Pulling in stereo radio on 500-C - very distorted

[thornev]

Ah OK, gadget. I understand. I depend on my ears for my music "career" so I trust them. I absolutely understand depending on repeatable results. My day job is a software tester and I depend on automation to test which is all about repeatable results. My MPX is in the hands of an expert so I should be getting back an MPX board that is optimized. This is my first foray into electronics and it has been a good learning experience. I'm going to need to find a new project. Maybe something on one of my guitars or amps (although they are not "tubed").

 

[dcgillespie]

For all who have tried to align a MPX unit by ear (I know I did many, many, many moons ago!), Thorne did better than most. I haven't actually aligned his unit yet, but I thought some might be interested in knowing just what the status of his adapter was when received before I do align it. The scope shot shows the stereo output from the adapter with the scope set for dual trace mode, when being fed a Left Channel Only 1 kHz Composite signal from the Fisher Model 300 MPX Generator. The sine wave with the greatest amplitude is in fact the Left Channel stereo output from the adapter, while the sine wave with lower amplitude is the Right Channel stereo output. Properly aligned, the Right channel should show no output at all (basically a straight line), or more practically, an output that is 35 db or more below the output of the Left Channel in this scenario. The measured separation between the two wave forms in the scope shot presented has the Right Channel producing a signal so great, that it is just 10 db below that of the Left Channel.

Like the early "Matrix" style decoder designs, proper operation of Time Division style decoders that Fisher uses depends heavily on proper cancellation (as necessary) between the main and stereo information sub-channel to produce the FCC mandated minimum separation of 30 db between the two stereo channels. No doubt, Thorne adjusted his adapter to perform much better than it originally did. But therein lies the problem: Adjustment by ear lets you easily enough adjust away from "horrible" performance, but invariably never lets you get to optimum performance, because the closer you get, the harder it is to distinguish the improvements made. The result is that the best ear adjustments invariably have the adapter ending up performing somewhere in between, but typically closer to the lower performance end of the scale -- exactly how this one performs as received.

Dave

 

[thornev]

Dave - I'm not sure if I'm getting a compliment or not. LOL It doesn't matter. I'm confident my MPX is in good hands and will perform wonderfully when you're done with it. Thanks so much for your willingness to work on it. Thorne

 

[thornev]

Larry - Back in post 35 you suggested replacing 2 resistors, R139 and R143. After having the MPX realigned and doing the IF transformer adjustments to optimize signal strength, do you still recommend the resistor replacements? I don't want to mess up work already done since I don't know how the various components interface with each other if at all. Thanks, Thorne

 

[larryderouin]

R143 and R139 are the Main power resistors between the 1st(C98) and 2nd(C91D) (R143) and the 2ndC91D and 3rdC91C(R139) sections of the Main filter caps. The originals tend to get heat stressed and operate at a high percentage of it's rating. Changing R143 to a 10W vs. a 7W puts the operating range back in the center of it's operating envelope. Changing R139 to a 2W vs. 1W does the same thing. IIRC R143 is dissipating 5W or slightly more. Increasing to 10w gives you some headroom wattage wise and puts it more in the SOA. SAME with R139. It won't change anything regarding the MPX/Stereo/FM reception as far as signal strength or sensitivity. It will help stiffen the POWER SUPPLY.

 

[vintelectra]

Hello , I would like to pose a question to this august panel: I am getting audio distortion( on scope) coming from the ratio detector stage V6:
IF strip well aligned, new tube, replaced 8uf cap. What suggestion would you have to proceed to troubleshoot this stage? Thanks Martin

 

[larryderouin]

I don't know about august panel. Maybe February or March Hare panel. Check the diodes inside of the Ratio Detector 1st. Take the coil out of circuit and note the ohms and compare with another(if you have one or just swap them). Check the condition of the slugs too. You might have a cracked one. Dave is on walk-about with new wife, will be back late in the week. Maybe. If he doesn't get lost in the outback or eaten. Don't know what size knife he's got either.

 

[Dave451]

Agree with Larry. When I had distortion on a 400 with an otherwise aligned IF it was because of a bad ratio detector coil--had to replace it. Diodes were OK in that case, but worth checking. If this circuit is not operating symmetrically or is otherwise out of adjustment/whack, you will get distortion in the FM.

 

[vintelectra]

Thank gentlemen, will follow advice and let you know. Martin

 

[deeezy]

I have had a similar problem with my 500c as well, with the stations sounding distorted on the "stereo" setting. I employed a foolproof 2 step method that should be much easier than the aforementioned solutions:

1. Set your knob to "FM Mono"
2. Don't give a crap so much about stereo

I'm a simple man. All kidding aside, I am kind of surprised no one has suggested this yet, even sarcastically. Anyway, good luck, I'll be keeping an eye on this thread for improvements to my suggestion.

