FM MPX Alignment tutorial for "dummies"

[davefr]

Just for the sake of discussion, let's assume someone wants to learn basic FM alignment for their vintage receivers/tuners. Where do you go to learn the theory, the procedures and what all is involved? (ie so that you better understand in more depth the various "cookbook" procedures in service manuals). Will a series of you tube videos be sufficient for someone with a decent knowledge of electronics?

How does one learn this stuff to be pretty much self sufficient?

 

[Pablodeux]

Just for the sake of discussion, let's assume someone wants to learn basic FM alignment for their vintage receivers/tuners. Where do you go to learn the theory, the procedures and what all is involved? (ie so that you better understand in more depth the various "cookbook" procedures in service manuals). Will a series of you tube videos be sufficient for someone with a decent knowledge of electronics?

How does one learn this stuff to be pretty much self sufficient?

There are some pretty informative youtube videos created by xraytonyb that you might find interesting.
He goes thru step by step repair and alignment on several tuners.

 

[chazix]

If interested in a written resource, the manual for the Sencore SG-165 "Stereo Analyzer" is pretty good, and should be pretty easy to find on the interwebosphere. (The instrument isn't all that great, mind you, though it might be good enough if the equipment being serviced is vintage enough.)

 

[rcs16]

There is the odd service manual that contains theory on op and adjustment
Refer to the Akai at-2600 for instance
Also certain rf sig Gen like the St1000a have use and tutorials on above.

 

[Punker X]

If interested in a written resource, the manual for the Sencore SG-165 "Stereo Analyzer" is pretty good, and should be pretty easy to find on the interwebosphere. (The instrument isn't all that great, mind you, though it might be good enough if the equipment being serviced is vintage enough.)

The SG-165 would be a good start in learning. That an a oscope, freq counter and distortion meter you could do novice alignments. Started this way 20 years ago doing it at home, but got my knowledge as a Navy Electronics Tech years before that.

 

[davefr]

Xraytony's videos have been great. I have yet to see him use a distortion meter. He seems to eyeball everything on the scope. So is a distortion meter optional? He also seems to use his HP 8657 AM/FM signal generator coupled to an audio signal generator for most AM and FM mono alignment. Is the Sencore ST-165 really a must have for the final FM MPX steps? Is there a lower cost option assuming you already have a HP8657 or another AM/FM signal generator? It sounds like these Sencores are very old and often need a rebuild. They are also expensive.

 

[kevzep]

So is a distortion meter optional?

No, a distortion analyser is one of the most important piece of equipment for doing alignments.
I have found most visual alignments are not that accurate to be honest. There are the odd things in IF word that are useful to "look" at but things like discriminators are much better using THD and a multimeter shooting for 0V.....
Stereo separation is good to do on a scope for example, but the time it takes to set up looking at a discriminator, its not worth the bother. There are easier and more accurate ways of doing it.
Mostly its about understanding what you're trying to and a LOT of experience.

Thats some of my 2 cents...

 

[chazix]

It sounds like these Sencores are very old and often need a rebuild.

True, that.

They are also expensive.

Not compared to an HP, I wouldn't think!? But it sounds like you already have an HP.

For most phase-locked loop MPX decoders, any generator that will put out a signal that gets the receiver's FM section into "quieting" should suffice to let you adjust the decoder's free-run frequency. And that, IMO, is the most critical adjustment. (Sometimes, it's the only adjustment. If there are other adjustments in a PLL decoder, they'll most commonly be for (1) optimizing separation, which you could probably ignore since (again, IMO) it's unlikely to be significantly out of adjustment; and/or (2) for some sort of minimization of 19KHz pilot leak-through, and you could fake your way through that if you have a way of modulating your generic FM generator with an accurate 19KHz signal.)

If you need to work on more old-school MPX decoders, that use 19KHz and 38KHz tank circuits, I think you're really better off with a proper stereo generator. You could peak-tune the tank circuits using 19KHz modulation from your generic FM generator, but you probably won't be able to get very near to optimal separation. Phase of the recovered 38KHz signal (relative to the 19KHz pilot) matters a lot, and I'm not sure whether you could optimize phase using a generic generator.

I confess I haven't checked out the xraytony videos, but I'm with kevzep in thinking that a distortion meter is really something you want to have for alignment work.

 

[Gleb375]

You could peak-tune the tank circuits using 19KHz modulation from your generic FM generator, but you probably won't be able to get very near to optimal separation. Phase of the recovered 38KHz signal (relative to the 19KHz pilot) matters a lot, and I'm not sure whether you could optimize phase using a generic generator.

