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I found a Fisher 500c receiver in excellent condition at a garage sale at a great price. I started to do a mild rehab on it and ran into a problem. (I was planning to replace the coupling caps, the selenium bias supply rectifier and caps, and the B+ rectifiers and caps) Without any tubes, with the B+ secondary disconnected, and with the bias supply rectifier and caps replaced, I brought the line voltage up with a variac. (My line voltage measures 122 V) The power transformer buzzes loud enough to hear in the next room. It was loud enough that I thought it was going to burn out. If I put a clamp across the end bells, or even squeeze it with my hands, the buzz drops to a low level, but doesn't disappear. I measured the primary and secondary resistances on the transformer, but didn't see anything obviously wrong. I am planning to measure the current through the primary with nothing connected, but haven't done that yet. Can someone offer some advice? Is there something else I should test? Is the power TX shot? Or is this normal, and I should find a way to clamp the end bells tight?
Follow Ups:
I disconnected all the secondarys (secondaries?) from the power transformer and hooked it up with a current meter in the hot line. The transformer is drawing 2.6 AMPS (!) with no connection. I suspect that this is not good. ;) I have wasted hours and parts on a boat anchor. From now on, every vintage piece gets the iron tested first before doing anything else.
Hi Floyd, looking at my Dynaco Mark3 transformers there are long screws with nuts and lock washers to tighten up the bells and keep every thing together and tight. Those should be your clamps in theory.Try tightening those if you see them.I use a Dyna 35 power amp and there was a little hum when I got near the amp,not at the normal listening spot. I put a cast iron 5 pound York barbell weight on the cage and any minute hum disappeared. I think that was what the VPI Magic Brick was all about. I'm not a Mike Samra but I got a book here that lists what to look for under (hum)....1. loose connections 2. loose hardware 3.Output underbiased 4. bad tubes 5. bad output transformer(I think you said it tested fine)6. bad filter cap 7. bad power transformer(I don't know which transformer you tested?)8. open filament resistors 9.bad input jack 10. arc on tube socket 11.open resistor 12. shorted capacitor. Those were from (How to Service your own Tube Amp).I left the faulty reverb circuit part out because I'm sure the Fisher does not have that and it is for guitar amps. here's some more; missing tube shields can cause hum.Loose nut on input jack can cause faulty ground connection resulting in hum.Under biased amp can cause hum.Watch output tubes,if they glow red rebias. Good luck! ...Mark Korda
Thanks, Mark. The power transformer is riveted together. No simple way to tighten it without taking it out of the chassis. The end bells have holes top and bottom. I am going to try's to find 3 inch bolts that will fit through the holes. First I want to test the power TX with no connections on the secondary to look for excessive no load current. I guess the moral of this story is to test the transformers first in any vintage gear before any modifications. Live and learn.
Edits: 03/29/14
Floyd
I wonder if the power transformer was changed because I own five 500c and one 800c receiver and not one is riveted together nor are the many I rebuilt.
Just disconnect the two power supply diodes in the doubler..You only have to lift one end of each. If the buzzing goes away,I have seen shorted or leaky filter caps in the chain cause loud transformer transformer oscillation on occasion.
Honest amplification is better than excessive 2nd order distortion anytime.
Thanks for weighing in. The transformer looks like it belongs. No extra mounting holes in the chassis.I removed all the tubes and disconnected the voltage doubler rectifiers before the first time that I plugged it in. Didn't want a bad cap to spoil the whole thing. Since I posted the original query I have put a bolt through one end of the end bells and tightened it down. Still vibrating like it wants to take off. Next step is to disconnect all the secondaries right at the transformer and check the current draw. Currently the tubes are out, the B+ rectifier is disconnected, but the filament wiring is still connected and the rebuilt (new rectifier and caps) bias supply is connected.
Edits: 03/29/14
Floyd
If you disabled the diodes and pulled out all tubes,that pretty much disables any load on the transformer secondary..The only left is if you want to disconnect the selenium rect bridge on the side..Can you post a photo of the power transformer?
Honest amplification is better than excessive 2nd order distortion anytime.
Amps that I've worked on with heavy mechanical buzzing or vibration sometimes benefit from re-lacquering the laminations. Some ppl have inserted rubber bushings to isolate the tranny from the chassis, reducing the buzz from resonating into the chassis.
Here's a photo of some such arrangement. Maybe, it'll help.
Steve
Yes,that can work..Ebay many times has Fisher 500C power trannies that people throw on there for 50 to 75 dollars from 500c and 800c carcasses they have parted out.
Honest amplification is better than excessive 2nd order distortion anytime.
Thanks for sharing.
Floyd, I have to test my Mark 3 transformers too. Here's a couple more from the (Hum) page.1. high voltage on tube cathode means open cathode resistor. 2.Low or no voltage on tube plate means open cathode resistor.3. Most common causes of hum are faulty filter capacitor,loose ground connection,bad output tube, and bad rectifier. Let me know when you find a fix and I'll pencil it in on this trouble shooting chart for future references...Mark.
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