I have a nicely working fisher 70rt but I’m concerned about the selenium rectifier overheating. It plays fine so don’t want to mess with it if I don’t have to. I’ve replaced a selenium rectifier in a Dumont am fm tabletop radio with great success. However looking at the diagram for the fisher - it is a little perplexing. Not the same as the dumont. How would I replace this using a diode? Do I need 4 diodes or am I reading this diagram all wrong?
Fisher 70rt Tuner Preamp Selenium Rectifier
I’m looking at bridge rectifiers on mouser. Not sure how .024ADC translates. Thanks for your help.
I’m not usually a huge fan of loudness but I really like the loudness setting on this device.
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.024 amps Direct Current. Find a Bridge rectifier rated for 400volts AC and 2 Amps.
This one will do fine. https://www.mouser.com/ProductDetail/Vishay-Semiconductors/VS-2KBP04?qs=sGAEpiMZZMtQ8nqTKtFS/ERhNCRzfcON5sjErUmz0Sw=
JB-Weld it to the side or bottom of the chassis close to the original and transfer the ~(AC)leads, then the +(Pos DC) and the -(Neg DC) leads. Put heatshrink tubing on the joints so if they should get close or touch, they are insulated from each other and nearby parts, or the chassis.
R82 will probably need to be increased as Tom said. Try 27 to 30 ohms 1st. This range has been usual for my Fishers, but may need some slight adjustment for the 70-RT. Before replacing the rectifier, write down the voltages on both sides of R82. Replace the rectifier and take another set of voltage readings. Use ohms law to figure out the new resistor value.
This one will do fine. https://www.mouser.com/ProductDetail/Vishay-Semiconductors/VS-2KBP04?qs=sGAEpiMZZMtQ8nqTKtFS/ERhNCRzfcON5sjErUmz0Sw=
JB-Weld it to the side or bottom of the chassis close to the original and transfer the ~(AC)leads, then the +(Pos DC) and the -(Neg DC) leads. Put heatshrink tubing on the joints so if they should get close or touch, they are insulated from each other and nearby parts, or the chassis.
R82 will probably need to be increased as Tom said. Try 27 to 30 ohms 1st. This range has been usual for my Fishers, but may need some slight adjustment for the 70-RT. Before replacing the rectifier, write down the voltages on both sides of R82. Replace the rectifier and take another set of voltage readings. Use ohms law to figure out the new resistor value.
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Sounds easy enough. I’m going to have to study this some before I jump in though and I very much appreciate the help.
I can see neg dc goes to ground and pos dc goes to C2. Is there any particular order for the white/brown? Also, excuse my ignorance but, wouldn’t I want to leave R82 alone and instead put a new resistor between the new M1 and C2A? It might be clearer if I was at home looking at the actual device and not just the diagram.
Thanks!
That'll work too, though you'll find Ohms law doesn't work, as the waveform here is pulsed DC and not filtered as it is later in the circuit. Try 10 Ohms, increase if needed. PM me an address and I'll send a small bridge rectifier for the postage - I have a bag of them.