There are some ways to align those old-school decoders for best separation without the use of a special MPX generator.
1) Visual method. Couple the inputs of a 2-channel scope with your 19kHz pilot source and with the last 38 kHz tank that drives the diodes. Align all the tanks to get both maximum amplitude and zero phase error:



2) 'Differential' method. Peak all the tanks as good as possible, then connect the scope's inputs to L and R outputs, and set the scope to display the channel difference. Receiving a music station, play with any of the 19kHz slugs just a little bit to get maximum amplitude and highest-frequency spectrum (meaning the sharpest spikes) of the difference signal.

3) Computer method. If your soundcard supports 192kHz mode, get a stereo synthesizing app, like Air-o-mate or Stereotool. Those apps can generate high quality test signals.

 

[davefr]

Would something like this do it all? (assuming one has the basics like a o-scope and a means of measuring distortion).

I see them for around $500/or BO which isn't much more then old SG165's which often need $'s thrown at them for rebuild/probes, etc. I recall seeing others made by Kenwood.

 

[Gleb375]

That's good, except for the cost((

 

[davefr]

There are a lot of these various Panasonics on Ebay. (mostly from Asia) Some are <$500 or best offer with free shipping. (ex: VP-8174A) Some sellers claim 100% tested. Buying from Asia is always a risk but Ebay sides with the seller if misrepresented unless the seller represents these as "parts or repair".

Wouldn't these be superior to the ancient SG165 and maybe more cost effective? SG165's aren't cheap.

Anyone have an opinion on these Panasonics?

 

[amptramp]

I'll copy here what I said on the antiqueradios.com site:

Many top-end FM tuners require alignment adjusted to minimize distortion and the SG-165 has too much distortion to be usable for this but it is good for most other units.

For those of you rebuilding an SG-165, check the voltage at the source of TR-316. If it is low, you may be OK, but mine was at 6 volts (well within the MPF-102 spec) and that puts 18 volts across 400 ohms (counting R368). When I got mine, the output was clipped in the negative direction for the 400Hz output. The simple reason was R365 was a 1/2-watt resistor carrying 22.222 mA and dissipating 0.8666 watts. The 1/2 watt resistor had gone up to 2195 ohms. I put in a 1 watt resistor and it looked OK but was running too hot to touch, so I then added a 2 watt 390 ohm resistor and turned down the voltage balance on the power supply so I would get 12.1 volts on the positive output but 11.4 volts on the negative output. There is no reason to balance the power supply voltages - nothing depends on it.

I replaced the filter capacitors (input and output) in the power supply because the 15 volt rating on the 12 volt outputs means the reliability will be low, especially if the unit is turned on and the output voltage rises before the regulator has a chance to stabilize it. If I was going to be totally anal about it, I would change the rectifiers to fast recovery types as ordinary rectifiers tend to ring at 30 MHz and harmonics thereof and the third harmonic would be in-band for FM. A snubber capacitor may do the same thing, just less effectively.

I had to replace TR302 and TR316 because they were blown. I also added a 1.5 meg resistor to ground from the base of TR301 because I am not comfortable running transistors with a 15 volt Vceo with the base open when the resistor will bring it closer to the 30 volt Vcbo. I also replaced TR314 because it appeared to be shorted going by the diode voltage test on my DMM, but that was a mistake - it was actually seeing R358 and the removed transistor tested OK.

I can't complain about the SG-165 I got for $10 with these defects plus one channel of the audio meter was butchered - leads cut and not reconnected for some reason, but I never expect to use that function anyway. It was a piece of test equipment designed to a price to enable the average TV repair and stereo store to have AM - FM - FM MPX repair capability and it succeeded in that mission. I used PN3563 transistors for the oscillators, which are plastic versions of the SE-3002 transistors on the schematic and the 2N3563 transistors that were actually installed. The Vceo goes from the 12 volts of the SE-3002 to 15 volts on the PN3563 / 2N3563, critical in this circuit.

The unit is a royal pain to work on. It is built like a ship in a bottle and there is a delicate plastic shaft that has to be protected from breakage. If you fix the design faults, it is a good unit if you have limited space and must choose one unit for checkout of radio functionality.

 

[kevzep]

There are a lot of these various Panasonics on Ebay. (mostly from Asia) Some are <$500 or best offer with free shipping. (ex: VP-8174A) Some sellers claim 100% tested. Buying from Asia is always a risk but Ebay sides with the seller if misrepresented unless the seller represents these as "parts or repair".

Wouldn't these be superior to the ancient SG165 and maybe more cost effective? SG165's aren't cheap.

Anyone have an opinion on these Panasonics?

Yeah I have a Panasonic VP-7633A FM Multiplex unit.
I got it from Asia somewhere, ebay seller. It was very cheap, and I thought why not, I'll have a go.
Shipping was cheap too.
I could not find any service information, or even a user manual, but as it turns out, the unit works perfectly after I cleaned the level pots.
Its all electronic switching and so forth.
Its also very low distortion.
I have had it for about 3 years and it always fires up, totally reliable.

