Adventures With A 500-C

notdigital · Oct 22, 2013

Several months ago I picked up a non-working 500-c from a fellow AKer. I recently decided to un-lazy myself and start the restoration.

All the tube sockets have been cleaned as have the switches, etc. The tubes have been cleaned and tested and except for the 6HA5 and nuvistors, all are functioning. There were a few poor solder joints, a couple of out-of-spec resistors, and several remaining caps that needed to be changed (including the power caps.) I powered it up and happily, the AUX and headphone jack are in working order.

When a CD player is hooked up, music is loud and clear in both channels; the balance control shifts left/right without a problem. Likewise, the headphone operates without issue. The problem lies in the FM:

When the selector is turned to radio, whether it be MONO or STEREO, audio is weak and tinny in both channels with one channel being clearly stronger. When the balance control is turned to the extreme of the weak channel, there is an obvious buzz.

Originally, one of the nuvistors was dead, the other nearly so, and the 6HA5 was shorted. I put new tubes in all three positions and the FM still sucked. Replacing the 12ax7's or swapping them around had no positive effect. Likewise, moving the MPX tubes around made no difference. (When I purchased the unit, I believe the previous owner removed ALL the tubes; I'm not certain if any care was given to replacing the MPX tubes to their original positions.) Finally, there is a loud hum whenever I come close to a ceramic cap near the balance control. When I turn the control, it kills the hum. Release the control, it returns. The hum is most obvious when the balance is turned to the extreme weak channel. (Sorry, I have the thing flipped on it back and front facing away so I can't report whether it is the left channel or right. I'm at work not working but I'll identify which channel is which later tonight.)

I've been staring at this thing now for 3 days with no idea on how to proceed with the diagnosis. There are a few pictures that I'd like to share and I'll post them some time tonight. I'll probably give my brain a break for a day or so before I begin the attack again. In the meantime, I'd appreciate suggestions.

BTW: I do believe an alignment will be in order. However, I don't think that will account for the imbalance in channel strength.

dcgillespie · Oct 22, 2013

Not -- it will be helpful if you can identify if:

1. In mono mode, is this a good, solid FM signal whose audio output is being poorly coupled into the audio circuits? (that is, no static, good quieting, but poor audio level and quality?) Or --

2. Is this weak audio accompanied with much static as would be the case with poor/weak reception?

The first scenario implies problems with the FM auto switching circuits, while the second implies problems with the tuner.

Some may find Fisher multiplex units sensitive to different tubes relative to the sound produced, but they are not sensitive to different tubes relative to proper stereo decoding, as long as the tubes are good of course.

Dave

notdigital · Oct 22, 2013

Dave: I'd say it's the party of the first part! The signal indicator is ALMOST at 5 but the volume level is way low. I have to crank it up to full volume to get it to normal listening levels. And that's just the "good " channel. The weak channel is very, very weak even at the highest volume level.

The sound is tinny and shallow. no bass to speak of. The only real distortion is when I turn the balance to the extreme left (or right, I forget which) when the buzz becomes obvious.. I tap around the balance control and it snap, crackles and pops but not all the time. When I touch the balance shaft, it kills the buzz. Take my finger off, and you can hear the buzz. I thought I had isolated it to a square ceramic capacitor just behind the balance control because when I touched it with a pencil, it squealed like a stuck pig. Move away and the buzz stopped However, like I said, it doesn't happen all the time. It buzzes but not stuck pig squealing level.

The tinny quality as well as the imbalance exists in both MONO and STEREO. There is no unusual static. Of course there is some (I am in my basement) but I can get to a good clear channel when I turn the tuner.

Mind you: the AUX and headphones are working fine; the balance control shifts left and right perfectly.

:scratch2: :scratch2: :scratch2:

When I get home I'll put up some pics.

dcgillespie · Oct 22, 2013

Not -- If the balance control works properly for your phono and high level Aux inputs, then it is not the problem with your concerns for FM.

In particular, take pics of the selector switch, and the Auto FM switching circuits, as the problem will almost surely reside in one of these areas.

Dave

notdigital · Oct 22, 2013

Here are some pics that I had already taken. Two are of the left and right phase inverter adjustment pots, one is of the antenna coil (L1 I believe.)

The inverter adjust pots obviously have had a bad time weathering the heat as they are both melted; the antenna coil is clearly damaged.

I'll get more pics up later tonight.....

notdigital · Oct 22, 2013

A few more pics.....

notdigital · Oct 22, 2013

And more....

dcgillespie · Oct 22, 2013

This could likely be a power problem to the auto FM Stereo switching matrix board, as without any power, the is no voltage to bias the diodes into conduction, so no signal passes through them. May I assume that the Stereo Beacon is operating correctly? Also, can you get some good shots of the matrix board behind the stereo beacon relay?

