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Refurbishing AC connections on vintage receivers

I just picked up a 1952 Philco radio that had a disintegrated power cord, just bare metal wire coming out of the strain relief. And someone just wrapped them with electrical tape and spliced a new cord on. Ugh...
 
blhagstrom said:
I’ve been down this hell hole before and gotten ripped up for arguing against grounding the chassis so I’m done.
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fred soop said:
Replacing with a 3 wire grounded cord now connects the chassis to earth and can cause all kinds of ground loop problems.
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Amen

I have a short tale to tell on a Nak CA-5 that I recently obtained. It was wired with a 2-prong, non-polarized plug that entered the chassis via a strain relief fused to a plate, and then directly to the switch PCB. However, photos of CA-5s I had seen on the web all had IEC receptacles on their back panel, so I figured my unit was retrofitted with a direct wired connection, although it looked amazingly professional. Since the cord had seen its better days (several segments of it had been wrapped with electrical tape), I decided to replace it and install an IEC receptacle, which I believed was the original stock configuration on the unit.

Consulting the SM for the CA-5, the ground tab on the IEC AC inlet was shown as grounded to the chassis. So I soldered a short wire from the ground tab on the IEC to the chassis. I also traced which wire from the IEC entered the switch, and made sure that wire was the hot one on the IEC. When I operated the unit, I got hum, and lots of susceptibility to hum depending on what I connected to the CA-5. These went away if I disconnected the ground tab on the IEC receptacle from the chassis.

I then searched more thoroughly for images of the internals of the CA-5, and where it could be seen, the ground pin on the IEC was clean and empty (i.e., nothing appeared to have been ever soldered to it). I also saw a few examples of CA-5s that lacked IEC receptacles and had strain reliefs and small plates on the rear panel, as mine did. So now I suppose my unit was originally like that.

End result: adding a chassis ground connection to the AC inlet ground, as per the SM, resulted in a ground loop.

I still have some lesser noise issues (higher freq than hum) on the phono circuit of the CA-5 (although one of my friends with a lot of audio knowledge and experience tells me it is normal), and some susceptibility to that noise depending on what the CA-5 is connected to, but that is a story for another day, after I chase it further.
 
northpaw said:
Amen

I have a short tale to tell on a Nak CA-5 that I recently obtained. It was wired with a 2-prong, non-polarized plug that entered the chassis via a strain relief fused to a plate, and then directly to the switch PCB. However, photos of CA-5s I had seen on the web all had IEC receptacles on their back panel, so I figured my unit was retrofitted with a direct wired connection, although it looked amazingly professional. Since the cord had seen its better days (several segments of it had been wrapped with electrical tape), I decided to replace it and install an IEC receptacle, which I believed was the original stock configuration on the unit.

Consulting the SM for the CA-5, the ground tab on the IEC AC inlet was shown as grounded to the chassis. So I soldered a short wire from the ground tab on the IEC to the chassis. I also traced which wire from the IEC entered the switch, and made sure that wire was the hot one on the IEC. When I operated the unit, I got hum, and lots of susceptibility to hum depending on what I connected to the CA-5. These went away if I disconnected the ground tab on the IEC receptacle from the chassis.

I then searched more thoroughly for images of the internals of the CA-5, and where it could be seen, the ground pin on the IEC was clean and empty (i.e., nothing appeared to have been ever soldered to it). I also saw a few examples of CA-5s that lacked IEC receptacles and had strain reliefs and small plates on the rear panel, as mine did. So now I suppose my unit was originally like that.

End result: adding a chassis ground connection to the AC inlet ground, as per the SM, resulted in a ground loop.

I still have some lesser noise issues (higher freq than hum) on the phono circuit of the CA-5 (although one of my friends with a lot of audio knowledge and experience tells me it is normal), and some susceptibility to that noise depending on what the CA-5 is connected to, but that is a story for another day, after I chase it further.
Click to expand...
I have a similar issue on my 1974 vintage Scott receiver.
It came with an IEC connector, but it only had a lamp-cord gauge power cord with a non-polarized two prong plug on the other end.
I replaced the power cord with a heavier gauge, but left the ground "floated" (not connected), is there any possible advantage to connecting the ground?
What are the potential risks?
 
I don't know if there are any risks, other than possibly creating a ground loop like I did, especially if the IEC receptacle is a later mod from an original 2-wire arrangement. Not sure when IEC receptacles were invented, but 1974 might be a stretch.

I would just make sure that the hot wire on the new plug (presuming your used a polarized one) connects to the leg with the switch and fuse.
 
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