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    PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

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    deathman68

    Posts : 10
    Join date : 2016-01-23

    PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by deathman68 on Sun Jan 31, 2016 3:24 pm

    Hi all,

    I'm still very much a noob in terms of tube amps, so please bear with me.  My father recently handed down his PAS-2 preamp and ST70 amp, both in dire need of repair.  After reading and re-reading several forums, websites, etc, I decided to take the plunge and start with the PAS by upgrading the Phono and Line PCBs with TubeNirvana's z-mod PCBs and threw in the upgraded power supply board for good measure.  I soldered all connections as indicated and triple checked that all capacitors/resistors/diodes were correctly placed/oriented before supplying any power to the unit.  I also ran continuity checks after each new soldered joint to ensure no breaks or ground faults.  For test purposes, I plugged in my two old 12AX7s and two new 12AU7s along with the 12X4 rectifier tube.  As I don't yet have a working amp (or know anyone local who does), I'm left to judge the success of my work based only on the voltages that I can measure.  All tubes glow faintly orange.  According to the voltages on the original schematic, I'm way off base.  Here is what I am measuring.  Note that I recorded any measured voltage be it AC or DC...can anyone with experience with these boards let me know where these voltages should be?:

    Wall: 122 VAC
    Power Supply Board:
    Transformer (blue wires): 10.5 VAC
    B+P (PC6-16): 146 VDC
    B+L (PC5-16): 205 VDC
    B+IN PIN7 12X4: 392 VDC
    Line F+ to Line F-: 23.9V

    12X4
    1: 363 VAC
    2: 0
    3 and 4: 10.5 VAC
    5: 0
    6: 362 VAC
    7: 392 VDC

    PC6
    Tube1
    1: 76VDC
    2: 0
    3: .45VDC
    4: 66.7VAC/31.2VDC
    5: 19.5VDC
    6: 67.8VDC
    7: 0
    8: .47VDC
    9: 53.9VAC

    Tube2
    1: 72VAC
    2: 0
    3: .45VDC
    4: 66.5VAC/31.2VDC
    5: 19.82VDC (intermittent)
    6: 78VDC
    7: 0
    8: 0
    9: 0

    PC5
    Tube 1
    1: 92.3VDC
    2: 0
    3: 4.4VAC
    4: 0
    5: 90.6VAC
    6: 91VDC
    7: 0
    8: 2.45VDC
    9: 36.7VDC

    Tube 2
    1: 187VDC
    2: 93VDC
    3: 0
    4: 0
    5: 65.5VAC
    6: 187VDC
    7: 91.6VDC
    8: 0
    9: 25.1 VDC

    Thanks in advance for any insights,

    Jeff
    avatar
    peterh

    Posts : 814
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by peterh on Sun Jan 31, 2016 3:38 pm

    deathman68 wrote:Hi all,

    I'm still very much a noob in terms of tube amps, so please bear with me.  My father recently handed down his PAS-2 preamp and ST70 amp, both in dire need of repair.  After reading and re-reading several forums, websites, etc, I decided to take the plunge and start with the PAS by upgrading the Phono and Line PCBs with TubeNirvana's z-mod PCBs and threw in the upgraded power supply board for good measure.  I soldered all connections as indicated and triple checked that all capacitors/resistors/diodes were correctly placed/oriented before supplying any power to the unit.  I also ran continuity checks after each new soldered joint to ensure no breaks or ground faults.  For test purposes, I plugged in my two old 12AX7s and two new 12AU7s along with the 12X4 rectifier tube.  As I don't yet have a working amp (or know anyone local who does), I'm left to judge the success of my work based only on the voltages that I can measure.  All tubes glow faintly orange.  According to the voltages on the original schematic, I'm way off base.  Here is what I am measuring.  Note that I recorded any measured voltage be it AC or DC...can anyone with experience with these boards let me know where these voltages should be?:

    Wall: 122 VAC
    Power Supply Board:
    Transformer (blue wires): 10.5 VAC
    B+P (PC6-16): 146 VDC
    B+L (PC5-16): 205 VDC
    B+IN PIN7 12X4: 392 VDC
    Line F+ to Line F-: 23.9V

    12X4
    1: 363 VAC
    2: 0
    3 and 4: 10.5 VAC
    5: 0
    6: 362 VAC
    7: 392 VDC

    PC6
    Tube1
    1: 76VDC
    2: 0
    3: .45VDC
    4: 66.7VAC/31.2VDC
    5: 19.5VDC
    6: 67.8VDC
    7: 0
    8: .47VDC
    9: 53.9VAC

    Tube2
    1: 72VAC
    2: 0
    3: .45VDC
    4: 66.5VAC/31.2VDC
    5: 19.82VDC (intermittent)
    6: 78VDC
    7: 0
    8: 0
    9: 0

    PC5
    Tube 1
    1: 92.3VDC
    2: 0
    3: 4.4VAC
    4: 0
    5: 90.6VAC
    6: 91VDC
    7: 0
    8: 2.45VDC
    9: 36.7VDC

    Tube 2
    1: 187VDC
    2: 93VDC
    3: 0
    4: 0
    5: 65.5VAC
    6: 187VDC
    7: 91.6VDC
    8: 0
    9: 25.1 VDC

    Thanks in advance for any insights,

    Jeff

    both PC5 and PC6 boards should have 12ax7 / ecc83 tubes, not 12au7. Wrong tubes
    will screw up voltages ( and sound). Go get some 12ax7 !

    Whoops, i now realize that this is not a PAS, it's a tube nirvana variant. Thus tube & voltage description from
    the Dynaco PAS manual does not apply.


    Last edited by peterh on Mon Feb 01, 2016 4:29 am; edited 1 time in total (Reason for editing : Reading the full text :-))

    mdye

    Posts : 3
    Join date : 2017-05-30

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by mdye on Sun Jun 04, 2017 8:52 pm

    I know that the original post is over a year old, but I am having the exact same issues with my recently modded PAS2 (with new Z-mod boards).  My voltages are nearly identical to the OP's, and I cannot figure it out.

    deathman68--are you still out there?  Curious as to what the solution ended up being for you.

    deathman68

    Posts : 10
    Join date : 2016-01-23

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by deathman68 on Sun Jun 04, 2017 9:05 pm

    Hi mdye,

    I'm happy to report that once I swapped the main transformer the problem went away and I was able to immediately dial in the bias voltages. I lost a lot of time and sleep over this one for a fix that in the end was so simple. Keep us apprised of your progress and shoot me a message if you need me to check/confirm anything since it would seem that very few ppl are running this particular setup.


    mdye

    Posts : 3
    Join date : 2017-05-30

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by mdye on Sun Jun 04, 2017 9:33 pm

    Thanks for the quick reply, deathman! Was your transformer shot, or is it that the stock transformer just isn't up to the job when matched with the Z-mod boards? Did you just get a new transformer from dynakitparts or similar place?

    Thanks again.
    avatar
    tubes4hifi
    Admin

    Posts : 1349
    Join date : 2008-11-30

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by tubes4hifi on Thu Jun 15, 2017 7:34 pm

    I guess none of you guys read my sticky post from a couple year ago.

    http://dynacotubeaudio.forumotion.com/t1728-fully-modified-pas3-or-a-new-sp12

    mdye

    Posts : 3
    Join date : 2017-05-30

    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

    Post by mdye on Fri Jul 21, 2017 5:32 pm

    After trying everything I can think of and going over the boards too many times to count, I finally gave up and purchased a new PAS power transformer from Dynakitparts.  Unfortunately, I am still getting the exact same voltages out of the power supply even with the new PT installed--about 215V to the line stage and 172V to the phono.  Anyone out there have any ideas?

    Really appreciate it!

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    Re: PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

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