Fisher 400 FM issue

RS Steve

RS Steve

Tube Junkie
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I was able to bring this 400 back from the dead, basically fully restored except for the big can caps which I will do last. I believe something is up with the MPX section, all seems well but as the unit is on longer, the signal strength goes lower and sound gets somewhat distorted. When in FM mono the signal is steady and strong, no difference occurs no matter how long it's on. I have swapped tubes, replaced the three film capacitors and one lictic on the MPX unit that are usually replaced. It makes no difference; the FM stereo loses signal over time. Any suggestions on what to look for next?
 
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Hey, Steve. Having been through a few of these MPX-65 MPX units, it sounds like the tuner and/or the MPX decoder itself may be out of alignment. The tuner IF, particularly the limiters and detector, need to be in good alignment to have good stereo operation, including noise and separation. You might try this first to see if stereo reception improves--if the tuner is out of alignment now, there's a good chance an alignment will improve things.

It's not unusual for the volume level to be slightly different between FM mono and FM stereo and for there to be a very slight amount of noise (hiss) in stereo mode (from the decoder operation), but neither should be pronounced. Certainly you should hear no perceivable distortion.

If aligning the tuner doesn't solve it, the MPX-65 itself may need to be aligned and this requires an FM stereo multiplex generator (like the Fisher 300, or the Sencore SG-80 for example) to complete. The instructions for both alignments are in the Fisher 400 service manual.

Dave Gillespie (of course!) is the expert on these matters. This link (and some of the work in other AK threads by Dave) may help:

Fisher 400 low right channel on fm stereo

Hi everyone. Recently picked up a 400, had the basic power supply work done. Sounds great on aux and phono and fm mono. But on fm stereo, the right channel volume is about 1/2 of the left. I’ve tried swapping around the 12ax7 tubes but no difference. Any ideas? Thanks in advance. Brent
audiokarma.org

Dave
 
Dave451 said:
Hey, Steve. Having been through a few of these MPX-65 MPX units, it sounds like the tuner and/or the MPX decoder itself may be out of alignment. The tuner IF, particularly the limiters and detector, need to be in good alignment to have good stereo operation, including noise and separation. You might try this first to see if stereo reception improves--if the tuner is out of alignment now, there's a good chance an alignment will improve things.

It's not unusual for the volume level to be slightly different between FM mono and FM stereo and for there to be a very slight amount of noise (hiss) in stereo mode (from the decoder operation), but neither should be pronounced. Certainly you should hear no perceivable distortion.

If aligning the tuner doesn't solve it, the MPX-65 itself may need to be aligned and this requires an FM stereo multiplex generator (like the Fisher 300, or the Sencore SG-80 for example) to complete. The instructions for both alignments are in the Fisher 400 service manual.

Dave Gillespie (of course!) is the expert on these matters. This link (and some of the work in other AK threads by Dave) may help:

Fisher 400 low right channel on fm stereo

Hi everyone. Recently picked up a 400, had the basic power supply work done. Sounds great on aux and phono and fm mono. But on fm stereo, the right channel volume is about 1/2 of the left. I’ve tried swapping around the 12ax7 tubes but no difference. Any ideas? Thanks in advance. Brent
audiokarma.org

Dave
Click to expand...
Yep, I pretty much know all of that. Tuner in mono works exceptionally well, but when in stereo the signal starts good, sounds clear, but goes lower and seems to distort as the receiver is left on for longer. Pretty sure it's the MPX unit. Having it start off good seems to point to maybe a flakey resistor losing value.
 
Also a sign of a weak tube (s), but you mentioned that you tried different. I have also seen a tuner that works well in mono but was not well-enough aligned for the MPX unit. How long does it take before the stereo goodness starts fading?

Also, does the stereo signal quality improve it you re-tune the tuner?
 
Another theory: you mentioned that you haven't changed out the can caps. If one or more of the sections supplying B+ to the MPX unit are starting to fail, the supply voltages may start to fall as the set (and maybe the caps) warm up, causing the voltage to fall off and affecting functioning of the decoder and IF. You may want to watch the plate voltages in the MPX-65 after switching on to see if that's happening. Just a thought....
 
Dave451 said:
Also a sign of a weak tube (s), but you mentioned that you tried different. I have also seen a tuner that works well in mono but was not well-enough aligned for the MPX unit. How long does it take before the stereo goodness starts fading?

Also, does the stereo signal quality improve it you re-tune the tuner?
Click to expand...
It takes 10 minutes or so for the signal seems to fade on the signal strength tube, and the sound seems a little distorted. The signal does not improve trying to tune it better like maybe it drifted some. Can voltages seem fine, and like I said, in mono the FM stays strong and doesn't change.
 
dcgillespie said:
If the can caps serving the MPX section — particularly the one serving the local 38 kHz oscillator (V101), it will cause all manner of weird problems.

Dave
Click to expand...
Thanks for chiming in Dave, looks like I'll do the cans and see if the problem persists.
 
So after installing the new HH cans, the issue still persisted. Turned out a total cleaning of the MPX tube sockets cured the issue. :crazy: I also added the 47K resister to stop the eye tube bounce. The two bars actually touch together on very strong stations now.

Now on to the last issue with this receiver. When turning off the receiver, sound is still present in the Right speaker briefly as the caps draw down. Would this be an issue with the volume control? Maybe contaminants in the pot when in the off position making contact? I have used Deoxit on it a couple times with no change, maybe compressed air might help. Any suggests on what would cause this will be appreciated.

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Steve, I have probably 10 or so tube amps and integrated amps laying about here. Most of them behave exactly as you described so I think it’s normal. I’m puzzled why you are only hearing briefly one channel after shutting down. In fact, I’m just now recalling my early childhood and our home tube radios, etc. did the same thing.
 
If the volume goes to zero right before deactivating the power switch, but comes back when the switch is deactivated, then the only thing it can be is the volume control.

Dave
 
dcgillespie said:
If the volume goes to zero right before deactivating the power switch, but comes back when the switch is deactivated, then the only thing it can be is the volume control.

Dave
Click to expand...
I believe when at zero before deactivation, I can hear just a tiny bit of sound in the left channel. After deactivation it switches to the right side a bit louder until bleed off of a second or two. I will verify this when back home from work. Thanks Dave!
 
RS Steve said:
I also added the 47K resister to stop the eye tube bounce. The two bars actually touch together on very strong stations now.
Click to expand...

Is there a thread that talks about adding the 47k resistor? Surprised I never saw that when I was searching for info for my 400.
 
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joebass3 said:
Is there a thread that talks about adding the 47k resistor? Surprised I never saw that when I was searching for info for my 400.
Click to expand...

Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

All of the transmitters in the Wash/Balt/Annapolis area are at least 25-30 miles from me and I'm on a peninsula oriented NW-SE. Towers are NW, N and SW of me, with my mostly listened to stations 45-60 miles away. The area I'm in is at sea level (
audiokarma.org
 
So after another Deoxit flush on the volume pots, I blew them out with compressed air. Now when the volume is turned all the way down, I don't hear anything in either channel (no bleed through), but when turning off I still hear sound out of the right side as the caps bleed down. It did seem to be less prevalent though. I'm thinking there might be some debris stuck in that pot, I guess. :dunno:
 
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