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

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    deathman68

    Posts : 2
    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

    peterh

    Posts : 681
    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 :-))

    deathman68

    Posts : 2
    Join date : 2016-01-23

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

    Post by deathman68 on Sun Jan 31, 2016 4:23 pm

    Hey everyone, Holger from TubeNirvana has been helping me to directly troubleshoot this problem and has already made some helpful suggestions to point me in the right direction. Nice to see such support in this day and age! I will report back in a few days with any progress.


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