Preamp rebuild - hum problem

[Sherman]

Yup! I can see the isolation washers on both sides.

Just a thought from someone definitely not an expert- I would remove the jacks and check that the washers are intact and that the metal from the jacks truly isn't touching the case.

I was modding an SI digital amp and one of the jacks wasn't properly isolated. It looked fine but I saw the problem when I finally removed it (after hours of trying to find out why the thing hummed).

 

[jeff mai]

around for any wires that may have been replaced, not just in the power supply but in the signal circuitry.

Do you have a schematic for the line stage?

Not spotting any stray wires. Here's a photo of the input output jack wiring:

http://members.iinet.net.au/~jmai/pix 448.jpg

As pictured from left to right:

Output - Tape out - Monitor - Aux - Tuner - Tape - Phono

With the exception of the Output and Phono Input - all are grounded at the point where the safety earth and chassis are connected to the circuit. You can see the little tab sticking up from the PCB where these are connected. The safety earth is connected on the other side of the PCB at the same point. The Output / Phono Input jacks are grounded on the PCB nearest to the output / phono input tubes respectively. It all looks pretty kosher to me.

Here is the preamp circuit diagram:
http://members.iinet.net.au/~jmai/pix 450.jpg

Of the line stage section, except for the 2uf output coupling caps and the plate resistor for the first 6DJ8, I had to replace everything after the volume pot but the valve sockets. I didn't touch the PSU (except for removing clearly non-original bypass and filter caps) or phono section and they look unmodified to me apart from the op amps in the regulator.

 

[SY]

Not so much stray wires as ones that have clearly been soldered after the fact.

Thanks for the other stuff. I'll look at it later today. Sherman's suggestion is an excellent one.

 

[SY]

I lied and looked first. The regulator is a total red herring. The line stage operates off the filtered but unregulated B+.

 

[jeff mai]

Not so much stray wires as ones that have clearly been soldered after the fact.

Yes, I know that's what you meant. Some of the signal wiring looks to have been replaced, but the ground buss wiring pictured is original (I have a picture of an relatively unmodified example from the web!) Aside from the RCA jacks, there are only two other ground wires: the safety earth coming in from the mains and a jumper from the bottom of the volume pot to the local ground plane where most of the connections between V4 and V5 are made. The jumper is connected to points labelled J1 at both ends and looks consistent with the original wiring.

 

[jeff mai]

I lied and looked first. The regulator is a total red herring. The line stage operates off the filtered but unregulated B+.

Yes, I can see that is the case if B+1 is not regulated. That's why I had asked specifically about B+1. B+2 only supplies the phono section.

 

[jeff mai]

Here's something interesting:

I was thinking 5mV of ripple was too little for the filtering present so I checked it out with PSU designer, which seems reasonably accurate most of the time.

5mV of ripple on B+1 is about 30 times *less* than PSU designer says it should be given the filtering and assuming a 30ma current draw (120V is dropped across the 4K resistor R37 according to the schematic.)

B+1 may not be regulated, but are we sure there isn't some sort of ripple reduction happening?