Fisher 400 low right channel on fm stereo

Here are the photos. Many if not all scopes will have an X-Y mode. This allows you to input one audio channel to the X axis and the other to the Y axis. The signal can be taken anywhere, even at the speaker outputs. Both scope inputs are to be set at the same input sensitivity.

The first image is a mono signal. If the channels were out of phase, the line would slope up to the left. Here we see that the left and right channels always have the same level.

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Next is a stereo signal with low separation. This could be the adjustment or the music source itself.

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Here we have high separation, or Beethovens brain on drugs. You will probably only see this with classical music and a full orchestra. Simply tune the separation control for the best separation visually. But, be aware that, at least on the solid state units, you can go too far and there will be good separation but the channels will be out of phase. That is easy to check. Switch to mono. If the volume drops noticeably, due to cancellation of the signals, then it's probably out of phase.

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Thanks, Fred and Larry. Looks like I need a scope. Any recommendations for a good model? There’s a place called Halted Specialties near me that has a lot of used ones for sale.
 
My Leader LBO-524L was available for the "right" price. Not perfect, not quite a Tektronix, probably could use some calibration, but it works and gets the job done for most things. I did eventually buy new probes because the old ones were getting "goofy".
 
Hey everyone, thought I would check back in with an update. I went through the mix section and replaced all the mylars there, as well as a couple in the phono section that were looking suspicious, and I can say that, while the phono section sounds better than ever, I still have a channel imbalance on the fm. However, it seems to take a minute for the right channel to lose volume.

I’m now looking suspiciously at the multi-section can cap sitting behind the magic eye. It looks like it’s connected between the fm section and the output tubes, and I’m pretty sure my tech didn’t touch it when he was updating caps for the power supply. Any thoughts from the Fisher wizards?
 
Rapscallion said:
Hey everyone, thought I would check back in with an update. I went through the mix section and replaced all the mylars there, as well as a couple in the phono section that were looking suspicious, and I can say that, while the phono section sounds better than ever, I still have a channel imbalance on the fm. However, it seems to take a minute for the right channel to lose volume.

I’m now looking suspiciously at the multi-section can cap sitting behind the magic eye. It looks like it’s connected between the fm section and the output tubes, and I’m pretty sure my tech didn’t touch it when he was updating caps for the power supply. Any thoughts from the Fisher wizards?
Click to expand...
Oops meant mpx section, not mix.
 
When the unit is operating and the channels are unbalanced, measure the plate voltages on V102 (pins 1 and 6). Are they relatively even?

Dave
 
Thanks for your reply Dave. Never measured plate voltages before, do I just measure off each pin separately against the ground?
 
OK, no apparent problems with V102 then. A determination needs to be made as to whether the problem is in the MPX sub chassis, or the main receiver chassis That can easily be done with the following procedure:

1. On the underside of the MPX sub-chassis, locate the vertical mounted tag board that has a coil connected to its bottom most center terminal (furthest from the underside chassis surface).

2. On either side of the identified center terminal, locate a terminal at each bottom most corner of the vertical tag board. These will have single resistor lead and a single wire lead connected to each terminal.

3. When the receiver is playing with unbalanced sound, short these two corner terminals together.

4. If the sound evens out, then the problem is in the MPX sub-chassis. If the sound does not even out, then the problem is in the main receiver chassis.

Let us know!

Dave
 
Test has been conducted, and the sound evened out. So, now, what next? I’ve replaced all the mylars within the mpx already with no change, and all 3 12ax7 tubes have tested out 65-70%.
 
Did you replace the 1 uF electrolytic cap in the MPX sub-chassis? If not, then by now it is typically bad, andt will cause all manner of weird problems when it goes south......

Dave
 
The blue control on the MPX chassis is the separation control -- is it set within a range of from about 4:30 to 7:30 on the clock face? If so, then don't touch it. If it is well off from this general range, then bring the control to within this range as it represents the normal setting of the control when the unit is operating properly. Generally however, the separation control won't control the individual sound level of either channel, although it can affect the overall sound level of both channels at the same time. If the control is well out of its normal range, that usually indicates that it has been accidentally moved while carrying the unit, or that someone tried to fiddle with the adjustments if the MPX sub-chassis was determined to not be operating correctly. Does the unit operate properly in all aspects when set to FM Mono? (good sensitivity, selectivity, non-fussy tuning, proper eye tube action, clear un-distorted sound, etc.)

Dave
 
Yes, the blue pot has been set to the middle zone all along, and fm sounds pretty good in fm mono, although it could pick up stations a bit better. It’s located in an inside wall, so reception isn’t ideal.
 
Residential walls normally aren't much of a barrier for FM. My lab is underground with garden variety dipole antennas mounted up at ceiling level (which is to say ground level) and I routinely listen to stations that are up to 100 miles away. Mono FM is static free with these stations, while FM MPX Stereo has noise, but is very listenable. Much has to do with how a station propagates its signal. But you can use this as a crude guide.

If the FM tuner portion proper has issues, it can definitely cause the MPX sub-chassis to not operate properly -- but not in such a way that one channel will play louder than the other. That kind of problem can only exist either in the main chassis audio circuits, or the MPX sub-chassis itself. The previous test proved that the imbalance was originating in the MPX sub-chassis. Since you've replaced all the usual suspects on that chassis, then either the MPX unit is out of alignment (but rare that that would cause a channel imbalance), or some other component has failed on the MPX chassis. Either way then, it would seem that your MPX unit requires service. If you send the receiver to someone local, make sure that they have the equipment to properly test, diagnose, and align the unit -- and that they have a proper understanding for how FM MPX Stereo operates. Many do not. Unless you've got the equipment and/or skills to address the issue further yourself, it would seem that you will need to seek a tech's services, as you've gone about as far as most diyers can go with a problem like this in attempting to resolve it yourself.

Dave
 
Thanks so much for your help chasing this, Dave. At least I know I’ve taken it as far as I can on my own. I guess I’ll stick to internet radio until I can bring this to a tech that has a deep understanding of mpx on these old units.
 
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