Dyna Pas

[TomMcNally]

I'm not sure what you mean by "channel" ... the word channel really has no bearing on the power supply board. It sounds like you have the transformer connected to the board properly. One filament lead from EACH board goes to one terminal on the SDS board, the other filament lead from each board goes to the other terminal. In other words, the boards EACH go across the 24 volt output of the SDS board. The filaments on the boards are in series, so they run on 24 volts DC which is NOT groudn referenced. It seems like you have them hooked up to the wrong spot. Can you send me some hi res pictures ?

... tom

[leadtower]

Yes; that makes sense. Thanks so much for having the patience to stick this out; I had a needle stuck in the groove there. The tubes are now lighting up nicely with 11.08v on each filament. Swapped the wires over and let there be light.
Looking at the schematic for the original Pas PS it is clear as daylight now.
Can I ask one last thing? I have two 25 uf 200v caps wired in parralell jumpering pin 16 on PC 5 to one of the unmarked solder points on the same board. I don't see that cap listed in the manual; do you have that set-up in yours; I don't see anything in the schematic for that either.
Again; much thanks.

[mesherm]

leadtower,
I emailed you some hi res pics of my factory wired PAS3 with the SDS cap board I installed. It was a factory PAS3x I got on ebay for a pittance from someone who had ripped the line pre amp pcb out and pulled all the componenets off only to get dishearted and quit. All I did was buy a SDS board and install componenets on a new line board I had and basically hook up all the wires the previous owner had unsoldered.
It is my backup preamp. I have one that was basically gutted and I had to completely rewire the whole thing and install new PCBs with new componenets. That one has far less hum than the sloppily (IMHO) wired factory unit.
Pin 16 on both boards is the B+ line so someone added some capacitance. I would remove them since the SDS has more than enough filtering.
BTW, the PAS manual has no pics showing the location of the various components so if you decide to replace the boards keep them long enough to use for reference. I got one with no boards and had to work it all out from the schematic...BIG PITA...whew.
Mike

[TomMcNally]

Hey very cool that you got it going !

Those extra capacitors across the B+ were prbably someones attempt at reducing hum. I have a PAS around here someplace, it needs to be restored.

How does yours sound ?

[leadtower]

Tom and Mike; I see now the two 25 uf caps must be as you say; hum reduction. This unit was built by a local engineer yrs ago; I bought it and the ST 70 from him. He did nice work but he moved to Florida to retire so I can't ask him why or what for.
The original boards are still in place and I only did a quick run-through last night. It sounds better than it did with the old stock PS; better bass for sure but I don't have a good listening session as of yet. I bought new exact replacement boards so thats next. On mine I removed the tone controls and that helped a lot. There is a lot of mods floating around on the net; I am wondering if and what is worth doing to the pre-amp; it is in nice condition and is the older Pas 2 model. I like the old retro-look.
Thanks to both of you guys again. I may have more questions as the project progresses if thats allright.

[TomMcNally]

Glad to help anytime ... I like that feeling when they work !
You might motivate me to dig mine out and mess with it. I bought a bare board for the power supply upgrade, I think it was an early CChong product. Gotta populate it and give it a shot.

[mesherm]

On the PAS I rebuilt, I tossed the stock rotary switch in favor of a nice new 6TDP one.
The original was crusty and had the tape/phone EQ switching componenets. I hard wired the PCB for RIAA EQ eliminating the tape EQ which is obsolete. I also made a new RCA jack panel with more room between jacks. The RCA pair for the phono now go directly to the circuit board. I think Roy Mottram at tubes4hifi was selling a nice replacement RCA board for a PAS at one time, you might shoot him an email. I was lucky enough to get a PAS3x with the special pots that pass through the signal when centered so all the tone controls are still operational although I rarely use them. I also put my best set of 12AX7s (Telefunkens) in the line board and a set of cheap Sovteks in the EQ board since I don't have my turntable hooked up.
Redoing the input wiring with twisted teflon wire and dressing the wires really cut down the hum compared to a stock PAS.

[TerrySmith]

I've often wondered about using shielded multi-conductor cables from the RCA jacks to the switch, but I'm worried about crosstalk between channels. Has anyone tried it?

Another idea I had was to mount the rotary switch on a bracket close to the rear panel and add a shaft extension to keep the Knob in the stock location.

[mesherm]

I haven't noticed any and I used red/white 26 gauge teflon twisted pair to carry left/right and bundled them all together from the RCAs to the switch in front. I'm sure the levels are too low to cause any noticable crosstalk. If you use shielded cable I suggest soldering the shield at the RCA end only and leave the switch end unconnected. Mounting the switch near the back would work too but probably require some creative rework.
Like I said, my leadins are all twisted, bundled and routed away from everything else close to the chassis and there is no noticeable hum even at full volume.

[leadtower]

Say Mike; how did that rotary switch fit in the chassis?
I bought the Mottram board for the RCA connections and it requires that you cut out some of the chassis bracing to install it so I haven't decided yet whether to do that.
Has anyone tried a volume pot they thought sounded better?
It's funny but after doing some reading on this pre-amp it seems there are two schools of thought. One groups says that Hafler designed the circuit to work properly as built while others claim to have all these mods to make it sound better. The first group says the mods are not really well thought out from a design perspective and that many of them produce unwanted problems. In my experience I have heard vintage units modded that I thought sounded worse than before. It makes you think.
So I think to replace the Controls; all filter and coupling caps and the plate and cathode resistors with better tolerance units. But keep the circuit basically stock. I have read some really exorbitant re-engineering schemes; I wonder how that works out? I am looking at the mosfet follies now for some interesting reading. I also have two pair of Bottlehead CCS circuits; I wonder if they could offer some advantage?
I listened for about an hour yesterday and truthfully it sounded really good with the ST 70/ DIYTube driver stage. Even with the old caps and resistors I liked it at least as much as my AES pre-amp. A little rounded at the freq extremes and somewhat soft but Traffic sure sounded great; 40,000 Headmen can't be wrong!
And no really noticiable hum unless you get real close to the speakers.

[mesherm]

The rotary switch fit in fine. It was much smaller than the stock unit.
I didn't know that the chassis has to be hacked to fit that input board.
I don't blame you for holding off on installing it. I am generally against hacking into an original chassis unless absolutely necessary. I made my own input board to fit the chassis with some phenolic laminate and RCAs.
I kept all the original control functions also with the exception of the tape head EQ. I cleaned the pots with tuner spray and lubed them with the red Caig stuff that used to be called Cramolin. They work fine. I kept the tone controls since they were the "special" ones that bypass the tone circuit at center. If it had been a regular PAS-2 or 3 I might have tried taking them out of the circuit ala Curcio. I think most of the hum comes from the input wiring and rotary switch as the one I rewired is dead quiet and the other stock factory unit has some hum even with the SDS board.

[kurtbauer]

I thought I'd toss my 2 pF in on this as I just fnished Van Alstine upgrade to a PAS3. As you may know, it has new line and phono boards and and cap board. It also features no tone controls, new volume and balance controls (stepped attenuators), and the original selector switch.

The latter point I thouht you might find interestiing. I'm using new JJ ECC83s and only using the phono input (the other obsolute inputs are converted to line level) has the slightest bit of hum. The line levels are super quiet. Therefore, I'm not sure that the selector switch is the source of hum aside from wiring to/from it, of course. Van Alstine views this as a "replaceable item" so if it works us it. I have heard from elsewhere, that the low level of crosstalk across active inputs can be reduced/elimateed with a modern selector switch.

It's a nit but wanted to toss it in.