Tuner alignment by ear (ya I know)

[Carbsandcaps]

So ive got a Harmon kardon st-350 tube tuner that im trying to get to work with my fisher MPX-100 multiplex adapter. I can’t get stereo no matter what I do. Now the history on the tuner…

When I got the tuner it had nothing. No am or fm. Replaced some bad tubes, still nothing. Decided to mess with the oscillators. could sometimes get a faint signal. I finally figured out that the fins were coated in crud and now I got some stations. But now the oscillators are all out of whack so I adjusted them by ear until I got the loudest output I could and it works just fine in mono.

I tapped into the multiplex adapter plug to put rca jacks on the back for the input and outputs to go to my MPX-100. But no stereo no matter what I do. I tried a soundcraftsman mpx adapter that I know is good a still no stereo so I know it’s in the tuner.

My question is how close could I have gotten it by ear? I’m assuming the mpx signal is carried separately of the audio signal so it could very well be my ear tuning. Is there a certain oscillator that grabs the stereo signal or does the whole thing have to be redone? I have a waveform generator and scope so I can mess with it but is that all I need?

 

[Bob@FM]

here is a partial HK ST-350 schematic from hifi engine - https://www.hifiengine.com/manual_library/harman-kardon/st350.shtml

looks to me like you need a series coupling cap from pin 1 of the MPX jack, (use gnd from pin 5), to go to the MPX decoder. I would use at least .47uf film cap. Otherwise you are DC coupled to the stereo generator.

Now tune in a loud stereo station, and measure the peak - peak volts on your scope. You likely need at least .5V ac peak-peak for the mpx adapter to work.
You can test the MPX adapter sensitivity for stereo - output a 19 kHz signal on your generator, (must be exact - use a counter) start at a low level, then turn it up slowly until the MPX decoder stereo works.
Now measure the peak-peak V on the scope. Typically the 19 kHz pilot will be 10% of the MPX out signal in amplitude. So if your generator 19 kHz signal triggers stereo at 100mv peak-peak, you will need a MPX out signal of 1V peak peak from the HK ST-350 to work in stereo.
Bob

 

[Carbsandcaps]

here is a partial HK ST-350 schematic from hifi engine - https://www.hifiengine.com/manual_library/harman-kardon/st350.shtml

looks to me like you need a series coupling cap from pin 1 of the MPX jack, (use gnd from pin 5), to go to the MPX decoder. I would use at least .47uf film cap. Otherwise you are DC coupled to the stereo generator.

Now tune in a loud stereo station, and measure the peak - peak volts on your scope. You likely need at least .5V ac peak-peak for the mpx adapter to work.
You can test the MPX adapter sensitivity for stereo - output a 19 kHz signal on your generator, (must be exact - use a counter) start at a low level, then turn it up slowly until the MPX decoder stereo works.
Now measure the peak-peak V on the scope. Typically the 19 kHz pilot will be 10% of the MPX out signal in amplitude. So if your generator 19 kHz signal triggers stereo at 100mv peak-peak, you will need a MPX out signal of 1V peak peak from the HK ST-350 to work in stereo.
Bob

So here is the hk multiplex unit on top (which installs in the chassis and I don’t have) and the input end of the fisher MPX. I did go straight to pin 1 of the adapter plug for the output and pins 4 and 6 for the input back to the tuner from the MPX. I am more than willing to try the. It looks like there’s some small caps on the input of the fisher MPX unit. I am getting sound mono sound out of the unit through the MPX adapter. I just wanted to show you how I hooked it up. I’m assuming the .01 caps aren’t enough to remove the dc voltage? I’m also kind of learning as I got so excuse any dumb questions.

 

[Bob@FM]

It may be OK on the Fisher input as the input impedance is so high.
I would use one on the tuner MPX output too - it's bad practice to put DC across a cable, especially with a plug connector. I would use 0.47 to cover maybe using a SS decoder later.
Some of the FM tuners have a very hot (high level) composite output, so it's good to know the levels coming out. Chances are the level is too low, as the alignment is probably off. You can do a lot better than ear with a scope, using off the air stations.

