My other 500c that I thought was dead...

Catmanboo

Analog guy, meet digital world - HELP!!
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...isn't after all. It hit the big desk yesterday for a nice, slow power-up so I could start working on it. Previously was sidelined upon purchace (or theft, considering price!) due to bad hum, i mustve had selector in phono or tape head position then. Hooked up antenna & speakers, switch on fm, started twisting up the juice, 150w bulb in the power source glowing orange- damn, I hear something besides hiss as I slowly advance the big knob on the throttle. It's alive! :banana:tuner working !
Getting to work, started poking about with the dvm,voltages as follows:
Siemens bridge- +4.2vdc, -21.3vdc.
375 volt bus to pin 8 on 7591's- 397.5vdc
Plate voltage (pin 3) 435vdc on v8 & 9, 434vdc on v10 & 11.
Bias volts on pin 6- -16vdc on all 4.
Voltages at c91a-212v, b-312v, c-362v, d-397v.
Voltages at c92a-164vdc, b&c- 188vdc, d- 214vdc.
Tuner working fine, except for too-high stereo capture threshold, hum when in stereo mode which is directly proportional to volume level. Not present in mono mode.
Schematic not clear where the diode bridge feeding to relay gets its power, but it's the same source that feeds to selector switch section 3.
Other major issue is LOUD hum in both phono inputs & HELLISH hum in tape head position. Being that this preamp, v16 & 17 ,is powered from 215v source at c91a/r133, shall I suspect that 20uF can cap section may be at fault? Yeah, I know-replace all can caps,right? Prior to that, I want to find out & fix what ails it before I shotgun ALL the caps.
This unit, s/n 78778am, is all original, right down to the tubes & light bulbs, which all still work. I'd like to keep it as original as reasonably possible, but will start out with upgrading bias resistors & caps, cathode ground resistors, mpx electrolytic, don't need the inrush limiter just yet, busted & bypassed power switch will be difficult to replace. Metal bone maybe still has a couple, but I don't deal in bitcoin, which is what it'll take to pry one out of him according to a recent visit to his site. (Dammit!) & maybe a 10k pot for basic bias adjust.
I don't think this unit ever got much use, due to condition of the tubes & bulbs. The 7591's all show in excess of specced mho on the 533a tester, they're electrically like new. Only cosmetic issue, not counting knob brights (I have them all in a safe place) is darkening of the signal strength meter & stereo beacon dummy faces, likely UV damage from sunlight exposure. Nothing I can do for that but eventually replace. Would be nice to have an em84a in place of that stupid bulb/relay set-up, it would just fit in that faceplate slot, too.:rolleyes:
So, Larry, Dave & company, your hallowed opinions & advice greatly appreciated! As I have two of these things plus the 400, they'll eventually all get the treatment, but one at a time.
Last question : are there any known discrepancies in the factory service manual for these things? And believe it or not, this one came mounted in an end table with a lift-off ceramic top. No pics of that yet, I'd need to dig out of storage.
 

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Man, that is one clean sucker!! And original to the bone!! Congratulations :thumbsup:

Being the eternal optimist, whenever I get music upon first turn-on I consider it a down hill operation going forward. It looks like you're in that position now. Given that you presently have an operating unit, albeit a humming one, I'd tackle those can caps first, including the internal bias dual cap. While there, pluck out the original rectifier in favor of a silicon bridge. After that, then pretty smooth sailing on replacing other components and adding modifications as desired. A leisurely stroll through the archives should give you enough to tackle.

Good luck with the restoration. It's a really beautiful piece!!
 
Thanks. I lucked out for a change, usually I watch everyone else make the big scores. Initial power-up was with my junker 7591's in event of crazy shiite happening & it sounded wonderous! Then, in go the originals after this thing ran for hours. About blew the test speakers, little Crimson Burhoe Acoustics, off the shelf! So, I've something solid to work with, and another one to use as reference /comparison. As us steelers fans say, "Here we go!":jump:
 
The rest of the pics... the table was modified by someone who knew what they were doing, used wood scraps in a crafty way, even offsetting the center support so as to not block ventilation. It's likely going back in service as-found eventually, but rest assured, I'm putting forced-air ventilation in there.
 

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HOT DAMN! A unit with Original Sylvania 12ax7's. Orange Glow isn't necessarilly bad on a 150W bulb. Considering the 500c draws 180w during operation, The glow is normal as the bulb is taking some but not all of the load. If it was bright white, I'd pull the cord out of the wall in a heartbeat.

I concur with NOTDIGITAL. P.S. rebuild 1st to get everything stable and quiet. For a test, take a 22uf 400v lytic and parallel it across C91A. If the HUM reduces or is eliminated, chances are good (>90%) that the can needs replacement. Before that tho, take some shorting plugs and install on phono and tape head RCA's, then turn selector to those positions. there should be no noise. If there is, more than likely you have P.S. problems.

With regards to Relay power, look just just above the S1 Section 4 Front part of the selector switch. There is a takeoff with a Triangle marked "C". Now go over to the power supply and find the end of the power supply takeoff's Marked the same way (triangle "C"). It's fed off C91A. The Stereo Beacon Lamp is fed off "E". Fisher did it that way rather than add more lines to corn-fuse the rest of us.
 
