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Fisher 500C channel output variance

davstev

davstev

Well-Known Member
My new (to me) Fisher 500C, which I love, has an annoying issue in that it outputs very different levels on the right and left channels. Same is true when listening by headphones. I need to play with the balance knob to compensate. It sounds quite even/balanced when the balance knob is at around the 10 o'clock position. This is really a problem for stereo recordings.

Any idea what might be the issue here? How to troubleshoot? I assume, maybe incorrectly, that the tubes might be the reason. Which tubes I cannot say. I think they are all good tubes, but perhaps some are getting tired. I don't have a tube tester.

Thanks for considering.
 
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Welcome to AK. Sounds like you have dirty controls is all and you need to clean them. A common issue with all things 50 years old but not hard to do at all. There's a good thread at the top of this forum on how to do it. Try that first and if that doesn't solve your problem there is tons of good help here. Good luck with it! Al
 
AlTinkster92 said:
Welcome to AK. Sounds like you have dirty controls is all and you need to clean them. A common issue with all things 50 years old but not hard to do at all. There's a good thread at the top of this forum on how to do it. Try that first and if that doesn't solve your problem there is tons of good help here. Good luck with it! Al
Click to expand...
Been on AK for awhile, but thanks for the greeting nonetheless! ;)

Dirty controls? You mean dirty potentiometers? I don't think (though could be wrong) that is the issue. None of the pots crackle at all when turned.
 
The fact that the pots don't crackle doesn't mean that your issue isn't dirty pot related. I too would start simple and go from there. Cleaning the pots and switches eliminates them as possible culprits (of which there are many.)

I had an imbalance issue that was related to a slider switch. Took me a while to figure it out. In this thread Dave instructs on how to build a simple signal tracing tool. Pretty slick and helpful in diagnosing the problem:

http://audiokarma.org/forums/index.php?threads/adventures-with-a-500-c.551206/
 
Thanks, Guys! I will follow this path for awhile. It is a fine amp, I just need to resolve this problem.
 
AlTinkster92 said:
Welcome to AK. Sounds like you have dirty controls is all and you need to clean them. A common issue with all things 50 years old but not hard to do at all. There's a good thread at the top of this forum on how to do it. Try that first and if that doesn't solve your problem there is tons of good help here. Good luck with it! Al
Click to expand...

I cleaned the pots with DEOXIT and it made no difference.

I might now try swapping tubes around to see if that makes a difference.
 
Perhaps its the volume pot not tracking linearly

Measure the ohms on each side (left/right) of the pot at 0:00 o'clock, 1:00 o'clock etc and see if they track close or if one (the bad side) measures out of tolerance
 
Also make sure the slide switches on the front are cleaned too. Signal goes thru all of them. AND multiple cleanings may be necessary with older gear.
 
larryderouin said:
Also make sure the slide switches on the front are cleaned too. Signal goes thru all of them. AND multiple cleanings may be necessary with older gear.
Click to expand...

A good suggestion, thanks. I did, however, clean the switches. I assume you are talking about the hi filter, low filter, etc., the four vertical switches on the front, two on either side of the headphone jack?
 
I am confused. I don't know how to take a reading for measuring the ohms/output on each side (L/R Channel) coming from the volume pot itself. I have a DMM but I don't know where to begin. Maybe I will look at the schematic to try to understand, but if anyone has any guidance...

See the attached pic. The volume pot is divided into three sections.

Thanks.
 

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Don't discount the possibility that the switches and pots may still be dirty even after 1 or 3 cleanings. I've had switches so bad after 4 cleanings with DeOxit, that they had to be opened up and manually cleaned with brushes and surgical picks. An especially the open type switches and pots that FISHER and everyone else used in the 50's and 60's. Multiple cleanings may be needed.

In your pic you have 3 sections which you denoted as #1, #2, and #3. #1 and #2 are the Left and Right Channel pots, #3 is the On-Off Switch. Right now we're concerned with #1 & #2.

UNPLUG the unit and bag & tape closed the plug end (this keeps you from inadvertantly inserting the plug in the wall socket and getting Zapped!

Switch the DMM on and move the selector to ohms (omega sign). Touch the probes together and write down the reading. This is your baseline for the meter.

THIS IS A POWER OFF AND UNPLUGGED TEST! (Bold, underline and caps to get your attention so you don't get zapped!)