 

[larryderouin]

deeezy. With the current state of Radio station mixing and transmitting, every thing is set to 11, with no dynamics to speak of, with the exception of very few select Classical or Jazz NPR stations. Mono is starting to sound better to me at times (except when I'm playing a tape (R2R) or on the phono).

 

[vintelectra]

Still working on the problem:checked detector stage diodes etc, OK.
I found there is no voltage @the matrix network, I am tracing this.
My problem is a distortion @ the CD-R out audio when feeding fm modulated signal. When switch selector is placed in stereo, open relay, the pwr returns to the matrix, but when the relay is closed, the pwr is grounded
And the matrix network does not return. Caps replaced, checking resistors.
Suggestions are welcomed.

 

[vintelectra]

Correction ..... distortion in recorder out jack.

 

[thornev]

deezy, Always a good discussion at parties. I enjoy both. I like stereo for the clarity of the individual instruments (it's actually dual mono, isn't it?) and the added variable of staging within the stereo image (which is up to the composer/engineer). I like mono for taking OUT that variable which provides for total focus on the music (and perhaps EQ and other effects) and not having to play with a balance control to ensure I'm getting all the audio content.

 

[fred soop]

Using the scope in x-y mode and looking at separation, the only station in the Chicago area that has program content with good separation is the sole remaining classical station, WFMT. Even though many other stations broadcast in stereo, the crap music has very little separation, so there is almost no need for stereo. In fact, there is little need for the FM tuner any more. One could just stream the one station from the phone into the amplifier.

1440

1441

 

[dbxdx5]

Thorne -- The four caps you replaced on the MPX sub-chassis are in fact the ones -- and the only ones -- you should replace on the MPX sub-chassis, as they will either deteriorate (C210) or potentially become intermittent (C214, C221, and C222) over time. The burgundy colored silver mica caps should never be replaced, and the remaining tan covered ceramic caps will remain quite consistent in their performance characteristics long after we are both gone.

Dave

Dave -- I'm pretty green when it comes to tuners. Will any good film cap, such as an orange drop, be fine in one or all of these four spots?

 

[dcgillespie]

I use and highly recommend the Panasonic line of 630 volt film caps -- yes they are a good cap from a sonic standpoint, but importantly in this case, they are also of a physical size that can easily fit into the allotted space, which orange drops don't do easily. The 1 uF electrolytic can even be a film if you like because there is so much more capacitance packed into modern caps today for a given capacitance/voltage value than in the day your receiver was produced. Therefore, a film cap in this position becomes viable, where as back in the day, it would have been impossible. I typically still use an electrolytic in that position unless requested otherwise by a client. Personally, I haven't been able to discern any difference between the two types in this particular application.

I hope this helps!

Dave

 

[jjmcf]

For all who have tried to align a MPX unit by ear (I know I did many, many, many moons ago!), Thorne did better than most. I haven't actually aligned his unit yet, but I thought some might be interested in knowing just what the status of his adapter was when received before I do align it. The scope shot shows the stereo output from the adapter with the scope set for dual trace mode, when being fed a Left Channel Only 1 kHz Composite signal from the Fisher Model 300 MPX Generator. The sine wave with the greatest amplitude is in fact the Left Channel stereo output from the adapter, while the sine wave with lower amplitude is the Right Channel stereo output. Properly aligned, the Right channel should show no output at all (basically a straight line), or more practically, an output that is 35 db or more below the output of the Left Channel in this scenario. The measured separation between the two wave forms in the scope shot presented has the Right Channel producing a signal so great, that it is just 10 db below that of the Left Channel.

Like the early "Matrix" style decoder designs, proper operation of Time Division style decoders that Fisher uses depends heavily on proper cancellation (as necessary) between the main and stereo information sub-channel to produce the FCC mandated minimum separation of 30 db between the two stereo channels. No doubt, Thorne adjusted his adapter to perform much better than it originally did. But therein lies the problem: Adjustment by ear lets you easily enough adjust away from "horrible" performance, but invariably never lets you get to optimum performance, because the closer you get, the harder it is to distinguish the improvements made. The result is that the best ear adjustments invariably have the adapter ending up performing somewhere in between, but typically closer to the lower performance end of the scale -- exactly how this one performs as received.

Dave

View attachment 1103652

View attachment 1103655

 

[jjmcf]

I have the distorted-fm-stereo problem on my Fisher 500, as reported here from time to time. After trying various remedies, I’d like to take a stab at aligning the stereo decoder. I have a fisher 300 multiplex generator, but have never been able to use it because the instruction manual, which is full of various information, does not actually explain how to do an alignment. Where can i find out how to do this?

 

[dcgillespie]

The 500C service manual has the instructions for aligning the MX-65 MPX unit with the Model 300 generator.

Dave