[Rono944]

Love my Cornet but I have a persistent hum that is getting to me.  I have tried all of the standard fixes and none have helped.  I am using an IAG chassis with Cardas RCA's isolated with insulators.  At this point I am running twin center conductor coax cables to the rca's.  I have attached the shield to the ground wire at the input rca jack and floated it at the board side, attached the shield to the ground wire at the board output and floated it at the output rca jack.  I used nylon standoffs.  The hum is present with and without the input jacks hooked up. Did constant current source and hum fix for C14 and use powerline conditioning.

Two things are confusing me.  Other than the coax cables from the PCB to the rca's there is no wire going from the PCB or anywhere else to the ground post between the rca's, however, if I use my DVM I have continuity from the ground post to the board or the rca's.  How is the PCB being grounded to the chassis?  If I run a seperate ground from the board to the ground post the hum increases five fold.  I checked voltages on the diodes and one side of 1 & 4 measure 8.49 the other sides 4.24.  Diodes 2 & 3 measure one side 4.24 the other sides measure 0. Is this a problem?  (By mistake I powered up and then realized that C11 and C12 were not installed, I had forgot to order them).  Voltage after R21 is 6.17V (used 2.5 ohm resistor).  Voltage off the rectifier reads 375V, 319V before R6 and 290 before R3.  Any help will be appreciated.

Ron

[hagtech]

Rono,

There should definately be a ground connect from PCB to chassis.  This is best done with a wire from ground jack to PCB gnd hole.  Then use the inner two conductors for the RCA jacks.  Tie the outer shield to board ground only.

Find the other connect from board to chassis ground.  The transformer provides isolation at that end.  A big loop will catch the magnetic field from the tranny.

Another way to get lots of 120Hz hum is to not include the electrolytic capacitor on the base of the transistor in the current sinks.  This RC filter is very important.  Check to make sure it got wired correctly.

It also helps to connect the earth wire to chassis.  But that is more of a buzz problem than hum.

jh

[Rono944]

Thanks Jim,  I pulled the board last night, removed the RCA's from the chassis and still had continuity from ground post to the board (.2 to .3 Ohms) or from the ground post to the rca jacks.  The only thing in contact with the board were the transformer wires, I am still confused as to how I can have a ground connection to the chassis.  I rewired the input and output wires with the shields connected to ground on both sides, I will remove the shields from the rca's.  

I checked voltages with a DMV with ground taken from the board and from the unwired ground post and all readings were the same.   I then hooked up a scope (I am not a genius with one) set it to AC .5 volt and checked the heater supply as well as the rectifier.  I did notice that I had  AC on one side of R2, no AC on the other side of R2 (both channels), maybe I have a bad capacitor.  I will try to post some pictures later.

Ron

[Rono944]

Jim, here is a photo of the board reinstalled.  
Thanks, Ron

[hagtech]

Yes, there should be quite a bit of ripple on one side of R2.

Board looks nice, great parts.  I would find that other ground connection to chassis.  That is likely it.  One big loop to catch the magnetic field from the power tranny.

With an open input, you should get some hiss and maybe a hum that is about the same loudness.  Depends a bit on construction and first tube.  But definately WAY quieter than a lead-in groove.

jh

[Rono944]

I finally found the other ground to chassis, now I need to know what to do about it.  As soon as I unsoldered the red/yellow ground wire from the transformer the chassis ground was gone.  Now when I go from the unwired ground post to the rca's or the board, no continuity.  If I go from the ground post to the red/yellow wire, I have ccntinuity.  Is the transformer being grounded from the screws going to the chassis?  Is there a problem with the transformer?

Thanks,

Ron

[Rono944]

Problem Solved!!.  I removed the tranny and found that the red/yellow ground wire was pinched and had sliced through the insulation.

Silly little mistakes can cause alot of apprehension, but we do learn more from our mistakes.   Thanks for the help.

Ron