Dave

notdigital · Oct 23, 2013

dcgillespie said:
This could likely be a power problem to the auto FM Stereo switching matrix board, as without any power, the is no voltage to bias the diodes into conduction, so no signal passes through them. May I assume that the Stereo Beacon is operating correctly? Also, can you get some good shots of the matrix board behind the stereo beacon relay?

Dave

"FM Stereo switching matrix board" That would be the board adjacent to the 1uf cap and which the 1800 cap is connected in pic #1??

"May I assume that the Stereo Beacon is operating correctly?" Yup.

I'll get up some shots tonight. I checked that board for mechanical connection faults; it looks good.

Thanks Dave :thmbsp:

dcgillespie · Oct 23, 2013

You got it -- both sides if you can!

Dave

notdigital · Oct 23, 2013

Dave: here ya go!

dcgillespie · Oct 23, 2013

Hi Not -- When you can get a chance, if you are comfortable with it, use your dvm to measure the DC voltage at the point where the 470K resistor connects to the front of the matrix board (black meter lead to ground). Use a 200 volt scale. Check when:

1. A stereo station is turned in (Stereo Beacon on), and

2. When forced mono operation is selected.

Of course, be careful, and let us know!

Dave

notdigital · Oct 24, 2013

dcgillespie said:
Hi Not -- When you can get a chance, if you are comfortable with it, use your dvm to measure the DC voltage at the point where the 470K YVYG resistor connects to the front of the matrix board (black meter lead to ground). Use a 200 volt scale. Check when:

1. A stereo station is turned in (Stereo Beacon on): 134.2

2. When forced mono operation is selected: 9.22

Of course, be careful, and let us know!

Dave

Here ya go.

notdigital · Oct 24, 2013

Hmmmmm......

Poking around tonight checking for loose wires, spotted this little puppy in the MPX: C-204 470pF, connected to pin 1 of V-100. One end is blackish. Is it toast and could that be the problem?

dcgillespie · Oct 24, 2013

No -- that would only affect FM stereo, where as you are having a problem getting audio out of FM mono as well.

Your voltages imply that the matrix board is likely working correctly. Time to check the wiring between that board and the selector switch. Also check that the contacts on the selector switch for the FM positions are in fact all making a complete circuit when turned to those positions.

Dave

notdigital · Oct 25, 2013

Went back at the selector switch with a spot cleaning as well as checking the contacts. All good, same difference. Traced most of the wiring and everything seems to be in order so far. I'll get back to finishing the wiring check tonight and/or tomorrow to see if anything pops up.

Assuming the wiring and selector check out, where should I poke around next? Can I assume the tubes are not the issue since AUX works fine? And the nuvistors and 6HA5 are new??

Anyway, thanks for helping me work through this. It is much appreciated :thmbsp:

notdigital · Oct 26, 2013

dcgillespie said:
No -- that would only affect FM stereo, where as you are having a problem getting audio out of FM mono as well.

Your voltages imply that the matrix board is likely working correctly. Time to check the wiring between that board and the selector switch. Also check that the contacts on the selector switch for the FM positions are in fact all making a complete circuit when turned to those positions.

Dave

Traced ALL wires coming off selector to their destination: all check out. Checked integrity of contacts as well: all check out.

:scratch2: :scratch2: :scratch2: :scratch2: :scratch2: :scratch2: :scratch2: ??

notdigital · Oct 26, 2013

OK, still at it and still at a loss. :tears:

After figuring out that V-14 V-15 snafu, I'm back at that part that I now identify as PC5. Whenever I come close to it, it buzzes loudly. None of the other PC's behave that way. If I touch one of its leads, it squeals deeply. If the part is indeed defective, would it account for the problem I am having??

dcgillespie · Oct 26, 2013

Nope. PC5 would be very sensitive if you bring your finger close to it. But normally, with the bottom cover on, that won't be a problem!

OK. Time to turn your Fisher into its own signal tracer. Are you good with a schematic?

Dave

notdigital · Oct 26, 2013

dcgillespie said:
OK. Time to turn your Fisher into its own signal tracer. Are you good with a schematic?

Dave

I do pretty good at throwing it across the room. Other than that, I struggle but eventually get there, I guess. Don't have much of a choice now. Let's give it a shot.

Lay it on me. If I get stuck, you'll be the first to know, I promise

(In-laws are coming in tomorrow so probably will take a break till Monday/Tuesday.)

Adventures With A 500-C

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