each board have their 12 volt filaments in series, so yes, if one tube is bad or loose in the socket, or bad solder joint, neither will light. You can easily check them by simply taking them out of the socket and putting your ohm meter across the filament pins (4 and 5) on the 12AX7's and you should get a reading, if it's open, the tube is bad. Swap them around and see if you can find a pair that work ... put them in the line preamp side and see how it works.

I'm not sure how you are measuring the capacitors, it's not real meaningful since they are in series.

... tom

[leadtower]

Say Tom; whats the chance that the position for the diodes are mismarked on the PCB? What happens if the diode is backwards in it's orientation and you apply power?
Thanks for your help BTW.

[TomMcNally]

It doesn't make sense that you are getting filament voltage at the board, but the tubes don't light. I would do this ... test the tubes filaments with an ohm meter, and test the voltage at the filament pins with the amp upside down. If everything is wired right and the tubes are good, they have to light. Did you swap the tubes around ? Find two that have filament continuity for sure and try them on each board.

I doubt the diodes are backward if you are getting DC on the board.

[leadtower]

I know something is mis-wired here. If I had a diagram of how exactly the SDS board is meant to be wired that would help. Even though it only has eight tags there must be something wrong. I get 29v on one channel filament output and nothing on the other channel filament output. The channel with 29v reads 29v on the tube filament pins 4&5 on the PC 5 line level board. The tubes are now brand new and checked by the vendor. Yet they still do not light.
On the SDS board I get 15v on each of the transformer/rec tube filament inputs. I get good B+ and good ground. I get good voltages at the Line outputs and phono outputs. Nothing on one channel of the filament outputs even though the diode is now new and tests good. I hope this is a lucid description because this is baffling to me.
Let me ask; one each of the transformer secondary wires are twisted together with one each of the rec tube filament wires then soldered into the double holes between the two filament filter caps on the SDS boards; one twisted pair for each channel? Thats how Triodeel told me to wire it.