Quote:
Originally Posted by stromberg6
Hi Bob, I'm curious to know how you resolved the dropping resistor situation. I also would have inserted it between the 4.5 amp fuse and the rectifiers, but upon looking at the schematic for my 4, I see that it would not be the best place to put it. I too am having voltage drop trouble with the original rectifiers, and need to replace them with 1N4007's, or equiv. B+ is about 345-350, not the 380 specified in the RCA lit, and when I crank up the variac the improvement in the pic is notable.
Thanks!
Kevin G.
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Sorry it took so long to get back to you Kevin. I have had the flu and a million things to do and I just got to figuring out the placement and value of the resistor needed.
This is what I have done. Using clip leads I inserted a 1K 50 watt wire wound pot into the power supply circuit. The best place to insert the droping resistor is on either one of the leads of the power supply choke. I chose this location because it is the best place to insert and have the droping resistror effect all of the various power supply voltages.
While adjusting the 1K 50W pot, I monitored the B+ supply voltage and when I reached 350V (the specification on the Sams schematic) I recorded the voltage across the pot and the current through the pot. I dropped 72 volts and had 440ma currrent. I calculated about 32 watts of heat energy will be generated. Then I disconnected the pot and measured the resistance. 150 ohms. I have ordered some Vishay 50W 150 Ohm Aluminum clad power resistors from Mouser. Mouser Part# 71-RH50-150 at $4.67 each. I will mount the resistor somewhere on the top of the chassis where the heat will be easily disipated.
If you want to go with the 380v B+ as specified in the RCA print, then these values are what you will need. 70 ohm 20 watt, gives me 380v B+ in my set. My set seems to be functioning about the same at either B+ voltage. I get a nice full bright raster at either 350v or 380v.
If you replace the original stack rectifiers with 1A 1000piv diodes I would guess you will need to drop a similar amount of B+ voltage to get the supply voltages back within specification. Good luck, and I will post the photos of the mod in a couple of weeks after the new resistors arrive and I install them, so you can see how I did it.