Pulling in stereo radio on 500-C - very distorted

[larryderouin]

R143 is the LARGE TAN Resistor between V9 and V10. R139 is to the left of R143 and sitting on the green block cap. This is the normal location for them.
R139 is striped BRN RED RED Silver.

Tube. Dave G. has fabricated a special jig for the FISHER MPX units to align them. They were built and aligned on their own line, and then installed in it's tuner, so it's possible to align them out of the tuner/receiver. So someone with a flaky MPX can remove the MPX unit from the tuner or Receiver, send it to him, it gets aligned on a FISHER 300 MPX alignment unit, and shipped back. You might be interested in some of the stickies, that pertain to the Hx and problems with these. There is a sub list called Reference Technical threads that has more than a few threads pertaining to the MPX modules.

 

[thornev]

larry - I've read all those sticky threads, but they're too technical for me to understand. I PM'ed (started a forum conversation with) Dave and am waiting to hear back.

UPDATE: So R143 is connected to pin 4 of V9, and R139 is connected to pin 8 of V10, correct?

 

[larryderouin]

Thorne; The 2nd 1/2 of that post was actually directed to tubologic.

 

[Tubologic]

.

Tube. Dave G. has fabricated a special jig for the FISHER MPX units to align them. They were built and aligned on their own line, and then installed in it's tuner, so it's possible to align them out of the tuner/receiver. So someone with a flaky MPX can remove the MPX unit from the tuner or Receiver, send it to him, it gets aligned on a FISHER 300 MPX alignment unit, and shipped back. You might be interested in some of the stickies, that pertain to the Hx and problems with these. There is a sub list called Reference Technical threads that has more than a few threads pertaining to the MPX modules.

Of course the MPX unit can be aligned removed from the receiver (provided you have the required power supplies and MPX generator) but I wasn't aware someone offered such a service today. I allways repaired/aligned the MPX unit in situ which (IMO) is preferable because it includes the whole I.F/detector chain which sometimes needs a final touch up to get the best performance and separation. For instance, the MPX unit will never work correctly if the ratio detector is out of alignment, and this coil is located on the receiver chassis. The only advantages I can see here are shipping related issues.
Regarding the FISHER 300 MPX generator, I have one but mostly consider it as a collector's item and no longer use it. (like my SCOTT 830) There are many better and more stable modern (solid state) MPX generators which doesnt require a warm-up period and pilot phase re-alignment each time you need to use them. I know the 300 can be improved/upgraded but still it's 50+ years old and has been superseded by more recent equipment with better specs.

 

[dcgillespie]

Without a doubt, it is always best to align an adapter in the receiver it will be installed in. The beauty of using the Fisher 300 generator, is that its characteristics, when used with the proper coupling network, closely mimic the phase and response characteristics of a properly aligned Fisher Wide Band IF strip. In fact, Fisher's own MPX alignment instructions have you align these adapters with their generator's signal directly coupled (through the coupling network) to the MPX sub-chassis. Only at the very end do you transmit the MPX signal through the complete tuner/MPX system to touch up the separation control. Of all the Fisher MPX units I have aligned in their tuner/receiver settings over the years, the separation control has required only the slightest touch up, if any touch up at all when the factory alignment procedure and equipment are used.

In the particular scenario of the OP, the basic integrity of the tuner proper's alignment has been established to be a non-issue at this point, allowing a high degree of out of receiver alignment success for the MPX sub-chassis.

There is no doubt that the design of the Model 300 has been improved on in succeeding years. But properly operating, the Model 300 is a very stable piece of equipment, requiring only minimal calibration adjustments at it's annual calibration events. For everything other than Fisher gear, I use the Sound Technology 1000A for multiplex alignments. It is a wonderful piece of equipment, and considered the standard bearer by many who do such work. But for all of its capabilities, it does no better job aligning a Fisher MPX adapter than the Model 300 does, so I use both regularly depending on the application.

