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    ST-70 rebuild with terrible hum

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    kens770

    Posts : 7
    Join date : 2012-08-01

    ST-70 rebuild with terrible hum

    Post by kens770 on Wed Aug 01, 2012 3:52 pm

    I have rebuilt a early ST-70. I have added a new motherboard, new choke, new bias caps and resistors. New power supply caps and rewired the entire amp. I added a 3 wire power cord and have the ground wire attached to the chassie with a star lug.

    All voltage test points are within spec. The problem is it hums like a toothless old lady at church.
    The thing I find odd is the input jacks are not physically grounded. When I have a cable plugged into the input and short it out the hum is worse. However when I take a jumper wire from the chassie ground lug in to the input jack ground it is dead quite. What am I overlooking?

    I bought the replacement input jacks from Dynaparts as akit and wired the stereo/mono switch in.

    I have not changed the GZ-34. I am not sure were to go now. Any help would be nice

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 rebuild with terrible hum

    Post by Bob Latino on Wed Aug 01, 2012 6:06 pm

    Hi,

    Did you connect the ground tabs on the input jacks to the two ground points on the new driver board? The two input jacks SHOULD be physically grounded to the driver board and then the driver board should be grounded through eyelet #9 (if your board is a stock type driver board) to one of the two main grounding tabs next to the quad cap.

    Bob

    kens770

    Posts : 7
    Join date : 2012-08-01

    Re: ST-70 rebuild with terrible hum

    Post by kens770 on Wed Aug 01, 2012 6:19 pm


    No this is were I was confused. The schematic I used for the st-70 was a manual from dynaco-doctor that shows two small wires from the input ground going to two unmarked connections on the driver board. That is the 2 ground connections. I have eyelet 9 ran to the ground shown in the drawing. I will need to look at the board and figure out the LR ground and have them properly grounded to the board. I am sure this will fix my humming.

    Thank you for your help.

    Ken
    Duluth, GA.


    kens770

    Posts : 7
    Join date : 2012-08-01

    Re: ST-70 rebuild with terrible hum

    Post by kens770 on Fri Aug 03, 2012 8:53 am

    I am unable to figure out where the two wires going from the driver board connect to the input grounds. The ST-70 manual, step 46, only says to refer to the two wires extending from the PCB ... It does not mention where they are attached to on the PCB. I looked at the schematic and think the two wires attach to each LR side of the 10 ohm resistor. Is this correct?

    I made the fatal mistake and connected the input ground wires to socket 8 and 10. That mistake took out a E34L, I believe my GZ34, and a coupling cap. I didn't think that misplacing a ground would cause that much damage.


    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 rebuild with terrible hum

    Post by Bob Latino on Fri Aug 03, 2012 9:03 am

    Hi Ken,

    On the photo below, the two ground wires for each RCA input jack are seen as the two bare wires sticking up from the driver board just above eyelet #9. These two ground eyelets do not have an eyelet number.

    Bob




    Last edited by Bob Latino on Fri Aug 03, 2012 4:46 pm; edited 1 time in total

    DynakitParts
    Admin

    Posts : 181
    Join date : 2008-11-30

    PC-3 Grounding

    Post by DynakitParts on Fri Aug 03, 2012 2:57 pm

    Ken,
    Grounding of the RCA jacks are connected to the (2) extended leads on the original Dynaco PC-3 PCB as shown in the photo posted above by Bob. Actually, these are the extended leads from each of the (2) 10 ohm 1/2 watt resistors on the top side of the board. Most new replacement PC-3 boards do not include these long lead resistors nor does the supplier explain these (2) critical ground connections. Our Dynakit replacement PC-3 board includes (2) solder pads marked [L] & [R] for these connections.
    If you can locate these two resistors then you will need solder a separate ground lead from each RCA ground lug to each of these resistors. This should be done from the bottom side of the circuit board. Trust this makes sense to you...

    Kevin

    kens770

    Posts : 7
    Join date : 2012-08-01

    Re: ST-70 rebuild with terrible hum

    Post by kens770 on Fri Aug 03, 2012 5:36 pm

    Thanks for the help. Yes I looked at the schematic and figured that the two LR ground leads come from one side of each 10 ohm resistor. Unfortunatly it was to late and took out a E34L and coupling cap. So far, Do not know what else. Bummer.

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