• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Fisher 500C Rebuild Questions!

P

Paul Fischer

Active Member
Hey Guys, I'm trying to figure out the specs on some of the Fisher 500C components but of course, cannot find the part numbers anywhere. For instance, CR5 and CR7 are listed as SR50411-1. Cannot find these doing an internet search. Does there exist some sort of cross-reference with more modern parts?
Thanks,
Paul
 
Register to hide this ad
based on the part number I actually would have assumed the originals were 500ma, but either way if those are the main voltage doubler rectifiers, a 1N5404 or higher is more than sufficient. I usually stock 1N4007 and 1N5408. Thats 1 and 3 amp at 1kv respectively. More than enough for basically any diode you're likely to find in tube gear.

those plus some 1N4148 for low power / switching diodes covers most things.
 
Hey Guys,
I've got this image of mods Dave G suggests for the Fisher 500C but it's too small for me to read. Wondering if anyone has an image I can make larger without loosing all the detail. Here's what the image looks like.
Thanks,
PaulDaveGillispiesFisher500Mods.jpg
 
Hey Guys,
Recently acquired a nice fisher 500 C. Seller is a 90 year old electrical engineer who has owned new from the factory. His nephew, who sold it to me, assured me it was working. I took his word for it and began updating the unit without trying it out first. Anyway, I have no replaced all the multisection caps with caps from Hayseed. Installed new bias capacitors, bridge rectifier, diodes, coupling caps, and installed screen resistors and cathode followers, and bias setting controls. Today I thought I'd do a dim bulb test. Well the bulb is not dim it is quite bright and the front panel does not show any signs of life. I checked the black leads coming out of the power transformer saw good voltage there. Are there any other obvious places to start checking? OH, the previous owner installed a new volume control/on off switch but I don't think that's the problem as it's been in there a while but it's not original. He had replaced one of the sections of a multisection cap with a stand alone cap but I pulled all that out when I put in the hayseed caps. So anyway, I'm stumped at this juncture. Suppose I should check the red output leads from the power transformer also. Haven't done that yet as I'm not at all sure which wires are what? Any advice would be greatly apprecaited.
Kind Regards,
Paul
I'll bet some pictures would help. Yes?
 
Hey Gadget, Thanks for getting back to me. Don't recall the wattage but it works great. Had this one a while used on other units with no problems. I think it's about 100 watts. Yes all the tubes are installed. When I used this on my 400 the bulb light up but just a bit. This time it just keeps getting brighter as I turn up my variac. Have not turned it up all the way yet cause the light keeps getting brighter. Should I use a bigger one? The 500C manual says it used about 205 watts at 1/3rd power. Is this bulb too small?
 
quite possibly. Pull the tubes and see what you get. That will tell us if there is a problem in the power supply.
 
Okay. So without the tubes we shouldn't be pulling any power. yes? Or at least very little. If there's a short the bulb will light up. Dang I hope not. So you suspect a bad tube?
 
Okay. Pulled all the tubes, connected up the dim bulb tester and it powers up just fine. Slowly turned the variac up to 120 vac and the bulb just barely lit up at all. Just like when I tested the 400. Does mean I have a shorted tube?
 
Or it's just too much current load with everything installed. Confirm the grid voltage at the output tubes is somewhat reasonable and see if it will behave with just the output tubes in if so. Guessing with no tubes, grids will be around -20 volts.
 
Went through installing all the tubes back in one at a time and checking with dim bulb. Worked fine. Now it fires up normally, however I did test all the tubes and found 3 very very weak 12ax7's which I replace. Don't understand why it's working now, however I'm only getting sound out of the right channel and can't find any bad connections anywhere. I haven't set the bias yet on the 7591's. Wonder if that could be the problem?
 
Improper bias probably won't either unless the tubes are glowing bright red on the plate. Check the space expander jumpers and the tape monitor switch. Problem there can kill a channel. If not, compare voltage channel to channel.

Cant really set bias on a dbt anyway
 
I installed a bias board but I screwed it up. There is zero bias going to the two left tubes. Nothing. Back to the drawing board. HAHA. What's a dbt? That 's new to me. And what space expander jumpers? Not tracking here gadget. Sorry.
 
You must log in or register to reply here.
Back
Top Bottom