where can i get a copy of the schematics for the Fisher 500C upgrade? I'd buy the kit, but i am too cheap...plus, I'd like to see the mods prior to attempting the work. Could one do only the mods in the output section of the receiver? I am told my 500C plays and sounds good on FM. I am a little more squeamish about working on that section, since i lack alignment skills....

Squeamish?

Don't be cheap, and let a pro do the job.
I recently had to "fix" a botched upgrade on a customer's 500C because they didn't understand proper servicing techniques.

The bias supply also needs addressing.

You either do the job to completion, or not - cutting corners with those great old receivers isn't an option.

Agree, this is a very nice set. Take care to have it restored properly and it will last another 50 years.

The upgrades vary. Most include a new silicon bias rectifier, new bias electrolytics, new B+ electrolytics, 10 ohm resistors in series with each output tube cathode, adjustments to the bias supply to set correct idling current for the output stages, and replacing the grid coupling capacitors to the output stages. Some change the grid circuit resistance for the output stages. Some also make changes in the multiplex decoder.

I strongly recommend adding a quiet fan for better cooling.

There have been debates as to what is the best output stage idling current and to whether the mpx decoder changes are significant. I will leave the namecalling and controversy to others.

--Chuck

30-35 mA.

Agree with Clay, 30-35 ma on the 7591's, and I prefer the low end of the scale. Push those tubes (and 7868's, for that matter) toward redline, make little monsters.

Also chalk me down for all the upgrades Chuck mentioned, apart from the MPX upgrades, which I've never tried (and had no reason to try). Furthermore, I like to add screen safeties (200-220 ohms, 5% tolerance at least) to those 7591A's and gear them down. The cooler those tubes run, the better.

Lastly, bucking x-former or Variac (and I prefer a Variac) to run such gear on 110v MAX. Cool running means conservation.

Good luck with your project .

Larry

Oh, but it's fun to do the Multiplex upgrades too!
That little shielded box is nothing to be afraid of!
Then the job's complete!

Mox nix!

I have no complaint with the way my 400 and 800 are handling FM now, and I've got better things to do.

Larry

gentlemen, thanks for the input. i too would like to try these upgrades. first, I'd like to see the schematic diagram for the upgrade. Does anyone have such a schematic showing these upgrades?

Just to make sure, in series with pin 8?

Hi, Clay, my last such job was my 800 overhaul. I'd have to go back and check. It's been a couple of years now....



OK, here it is. They fed the screens with a single wire straight across pins 4 and 8, serving all four tubes in a line. I clipped the midsections and tied the safeties to the closest pins. With the two on the right, it looks like pin 8; on the others, pin 4. However, as you can see, the voltage still goes to both pins on all four tubes.

You can also see the replacement control grid resistors (270K) and the cathode safeties (12 ohms) in this photo. The bias supply is out of view to the left.

Hope this helps. By the way, does your Fisher forum over at AK have the schematics Wilbur wants? I have nothing on the 500.

Many thanks and best regards ,

Larry

I was thinking 500C. The 375v is to pin 8.

I just pulled an 800C schematic, and the 375v screen voltage goes to pin "4/8", so I assume they're tied internally and either pin will work. If you have a chance to check that would be great.

Another edit...I assume 1/2 watters would be fine?

Hi Clay, you posted while I was editing. See above...thanks!

Larry

For screen safeties at 200 or 220 ohms, they should be. Those one-watters in my 800 get somewhat warm over time, but not very. The half-watter would also have the advantage of blowing faster, if there was a screen short.

Larry

Either one, 4 or 8, both pins are supposed to be connectedto screen grid. May not be true for reproductions made recently!

I have a standard 500-C schematic, but Wilbur was asking for one showing the upgrades. I've never seen a schematic with the changes documented. Might be an interesting project!

I'm leaving on two weeks vacation on Sunday, so I won't be able to take a run at it, but I'll see what I can do later in June.

If you mean 7591 tubes made recently, you'll never catch me using them anyhow....

Larry

Ooohh, interesting thought! I would hope they wouldn't do that, but you never know!

Some were pretty bad, no doubt, like the original JJ's. I'm hearing that the current ElectroHarmonix have been showing no problems at all, sound great, and have good longevity.

Vacation? I remember such things...vaguely...

Seriously, have a good time and safe trip, and get back home safe.

Larry