• 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

Another Fisher 500c project

I

IthinkIcanfixit

New Member
Hello,

I just registered with this site and have read the various posts on restoring Fisher receivers. I recently retrieved the dust-collecting 500c i grew up with (bad power switch, old caps, and an output hum), and I am ready to tackle job. I've gone through the schematics and underside, but before I start, I would like to go over them again with a decent set of instructions, just to be sure. I've tried contacting Al over tha past 2 months (email, phone, and snail mail), to no avail. Does anyone have a copy of the Fisher Doc's restoration instructions that they can email/send me? Many thanks.

Louis
 
Register to hide this ad
I'm at the same point as IthinkIcan --
if someone could send me the mentioned restoration instructions I would be very thankful.

Thanks,
Reed
 
I took the on/off switch off and am ordering the new one on Monday.

Question: I willstart replacing the selenium rectifier with a new bridge. Where do the two silicon diodes (1n4007) go ?

Thanks,

Louis
 
So far, I replaced the on-off switch, bridge, power caps, diodes, and capacitors in the amp portion of the 500c with new parts (BTW, VTV still owes me the schematics after almost 2 months), but I am not happy with the sound. There is a lot of noise, and the clarity and loudness is just not there. There are other issues with this receiver, which hasn't been operated in over 15 years and which shows a moderate amount of chassis corrosion.

I tested the voltage at different locations after the new parts and the results are as follows:

1. Bridge voltage: -25.5v
2. Voltage after R132 (3.3k): -22.7v
3. 7591 - Pin 5 of all tubes show only 0.01 to 0.03 volts across the 10 ohm resistors. I am not sure what is going on here, although the 7591s tested weak.
4. 7591 - Pin 6 of all tubes shows -17v
5. 7591 - Pin 8 shows 329 volts (should be 375)
6. 7591 - Pin 3 of all tubes show 352 volts (should be 430)

I also tested the voltage coming off of the can caps (C91, C92, and C98). Voltages off C91 are below specs by up to 45 volts. Voltages off the last cap of C91 is 173 volts. The voltage off of C98 is 354v, which is way below the 430v volts in the schematic. It looks like the can caps need to be replaced.

Does anyone know where I can get new can caps? I don't want the underside to look more chaotic than it already does. I suppose I can hollow out the old ones and put new parts in, but that seems like a lot of trouble.

Thanks,

Louis
 
Weak 7591's will sound flabby (in my limited experience) and bias current may be all over the place. Bet they have pretty bad grid leakage, to boot.

BKville's 400 sounded pretty bleh on its original 7868's. A match quad of the EH 7868's got it singing (plus the bias supply rebuild, cathode resistors,and coupling caps, of course).

www.tubesandmore.com and Radiodaze (not sure of their URL) have some new production can caps, but I never looked to see if any are close to the Fishers. The very first cap in the voltage doubler is probably the most vulnerable. I "always" (well, twice out of two) change the bias supply caps, FWIW.

EDIT: Good, free advice: at least :-) two guys here, EL-37 and NOSValves, know far more about these fine receivers than I do. Search "Fisher" and their monikers as posters, and you'll get lots of good info and advice. More than I can provide.
 
Source for custom can capacitors

IthinkIcanfixit,

When I was looking into parts for rebuilding a Dynaco PAS3 and ST70- each of which has a multipart can capacitor, I found this company " Vibroworld " in Portland, Oregon. Zack will make cans with any values- you choose the type of capacitor, style of mount, can diameter, height and etc.

I never did order one, but a couple of years ago a 4- part ST70 capacitor with my choice of values was $40. I was told on various audio forums the results are good, they are great to deal with, and reliable.

I notice on their site- http://www.vibroworld.com - that they are unfortunately currently moving shop and no definite schedule,...

The Fisher receivers are excellent. I have a 1965 800D with all original tubes and I used it happily for three years 10 hours per day as the centre of my office system.

Cheers,

Bambi B
 
Well now, the 500c finally plays pretty darn good. In my last post, I mentioned that the sound was crappy. The problem ended up being C98, that can cap was bad. it must have been leaking voltage all over the place. I ordered a 220uf 350v nichicon with the intention of mounting in under the chassis and bypassing c98. But the chassis was just way too crowded, and the nearby bunch of wires precluded a clean mounting. I decided to hollow out c98 and replace the internals with the nichicon. Problem was that the new cap was too long to fit the c98, so I hollowed out the 1000uf/1000uf cardboard-covered can cap from the power supply and used this to replace c98 up top. It is amazing to hear the difference in sound quality with just that one cap replacement, the receiver has regained its loudness/output, and its clean warm sound is a pleasure to listen to now (don't know why I waited 20 years to do this...).

I also replaced the original two driver 12ax7s with new JJs and the 7591s with new EHs. Funny thing is, the old tubes tested just as strong as the new one, although the top coating looks like they are on their last leg.The thing I don't like about the new EHs is that they are a lot taller than the original, and extend above the transformers, which means I have to be very careful and use risers under the transformers and the faceplate to work on the underside. Can anyone recommend another brand whose height is below the transformers?

I still intend to replace c91 and c92, but the urgency is not there since the receiver sounds so good. I found some replacement nichicons, but only up to 450v, I need to find a source for 500v 40uf caps. At some point, I intend to repack the originals.

Also, can anyone provide information on a good in-rush current limiter to protect the on-off switch (part no. and maker would help), including good mounting options?

OK, to summarize, this 500c received the following

1. new on-off switch
2. recap of the amp section
3. new bias power supply
4. recap of the b+
5. 10-ohm ground resistors from the 5 pin
6. new c98

I'll try to post some pix over the next week or two.
 
JJ 7591s are smaller, but still a bit larger than NOS. I used a Keystone (Thermometrics) CL-70 in my 800C, I think (don't feel like turning it over just now...). Mouser and Digi-Key stock them. Where did you get the power switch?
 
For the inrush limiter, I also use the CL-70, purchased from Digi-Key. The Digi-Key part number is KC007L-ND. I mounted mine in series with the power switch, connecting the return wire from the power switch to one side and the other side to the terminal strip (where the transformer normally connects to the switch)to support the CL-70. The CL-70 gets hot (by design) and so make sure it is not touching anything. I make sure there is room around the CL-70. I have been using these for years in old radios and also have them mounted in my 800B and 800C.

John K.
 
You must log in or register to reply here.
Back
Top Bottom