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Adventures With A 500-C

"Last Chance Power Drive"

I've come to the home stretch on my 500C restore with just a couple of replacement parts left to be done and simple cosmetic issues. One major item remains however, and that is a buzz in the right channel.

When I received this unit it was non-operational. After sorting through some dead caps, mis-wires, shorts, etc. I got it going EXCEPT for a nagging staccato buzz, mostly in the right channel. It is not there when in AUX, (or AUX 2, tanx D :D), or in FM mono. It only presents itself when the STEREO BEACON light is on. So, when "FM STEREO" is selected, it's there; when FM AUTOMATIC" is selected, it jumps in when STEREO BEACON kicks in.

I've swapped power tubes as well as signal tubes: same. Sockets cleaned, checked for cold joints, shorted speakers: same.

Now when I purchased this unit I know that the previous owner had pulled ALL the tubes. It is not certain that when he received the unit it came with tubes, and/or if, when he replaced the tubes, he put the MPX tubes and/or the 6AU6's back in their original places. I'm thinking, therefore, it is an alignment issue in which case I am out of my league. I just want to rule out anything else before I send it off to techland.

Any ideas?????? :scratch2:
 
No problems on moving the 6AU6s around in the FM section of any late model Fisher FM Stereo receiver.

As for the buzz, it is most likely coming from the MPX unit, and could very likely be alignment related. Here is a simple test you can try: With an FM stereo station tuned in and the stereo indicator on, remove the center tube on the MPX board (38 kHz oscillator). The sound will immediately go to mono (no stereo decoding), but if the buzz goes away with that tube removed, then the problem is almost certainly alignment related with the MPX board.

Let us know!

Dave
 
Well, it's been a while since I posted on this thread.....

Last month I finally came back to this unit. I decided to drag it over to a local AK member to check out the alignment. During the process he contacted me and reminded me of the distortion I was having, mostly in the right channel, and that it probably didn't have anything to do with the alignment. He was able to align the full sweep of channels from the low end through the high end, WBLS 107.5 here in NYC. Still, that staccato buzz remained....

Yesterday I popped it back on the bench to begin an analysis. I checked the diodes on the matrix board and the remaining diodes associated with the MPX. All good. Likewise the resistors checked out within spec. Then I noticed that C-214 had been replaced. Like several other botched replacements in this unit, it had simply been "tack welded" onto terminals 1 and 7 of V-101. Never spotted it before so I took a closer look: what a crappy job. I mean, I'm not perfect but I always believed attention to detail is important and, in order to avoid having to do something again, why not do it right the first time? Anyway, in the process of sucking away the globs of solder on the aforementioned terminals, something told me to check the schematic against the component installed. What did I find??? Well, I'm not the brightest bulb and I sure ain't a math whiz BUT something told me .0047uf doesn't equal .47uf!!!

I managed to find and install a proper valued mylar in the can of junked components I keep on the bench and: no more distortion! After I install the silver mica I have coming, this puppy will finally be ready to deliver nothing but :music:

Special thanks to Tom B who straightened out the screwy alignment issues :thmbsp:
 
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