Dave

 

[Tubologic]

Maybe I should dust-off and realign my old Model 300. I remember someone posted about it here some time ago mentioning an error in the manual alignment procedure but I can't find the info anymore. Anyone knows ?

Last time I used it (many years ago) to align a Fisher tuner I was less than satisfied with the final results (poor separation) though I used the correct Fisher's recommended procedure (including the RC phase correction network). Also, I consider the need for constantly monitoring the pilot phase with a scope a major inconvenience but it's probably typical for old (tube) test gear to drift a little bit.This is important because even a few degrees of phase error in the pilot signal can't be tolerated or you'll end up with poor separation in the aligned tuner. Maybe I'm too picky about this but I don't consider the alignment satisfactory if I can't get (at least) 30 dB separation @ 400 Hz and 20 dB @ 10 KHz (for the complete tuner), a performance any good tube decoder (Fisher, Scott,...) can reach. Using one of my modern MPX generators (Panasonic, Philips,...) is much faster and easier and they are stable as rock. (no warmup time, pilot phase drift,etc...). My last (and ultimate) test is with a Catel broadcast stereo encoder/modulator whose specs are an order of magnitude better than the humble Fisher Model 300.

 

[thornev]

Here's a list of everything in my 500-C that I can observe has been replaced on the main chassis:
1. C16, C17 replaced.
2. C51, C52, C53 replaced.
3. C57, C59 replaced.
4. C61 replaced.
5. C69, C70 replaced.
5a. C72 replaced with .1uF 630v.
6. C77, C78 replaced.
7. C79, C80 replaced.
8. C85 replaced with 10uF 50v.
9. R121, R122, R123, R124 replaced with 220K ohm.
10. 10 ohm resistors added to pin 5 and to ground on all power tubes.
11. C91 removed, replaced with SDS Labs board.
12. C97 removed, replaced with SDS Labs board.
13. C98 replaced.
14. CR5, CR7 removed, replaced with SDS Labs board.
15. CR6 replaced.

Alterations to the MPX chassis:
1. Added 47K 1/4watt resistor across R204.
2. C214 replaced.
3. C210 replaced with nichicon 1uF 400v film cap.
4. C221 replaced with .047 uF 400v.
5. C222 replaced.

Other:
Replaced three 12AX7 tubes on MPX board with new Tung-Sol tubes.

That's everything. Thorne

 

[dcgillespie]

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

 

[thornev]

Can someone tell me... on a Fisher that has both FM AUTOMATIC and FM STEREO settings... if you turn to a station that doesn't register in stereo but is fairly strong, when you set the receiver in FM STEREO mode, do you get distortion, or do you get the same mono signal you had in FM AUTOMATIC mode? I'm getting a horrible distortion when I do that and I'm wondering if maybe that's just the way these old receivers operate. Thorne

 

[dcgillespie]

When a Fisher unit employs an FM Automatic feature (setting), then the accompanying FM Stereo and/or FM Stereo Filter settings are always a "forced stereo" setting. That is, the unit is locked into always producing audio from the output of the MPX sub-chassis. In these particular non-automatic settings, if the unit is tuned to a station known to broadcast with (only) a classic mono signal, then there will in fact be a slight amount of distortion present when tuned to those stations. In this scenario, the sound will (should) be equal from both channels, and the amount of distortion present depends on how accurately the MPX sub-chassis is aligned. A MPX sub-chassis that is perfectly aligned will display only the slightest amount of distortion in its audio when passing a mono signal through it, which becomes worse as the alignment of the sub-chassis deteriorates from optimum. It is because of the less than optimum audio quality that an FM Mono signal displays at the output of the MPX sub-chassis that some means of switching from the audio appearing at the output of the MPX sub-chassis to the audio taken directly from the output of the ratio detector is required. In units like the 400 receiver, this must be done manually via the front panel selector switch, while in the upscale 500C and 800C, the receiver does this for you automatically when set to the FM Automatic position. When units with the FM Automatic feature trip to mono, then the audio heard is being taken directly from the output of the ratio detector transformer, and the MPX sub-chassis in completely out of the picture.