I also use it to check the frequency response of tuners if I need to as it has the Pre-Emphasis options in it, 25,50,75µS...I use a FFT analyser with a pink noise source. It has Audio inputs on it to be able to do that.
I have found this very useful, and I in fact modified a Sansui TU999 which had some dodgy filtering, I got that tuner sounding absolutely amazing just by being able to look at the frequency response.

I also bought a HP8657A RF signal generator from Asia as, that was also cheap and a bit of a crap shoot, but it also works perfectly, I really like that unit. So I run the Panasonic into the external modulation input. I even bought another HP8657A very cheap for parts, it actually works apart from it has some kind of level attenuation problem, so I am going to just repair that one day, its too good to use for parts!!

I have a TEK495P spectrum analyser I use for looking at IF filters and anything I need to reference levels on and so forth, its been really useful for looking at various things, I was using it when I was diagnosing a problem on my McIntosh MR80 tuner, it helped me diagnose and eliminate to track the issue.

I have a Frequency Counter too, its a HP5834, its really handy too, simple to use. Really good for your PLL MPX sections.

As previously mentioned, I also have a THD analyser, and again, I cannot recommend enough how important it is to have one.
Its a very valuable tool to have on the bench...

 

[Punker X]

Tried getting one of those cheap generators from Asia, nightmare story and lost my ass on it. Arrived and wasn't working properly. Said they would replace it if I shipped it back. Not cheap. Then their customs opened it and broke the cal void stickers, they said I opened it and wouldn't refund or replace. Ebay and PayPal were unable to help due to the time it involved shipping.

 

[PE9ZZ]

A good alternative for a distortion tester is an Si4713 board. Feed it with I2S (from a CD player or somesuch) and you have an almost distortion free source. Get it from adafruit.com. I have yet to try it but it is very promising. Then use an external distiortion analyzer to do your adjustments. Or use analog inputs of the Si4713 for the oscillator of your distortion analyzer with the penalty of the built-in compressor. I have a broadcast quality modulator that serves the same purpose.

 

[chazix]

My take is that the OP doesn't have a distortion analyzer at present, and is wondering whether one is needed. Having a low-distortion generator is important, but it may not be of much help without an equally-capable analyzer.

I think most HP FM generators of the last few decades will have both a modulation oscillator, and a modulator, with distortion low enough to suffice for aligning most vintage tuners.

I'm not familiar with the Si4713 chip - is there a spec that addresses its modulation linearity? (I'm just curious, not doubtful; and I don't overlook the chip's advantage of being able to do stereo.)

 

[PE9ZZ]

Si4713 is a consumer grade chip. It specs (attached) say 0.1% typical/0.5% max. THD. My broadcast mod has no spec but I would reckon it is similar.

For basic sensitivity you don't need a distortion analyzer. Even just a multimeter and off-air signals (but with a stable antenna) can do the job. It is important to use frequencies around 90 and 106 MHz. Most manuals use these too. This is because the tracking is worst at the band ends and around zero at these frequencies.

 

[kevzep]

Tried getting one of those cheap generators from Asia, nightmare story and lost my ass on it. Arrived and wasn't working properly. Said they would replace it if I shipped it back. Not cheap. Then their customs opened it and broke the cal void stickers, they said I opened it and wouldn't refund or replace. Ebay and PayPal were unable to help due to the time it involved shipping.

This is exactly what I was afraid of when I made the purchases.
I got the Panasonic first, which was cheap so I wasn't too worried about it not working or not turning up and so forth.
So once that turned up in good condition, well packed I thought I'd take a chance on the HP RF gen.
Looks like I got lucky, the seller was a good communicator and seemed honest enough.

So we can say it is a bit of a crap shoot, you might get lucky and end up with some nice kit, or you might end up with a disaster...

 

[rcs16]

Yes the si 4713 is a great chip to use as a rf sg and fm transmitter. Fun soldering it. It even has rbds/ rds. I have built the si4735 receiver. One of my projects on the shelf for another day
I agree you can adjust the tuner for quieting sensitivity and not even need modulation. Pll free run freq can be tweaked between capture range extremes by adjusting the pot centred between the two extremes.
Yeah it would be taking a chance getting a Panasonic from Asia. Need to trust the seller. Insist to Have it packed very well floating/ suspended in foam.
I use a Hp 8656b but it does not have the lowest thd, for that I use a hp 11715a, rf attenuator, amber 3501 audio test set. For stereo a hp 8904a. A 3325a function Gen, Hp 5316 counter, hp 3478 dmm’s. Mostly eBay stuff, about as cheap as one can get. Old stuff can have its challenges with age, but having service manuals and spares will add life to your investments hopefully