Fisher 80-C Restoration

[multichamp12]

Thanks! It's just odd that there was really no change between the 50 and 200 ohm resistor. Since my voltage readings are good I'm gonna leave it for now and compare it to the other 80c when I start on that one

 

[s-petersen]

According to the schematic you have 7ma (6.5 actually) current current flowing, and you need 3k to drop 20v
If you follow the schematic you'll see there are (3) 3.3k in series (10k), dropping 65v (250 to 185)

 

[multichamp12]

That's what I saw on the sams photofact (although I don't really trust this sams folder when comparing it to the fisher manual) but 3k seems very for a dropping resistor.

I guess it makes sense though. I'm getting 1.5v across 200ohms.
1.5/200=.0075 or 7.5ma

20/.0075= 2.6k

 

[s-petersen]

In theory you should see 25v? across each of the 3.3k resistors already in circuit.

 

[multichamp12]

Makes sense. I will double check the voltage across the 200 ohms resistor, because it was around 1.3-1.5v, which gives me around 6.5-7.5ma. And ill check the voltage across the 3.3k resistors. Thanks!

The readings I took with the 50ohm resistor in through me off, and with that info I gave gadget we got 200 ohms, but I realize that was due to a faulty multimeter.

 

[s-petersen]

I know the feeling, my multimeter just died on the resistance range, I had to buy another.

 

[gadget73]

usually the resistor between the rectifier and first cap can be smaller since there is AC ripple current + DC current involved. Ultimately as long as the voltage ends up right, it doesn't matter all that much how you get there.

 

[multichamp12]

I got the 2nd selenium rectifier replaced today. Instead of using 4 diodes mounted on a terminal strip, I opted for a bridge rectifier rated at 10a and 1000v. Its a little overkill, but it should outlast me! It is basically the same size as the original selenium rectifier, and I was able to mount it directly in its place using the existing hole and hardware. Here is the part # for anyone interested: KBPC1010-G

When using a bridge rectifier like this one, there will be 4 solder terminals:

2 are marked with "~" or "AC" (The 2 wires from the power transformer go to these terminals)
1 is marked with "+" (This terminal is turned 90 degrees from the others and goes to your PS capacitor)
1 is marked with "-" (This is negative, goes to ground)

The only thing that had to be modified was the length of 1 of the power transformer wires. I had to extend it about 1", which I really don't like to do, but with a bit of heat-shrink, and hiding it behind the rectifier, it looks very clean and hardly noticeable. For anyone replacing this selenium rectifier in their 80 or 90C, I used a 5 ohm 5w dropping resistor to drop the voltage needed to match the 28v on the schematic. Depending on your line voltage, anywhere from 4-7 ohms, and 2-5 watts, should be perfect.

I am still waiting to hear back from hayseed about my can capacitors, which will be the last thing needed to finish. Then I will start on the 2nd preamp.

 

[multichamp12]

I got my new can caps from Hayseed, and as always, they came through with flying colors. The voltage on the 40/40/40/40 cap was increased from 250v to 450v, and the 1000/1000uf can was increased from 35v to 50v. The original 15 ohm resistor between the 1000uf caps was changed to a 20 ohm because an extra 5 ohms was needed to drop the voltage for the heater supply after the selenium rectifier was changed for a diode bridge.

I still have to put new cans in the other 80c, but these preamps sound great!