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500-C Hit's a Snag!

Discussion in 'Fisher' started by notdigital, Dec 7, 2017.

  1. notdigital

    notdigital AK Subscriber Subscriber

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    Not too long ago I picked up a junker 500-C because Number One Son began expressing interest in lugging one of my darlings off to college with him. Well, not that I wouldn't do anything for him but the thought of him or one of his cute little drunken buddies splashing suds all over it with the resultant sparks and smoke gave me the shivers. So for what was a very good deal, I picked up the junker.

    The unit was sold "with a weak but working channel" so I thought nothing of it attributing the weakness to a cap or something dirty. I went about tackling the usual mods and exercising the usual gremlins along the way: new bridge, can caps, socket and switch cleaning, and misc. cap replacements. As I made changes I checked to see if the weak channel was corrected. Sadly, not. Now it can't be the volume pot since that was initially completely shot so I popped in a replacement from my stash. Still no LEFT channel. It's not the dastardly TAPE MON switch that has given my grief in the past since I removed it and thoroughly cleaned it. Still no good. (On deck is the MPX FILTER switch which has me wondering.....)

    To be clear about what's now going on: With the selector set to MONO, I get full sound from both channels. With AUX engaging a CD player-- both channels strong. With the selector set to forced STEREO, STEREO BEACON indicator comes on and left channel drops right out! With selector switched to FM AUTOMATIC, STEREO BEACON indicator remains lit regardless of signal strength. That is, when turning the tuning dial, the relay doesn't click and the BEACON light stays lit regardless of station signal strength. With the beacon lit as described, NO left channel (or extremely weak one.)

    As part of the recommended restoration protocol, I've replaced the 1uf cap off the MPX. Likewise, C 221 & 222 as well as C-214 within the MPX module. No impact. I've checked the diodes on the matrix board and all have one way conductivity. The associated resistors are good as well.

    I haven't yet checked voltages within the MPX but my thinking is IF the MPX is the problem, then neither channel would be working and, as I've mentioned, the right channel is fine. I'm bothered by the relay being always on but my thinking is that would not be channel related. Oh yeah, it's not the 6gk5 since it like all the other tubes that came with the unit have been changed. I've hit a wall!

    Anyway, any new thinking would be appreciated cuz my thinking module is exhausted!!
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Have you checked all the 12ax7's for a weak triode. I've had more 12ax7's with the "B" side (pins 6 thru 9) damned near dead with the "A" side showing as near new on both the EICO emissions tester and the Philco gm tester, than Carter's got Liver Pills. Mainly the tone amp tubes, but occasionally the MPX and phono tubes.
     
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  3. Dave451

    Dave451 AK Subscriber Subscriber

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    Audio amp 12AX7 in the MPX-65?
     
  4. notdigital

    notdigital AK Subscriber Subscriber

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    I don't think it's the tubes Larry.

    When i got this junker it was full of all original Fisher tele's and westies (xcept for 2 7591's which were GE's.) Since the tubes were the best thing about the receiver and since there would be no freakin' way I was going to leave them in for the kid, I swapped them all out for LPS's and JJ's, all randomly placed. Since the problem existed with the original compliment, and since the replacements have all been cleaned as well as the accompanying sockets, i'm sure it ain't a crippled tube. Just for its and iggles, I played musical chairs with the tubes to no affect. Now it might be a socket since this thing was crudded up but i don't think so. I'll go at it again with d-ox but I'm prepared to rule that out. It seems like a short somewhere but I can't narrow it down. It's not Clippy's negligence because this thing was a untouched before I got to it. I'm going to put it aside and get back to the CX-2 which needs some filter cap refreshing. I know me tho and I'll probably not put this one aside for too long :idea:
     
  5. notdigital

    notdigital AK Subscriber Subscriber

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    Dave: That thought didn't escape me and I not only super attacked all the MPX sockets, but I popped in sub tubes to see what effect it would have. No cigar! Same difference......
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Not -- Since you're getting good mono on all sources, the problem is not in the audio circuits, and probably not in the matrix switching circuits, either.

    Check that the relay contacts are not stuck together keeping the Beacon Lamp on. As for the missing channel, use a jumper clip lead, and jump the Left and Right outputs from the MPX sub-chassis together. If you now get audio from both channels then, then the problem must be in the MPX sub-chassis. If you still don't get audio from the dead channel, the the matrix switching circuit becomes the prime suspect.

    If the problem shows itself to be in the MPX sub-chassis, then with all that you've already done, you may likely be into the ring diode demodulators. When they are bad however, you will usually get some audio from the affected channel. Therefore, it will help to know how much of the bad channel is missing: All of it? Or enough to make for an obvious imbalance?

