Multiplex Decoder equipment

[cferry1]

Hello -

Fisher 440 Receiver

Would like to perform the Multiplex Decoder performance and Alignment procedure per SM.
I heard that using a HP 8656B or similar you may be able to work around the SM suggested "Multiplex Generator" piece of test equipment.

Is their a work around, using a featured RF Generator, so has not to require a test equipment "Multiplex Generator " ?

Any suggestions, appreciated.

 

[rcs16]

I do not know the FM stereo decoder ckts in a Fisher 440 rx.
The 8656B will only modulate in mono. Depending on the adj procedure, you usually need a composite that consists of a pilot, L+R, and the L- R side band modulated on the 38KHz suppressed carrier. With external mod on the 8656B you can provide the 19KHz pilot, if that helps.
A 8904A has this stereo encoder feature in its later revisions of firmware, so do a lot of other instruments.

 

[cferry1]

Thanks You All For The Replies and Information.

It Is Beginning To Look Like I May Be Purchasing A New Toy For This Project . . .
Looking at the Leader LSG-231.

IF you all have any other equipment suggestions, it would be appreciated.

 

[Tubologic]

Most cheap (repair shop grade) multiplex generators (including the LSG-231) must have the 19KHz pilot signal phase accurately calibrated before each use, or the separation of the tuner to be aligned will be severely compromised. This calibration is very critical and can only be done with a oscilloscope. HF separation specs of these generators rarely exceeds 20 dB, which is barely enough. Only synthesized laboratory grade MPX generators have stable locked pilot signal and good separation specs but they're still expensive on the 2nd hand market. (Look for Panasonic,...)
I don't like the inconvenience of calibration generators which need to be calibrated...

 

[chazix]

Most cheap (repair shop grade) multiplex generators (including the LSG-231) must have the 19KHz pilot signal phase accurately calibrated before each use,

I'm surprised to hear that. I would have thought that the only thing that's important about the 19HKz phase is its relationship to the 38KHz phase, and that it wouldn't be difficult to achieve stability of that relationship even in a "cost optimized" design. What am I missing? Does the phase relationship get screwed up in modulation, maybe?

Thanks,

chazix

 

[Tubologic]

I'm surprised to hear that. I would have thought that the only thing that's important about the 19HKz phase is its relationship to the 38KHz phase, and that it wouldn't be difficult to achieve stability of that relationship even in a "cost optimized" design. What am I missing? Does the phase relationship get screwed up in modulation, maybe?

Thanks,

chazix

Yes, that's correct: the phase relationship between the 19 KHz and the 38 KHz switching SC is very important as it dictates the stereo separation performance of the generator. But the allowable phase error is extremely small as even a few degrees error will seriously degrade the stereo separation. And this latter requirement is NOT easy to achieve with simple analog circuits where some (thermal or other) drift is allways present. And this is the reason why most service shop MPX generators have an external (user adjustable) 19 KHz phase control which need to be calibrated before starting any serious alignment work. Tube-based generators are especially bad in that respect and have considerable thermal drift of the 19 KHz pilot signal phase.

 

[KentTeffeteller]

Most cheap (repair shop grade) multiplex generators (including the LSG-231) must have the 19KHz pilot signal phase accurately calibrated before each use, or the separation of the tuner to be aligned will be severely compromised. This calibration is very critical and can only be done with a oscilloscope. HF separation specs of these generators rarely exceeds 20 dB, which is barely enough. Only synthesized laboratory grade MPX generators have stable locked pilot signal and good separation specs but they're still expensive on the 2nd hand market. (Look for Panasonic,...)
I don't like the inconvenience of calibration generators which need to be calibrated...

And these instruments are aimed at general Radio/TV service shop needs (where an average TV brand console stereo tuner section is what is on their benches on average). Not something well suited to the needs of higher end FM tuners with much higher standards of performance which demand much better equipment to align and more advanced techniques to get the performance the owner paid for.

 

[cferry1]

Could some DIY person tackling this FM Stereo Multiplexer Alignment calibrate the Leader LSG-231 FM Stereo Multiplxer Generator and feel comfortable with both results, expect positive results, positive expectations ?

 

[ConradH]

I use the LSG and find it holds calibration just fine. The old tube Heath units, maybe not as good, and nobody's complained much about the SG-165, which is pretty primitive, yet has probably aligned more FM MPX units than anything else.

I've always been tempted to build one of these- http://pira.cz/eng/stk2en.htm as the specs seem really good. Feed that to your RF sig gen and have at it.

 

[Tubologic]

Most of these old (repair shop grade) MPX generators are adequate for basic alignment of tuners of the same era which have limited performances anyway, and if you're not too demanding about extracting the top quality performance your tuner is capable of. As far as modern high end tuners are concerned they are inadequate (including the LSG-231 and SG-165, both generators I own) due to their very poor high frequency separation capabilities and unstable (drifting) specs.

You should allways remember a basic rule in (electronic) metrology: the equipment used for alignment must have much better specifications (an order of magnitude of 2 to 3 should be considered as a minimum) than the equipment being aligned with it, or the whole process is useless.

Don't confuse basic testing (does it work, more or less ?) with laboratory alignment (achieve the best performance the unit is capable of, meeting or exceeding the original factory specs.)

 

[ConradH]

There is one thing and one thing only where the FUD about MPX generators is warranted, and that's the separation adjustment. Every other adjustment will be as good, regardless of the equipment used, assuming it functions and hopefully has a frequency counter attached. One has to go back to vinyl and cartridge discussions to get an idea of how important separation is (or isn't). The conclusion back then was that anything better than about 25 dB was inaudible. If you've got a TOTL tuner that's capable of 40+ dB, yes, it will take something decent to confirm the spec. The LSG-231 is specified at 50 dB @ 1kHz, and few tuners will come close to that. It's likely worse if you modulate it with some other frequency, but few tuners provide other than a single adjustment. With most tuners my experience is that the resolution of the adjustments, plus the noise floor, unwanted signals would be more accurate, at any given test point, is the limiting factor on how well you can align them, and drift in shipment is a big concern- temperature cycling through a hot or cold UPS truck probably negates those last few dB of tweaking anyway.

 

[ConradH]

Forgot a topic- THD. The "lesser" MPX generators have high THD specs, nowhere near as good as the TOTL tuners. Can't remember, but I think the 231 is about 0.01%, still either not fantastic or not multiples better than the tuners. There's a not-crazy scenario where the distortion products of the generator and tuner may add or subtract resulting in a non-optimal setting of the tuner. I haven't actually seen a situation where one generator results in a different setting of the tuner, IOW, where the distortion minimum changes, but it seems like a potential issue to watch for. The other aspect is not being able to verify the THD, even if it's been minimized.