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    problems with PAS-2 Phono (one tube not heating)

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    greggoad

    Posts : 7
    Join date : 2013-10-08

    problems with PAS-2 Phono (one tube not heating)

    Post by greggoad on Tue Oct 08, 2013 7:08 pm

    Hello.

    I have a PAS-2. The PC-6 Phono Preamp is not working correctly.



    I noticed yesterday that one of the 12ax7's is not heating.

    When I swap a warm 12ax7 into the socket, the channel functions correctly until the tube cools off.

    I am trying to troubleshoot why that tube is not heating.

    I have noticed that where the Power supply connects to the PC-6 board at 14 and 15 (as labeled on the board) I get a reading of 22V DC at 14, yet 0 at 15. The tube that is connected to 15 is the problem.

    However, the PC-5 Line stage seems to be functioning properly. (has stereo output) Again however, on PC-5 i get 22V DC at 18 and 0 at 19 (as labeled on the PC-5 Board)

    Any Troubleshooting Ideas anyone may have would be helpful.

    Thank You

    Greg


    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by hawaii.ken on Wed Oct 09, 2013 7:26 am

    Sounds like there's a broken connection.

    Are the tube sockets in good condition?

    Try re-soldering the wire at eyelet 15. If no change, next:

    Solder a layer of new solder on the trace from eyelet 15 to the tube socket and freshen the solder at the socket also. If no change, next:

    On the same socket, go counter-clockwise to the next pin on the tube socket and re-solder that pin and put a new layer solder on the trace from that socket pin to the other tube and and freshen the solder for that pin also.


    greggoad

    Posts : 7
    Join date : 2013-10-08

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by greggoad on Wed Oct 09, 2013 11:28 am

    Thanks! I've tried all those things.
    Do you know what the voltages are supposed to measure at 14, 15, 18 and 19.

    I'm wondering if it could be a power supply issue. Thanks

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by peterh on Wed Oct 09, 2013 12:41 pm

    greggoad wrote:Thanks!  I've tried all those things.
    Do you know what the voltages are supposed to measure at 14, 15, 18 and 19.

    I'm wondering if it could be a power supply issue.  Thanks
    More likley a bad tubesocket; bad solder ; broken trace on the board.

    You should be able to follow the filament voltage on the underside of the board,
    and verify the voltage with the manual where there is an excellent description.

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by GP49 on Thu Oct 10, 2013 2:17 pm

    Not a power supply issue, since the same filament supply also feeds the tubes on the linestage (PC-5).  If this PAS-2 was built from a kit, over fifty years ago, and the original builder didn't flow the solder at eyelet 15 from the wire, over the eyelet and onto the trace on the board, the connection could easily have gone bad from corrosion.

    greggoad

    Posts : 7
    Join date : 2013-10-08

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by greggoad on Thu Oct 10, 2013 2:50 pm

    Thanks. I've decided it's just time to upgrade to newer boards. Somebody's been in here doing stuff and voltages do not match manual.

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: problems with PAS-2 Phono (one tube not heating)

    Post by GP49 on Thu Oct 10, 2013 3:08 pm

    greggoad wrote:Somebody's been in here doing stuff...
    You didn't mention that!!  Very Happy 

    Putting something back to stock sometimes is the only way to undo what's gone wrong before.  There
    are so many crackpot modifications for the Dynacos by every Tom, Dick and Pothead (I started my repair
    career in Berkeley, CA!)  that unless a unit is totally stock (and even then there are the usual kit-build
    and aged-parts issues), you really DON'T know what you're dealing with.

    Nice new boards are a lot nicer to work with, too; and they're not expensive.  I always do advise, though:
    build them up stock first, then make sure it all works, before starting on modifications.

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