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    Adding ground jumper eliminated hum from PAS3

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    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Adding ground jumper eliminated hum from PAS3

    Post by MexicoMike on Fri Nov 21, 2014 8:55 pm

    I have a newly acquired Pas3x that I've been doing a bit of work on.  One item was a bit of low level hum from the right channel that did not change volume with the volume control.  It was reduced somewhat by going through the Pas per the instructions and eliminating some twisted pairs that should not have been twisted.  I also re-flowed a lot of solder joints just to be sure but a bit of low level hum remained.  The hum was barely noticeable and totally covered by any signal source playing.  Still, I didn't like the faint hum with no signal.

    I totally eliminated the hum with a jumper between pin 1 and pin 8 of PC5.  I have no idea why that would eliminate the hum but it did.  The preamp is dead-quiet now.  I'm sure there is some problem in the original wiring by the kit builder that makes this jumper do what it does but I can't find anything that seems wrong other than that twisted pair issue.

    The Pas is oem except for Roy's 1st level power supply and new caps/tubes and I want it to function as original with all filters/switches/tone controls.  I have ordered a new jack board and volume control for it but for now that will be the extent of the changes.

    Dale Stevens

    Posts : 77
    Join date : 2014-07-06
    Age : 67
    Location : Loris, SC

    Re: Adding ground jumper eliminated hum from PAS3

    Post by Dale Stevens on Wed Nov 26, 2014 5:27 pm

    Could someone chime in here; I'm interested in a reply. Thx, Dale

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Adding ground jumper eliminated hum from PAS3

    Post by sailor on Wed Nov 26, 2014 9:09 pm

    Somewhere you had a missing ground wire or bad solder joint or you have a bad resistor or cap that goes to ground. Obviously, it's somewhere in the channel that hums so all common parts can be eliminated as possible culprits. I don't have a schematic in front of me so other than this I can't be of any help other than to say whatever the problem, the jumper wire completed the necessary circuit.

    peterh

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

    Re: Adding ground jumper eliminated hum from PAS3

    Post by peterh on Thu Nov 27, 2014 3:02 am

    Dale Stevens wrote:Could someone chime in here; I'm interested in a reply.  Thx,  Dale

    Both point 1 and 8 is "ground" and _should_ be interconnected.
    By connecting a wire you changed something :
    - the board has a weakness that has a non-zero resistance
    - some other trace/wire will pass close to the original cupperstrip and induce hum ( should be
    i wire that carries some hum-info, filament is on my mind)
    - there is a non-zero resistance in the point where signal ground and chassis is interconnected,
    usually at the big electrolytic cap
    - filament-to-cathode leakage on one of the tubes
    - a ghost in the machine ( unlikley but included here for comlpeteness)

    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Re: Adding ground jumper eliminated hum from PAS3

    Post by MexicoMike on Sat Nov 29, 2014 3:26 pm

    FWIW, I found the problem...the trace that goes between 1 and 8 has a "disruption" about 1/2 way along the trace. I needed a magnifying glass to see it. There's a tiny bit of solder there so it appears that someone - perhaps the original builder - damaged the trace but didn't repair it well enough to ensure sufficient continuity under power. I suppose I could repair the trace/removed the jumper but I don't see any reason to bother.

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