If the detector is not in full limiting, the output level will be weak. That may involve tweaking the IF cans. Did you mess with them? And for sure the detector, upper and lower slugs on all those. Usually you have to get at lower on the bottom. There should be a spot on the detector for your scope probe to monitor your adjustments.

 

[Carbsandcaps]

Y

It may be OK on the Fisher input as the input impedance is so high.
I would use one on the tuner MPX output too - it's bad practice to put DC across a cable, especially with a plug connector. I would use 0.47 to cover maybe using a SS decoder later.
Some of the FM tuners have a very hot (high level) composite output, so it's good to know the levels coming out. Chances are the level is too low, as the alignment is probably off. You can do a lot better than ear with a scope, using off the air stations.

If the detector is not in full limiting, the output level will be weak. That may involve tweaking the IF cans. Did you mess with them? And for sure the detector, upper and lower slugs on all those. Usually you have to get at lower on the bottom. There should be a spot on the detector for your scope probe to monitor your adjustments.

Ya I messed with all of them when I got it and had no am or fm stations. I got it cheap, had no plans on selling it because I love the look so I decided to make an experiment out of it to see if I could align it by ear because I’ve never aligned a radio. I started with the 1st IF can and went onward then back to the antenna adjustment. I was thinking about using the scope to adjust the output of each can individually to its maximum amplitude.

 

[Carbsandcaps]

Well the most I got out of a good channel is .2v after adjusting all the cans to max volume so there’s something wrong there. I did have 5vdc on the output to the MPX so I put the .47uf cap in and it took care of that. I’m wondering if this is just not a good tuner. I don’t have an fm signal generator all I have is a cheap sine/square wave generator to make sure it’s at maximum

 

[gadget73]

exactly what did you turn? There are two oscillator adjustments, high and low, up in the front end. Messing with that mostly only skews where stations appear on the dial. Those are easy to put back if you have a station around 90 and another around 106. If you messed with the IF or discriminator adjustment, getting that right by ear is really not likely. Might get it close enough to pull stations in, but its not gonna be right.

in the MPX unit there is a 19kc pilot tone detector which gets peaked, that locks in the 38kc oscillator. Normally that one has to be adjusted for proper phase and separation. It really needs an FM stereo generator to get it all set properly.

If the tuner isn't properly aligned though, you won't get the pilot tone and the stereo part isn't going to work.

a related question, do you know if that MPX-100 actually works like it should?

 

[Bob@FM]

Not clear what you measured - you report .2V and 5V - You need to measure the MPX out AC level - what is it?
Did you try testing the MPX decoders with a 19 kHz sine wave? as above? you can use your computer or cell phone to generate 19 khz...but you need to know the level.
Never reported how you know the MPX-100 actually works OK. The 19 kHz test does that. If you tried to align the MPX decoder by ear, you are out of luck...can't be done
It's not a "bad" tuner - it should trigger the decoder to turn on with a composite level within its specs...but it may not have great sound in stereo if the tuner is not properly aligned - poor separation, etc.
You can hook an FM table radio to a MPX decoder and get stereo sound if the MPX input level is OK...

 

[Carbsandcaps]

Ok sorry for he confusion. I messed with the front end and the IF cans. The .2vac was measured at the output to the mpx unit. The 5vdc was also measured at the output to the mpx unit before I put the .47uf capacitor inline with it.

I don’t know for sure that the fisher mpx unit works but I have a working soundcrafstmen mpx unit that I’ve used before and that doesn’t pick up stereo with this tuner. I have an inexpensive general wave form generator but I don’t think that will do it. I’ll have to save up for a stereo fm alignment tool.

 

[Carbsandcaps]

I do have this old tv/fm sweep generator but I don’t know if it will send out the signal I need.

 

[gadget73]

if its accurate and stable, that will work for an IF and detector alignment if paired with a scope. FM IF is 10.7 mhz. Should be pics in the tuner service manual of what shape you ought to see on the scope and where it needs to be connected for all that, its basically an inverted U shape for the IF and an S sort of shape for the detector.

exactly how that one is hooked up and adjusted I don't know but the manual is probably online somewhere.

its not going to give the outputs to check the MPX decoder though.

 

[Carbsandcaps]

Ya I’m just gonna wait until I can get the stereo alignment generator and do it all right. Here’s the instructions anyway.