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    Hum when input disconnected


    Posts : 217
    Join date : 2012-03-19

    Hum when input disconnected Empty Hum when input disconnected

    Post by wildiowa on Mon Mar 19, 2012 12:47 pm

    New guy here. Have or had many Dynas including Mark IIIs stereo 120 solid states and current project an ST70. To get it going a 'tech' only replaced the two tubes on the preamp board and it did work, to a point. But $100 and two tubes later I found when both RCA jacks are connected, it idles great and plays great with no noise etc. But unplug the LEFT channel RCA jack, and there is a huge hum that comes from the left side, and the right side is there but goes quite weak. When you unplug the RIGHT channel RCA, the left side sounds great with no hum or weakness, as it should. Another hint: Flipping the mono/stereo switch appears to make no difference in the sound of playback, although it does sound like it's currently in the "stereo" mode. Also makes no difference in the hum situation when switched back and forth.

    I'm no expert and the techs around here just don't get these older amps and seems I just pour $$ out with no real results. Since it got back I have replaced the rectifier with the recommended diode, found and replaced a funky tube that was messing with the bias and added two phone plug inputs on the front panel to bypass the existing narrow and fragile RCA's. Old RCA inputs are now unhooked. The symptoms persist even with the new input jacks so feel it is a ground, something in the mono/stereo switch or other mystery.

    I plan to replace the caps on the PC board and the bias caps but this hum has me frustrated and stumped. Guess it works OK in the end when both inputs are connected but just not right and I'd like to get it fixed correctly. Seems so much knowledge and experience on the board, any suggestions? Thanks...IA.

    Posts : 1624
    Join date : 2008-11-30

    Hum when input disconnected Empty Re: Hum when input disconnected

    Post by tubes4hifi on Mon Mar 19, 2012 2:04 pm

    yes, most likely in the input jacks or mono switch.
    since that's such a small area, would be real easy to take a close-up photo of the wiring in that area (preferably with lots of light, not on camera flash) and then we can probably easily see something not quite right . . .

      Current date/time is Mon Jan 27, 2020 12:35 am