    Dave
     

     

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  7. notdigital

    notdigital AK Subscriber Subscriber

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    I just knew I couldn't stay away from this thing!!! And Dave and Dave: it WAS the audio amp/V-102 in the MPX module! Went back in, put a loop to it and lo and behold, many of the pin terminals were still cruddy deep in the socket making for non existent contacts. A third really really aggressive cleaning cleared it all up and bingo, channel back! Now on to what appears to be a problem independent of the channel issue.

    As I've mentioned, the Stereo Beacon stays lit when in FM Auto regardless of station strength. And yes Dave G it looks like the relay is a bit sticky. But something else now became apparent: sometimes there is no indication on the Signal Meter. I mean sometimes it's working just as it should. Other times it remains at zero. For me it's late so I'll study that little problem over the weekend, content for now having solved the channel problem.

    Thanks guys!!!
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Glad you found it! I hope your intermittent meter reading isn't due to the meter itself. Clip your DVM onto the meter, and then watch both. If the DVM goes to zero when the meter does, then the problem is in the set wiring somewhere. If the DVM maintains an indication when the meter gores to zero, then the meter is the issue.

    Dave
     
  9. notdigital

    notdigital AK Subscriber Subscriber

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    OK, so I'm back doing battle with the Stereo Beacon Lamp-Always-On condition. Taking a closer look I see evidence of Clippy at work again. The unit I'm working on is a late model 77,XXX version. As I was examining the relay, I saw a ceramic that I couldn't find on the schematic. Then again, it is late 500-c so maybe components were added. Just dunno. Can someone verify whether this thing is wired up correctly?? I see evidence of a component being either clipped or added to pin 5 of the 6GK5 tube. R-60 is good to the relay but that .01 ceramic looks to have been added. Likewise an additional 1k resistor is there but it looks to be stock item.

    DSCN0094.JPG


    EDIT: Looking at the pic up close now, it looks like Clippy hit pins 2 and 5 :idea: Maybe the intention was to purposefully cause the always on condition??
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Not. Check the cap pin locations one side of the cap should be to the side of R-58 that is NOT connected to PIN 2 on the 6GK5 and ground. Number is C107. The resistor should be a 10K coming off pin 5 to the RELAY. R60. R58 should come off pin2 and go to the junction that has C-107 and 2 other lines together. One goes to Pin 11 on the Selector switch Pin 11 and the other to power connection "D" which is on pin 6 of the MPX connector row (This is for the Multiplex indicator ). This is based on the 30001-49999 manual schematic.

    I see the 1K going from Pin2 to the terminal strip and it connects to the cap which I presume grounds near the relay. I don't see R60 coming off pin5 to the relay unless it's hidden from view by C-107. This may have been a change in later manuals, but the highest I've ever seen is the 30001-49999 manual for the 500c. You might try the 51001 and up 800c manual and look at that section of the schematic. (I found the 1800 manual / 51001 and up 800c on HIFIENGINE.) See below.

    There is a cap somewhat hidden in the wiring on the left side of the pic. I presume that it may be a 5K pf (005uf) ceramic.) This would correlate with C-101.

    The 800c setup for the 6GK5 is identical EXCEPT that there is no SECTION 5 that I can find. So no Ground thru a 100ohm resistor. Otherwise the caps and resistors are the same even tho it looks different. It's the same. "U1" on the 800c is the same as "D" on the 500c. Both go to pin6 on the MPX terminal.

    DIAGRAM from the 51001 and up (covers the 1800) manual.
    ScreenHunter_164 Dec. 16 20.49.jpg
     
    Last edited: Dec 16, 2017
  11. notdigital

    notdigital AK Subscriber Subscriber

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    Larrry: R-60 is there and it's good. It's not in the pic but it is situated correctly.

    Out of curiosity, I checked through my library of 800-c pics and found this one. Looks like the same components but one of the 1k resistors is hooked up differently. Hmmm......

    DSCN1969.JPG
     

     

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  12. notdigital

    notdigital AK Subscriber Subscriber

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    Just found another pic confirming the 1k was moved:

    [​IMG]
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Not -- The .01 uF and 1K resistors -- as well as the variations on that theme -- are in fact add-ons, but by Fisher in later versions. All an attempt to snub transients from the relay switching on and off. These variations would not be the cause of your always on stereo beacon.

    To trouble shoot the condition:

    1. First, manually push the relay into a pulled in state. The Stereo Beacon should go out. If not, the wiring from the relay to the lamp, or the relay itself is suspect.

    2. Check that there is plate voltage on the Stereo Beacon tube (pin 5). If not, then either the relay or the 10K dropping resistor is open. This will leave the lamp always on.