On Sections 1 and 2 you'll see 3 connections. LEFT to RIGHT A-B-C. A and C are connected together with a piece of resistance wire or a variable resistance film that is approx 500,000 ohms (500Kohms). B is what's called a Wiper and is connected to the knob shaft. As you turn the shaft, the wiper touches different parts of the resistance wire and depending on which way you go and where you start the resistance can be as high as the A-C reading and as low as the baseline reading. What you are looking for is approx the same reading from the A-B connections at the same place on both #1 and #2 sections.

With the switch just turned on,(knob pointing to 8:00) take a reading on #1 A-B. It doesn't matter which probe goes on which connection. Write this down in oHms.
Without moving the knob, repeat the reading on #2 A-B. Write it down.
Turn the knob to 12:00 position. Repeat the readings again for #1 A-B and #2A-B. Write down the readings.
Turn the knob to full on (5:00) and repeat the readings again.
Now do another baseline reading on your meter. write it down.

Initial Baseline reading.

8:00 reading #1 A-B (your reading here) #2A-B (your reading here)
12:00reading #1A-B (your reading here) #2A-B (your reading here)
5:00 reading #1A-B (your reading here) #2A-B (your reading here)

Don't worry about taking readings from B-C as they will be the direct opposite of the A-B readings.
 
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larryderouin said:
Don't discount the possibility that the switches and pots may still be dirty even after 1 or 3 cleanings. I've had switches so bad after 4 cleanings with DeOxit, that they had to be opened up and manually cleaned with brushes and surgical picks. An especially the open type switches and pots that FISHER and everyone else used in the 50's and 60's. Multiple cleanings may be needed.

In your pic you have 3 sections which you denoted as #1, #2, and #3. #1 and #2 are the Left and Right Channel pots, #3 is the On-Off Switch. Right now we're concerned with #1 & #2.

UNPLUG the unit and bag & tape closed the plug end (this keeps you from inadvertantly inserting the plug in the wall socket and getting Zapped!

Switch the DMM on and move the selector to ohms (omega sign). Touch the probes together and write down the reading. This is your baseline for the meter.

THIS IS A POWER OFF AND UNPLUGGED TEST! (Bold, underline and caps to get your attention so you don't get zapped!)

On Sections 1 and 2 you'll see 3 connections. A-B-C. A and C are connected together with a piece of resistance wire that is approx 500,000 ohms (500Kohms). B is what's called a Wiper and is connected to the knob shaft. As you turn the shaft, the wiper touches different parts of the resistance wire and depending on which way you go and where you start the resistance can be as high as the A-C reading and as low as the baseline reading. What you are looking for is approx the same reading from the A-B connections at the same place on both #1 and #2 sections.

With the switch just turned on,(knob pointing to 8:00) take a reading on #1 A-B. It doesn't matter which probe goes on which connection. Write this down in oHms.
Without moving the knob, repeat the reading on #2 A-B. Write it down.
Turn the knob to 12:00 position. Repeat the readings again for #1 A-B and #2A-B. Write down the readings.
Turn the knob to full on (5:00) and repeat the readings again.
Now do another baseline reading on your meter. write it down.

Initial Baseline reading.

8:00 reading #1 A-B (your reading here) #2A-B (your reading here)
12:00reading #1A-B (your reading here) #2A-B (your reading here)
5:00 reading #1A-B (your reading here) #2A-B (your reading here)

Don't worry about taking readings from B-C as they will be the direct opposite of the A-B readings.
Click to expand...

Larry, this is incredibly helpful! Thanks for taking the time. I will get back to you with the results! Cheers.
 
larryderouin said:
Don't discount the possibility that the switches and pots may still be dirty even after 1 or 3 cleanings. I've had switches so bad after 4 cleanings with DeOxit, that they had to be opened up and manually cleaned with brushes and surgical picks. An especially the open type switches and pots that FISHER and everyone else used in the 50's and 60's. Multiple cleanings may be needed.

In your pic you have 3 sections which you denoted as #1, #2, and #3. #1 and #2 are the Left and Right Channel pots, #3 is the On-Off Switch. Right now we're concerned with #1 & #2.

UNPLUG the unit and bag & tape closed the plug end (this keeps you from inadvertantly inserting the plug in the wall socket and getting Zapped!

Switch the DMM on and move the selector to ohms (omega sign). Touch the probes together and write down the reading. This is your baseline for the meter.

THIS IS A POWER OFF AND UNPLUGGED TEST! (Bold, underline and caps to get your attention so you don't get zapped!)