Aw leave the end table as is. With the 500c running, the stone top should get up to temps enough to keep your coffee warm:rolleyes::rolleyes:. Seriously tho. It does need some ventilation. I'd open up the back,(side closest to wall) and screen it (paint the mesh black) . With the bottom open, there should be plenty of natural convection. Put Dave's EFB in it, and you won't need a fan.
 
And the rest...
 

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One more of the table, with the document side-pull out. Won't load pic, file too large. Dammit! :mad:
Larry, my schematic shows c92a feeding relay circuit, not c91a. I did overlook that though, I was up until the wee hours doing that post last night. Thanks for the other tips, will get right on it & post the results. Rest of the'ax7"s are german, 6ha5 is brit, 6hr6 is red labeled fisher, 6au6,all 3 are yellow label fisher. The nuvistors labeled fisher but I think are really RCA's. Golden synchrode label on tuner needs glued back on,too.
This EFB, I've seen reference to it, have yet to find out what it is. Enlighten me, por favor...:bowdown:
 
You're right it is 92A. I didn't have the magnification high enough. Sorry.

EFB Enhanced Fixed Bias. A development of Dave's that will improve the 500c's output and lower distortion, plus keeping screen voltage at a set percentage of the Grid ALL the TIME. It runs a lot better and cooler to boot. This will explain it better than I can. http://audiokarma.org/forums/index.php?threads/efb-and-fisher-800-500-early-version-400-receivers.539265/

The red and yellow labeled IF STRIP Fisher's are Sylvania tubes. The Nuvistors are RCA's.
 
A few more pics. Table top will make a great cat-warmer, no doubt.
Larry,on my schematic,"c" is feed from c92a, and due to late night droop-eye, I missed that section of selector switch entirely. Closest to a 22uf cap I've dug up here is a 33uf 400 to test that suspect cap. Phono preamp noise is internal, so I know what that means. Re: oem tubes, rest of the 'ax7's are west-German white label fisher as is the 6gk5 , nuvistors labeled fisher but I suspect are rca, the 6au6's are yellow label fisher, rf amp tube is fisher/great Britain, 6hr6 is red label fisher, may be tung-sol, number framed by octagonal frame. Relay tube just says "the fisher" & under that, "fisher radio corp., new York, last line below says 1022-615. I'm not up to speed yet on who made what for who regarding tubes.
I've yet to glue the golden synchrode sticker back on tuning cap cover.
Faceplate & glass carefully removed,& washed. 2 coats of wax on faceplate, foam dust seal between the glass & back of faceplate carefully reglued onto glass, it fell apart into several sections. On/off switch was bypassed (by me) with a wire nut, there was nothing left of it to fix. Judging from smoke deposit on chassis right above it, moisture may have caused it to self-destruct in the distant past. Least of my worries. ..
 

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I thought I lost that last post! I just re-composed & sent it. My bad...:dunno:
 
I just had an idea. Put the paper info in an envelope and store it somewhere else. When the 500c is operating just open the drawer. Problem solved. Air comes in the bottom and exits via the drawer from natural air convection.
 
1022 is FISHER's EIA #. 615 is 1966 15th week. March of 66 the tube was marked. The 500c probably made between May and June. .
 
Can you put up a few close-ups of the switch? I see the shield and it looks like the tabs of the switch are gone....but what is that inside the switch shield?????
 
Nothing but air in that switch housing. When I got it, I found rubble & crispy shiite that used to be a switch, see smoke deposit on underside of chassis. I'll go upstairs, flip it over & get another pic for you in a bit, the rcvr. is currently helping heat the room.
 
Catmanboo,
That sure is a clean unit, and I love the end table cabinet! Good luck with the restore. I've been sitting on my butt for the last two hours listening to my 500c and it just blows me away. Even my sweet wife commented on how good it sounds.
Rick
 
That's the one with the tabs. I might have one I can send you, if you feel up to taking apart the pot to get to the switch and replace it. Let me check my extras junk box.
 
Rick, that was me when this thing came alive, with my utility /test/junker 7591's (they all test low mu & emissions), I was pleasantly surprised to the point that I left it playing the rest of the day & night. Only plan for that one right now is to replace the 20uF section of can cap c91, feeding ac component into preamp, maybe put a pot in the bias line for ballpark adjustment ability, maybe replace the power switch if I come across one, & listen to it do its thing as I work on the other 500c. Already did bias caps & resistors on that one, bias voltage is 6 volts low (closer to 0) so the Siemens bridge likely has to go. I have a couple of suitable replacements, just have to find them in the clutter.
Larry, if you have such a thing, I have toobs to trade, none are repops/new production, either. I feel that I oughta send ya something anyway, you've been an immense help, with your info & patience.
So. Happy Sunday to you both! It's blowing & snowing here in my part of n.e. ohio, a good day for tinkering & turning vinyl. :cool:
 
Larry, regarding the pot switch- is that something particular to fisher units? One would think that those would've been used in a lot of other things besides, or did fisher make those themselves? This one looked to have arc-flashed itself into oblivion. When I first got it home years ago, I pulled the bottom plate for cleaning, pieces of that switch were falling out. Even the fiber back plate was cooked& in pieces, so I just shit- canned it all, didn't know any better, nor did I know at the time that this old tube gear had much value to anyone else besides me.
Pleasant surprise, wish I'd discovered this site years ago. Likely wouldn't have, if not for being winter, with time on my hands, practicing retirement. .. not officially there yet though, got a couple more years to go. Just waiting for the call from the hall to get back to work. But meanwhile, back to the play pen!:)
 

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