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Finished rehab to a stock Fisher 500C

JimWhit

JimWhit

Traded San Diego for The Woodlands in Texas
Subscriber
I completed my rehab thanks to all the help from this forum specially Dave. I used an online kit to upgrade the power supply, assorted capacitors, diodes, and resistors (I deviated from some of the kit instructions based on advice from the forum). I also added the Hayseed-Hamfest kit. I am still working on matching the output tubes but the stereo sounds great and completely functional with outstanding reception for my area in The Woodlands Texas. I am not sure if things could be better with IF, RF and MPX alignment and it isn't something I could do anyway.
 

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I still need to replace the two mismatched output tubes I have old Fisher on V8 & V9, and new Tung Sol tubes on V10 & V11. So far I have:
1. Out put bias pin 5 are 0.28, 0.24, 0.32, and 0.37 V
2. Grid bias voltage pin 6 are -26, -26.78, -26.5, and -26.2
3.B+ High Voltage supply pin 3 avg 470 V DC
4 Bias Supply Negative Voltage -27.13

I took these while on the variac and will do over when I get new tubes and do it on line voltage.
 
Nice receiver. Now you can consider some of the modifications that DaveG devised. The individual bias/balance board (IBBA), the volume control buffer (this one makes a nice difference to frequency response) and Enhanced fixed bias (EFB). These are a bit of work but I think worth it. I've done a 400 and a 500c. I also built a 500c power amp only from scratch using original transformers and installed the IBBA and EFB in that unit.
Good luck,
John
 
Thanks John, I am definantly considering those mods for the future I am monitoring this unit and learning more about it before I go further.
 
An AK member @ncwalz has kindly posted his design and Gerber files for the IBBA, EFB and volume buffer boards. I had them made by JLCPCB at a very reasonable price. @RS Steve has also made some pc boards for these functions and I have used his stuffed and ready to run boards. They are very nice as well. Not sure if he still have any but wanted to mention.
 
nerdorama said:
An AK member @ncwalz has kindly posted his design and Gerber files for the IBBA, EFB and volume buffer boards. I had them made by JLCPCB at a very reasonable price. @RS Steve has also made some pc boards for these functions and I have used his stuffed and ready to run boards. They are very nice as well. Not sure if he still have any but wanted to mention.
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I just installed one of my adjustable bias boards in a 800C.

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I have the screen grid resistors installed, I couldn't get the glass wire wound 7-ohm resistor to reach and all I had was this huge 1.2K ohm 10-watt resistor (the lead is exposed but it cant touch anything right now and I may replace when I make my next order. So far everything seems happy. I am getting ready to install the IBBA.
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JimWhit said:
I have the screen grid resistors installed, I couldn't get the glass wire wound 7-ohm resistor to reach and all I had was this huge 1.2K ohm 10-watt resistor (the lead is exposed but it cant touch anything right now and I may replace when I make my next order. So far everything seems happy. I am getting ready to install the IBBA.
View attachment 3763489
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Notice in my post above yours how I do my screen buss that my 100 ohm stability resistors attach to and the two 1.2 K resistors attach to also. This was on a 800-C though, not a 400.
 
RS Steve said:
Notice in my post above yours how I do my screen buss that my 100 ohm stability resistors attach to and the two 1.2 K resistors attach to also. This was on a 800-C though, not a 400.
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I couldn't reach the buss with the old one but I can attach this one to the buss. My new buss wire is one continuous wire and i added the shielding between each section I guess I was too lazy to strip it back.
 
JimWhit said:
I couldn't reach the buss with the old one but I can attach this one to the buss. My new buss wire is one continuous wire and i added the shielding between each section I guess I was too lazy to strip it back.
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I have Romex that I strip and use one leg cut to length for the buss, stripping the areas where the solder connections are made. Something I have always done on my restorations; I like the very solid connections and not snaking a wire through everything like they did stock.
 
I have my board installed and I can balance both L and R channels to 0.001 but my current when measuring at pin 5 (500C) and i can only get my current to -46mA (all tubes similar). I soldered in test points at each pin, but I also confirmed the values across each resistor. The board is working in the sense that all the tubes are the same bias just way to low. These pictures show how I ran A, B, C, D, and the - bias voltage.
 

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JimWhit said:
I have my board installed and I can balance both L and R channels to 0.001 but my current when measuring at pin 5 (500C) and i can only get my current to -46mA (all tubes similar). I soldered in test points at each pin, but I also confirmed the values across each resistor. The board is working in the sense that all the tubes are the same bias just way to low. These pictures show how I ran A, B, C, D, and the - bias voltage.
Click to expand...
I measure my bias in MVs. you must have your positive and negative leads from your meter backwards or something.
 
RS Steve said:
I measure my bias in MVs. you must have your positive and negative leads from your meter backwards or something.
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It is 46 mV. I am measuring directly across each of the 10-ohm resistor? I am wondering if my bias voltage going into the board is off? I need to pole around and measure more.
 
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While measuring voltage at pin 6 output tube V8 started to red plate (actually did red plate) when i turned my back to talk to my wife, that quick. I am taking the test point iI solder of and going to check installation of A, B, C, D, E and -V and Gnd. V8 is shot but i have a couple of substitute tubes I thought I would use just to dial in the board. I did check -V from the bias filter I believe it was -26 V and it was -20V at pin 6. I didn't get to finish so were n umber I remember when I started to poke around. Only V8 (the tube I was measuring red plated (I did have a test lead from DMM connected to it at the socket). I would greatly appreciate any advice moving forward. Thanks in advance.
 
46mv is too high, need to get that down to 35MV. What output tubes are you running?
JimWhit said:
It is 46 mV. I am measuring directly across each of the 10-ohm resistor? I am wondering if my bias voltage going into the board is off? I need to pole around and measure more.
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I had 4 Tung Sol, one is toast now. I replace the burned Tung-sol with two old Fisher tubes on the left channel with tow Tung Sol on the right channel.

Here are some readings after balancing L and R (the numbers were bouncing up and down around 0.0)

Bias V= -26.8V
PIN 3, V8-V11 all approx. 464V
PIN 4, V8-V11 all approx. 420V
PIN 5 V8=40mV V9=42mV (these are old Fisher Tubes) V10= 37.5mV and V11=34mV (these are Tung Sol)
PIN 3 and PIN4 High?

I can change the old Fisher Tubes but I'm not going to get to 30–32 mA without tweaking negative bias supply? Open to all suggestions.
 
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RS Steve said:
I have Romex that I strip and use one leg cut to length for the buss, stripping the areas where the solder connections are made. Something I have always done on my restorations; I like the very solid connections and not snaking a wire through everything like they did stock.
Click to expand...
I keep spools of 18ga and 14ga tinned bus wire around for such uses.
 
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