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    Please help me finish my PAS-2 upgrade

    Peter W.
    Peter W.

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    Post by Peter W. on Wed Dec 12, 2018 3:00 pm

    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.
    PeterCapo
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    Post by PeterCapo on Wed Dec 12, 2018 3:06 pm

    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+?

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

    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.

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

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


    Last edited by PeterCapo on Wed Dec 12, 2018 3:16 pm; edited 1 time in total
    billinrio
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    Post by billinrio on Wed Dec 12, 2018 3:15 pm

    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):

    Please help me finish my PAS-2 upgrade - Page 6 Z-PSU%20EA

    Two 10K and one 47K.


    Last edited by billinrio on Wed Dec 12, 2018 4:51 pm; edited 2 times in total
    Peter W.
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    Post by Peter W. on Wed Dec 12, 2018 3:22 pm

    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.
    PeterCapo
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    Post by PeterCapo on Wed Dec 12, 2018 4:03 pm

    Why would tekdevice think that PC5/6 would not make sufficient offset in voltages, if the cap board's outputs were the same as in the original B+ circuit?  That would be quite a leap for them - over analyzed or mis-analyzed.  Doesn't this suggest that reinstalling the 10K at the R3 position would actualize such an error on their part?


    Peter W. wrote:The OEM power-supply feeds the same voltage to the PC boards, the different resistors to the plates provide the offset.
    The original circuit supplies 330VDC under [the correct] load to E16 on PC5 and 210VDC under [the correct] load to PC6 E16.  So, there should be this difference to each board, right out of the B+ supply.  The 47K is needed to drop the voltage from the node indicated on the original quad cap with the triangle (355VDC).  Further offset occurs on the PC boards, which is why in an earlier post I asked Bill to post images of the resistors off of pins 1 and 3 on PC5.

    The bizarre thing about it is that this power supply circuit is very simple.  Difficult to imagine tekdevice screwing it up, especially when it is correctly represented in their schematic.  But, I suppose it's possible, somehow, that the circuit traces on the tekdevice cap board do not match up with their own schematic, and this might possibly be causing the problem and might be the reason they used three 10K resistors instead of two 10K and one 47K(?).  Some mis-arrangement of the voltage divider in the traces?  I would have long since depopulated the tekdevice board and traced out the circuit to see if it is correct, but, unfortunately, this is not feasible in this situation.

    Since we cannot really dig into it the way it needs to be dug into, I would think that, again, higher-than-component-level swaps need to be made.  Even then, I am not sure that the feedback we receive will be definitive.

    We're just spinning our wheels. Okay, fine, put the 10K back in for R3 and let's see what happens. It will raise the voltages and might be a compromise we'll have to accept.
    Peter W.
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    Post by Peter W. on Wed Dec 12, 2018 4:49 pm

    I have a single datum: Mine works with the 10K resistor ILO the 47K.
    PeterCapo
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    Post by PeterCapo on Wed Dec 12, 2018 4:51 pm

    Did you change the resistors on the PC boards to compensate? If not, then I am stymied, if your voltages on the nodes given in the chart are right. But, that's okay.

    Gotta run for a while.
    billinrio
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    Post by billinrio on Thu Dec 13, 2018 5:12 pm

    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).
    Peter W.
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    Post by Peter W. on Thu Dec 13, 2018 5:58 pm

    OK. We have plumbed the depths of remote diagnostics. So. After an hour or so, how warm is the transformer?
    billinrio
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    Post by billinrio on Thu Dec 13, 2018 6:04 pm

    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?
    Peter W.
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    Post by Peter W. on Thu Dec 13, 2018 6:38 pm

    Only warm - such that you would let a baby touch it.
    billinrio
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    Post by billinrio on Thu Dec 13, 2018 8:29 pm

    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.
    Peter W.
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    Post by Peter W. on Thu Dec 13, 2018 8:54 pm

    Now put it in service and see what happens.
    billinrio
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    Post by billinrio on Thu Dec 13, 2018 9:02 pm

    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.
    billinrio
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    Post by billinrio on Fri Dec 14, 2018 11:08 am

    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:
    Please help me finish my PAS-2 upgrade - Page 6 IMG-0399_zpsurqlkhjh

    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?
    Peter W.
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    Post by Peter W. on Fri Dec 14, 2018 11:25 am

    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:
    Please help me finish my PAS-2 upgrade - Page 6 IMG-0399_zpsurqlkhjh

    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.
    billinrio
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    Post by billinrio on Fri Dec 14, 2018 11:38 am

    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.
    PeterCapo
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    Post by PeterCapo on Fri Dec 14, 2018 3:39 pm

    Glad to hear you have it up and running and that you like the way it sounds.

    Bear in mind that your last readings on PC5 pins 1 are about 36% low.  And, on PC6 pins 1 about 13% high (should be unlikely given the output from your power transformer is on the low side).

    As I had mentioned earlier in this thread, and consistent with the contents in the original Dynaco manual, this is a compromise you might have to accept given you do not have other parts to drop in to try to isolate the problems.  One possible and unfortunate outcome might be that you arrive at a point where you feel like you wish it sounded better after-all and decide to drop in an altogether different circuit.  Or, you just keep it as it is and conclude that the original PAS wasn’t all that good-sounding, after all.  This kind of scenario is probably a frequent reason the original Dynacos get so beat up on the Internet and part of the reason why the cottage industry to replace the original circuit with a supposedly superior circuit does so well.

    BTW, while I do agree that heat can be an enemy, I really don’t think you lost your original power transformer to normal operating heat.  AFAIK, the business about mounting the transformer outside the chassis had its origins with the old Van Alstine "mod" that put the transformer on the back panel, IIRC in an effort to reduce hum and possibly also because there might not have been enough room inside after doing the modification.  You probably lost your original power transformer because there was some kind of electrical short that destroyed it.

    Feel free to disagree with any of the above.


    Last edited by PeterCapo on Sun Dec 16, 2018 1:56 pm; edited 2 times in total (Reason for editing : Clarification)
    billinrio
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    Post by billinrio on Fri Dec 14, 2018 3:47 pm

    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.


    Last edited by billinrio on Fri Dec 14, 2018 3:55 pm; edited 1 time in total
    PeterCapo
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    Post by PeterCapo on Fri Dec 14, 2018 3:54 pm

    billinrio wrote:I'm not sure just how hot it may get as a result of "normal operating heat" and still be OK.

    I think it is fair to say that the original designers knew well the answer to this question almost sixty years ago and released the PAS in its familiar configuration.  If the PAS is operating properly, long life can be expected without trying to compensate for things the original designers had already taken into account.  And, if it is any consolation, note that they did include a vent in the back panel.
    billinrio
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    Post by billinrio on Fri Dec 14, 2018 3:57 pm

    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.
    billinrio
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    Post by billinrio on Sat Dec 15, 2018 2:05 pm

    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.
    Peter W.
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    Post by Peter W. on Sat Dec 15, 2018 2:34 pm

    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.
    billinrio
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    Post by billinrio on Sat Dec 15, 2018 2:56 pm

    Yes, switching the tubes between the boards does improve the sound of the phono input considerably.
    Peter W.
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    Post by Peter W. on Sat Dec 15, 2018 7:31 pm

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

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