    3. Without an antenna connected, tune to the left end of the dial, and check the voltage on the grid of Stereo Beacon tube (pin 2). This should be only slightly negative -- much less than the -4 volts required to trigger the Beacon ON. If there is -4 or greater negative voltage at this pin, that is what is keeping the lamp on.

    4. Regardless of the voltage measured at pin 2, momentarily ground pin 1 or 7. The Beacon (if on) should immediately go out. If not, and #2 above checks out, check the 100Ω cathode resistor. If it is good, replace the tube.

    Let us know!

    Dave
     
  14. notdigital

    notdigital AK Subscriber Subscriber

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    Larry/Dave:

    Thanks for your input so let me share where I'm at: The relay is good. When I was first perturbed by the "always on" condition, I thought it was a stuck relay and decided to catch up to it later. Yesterday, as I got back to the unit, I focused on the condition of the relay, its related components, and the voltages involved. I had cleaned the contacts of the relay previously, as well as the tube and sockets to the 6gk5 checking the run of wiring to the everything. All checked out EXCEPT the voltages to the tube which brought me back to the components and their tolerances. All the resistors were spot on which then brought me to the schematic. That's when I realized that the version of receiver I had had components not documented in the paperwork. Ahhh, another sneak in by the factory....

    Before doing anything drastic I manipulated the selector switch to see if I could find anything quirky. Starting from AUX and turning through to PHONO the Beacon Lamp displayed as follows: OFF, OFF, ON, ON, OFF which is correct. Going in the opposite direction starting with PHONO it would be: OFF, OFF, ON, OFF, OFF. That told me the relay was good but once AUTOMATIC was selected, it remained in the normally closed position regardless of where on the dial the unit was tuned.

    When I didn't see anything in either the 500-c or 800-c schematics, I went to the pictues of units I've owned and that's where I discovered that one of the two 1K resistors had been moved in the unit in front of me. I moved it as per the pic above and the relay behaved more normally, i.e. it would switch on and off as I advanced the tuning dial. The only odd thing now is that it behaves normally only in the center third of the channels. The extreme left and right of the dial the Beacon remains lit. At the extremes the reception is powerful so I think I'm content with leaving it as is. I haven't played around long enough to indicate an MPX alignment is in order but we'll see.

    Thanks guys.....

    Now on to the Signal Meter..........
     
  15. notdigital

    notdigital AK Subscriber Subscriber

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    Stole some time today to fire up the 500-C and check into that funky signal meter. Weird: put clips on the meter lead and after an indication on meter between 3 and 4, I watched it drop in jumps till it got to zero, both on the meter and on the DVM. Circuit issue. Left it on to finish up prepping for the holiday only to return a couple hours later to see it back at 3-4. Hmmm..........

    I turned the unit off with the hope of duplicating things and again, watched the meter drop. I cleaned the tube and socket, again, check the resistors and solder joints.......then stared at Z4. Stuck a tool in there and gave it a slight nudge and BAM---meter back up and responding as it should. Luckily, another easy solution to an annoying little problem.

    Gonna take a break on this one for a bit but next up: an IBAM.

    Merry Christmas and Happy New year all!!!!
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Not -- Merry Christmas and Happy New Year back at cha!

    I hate to be a bah humbug, but dollars to donuts Z4 has developed silver mica disease -- either that, or there is a loose connection or intermittent component connected to one of its terminals -- both of which are unlikely. Check for that possibility, but if all is good with the connections, the SM disease is the most likely cause of the intermittent meter operation -- this is exactly how SM disease displays itself.

    Dave
     

     

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  17. notdigital

    notdigital AK Subscriber Subscriber

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    Kinda figured something was wrong with the transformer itself Dave, just didn't know what. It's certainly not a connected component since I've checked all those. I'm loathe to remove the can and check inside. (Although rumor has it that Santa dropped of a Hakko desoldering gun last night :biggrin:) Maybe down the line sometime if it acts up again. I'm hoping something just worked loose in there.

    Anyway, back to getting ready for the arriving mob. All the best!
     
  18. notdigital

    notdigital AK Subscriber Subscriber

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    Had a chance to pull Z4 and couldn't find anything wrong. The only thing I could think of is that there is a small break in the coil itself. It's not readily apparent as there is continuity when cold. Still, I decided to not mess with it and, thanks to rufleruf (Matt) I'll be replacing it altogether. In the meantime, I've put together another IBAM with external test points. The only thing left to do is to have Number One whip up a decal labeling the pots.
     

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  19. notdigital

    notdigital AK Subscriber Subscriber

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    Received the replacement Z4 today thanks to Matt and, voila, problem solved!! THANKS MATT :thumbsup:
     

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