500C tuner L-R channel balance issue

[KelvinMC]

I recently purchased a 500C receiver to use in an old console my father built. I am having an issue when listening to FM. The right channel is not as strong as the left channel. To make it sound correct, I have to turn the balance knob to the right to position 2. Weirdly the balance is fine if I play through the Aux input, so it has to be something in the tuner circuit. Anyone have any thoughts as to what this could be?

Kelvin

 

[rufleruf]

If the tuner sounds good other than the channel imbalance, check out V102, the audio amp for the tuner section according to the 500-C manual. This is on the MPX unit, the tube closest to the back of the receiver. Maybe try swapping the 12ax7 from V16 or V17 into this position? V16 is up by the tuning pulley.

 

[audmod01]

Matt is right, always try the most simple things first.

Joe

 

[dcgillespie]

Kelvin -- Welcome to the AK Fisher Forum! As a first question to help further localize the issue, we need to know -- is the imbalance present in ANY FM selector switch positions? Or is it just when a FM Stereo station is being received (either in the Automatic setting or in the FM Stereo position)? Knowing the answers to these questions can really help to pinpoint where to look for the problem.

Let us know!

Dave

 

[KelvinMC]

So I have just got around to trying a few things, been a busy evening. I think Ihave an early model 500C, it is serial number 10925A.

Fisher 500C drawer

• KelvinMC
• Nov 30, 2017

The MPX section looks like this:

500C MPX early

• KelvinMC
• Nov 30, 2017

No tube numbers. I re-seated all three, but no help. I also checked; FM Auto and FM Stereo both act the same. I am curious about the blue pot, but I didn't want to mess with it until I knew what it did. It is possible I moved one of these pots, I did do a little cleaning when I got it. I have also had the bottom off troubleshooting a phono issue -which turned out to be user ignorance. I was running an MC cartridge.

Kelvin

 

[KelvinMC]

I just swapped all three tubes with V12, V13, and V14. The problem stayed the same. Please tell me the blue pot is the MPX Separation control, cause that sounds a lot like it might adjust the balance?

MPX Separation Control schem

• KelvinMC
• Nov 30, 2017

 

[rufleruf]

That is an early one!

I don't think you want to mess with the blue trim pot unless you have alignment gear.

 

[larryderouin]

Take a picture of the Front of the Sep. Pot. A rough seat of the pants adjustment can be made if you know where the optimum area is.

But we'll get to that in a minute. Does the balance change when the Selector is changed from either of the STEREO POSITION's to FM MONO. Describe in either case.

The separation control in a basic explanation adjusts the amount of STEREO SEPARATION you get. This affects balance a bit in STEREO ONLY. If the control is thrown to one side to the stop you get minimal or no stereo separation. The other stop you get an exagerated sound from one of the channels with a lot of distortion. If the distorted stop is on the right side of the control (full anti-clockwise), you will want the control (the tab on the knob) to be at approx 5:30 to 6:00. If the distorted stop is on the LEFT side of the control (full clockwise) you will want the control (the tab on the knob) to be approx 6:30 to 6:00. This is as close as you can get it without sending to a shop for an alignment.

Get a copy of the Early 500c Service manual. It will have a Range of 10001-19999 on the cover. Owners and service manuals here.....http://www.fisherconsoles.com/service owner manual list.html

The service manual should have a tube layout diagram, including V100,V101,V102. In order LEFT to RIGHT on your MPX Picture.

 

[dcgillespie]

Agreed. Larry is asking the same question I did: We need to know if the imbalance exists in the FM Mono setting. If it does, then the problem for that likely rests in the diode matrix audio switching circuits. In that particular setting, the MPX unit cannot have anything to do with the problem. The MPX unit can only be a (potential) problem if the imbalance exists when the selector switch is set in:

1. The FM Stereo setting, or

2. FM Automatic -- but only when the Stereo Beacon is lit.

When the Stereo Beacon is not lit in the Automatic setting, the unit automatically switches to mono operation, which is electrically the same as turning the selector switch to FM Mono.

Let us know!

Dave

 

[KelvinMC]

So, I checked the FM Mono this morning and it has the issue as well. Also, in an interesting turn of events, as I was listening to the Aux in last night, I got a loud crack in the left speaker and then a 60 cycle hum, the music was still playing, but the hum is there in the left channel even with the volume all the way down.

 

[rufleruf]

Bummer. Any cooked looking capacitors? Both channels?