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Fisher 500-T MPX Decoder Issue

mrphilco

mrphilco

Well-Known Member
I have a Fisher 500-T that I have partially recapped.

It is playing well...except for the fact that the left channel is very low compared to the right.

Using my signal generator producing a 1000 cycle modulated stereo signal, I traced the audio in both channels with an oscilloscope back to the output of the MPX decoder where the left channel audio is much lower than the right channel.

I've replaced the 0.22 uF tantalum cap that couples audio from the chopper to the output. No dice.

Oh, and other than the unbalanced outputs, the MPX decoder seems to be working - at least, it lights the Stereo Beacon on an FM Stereo signal.

Any suggestions on how to proceed from here? Am I correct in thinking that the problem must be in the chopper? And if so, any suggestions on how to repair short of finding another 500-T with the same chopper?

Thanks.
 
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Ron -- If you will, post a schematic. Short of that, and making (obviously) some assumptions -- one being that the decoder employs diode ring detectors rather than a chopper -- then the diode ring detectors are likely good, as a failure of the left channel detector ring would simply leave the left channel as producing a mono L+R signal, which would be similar in amplitude to a L only signal. As a result, I would be looking at the output of the matrix circuit (or summing circuit, whichever is employed) for the loss of signal -- possibly at whatever network is being used as a 38 kHz filter at that point. Short of this, I'd need a schematic.

Good luck with it!

Dave
 
Hi Dave

Thank you for the reply. You can see a schematic of the 500-T's multiplex decoder here:

http://www.philcoradio.com/images/500t_mpx.jpg (1 MB)

Audio in the right channel is nice and loud. Audio in the left channel is audible, but very quiet in comparison to the right.

Thanks again...I look forward to your reply...I appreciate the help.
 
First, we need to determine if the problem is the MPX decoder or in the audio path afterwards. It is possible that something after the MPX output is pulling the signal down. Swap the wires on the MPX board at terminals 4H and 4K. If the left channel is still defective, then the problem is after the MPX. If it moves to the other channel, then we are in the MPX board.

The stereo light only indicates that a 19 kHz signal is being received. If there is misalignment at Z403, the light could still be on but with a channel mismatch.
 
Here is a copy of a service bulletin for replacing the chopper with discrete parts AFTER the chopper became NLA shortly after production stopped. This was for the 700-T but the 500-T used the same chopper.

700-t%20chopper_zpslatielwh.jpg
 
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Many thanks for the replies, Fred and Larry! I've been out of town all weekend. Fred, I will try switching the L and R wires at 4H and 4K, and will get back with you here later today. And Larry, I really appreciate the drawing - great info!
 
OK Fred

I switched the left and right outputs (4H and 4K) at the multiplex decoder board. Now the left channel is loud and the right channel is soft.
 
The next stop then would be to check C423 if it is used on your model (the asterisk denotes it is not used on all models).

Let us know!

Dave
 
OK. Can you check the signal level on the other side of the filter/de-emphasis network? Of course, for greatest accuracy, you may need to lift the ground connect to it, because IT may be the problem.

Dave
 
I have tested right and left output just ahead of the filter network on each channel.

Right channel audio:

right001.jpg~original


Left channel audio:

left_001.jpg~original


I will next remove both filter networks (it does not look possible to lift just the ground terminal without causing damage to the integrated ceramic unit), and try the test again.
 
Dave

I think you pinpointed the issue.

I removed the ceramic filter networks (PC402 & PC403), and temporarily soldered a wire to where terminal 1 of each filter connected on the board.

I then ran the test again - a 10.7 mc IF signal with 30% modulation, left and right, and a 19 kc pilot; injected into the input of the IF board.

Right channel output from the chopper:

right002.jpg~original


Left channel output:

left_002.jpg~original


The left channel waveform, as you can see, has weird artifacts riding on top of the audio wave.

But I decided to then perform a listening test. I connected two 24K resistors, one each in series with both outputs, and tied the free ends of each resistor into the 500-T's audio system.

Hearing is believing...both channels sound equal now...

So what do you recommend as a replacement for the ceramic filter network? The 500-T service info does not give the details of PC402/PC403...only a Fisher part number...

And, any thoughts on the strange artifacts riding on top of the left channel audio?
 
I would use the good network to measure the resistance between the input and output, and also the capacitance between terminals 2 and 3.

With that information, you could then build a jig using the capacitance you measured between pins 2 and 3, and a control for the total resistance between pins 1 and 3. Then, try different values of capacitance between the wiper of the control and pin #3, varying the control and the capacitance until you make the jig network show the same response as the good network does. You may need to keep a 19kHz pilot present to produce a 38 kHz output if the decoder uses a doubler instead of an oscillator.

As for the strange artifacts, I don't think I'd be too concerned about them at this point. That could be a product of how you are injecting the signal. It is interesting however that you used a complete composite FM stereo MPX signal to modulate a 10.7 mHz signal for injection right into the IF strip -- bypassing the RF front end? I know it can be done -- I'm just not aware of any single piece of equipment that can do that. I could take the composite signal of a stereo generator, and use that to modulate a good RF generator for injection into the IF strip -- but I would not recommend alignment using this procedure.

I hope this helps!

Dave
 
What I have is a Sencore SG165 AM-FM Stereo Analyzer. It does modulate the IF signal in stereo, or left channel only, or right channel only, or mono. It all depends upon the settings of the left channel, right channel, and pilot switches.

I measured roughly 13K between pins 1 and 3 of both ceramic filters, and I was getting 7 nF (7000 pF) between terminals 2 and 3 of both. I studied several other Fisher MPX decoders, and decided to use 15K with 1500 pF to ground as 7000 pF seemed excessive to me based on the values I found in other decoders' filter networks. It seems to be OK listening through headphones so far. I have not yet tried it through speakers again.

The left channel just sorts of drifts in after a stereo station is tuned in. This takes about 1/2 second for the left channel to fully make its presence known. I think the MPX decoder could probably use a good alignment as well as the IF strip. I'll have to construct some more test probes for my scope, as I currently have only one pair for it (plugged into the vertical input) and do not have any for the horizontal input as up to now, I've never used my scope's external horizontal input.

Thanks to all for the help, I really appreciate it! :thmbsp:
 
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Ron; I have a 700-T that uses the exact same MPX DeCoder. If you want I can measure the PC's for you (mine is working). Also have an extra unknown condition board, I can pull the PC's out and check for values.

Larry
 
You might check the condition of C421 and C424. C424 is part of a network that biases the chopper diodes during stereo operation. If it has a high power factor, it might takes its sweet time allowing the network to come up to speed.

Interesting about your Sencore generator. Will it also allow you to inject the signal RF style into the antenna terminals? Injecting it via the IF stage is unique to say the least based on all the models I am aware of.

Dave
 
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Larry - I would appreciate that, thank you! If I need to change the 15K and 1500 pF, I will...

Dave - C421 and C424 are new. All of the electrolytics on this board are new save for C418, and only because I ran out of 25 uF axials.

The Sencore has an RF generator as well which can be tuned to any point on the FM (or AM) band.
 
Sounds like a nice unit!

You might check that tant cap on the left output of the decoder board. I know its new, but tants can be temperamental, too. For the left channel to come up slow would seem to indicate some kind of leakage issue -- which happens with the bias applied to the chopper networks in this design. Just a thoght.

Dave
 
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