GE Model 402 Power Supply Hum Even With Filters Replaced

[Isja]

I’ve been working on this old GE Broadcast Band Superhet AA5 receiver from the late 1940s, early 50’s. I decided to get into a full restore instead of just simply servicing it and for a while, all was going well. The light socket had to be replaced as it had internally shorted, which was no big deal. There was the normal 60 cycle hum on the speaker so I knew it would have bad filter caps. I made a list of all the caps and resistors and replaced them all one by one and it really sounded great afterwards.

Being a perfectionist, I had one more major gripe before I could put the project down and that was the chipped old power cable. First off, it was non polarized, which worries me as to both excessive possible hum due to the transformerless design as well as a hot chassis. It was also chipped on the plug exposing a lead when plugged in so it had to be replaced. I wired a new polarized plug in, but while I did, I noticed a wire coming off the Rectifier resistor that seemed to be tucked under the power switch. I had assumed it was meant to be on there and had popped off from old age, so when I plugged the unit back in, I got a light show between both that resistor and the tube filament. It took a while to realized this “nowhere” wire was looping the power in on itself and was in fact never supposed to exist in the first place, claiming the filament of another 35Z5 tube in the process.

I didn’t have a high enough watt 22ohm resistor to replace with, so I salvaged a 33ohm with long enough leads out of an old crt. When I turned the unit back on to do the alignment, I immediately noticed that the hum was back! Regardless of volume, so most certainly lie in the power supply. I thought “oh, easy. The short must’ve cook my 150V filter caps.” So I just replace them with 450V replacements at 100uf instead of 50 since it was the closest I had in stock, but it still hummed! I’ve checked around for shorts in C16,17, & 18 as well as all the resistors relating to the supply. I’ve switch polarity on the power by clipping in an external plug, but got nothing. I’m measuring full ac across both B+ and B- (floating ground) as well as the rectifier cathode, however there does seem to also be around 60V of DC. The b+ has gotten up to as high as 70v.

What I don’t understand is that nothing should have changed within the circuit between when I had it working and the new power cable/filter being put in. Unless either my new filters have also been recked or I’m out of good rectifiers, I can’t understand why the floating ground would suddenly start causing an issue now. I’ve spent weeks on this and am ready to throw in the towel and just chock it up to a loss.

 

[gadget73]

you're at 70 volts DC across the first filter cap? Thats definitely not right. Should be around double that much. if the caps aren't open about the only thing that would make sense is extreme load on the power supply. Voltage drop test across the 33 ohm would give a clue there. Offhand not sure what it should be but I'm guessing not more than 10 volts or so.

excessive load will produce extra hum with a half wave rectifier setup like this.

 

[Isja]

you're at 70 volts DC across the first filter cap? Thats definitely not right. Should be around double that much. if the caps aren't open about the only thing that would make sense is extreme load on the power supply. Voltage drop test across the 33 ohm would give a clue there. Offhand not sure what it should be but I'm guessing not more than 10 volts or so.

excessive load will produce extra hum with a half wave rectifier setup like this.

hmmm, so after pulling all the caps out and testing them I'm even more confused here. The filters being the only ones that had even questionable behavior, having usually started at 4 mega ohms before slowly climbing up, whilst all the other ones would just fill right up from zero to max. I checked the voltage drop across R10 and it only gave me about 1.5V at most, so could hardly be overcurrent. The Rectifier tube was also not inspiring any hope in me from the readings I was getting on my Eico tester so I put a 1N4007 in between pins 5 and 8 with a series resistor to protect against inrush current and that seemed to make the hum quieter, but not a viable solution as the heat dissipation was still a ton even at 5 Watts. Plan now is to find a new 45Z5 to replace the rectifier and run all the filaments a little lower and get some actual nichicons to replace the filters with, otherwise I'm totally out of answers as I can't see it being a wiring issue since nothing has changed since I had it working.