Please help me finish my PAS-2 upgrade

PeterCapo wrote:The schematic the tekdevice cap board came with duplicates the original Dynaco circuit.  R3 should be 47K.  If you put the 10K back, the voltages may come up some, but it still isn't really correct.  That circuit is supposed to work with the original values in the original places, as shown in the tekdevice schematic.

All true - but the resistors on-board PC 5 & 6 will handle the differential. I believe that the 10K resistor was a downline modification after introduction.

I did look at mine (installed and all voltages correct). It does have the 10K resistor.

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Well gee, that puts me in a bit of a quandary.
I note that the board sold by Erhard Audio looks very much like the one I have (should have purchased one from them; it's even less expensive, and I would have some support).  Here's an image from their web site (http://www.erhard-audio.com/Power_Supply_Kits.html)

(sorry about the size):



Two 10K and one 47K.

Peter - I will interpolate.

PeterCapo wrote:Your what?  You have a tekdevice cap board, or another cap board, or did you just use 10K for R3 when you built your B+?

I have very nearly, if not the same board as Bill does. And, it has, OEM, the 10K resistor.

Down-line modification because they ran out of 47K resistors?

Because the developer over-analyzed his first design, and attempted to create the same offset on the cap board as the PC 5 & 6 boards do directly. The OEM power-supply feeds the same voltage to the PC boards, the different resistors to the plates provide the offset.

I think the difficulty here is that we need to be able to swap out parts at a higher level than component-level troubleshooting.  We need a known good PC5 to swap in, or a known good cap board, or a known good power transformer - I understand you feel his PT is okay, but I can't help but still be skeptical of it.

His PT provided acceptable voltages without going over in the last configuration. This suggest that it, at least, is not the problem. There are so many other contributing factors that even a known-good board may not be good in this circumstance. I am tempted to send Bill a lash-up to go back to the OEM B+ power-supply and filament supply. But as you are skeptical of the PT, I am equally skeptical of the new boards. Not from any direct evidence, but from the POV that it has to be something.

We are trying to do this with both hands tied behind our back.

And one eye closed.

Okay, fine, put the 10K back in for R3 and let's see what happens.

In 24 hours.

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I have a single datum: Mine works with the 10K resistor ILO the 47K.

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Well, i placed the 10K resistor back on the cap board, and the pinout readings are not encouraging (for example, still around 112VDC on pins 1 of PC-5, and 130VCD on pins 1 of PC-6).

OK. We have plumbed the depths of remote diagnostics. So. After an hour or so, how warm is the transformer?

I'll turn it back on, wait for that amount of time, and let you know. How warm (or cool) should it be? Is there a way to measure its temperature?

Only warm - such that you would let a baby touch it.

Well, it's warm to be able to keep your hand on it without being uncomfortable.
10:30 p.m. here, and the ambient temperature is 70ºF.

Now put it in service and see what happens.

I need to get the two MKIV's out of the closet and hook them up to speakers (AR2ax); so given the hour, I'll do that in the morning and report back.

OK.  All set up.  Playing from my DAC (inaugurating with my favorite Schubert sonata, the D.959 played by Charles Rosen).  120VAC into the preamp. Amplifiers powered from its rear jacks.  Volume up. No hum and no distortion whatsoever.  The recording sounds very clean.  I had previously been listening to these speakers through my Marantz 2275, which is excellent, but this all-tube sound, with its warmth, is  much more musical.  I'm keeping the cover off the preamp due to the heat.  
Here's a picture:


This has been a very long thread, and I'm extremely grateful for all the kind help I've received, as well as your Job-like patience.  Any recommendations?

billinrio wrote:OK.  All set up.  Playing from my DAC (inaugurating with my favorite Schubert sonata, the D.959 played by Charles Rosen).  120VAC into the preamp. Amplifiers powered from its rear jacks.  Volume up. No hum and no distortion whatsoever.  The recording sounds very clean.  I had previously been listening to these speakers through my Marantz 2275, which is excellent, but this all-tube sound, with its warmth, is  much more musical.  I'm keeping the cover off the preamp due to the heat.  
Here's a picture:


This has been a very long thread, and I'm extremely grateful for all the kind help I've received, as well as your Job-like patience.  Any recommendations?

If you know a machinist somewhere who can machine slots in the cover of the PAS, that will help. Or, you can drill using an end-mill on a drill-press or similar for clean holes. In a pinch, and if you are so-tooled. you could cut out a square hole and insert some punched metal, secured with epoxy or similar. Harder to make it neat, but entirely possible. But, DO keep checking for temperature. That transformer should never get too hot to touch for a sustained period.

Otherwise, the "lesson" here, if any, is that Dynaco products are quite resilient and will operate cross a wide range of conditions without protest. If heat is avoided, most other conditions are tolerable as long as they do not affect sound. But in all-and-every-case, HEAT is the enemy.

i suspect that heat is what destroyed the original transformer, after I had inadvertently left the preamp, with the cover on, overnight. i've seen recommendations about mounting the transformer on the rear panel, but I think that even with that, the preamp really needs a vented cover due to the heat generated by the tubes.

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Very likely that was the case (a short) with the original transformer.  I'm not sure just how hot it may get and still be considered to be at "normal" operating temperature.  At the time of the transformer failure, the preamp still had the selenium rectifier and four section cap as well. It was built from a kit by someone with marginal soldering skills.  I got it while living in Chile from eBay (which in that country, much as in this one, is really the only vintage equipment market in town), and still during bygone times when U.S. international mail rates were reasonable. As I recall, the shipping cost to Chile was less than $25.

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Yes, but right now, just for belt and braces,  I now have a very small and silent fan blowing right at the transformer. As a result it's quite cool to the touch.

Update:  I had only listened to the preamp with the audio signal going from my DAC into the "Spare" input, and it sounded quite good.  But this morning I tried the phono input, and it sounds really awful, with high frequency distortion. So apparently PC-6 isn't as tolerant of out of spec voltages as is PC-5. This is serious for me, since I have a rather large (over 3,000 LP) vinyl collection.

billinrio wrote:Update:  I had only listened to the preamp with the audio signal going from my DAC into the "Spare" input, and it sounded quite good.  But this morning I tried the phono input, and it sounds really awful, with high frequency distortion. So apparently PC-6 isn't as tolerant of out of spec voltages as is PC-5. This is serious for me, since I have a rather large (over 3,000 LP) vinyl collection.

First, switch the tubes between the boards.

Yes, switching the tubes between the boards does improve the sound of the phono input considerably.

Suggests that you have some bad tubes. And if one (or more) is fried, that could be the "first cause".