I hope this helps!

Dave

 

[thornev]

I bought a cheapo FM dipole antenna, you know the kind that comes with an FM receiver, but the package arrived Saturday and it was empty ! I just found one lying around in a crate. I hooked it up and wowser - I'm picking up a dozen more stations in stereo !

Also, against all advice, I played around with the IF ("intermediate frequency" - thank you Dave) transformers on the MPX with headphones on. I've got bionic ears when it comes to listening to music so I thought I could adjust the 2 transformers to get good stereo separation. I was successful. I don't know what else I may have messed up adjusting the transformers, but between the adjustments and the components I replaced or that were previously replaced, I'm getting some good stereo radio now. (I may still get the MPX professionally realigned because I still get that ugly distortion when I turn on FM STEREO mode on a mono station) The position of the dipole antenna is very picky. Even moving it just a few inches in any direction can mean the difference between getting a station in mono versus stereo. Anyway, I wanted to thank everyone for your support and let you know where I'm at.

 

[KentTeffeteller]

This tuner section is not alignable by ear, the IF and MPX is not a beginner alignment and you need a really good alignment generator and the rest of the bench. You need to get it aligned properly by someone who knows how to align high end tuners who has the skills and proper equipment for alignment. TV repair shop generators not recommended.

 

[gadget73]

yeah I've tried to do that before, its never gone very well. I bought an ST-1000a specifically because I've had no luck trying to do "by ear" adjustments. Now I just need to learn how to use that properly. AM is cake, most things you can do by ear with a good clear station and get perfectly good results but FM is another animal.

 

[larryderouin]

Here's the offish'ul word from the 500c Owners manual. It backs Dave up.

Note that the 500c/800c switches automatically between MONO AND STEREO in FM Automatic. FM AUTOMATIC is the PREFERRED POSITION when listening to FM STEREO.

And here's the same page from the 400. As the 400 doesn't automatically switch back and forth btwn MONO and STEREO you have MONO FM, STEREO FM, and FM STEREO FILTER (which attenuates the noise from the AM carrier on a weaker strength signal).

 

[thornev]

gadget - When you tried to adjust by ear, what went wrong that you say that it didn't go very well?

 

[gadget73]

It just ended up fuzzy to some degree. IF strips in an FM receiver need to be adjusted for symmetrical response, and the detector also needs to be done the same way. Off-center at all results in more distortion even if you end up with a more sensitive receiver. The best I've gotten without a proper sweep generator was to use a good fixed frequency generator with FM modulation and pair it with a distortion analyzer. Do the fixed frequency peaking with a VTVM, then go back and re-tweak everything for minimum distortion, bouncing back and forth between peaking and distortion tuning for best compromise results on both. Comes out ok but not always perfect.

I've never gotten an MPX decoder to work properly by ear or even just injecting a 19khz tone into one and adjusting per the instructions. They always oscillate, have weird phase locking problems (it warbles for lack of a better way to describe it, its a problem in the 38khz adjustment mostly), bad separation, or some mix of all of that.

 

[thornev]

gadget... Did you try with headphones and adjusting just the 2 pots on the MPX board? I did that and I got a good stereo signal with good separation.

 

[larryderouin]

One question I don't think was covered. The separation control pot on the MPX. Where on the control is the pointer sitting< at one of the ends or in the middle??? IT should be in or just off the middle setting toward the side that gives the most distortion. Turning the pot to the stop will either give you a MONO sound or a DISTORTED sound with an increase in one side depending on which way it's turned. If it's on one of the stops, turn it back to 1/2 way. This is the NORMAL position or as close to normal as possible by ear. I get best response with the pointer slightly past the centerline toward the distorted side.

 

[thornev]

larry - I used my headphones to turn it off the maximum just enough to get no distortion. With that control and the 2 IF controls on the MPX, I had really good stereo going.

 

[gadget73]

I don't own headphones. I gave up on using my ears for adjustments. Instrumentation is much easier to get repeatable results with.