On Sections 1 and 2 you'll see 3 connections. LEFT to RIGHT A-B-C. A and C are connected together with a piece of resistance wire or a variable resistance film that is approx 500,000 ohms (500Kohms). B is what's called a Wiper and is connected to the knob shaft. As you turn the shaft, the wiper touches different parts of the resistance wire and depending on which way you go and where you start the resistance can be as high as the A-C reading and as low as the baseline reading. What you are looking for is approx the same reading from the A-B connections at the same place on both #1 and #2 sections.

With the switch just turned on,(knob pointing to 8:00) take a reading on #1 A-B. It doesn't matter which probe goes on which connection. Write this down in oHms.
Without moving the knob, repeat the reading on #2 A-B. Write it down.
Turn the knob to 12:00 position. Repeat the readings again for #1 A-B and #2A-B. Write down the readings.
Turn the knob to full on (5:00) and repeat the readings again.
Now do another baseline reading on your meter. write it down.

Initial Baseline reading.

8:00 reading #1 A-B (your reading here) #2A-B (your reading here)
12:00reading #1A-B (your reading here) #2A-B (your reading here)
5:00 reading #1A-B (your reading here) #2A-B (your reading here)

Don't worry about taking readings from B-C as they will be the direct opposite of the A-B readings.
Click to expand...

Here's what I found (and also found quite confusing)

8:00 reading #1 A-B (116 kilohms) #2A-B (138 kilohms)
12:00reading #1A-B (126.5 kilohms) #2A-B (143 kilohms)
5:00 reading #1A-B (2.3 OHMS) #2A-B (1.55 OHMS)

Note that the final readings with the volume pot wide open seem to be lower than the other readings.

I am far from being an electronics whiz but I found this confusing. As for the discrepancy between the different L and R channel outputs (corresponding to pots #1 and #2), I see some discrepancy, and am wondering if this is the issue.
 
When the Volume pot is at the minimum spot you have the most resistance (hence lowest volume). And with it wide open there is almost no resistance and you get pretty much everything the pre-amp can give to the amp(hence full volume).

The differentials aren't too bad. I've seen worse with no discernable difference in hearing the channels. there will be a slight difference on a scope, but probably not 3db or more worth.
 
Thanks for clarifying. That makes sense now.

However, I found it strange that at the final 5:00 reading, pot #1 became the bigger number and pot #2 became the smaller number. The pattern that was established previously was that pot #2 had higher values, but this was reversed at 5:00. Double checked the reading to be sure, and I got the same values.

In any case, I think my next step is to switch the tubes in the audio path that correspond to the left and right channels. Maybe one of the 12ax7's is off. If I switch them, and if there is a bad tube in there somewhere, maybe the weaker channel will correspondingly become the right channel, not the left as it is now. Does this seem like sound logic and troubleshooting for someone with no tube tester?

Again, you might be right about cleaning the pots again. Think I'll do that too....but my hunch tells me that's not the issue. I gave it a real soaking with deoxit the first time around, and there was zero improvement.
 
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Hey, Good news! I think that maybe, just maybe, a second heavy round of Deoxit on the pots and switches may have done the job. It is late, I'm tired, but I will give it a listen tomorrow to confirm that my tired ears are hearing an improvement in the balance. But it seemed that things were better. Thanks for the insistence on this possible problem.

I've never used faderlube before. Do you think that maybe it's a good idea after deoxiting?
 
Faderlube has some lube in it, but it's formulated more for sliders rather than pots and switches. DeOxit GOLD, or CRC 6-26 electrical lubricant is better. The CRC is cheaper and is all lube. DeOxit Gold is part lube and protectant. But it's as expensive as DeOxit D-5.
 
larryderouin said:
Faderlube has some lube in it, but it's formulated more for sliders rather than pots and switches. DeOxit GOLD, or CRC 6-26 electrical lubricant is better. The CRC is cheaper and is all lube. DeOxit Gold is part lube and protectant. But it's as expensive as DeOxit D-5.
Click to expand...

Is there a risk in not lubing at all? Just D-5'ing and then using it as is?
 
When I De-Oxit pots I "360" the whole receiver in order to get the juice everywhere. Upside down, sideways leaning front, leaning back
you get the idea

At least now you know how to check the pots for linear performance

I hope that fixes it

I believe the preferred method is D5 followed by Faderlube

There are threads that detail completely rebuilding the pot
 
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