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peterh
Pablo
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    Dyna PAS3 voltage question

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    Pablo


    Posts : 13
    Join date : 2021-04-21

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    Post by Pablo Sun Aug 22, 2021 8:24 pm

    Hello:

    I found a PAS3 preamp I forgot I had, that has been residing in storage  for the last 30years.

    It's in decent cosmetic condition, so I decided to clean it up, give it a set of tubes from my stash,  give it a gentle wake up with a Variac, and check out the voltages.
    My goal is to sell it in good operating condition.
    I didn't expect the can cap to still be good, but with a very gentle awakening  it responded without heating up, and the background noise level is very low without any hum present.
    I replaced the selenium rectifier with silicon diodes.
    I cleaned the sockets, controls and jacks, and the tubes I put in all test strong.
    I've installed upgrade packages from Curcio and Van Alstine back when I acquired this preamp, but my goal is to have it original, presentable and running properly when I sell it.
    It sounds as I expect a PAS3 to sound, in fact none of the control pots exhibit any scratchiness after I gave the a Deoxit Faderlube treatment.

    So I have finally gotten around to my question:
    I am using the voltage chart  from the original Pas3 assembly manual to check voltages at the can cap and the tube socket pins.
    I did the check with 120V line voltage since the voltage chart specifies 120V.

    The 1st can cap terminal is just about at nominal voltage (405V).
    I installed new dropping resistors on the can cap per the schematiic.
    The 2nd can cap terminal is about 25V below nominal (355V).
    The 3rd can cap terminal is about 45V below nominal (330V).
    The 4th can cap terminal is about 10V below nominal (210).

    I realize the low voltages are near the +/_ 10% tolerance often cited for tube equipment, but when I check the plate voltages for the line and phono sockets I am well beyond the 10% tolerance.
    On the line board I was 65V below nominal (200V) for pin6.
    I calculated what the 10K dropping resistors for the 1st two terminals on the can cap would need to be to get nominal values and added parallel resistance needed to get there.
    So I now have plate voltages on the 12AX7's at nominal or within 10% high.
    I'm thinking that the schematic either has the dropping resistors wrong or the pin voltages wrong.
    I've done some searching on the web and don't see anyone else mention this issue.

    So there are lots of folks out there who have worked the PAS3, has anyone seen the voltage anomaly with the schematic?
    peterh
    peterh


    Posts : 1599
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    Location : gothenburg, sweden

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    Post by peterh Mon Aug 23, 2021 1:11 am

    Pablo wrote:Hello:

    I found a PAS3 preamp I forgot I had, that has been residing in storage  for the last 30years.

    It's in decent cosmetic condition, so I decided to clean it up, give it a set of tubes from my stash,  give it a gentle wake up with a Variac, and check out the voltages.
    My goal is to sell it in good operating condition.
    I didn't expect the can cap to still be good, but with a very gentle awakening  it responded without heating up, and the background noise level is very low without any hum present.
    I replaced the selenium rectifier with silicon diodes.
    I cleaned the sockets, controls and jacks, and the tubes I put in all test strong.
    I've installed upgrade packages from Curcio and Van Alstine back when I acquired this preamp, but my goal is to have it original, presentable and running properly when I sell it.
    It sounds as I expect a PAS3 to sound, in fact none of the control pots exhibit any scratchiness after I gave the a Deoxit Faderlube treatment.

    So I have finally gotten around to my question:
    I am using the voltage chart  from the original Pas3 assembly manual to check voltages at the can cap and the tube socket pins.
    I did the check with 120V line voltage since the voltage chart specifies 120V.

    The 1st can cap terminal is just about at nominal voltage (405V).
    I installed new dropping resistors on the can cap per the schematiic.
    The 2nd can cap terminal is about 25V below nominal (355V).
    The 3rd can cap terminal is about 45V below nominal (330V).
    The 4th can cap terminal is about 10V below nominal (210).

    I realize the low voltages are near the +/_ 10% tolerance often cited for tube equipment, but when I check the plate voltages for the line and phono sockets I am well beyond the 10% tolerance.
    On the line board I was 65V below nominal (200V) for pin6.
    I calculated what the 10K dropping resistors for the 1st two terminals on the can cap would need to be to get nominal values and added parallel resistance needed to get there.
    So I now have plate voltages on the 12AX7's at nominal or within 10% high.
    I'm thinking that the schematic either has the dropping resistors wrong or the pin voltages wrong.
    I've done some searching on the web and don't see anyone else mention this issue.

    So there are lots of folks out there who have worked the PAS3, has anyone seen the voltage anomaly with the schematic?

    My guess is that you have a leaky cancap.
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    Pablo


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    Post by Pablo Mon Aug 23, 2021 11:40 am

    I was thinking that was a possibility, but other than the can cap getting hot, I don't know of any way to check for electrolytic cap leakage.
    The can cap is running close to body temperature after an hour of operation with the preamp case removed, and since it's sitting next to the rectifier tube, that doesn't seem abnormal.
    I have a variac, a couple of multimeters, and an oscilloscope.
    Is there ready way to check for leakage using the equipment I have ?

