• 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

Newbie Fisher 500C Rebuild ?'s

D

davefr

Active Member
I have a minty/working Fisher 500C as an upcoming rebuild project and want to start doing my homework. It's pretty much untouched except for substituting a new voltage regulator in place of the old germanium component. My 7591 tubes are a pair of RCA and a pair of Realistic in good condition. Here are some get started questions:
1. Can these two different tube brands co-exist or do I need a new set of matched quads? I thought I read that Fisher needs all 4 same brand/closely matched.
2. I've watched FisherDoc's youtube video. He says don't touch the big top side electrolytics unless they're leaking because they last forever. Do you agree with this approach?
3. Has anyone done a comprehensive Fisher 500C rebuild thread here on AK or is FisherDoc's video the best source?
TIA
 
Register to hide this ad
With individual bias adjustments this isn't a problem. Its more important that the pair in each channel match one another.

In stock form they did need to be all well matched and to very specific parameters because there is no mechanism to adjust the bias. This is easy to change. If you look for IBAM or IBBA (individual bias adjust mod) or (individual bias/balance adjustment) you'll find info on how to do either. Personal preference is the IBBA method but both get you to the same place.

Caps go bad. Have had plenty of them come to me bad, and I've had others decide to nope out with no warning. On anything I actually wanted to just run without stressing about it they would get changed. Caps can also fail in not so obvious ways. The obvious one is hum, or it gets hot and starts puking goop out of it or blowing fuses. The not so obvious are strange circuit problems and weird noises that happen because the cap is too degraded to work properly, but not quite enough to make it hum.

THere are enough threads on various Fisher model overhauls that i'm certain a comprehensive 500C thread exists. I've never done one personally so I have no idea which to suggest specifically. @RS Steve seems to have done at least one of every model though, so if he doesn't have a thread on here I'd bet @larryderouin does, or can suggest one to check out.
 
Having vintage era output tubes can be a plus, new production 7591's require a lot more negative bias voltage applied to run at the proper bias needed. If you are able to test yours and determine which match up better as pairs, there might not be a need for installing a balance / bias adjustment mod. There are many threads on restoring 500-C's that you can read and really get a good direction on what to do. Just know that if it's in original condition, there are many components that are considered way beyond their expected life span. Add in the higher wall voltages we see today, and it's a recipe for failure.
 
FisherDoc is an unpleasant subject to many of the older members here. The collective knowledge on AK, along with the number of upgrades (both for Safety and Reliability) produced for the FISHER brand tube gear in the last 10 years is unparalleled on the "open market"(read as You tube). I would take anything FisherDoc says with a LARGE BOX of Salt in a Deep OPEN WOUND! Then look for other sources of info.

Stickies to Read.
1.) https://audiokarma.org/forums/index.php?threads/most-common-parts-needed-for-fishers.327561/ Lot's of information here that pertains mainly to the 400-500c-800c in the main stickies section. The 500-C and 800c are essentially the same unit with 1 major exception. The 500c DOES NOT have an AM Band while the 800c does. Other than that they are essentially the same.

2.) ANY 800c sticky will pertain to the 500c (with exception of aforementioned AM Band).

3.) OLD Main caps can/will short, overheat, and will PUKE! Replace them. You wouldn't buy an unrestored 60's vehicle and drive it cross country without basic maintenance and some replacement parts. Same thing. These units are now in their 6th decade, higher bias voltagethey're tired........VERY TIRED!

4.) Output tubes. Old Stock tubes if matched for load (tube draw) can be mismatched for brand. Of the replacement Russian Tubes, only the Tung-Sol Re-Issue 7591's require a higher (more negative) bias voltage. EH 7591's usually don't, in my experience. But the EH's have a larger bottle which can be disconcerting aesthetically, where the Tung-Sols are closer to an Old Stock 7591 in size. Both are good tubes and reliable, provided the individual bias mod has been done.

5.) Lot's of 500c and 800c restoration threads. HOMEWORK ASSIGNMENT! :) And you thought "HOT DAMN, no more school" when you got that Sheepskin. :naughty::naughty:

We can guide you along to get the most out of your 500c. @thornev made a couple of overlays of a 500c with locations of components that have been extremely helpful to new guys. He can pull up the link for ya on that. When you get done, the next time it'll need another overhaul, your grandkids will have Gray Hair.
 
Last edited:
Hey Larry. I don't get notified when someone @thornev's me. I don't know why not. But I'm glad I decided to review new posts today.

To the OP, essentially, I made a PHOTOFACT for the 500C because I spent so much time fiddling around in it with the help of everyone on AK. Let me see if I can find where I posted them. "I'll be back"

Here ya go:
https://audiokarma.org/forums/index...nd-modification-project.970646/#post-15041717
If that's too small to view, I can find a way to get you a bigger one. Just let me know via private message as I'm not as active on AK as I used to be. Been busy doing music.

Found this link too:
https://audiokarma.org/forums/index...ate-model-project.912239/page-5#post-14398601

This post explains how to use the grid model:
https://audiokarma.org/forums/index...ate-model-project.912239/page-4#post-14393606
 
Last edited:
Dave,
I completed my first 500c restoration earlier this year. Here is a link to a synopsis of the work I did: https://audiokarma.org/forums/index.php?threads/500c-restoration-chronicle.1000188/

I am including this, not as a strict recipe to follow, but to give you ideas on some of what can be done, so you can decide what you want to do. Not all the mods I did are necessary, or need to be done on the first pass through. There are other mods, that I chose not to do, but may be of interest to you. Replacing the main filter caps, providing stabilization and protection for the output tubes, and providing the ability to individually bias the four output tubes are probably the most important things to address. I used the IBBA (Individual Balance/Bias Adjustment) approach. The easiest way to replace the three can caps on top of the chassis is to order them from Hayseed Hamfest - not cheap, but they are essentially direct replacements - this was the first thing I did as the power supply provides the foundation for all the other circuits. Hayseed can also supply the bias caps - either one unit containing two, or two separate caps - these go under the chassis and replace the large, usually yellow, bias cap.

The folks who chimed in above, LarryD, Gadget73, Thronev, RSsteve, and DCgillespie are a font of knowledge and who I learned from by reading their MANY posts here, often more than once. It took a while, but I learned what was needed. READ, READ, and read again. Post questions, go slow. Your efforts will be rewarded with a beautiful sounding unit.
Good luck.
Gary
 
Your synopsis and the Fisher Stickie thread are exactly I'm looking for. What's purpose of the 100 ohm screen stopper resistors as part of the IBBA mod? Are they like fuses to protect the 7591's from a runaway thermal condition? All in all it looks like a 500C rebuild won't be too bad with a Hayseed caps. It looks like a pretty modest BOM compared to solid state. I'm finishing up my Sansui 7000 and would like to bring this 500C up to date as my next project. Thanks for sharing.
 
I think you have the function of the 100 ohm resistors correct. There are several posts from LarryD and DaveG on exactly what to do on the underside of the output tube sockets. The stickies are a great place to start, they explain the why and how of the mods you need to do.
 
Everything you wanted to know about Screen Stability resistors, but haven't asked ..........yet. http://www.tronola.com/Gillespie2544.pdf They minimize and or prevent arcing in the tube causing a Chernobyl event in the tube. The Cathode resistors are the "fusistors" that can shut down the tube and save the transformer.

Post 40 of this thread in the stickies has diagrams on 7591 & 7868 install of Screen stab's. https://audiokarma.org/forums/index...needed-for-fishers.327561/page-2#post-6480170
 
You must log in or register to reply here.
Back
Top Bottom