    The reason I am questioning the schematic is that I have the assembly manual from the Van Alstine Super Pas3 I assembled 30 years ago, and the PC5 board shows the same components as the original Pas3, but power supply delivers 240V to the PCB as opposed to the 330V shown on the Pas3 schematic.
    The voltage check after assembly says to look for 125V to 150V on the 12AX7 plates.
    The power supply on the super Pas3 is enhanced over the original Pas3 so I can't really compare the can cap voltage with it.
    peterh
    peterh


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    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh Mon Aug 23, 2021 12:08 pm

    Pablo wrote:I was thinking that was a possibility, but other than the can cap getting hot, I don't know of any way to check for electrolytic cap leakage.
    The can cap is running close to body temperature after an hour of operation with the preamp case removed, and since it's sitting next to the rectifier tube, that doesn't seem abnormal.
    I have a variac, a couple of multimeters, and an oscilloscope.
    Is there ready way to check for leakage using the equipment I have ?

    The reason I am questioning the schematic is that I have the assembly manual from the Van Alstine Super Pas3 I assembled 30 years ago, and the PC5 board shows the same components as the original Pas3, but power supply delivers 240V to the PCB as opposed to the 330V shown on the Pas3 schematic.
    The voltage check after assembly says to look for 125V to 150V on the 12AX7 plates.
    The power supply on the super Pas3 is enhanced over the original Pas3 so I can't really compare the can cap voltage with it.
    The best you can do now is to revert to the dynaco manual. Without proper documentation anything can look suspicious.
    Instead of replacing the can cap a power board like the z-psu from erhard would get you back on track. This board is a direct copy of the PAS powersupply but with modern components.
    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi Thu Aug 26, 2021 10:36 am

    if the can cap is hot, it's bad.
    You can get a new one here . . .
    https://www.dynakitparts.com/shop/multi-section-capacitor-1-2-3-4-5-6/
    or better, get a replacement power supply board, many suppliers
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    ELBill34


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    Location : Chicago

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    Post by ELBill34 Thu Aug 26, 2021 4:24 pm

    Check the other resistors throughout the circuit. Something may be out of tolerance enough causing a tube to draw more plate current than it normally should loading the supply and bringing down the voltages.
    Also I've seen one section of a supply cap read very low in value and cause voltages to be lower than they should.
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    Pablo


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    Post by Pablo Thu Aug 26, 2021 6:40 pm

    When I originally woke the amp up with the low plate voltages,  and took a look at the output with my oscilloscope, the wave form was distorted, and channel balance was off with controls centered.
    I could clean up the wave form and bring the balance back by skewing the treble, base, and balance controls and watching the scope.
    When I adjusted the can cap voltages I got the output to look good with controls centered.
    Although I wasn't sure that a can cap running at body temperature was a symptom of a leaky cap, everything else seemed to point to it so I have ordered a new one.
    Even though it's running well with revised dropping resistors, I was concerned about putting an extra load on notoriously marginal PA211 power transformer.

    Thanks to everyone for the responses.
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    Wharfcreek


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    Post by Wharfcreek Thu Aug 26, 2021 8:06 pm

    Because a multip-cap can doesn't get hot does NOT mean that the can is good. On the contrary, there can be open sections where no filtering at all is in the circuit, and this can throw voltages off significantly. I'm sure there are other ways to test a multi-cap, including ESR meters, etc. BUT, from my perspective, the best way is to pull it off the board and use a genuine capacitor tester that actually loads the cap at it's rated voltage and measures internal leakage in terms of milliamps. I believe several manufacturers made such devices, and I use a Sprague TelOMike unit for doing just this. There is also a chart associated with testers like this that show what caps of varying voltages and uf ratings 'should' have as a maximum leakage number. So, when pulled from the unit, you can simply check each section for proper operation. Honestly, I think it's just as easy to just replace an old can-cap if for no other reason than simply because it's good 'preventive' measure. The cap-board systems are great too, and I've used many of those made by Sheldon Stokes on a number of different amps, including Dynaco, Eico, Heathkit, and others. Good stuff!
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    Solder Slinger


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    Post by Solder Slinger Fri Aug 27, 2021 9:13 am

    I agree with Wharfcreek, a 60 old electrolytic is most likely leaking in one or more of its four sections. I would cut off the three resistors, test the cap with a Sprague Tek-O-Mite or Heathkit IT-28 (or equivalent), then, assuming the cap tests good, replace the resistors later with metal film units which will be much quieter anyway.

    The Carbon Comp resistors used in these older units shift values, usually going to a higher resistance, so that MAY be the issue as well, you can't really test the values when they are hooked up to a cap.

    Just some thoughts,

    -Ed
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    Solder Slinger


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    Post by Solder Slinger Fri Aug 27, 2021 9:39 am

    And forgot to add that the reason to use the Sprague or Heathkit types of test unit is you need to test at the rated voltage of the cap, a transistorized unit can't test at 300 or 400 volts, the Tek-O-Mite and Heathkit can. The cap MAY test fine at a lower voltage, then start leaking when the voltage reaches a certain point. Note these units can also be used to "reform" the cap, which in some cases resolves the problem.

    -Ed
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    Wharfcreek


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    Post by Wharfcreek Fri Aug 27, 2021 11:40 am

    I dare say most of the leakage issues I find with caps are generally noted as the cap reaches at minimum a 50% of rated voltage level. Sometimes it takes going up to 75%, but rarely do I see it at minimal voltages.....and I would guess that no 9-volt-battery operated hand-held tester is going to reliably show those kinds of results. I say this as owner of a few of these types of testers, and I've found that if a cap is 'shorted' and essentially useless, the hand-held units may still register with a 'uf' capacity in the general range of what the cap shows as being it's specified rating. However, even then, it can still end up being basically 'significantly leaky' when you get it up to 100 VDC. If it's rated for 450VDC, it's clearly shot! So, I don't trust those hand-held meters as far as I can throw them!!

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