Technics SA-400 FM Stereo issue, tuning calibration help needed

rocknroller

Active Member
Problem statement - The FM tuner was not getting any (or hardly any) FM Stereo signals identified with a good 300 ohm antenna connected. Mostly stations coming in mono with no FM stereo light (lamp is good) Also the Tuning indicator is not anywhere near center, quite a bit left (about 7 or 8 o'clock). These two issues may be related or not, I'm not sure.

Background - I don't have the proper tools to be calibrating the FM circuit, no oscilloscope or freq generator so please postpone all the comments to that regard and focus or what I can do anyway with making the calibration worse than it is. I do have the service manual. In a environment where I test lots of receivers so it's not a coverage/antenna related issue. It's specific to this unit.

Action taken - Checked the electrolytic caps on the board, many out of tolerance so replaced all but a couple that were good tolerance and esr . In general, the FM Stereo improved. Now get many stations decoded with FM stereo, but still a fair number identified by the FM light, but highly garbled, like two stations on top of each other. Also, no improvement to the tuning indicator as a result of the cap changes. It matters not what station I'm tuned to, FM mono or stereo, I cannot achieve center of this gauge (or even close)

The Service manual says VR101 can be adjusted for the signal meter. As I rotate this counterclockwise the tuning indicator moves slowly right towards center, but never reaches the center before I run out of range on the pot. Further, as I rotate it, the signal audio gets weaker and weaker. Returning it to it's original position restore the signal audio fully, but also returns the tuning indicator back to it's 8 o'clock position, so this control does not resolve anything, at least by itself.\

Thoughts as to the cause of the FM heavy overlap on some station points and the off center tuning indication? Are they related? Any components I should specifically be checking?
 
What constitutes a good 300 ohm antenna? Sounds like pro alignment is overdue. Without pricey test gear, expertise & experience, you might as well send it to the Lighthouse for alignment. There are no free or cheap lunches in alignment land.
 
First thing first.

In the center tuning meter centered when the receiver is turned off?

It is centered when the receiver is tuned to blank spot on the dial (no station).

If it is centered when the power is off and it is not centered when the receiver is tuned to a blank spot on the dial, it may be a simple adjustment.

The following are from the service manual.

The procedure for this per the service manual does not make use of any test equipment. See picture below.
This is the procedure to center the tuning meter (adjust the RF discriminator). Note you just tune the receiver to a blank spot one the dial and make the adjustment until the tuning meter is centered.
Capture30A.JPG

Be sure the use the correct non-metalic tool.

This may help with the no FM stereo issue.

The adjustment that you made is for the other meter, the signal strength meter. It is not for your problem.

After you perform the above adjustment, the following adjustment may also help with FM stereo. Turning this control too far one way or the other will cause the FM stereo to stop working. Find these two points and put the adjustment in the middle of these two points.

Capture30B.JPG
Note both of these adjustment can be done without test equipment. Mark the position of the adjustments before you start and you can always return them to original.

This may not take care of your issue, but it is simple to try, just be sure and use the correct alignment tools.
 
^^^^^^^
Agree with above...

Sometimes it's hard to find a dead spot. If you spin the needle down the dial. Is should mostly be in the center with swings as you reach the stations. If while traveling it stays to the right or left the discriminator zero is off and depending how far off it is it will not receive stereo, it is usually tied into the muting circuit also. So if it isn't zeroed it will mute the output and/or disable the stereo.
 
I should have mentioned disconnecting the antenna and even shorting the antenna terminals for the center tuning meter (FM discriminator) adjustment and reconnecting the antenna for the VR301 adjustment.
 
First thing first.

In the center tuning meter centered when the receiver is turned off?

It is centered when the receiver is tuned to blank spot on the dial (no station).

If it is centered when the power is off and it is not centered when the receiver is tuned to a blank spot on the dial, it may be a simple adjustment.

The following are from the service manual.

The procedure for this per the service manual does not make use of any test equipment. See picture below.
This is the procedure to center the tuning meter (adjust the RF discriminator). Note you just tune the receiver to a blank spot one the dial and make the adjustment until the tuning meter is centered.
View attachment 865666

Be sure the use the correct non-metalic tool.

This may help with the no FM stereo issue.

The adjustment that you made is for the other meter, the signal strength meter. It is not for your problem.

After you perform the above adjustment, the following adjustment may also help with FM stereo. Turning this control too far one way or the other will cause the FM stereo to stop working. Find these two points and put the adjustment in the middle of these two points.

View attachment 865667
Note both of these adjustment can be done without test equipment. Mark the position of the adjustments before you start and you can always return them to original.

This may not take care of your issue, but it is simple to try, just be sure and use the correct alignment tools.

1st - Thanks for the helpful reply! It is centered when turned off, but not centered ever no matter the dial location. I had down the 1st step previously but not on a blank spot on the dial so I redid that. Unfortunately, it made no difference the the centering of the dial, the range of the control simply will not reach it there which make me wonder if there is a specific part drift going on (that wasn't one of the many caps I changed out) I will say though with some additional tweaking of that control the FM Stereo is working pretty darn well. Lots of stations tuned in, more than before even with only a few trouble ones which might simply be multipath interference. Actually, I'm quite happy with the FM stereo portion so did not move onto the second step, since that doesn't appears to have and resolution to the tuning centering.
 
I should have mentioned disconnecting the antenna and even shorting the antenna terminals for the center tuning meter (FM discriminator) adjustment and reconnecting the antenna for the VR301 adjustment.
I just tried that as well for the tuning centering issue but no change. Honestly, if it weren't for the tuning meter I don't know that I would know anything was wrong. I wonder if some parts measurements are in order to see if they are in spec? Problem is, I'm not familiar with tuner circuits or which specific parts directly relate to the tuning meter logic. Perhaps some of the non electrolytics that I didn't look at or a resistor
 
What weird is I just took voltage readings around the AN377 chip and nothing is even close to what the schematic shows (see attached)
My pin reading sare far off. Using pin 16 as my ground reference I get
Pin 1 -2.2
Pin 2 3
Pin 3 -2.2
Pin 4 1
Pin 5 -2.1
Pin 6 9
Pin 7 3.8
Pin 8 3.8
Pin 9 3.8
Pin 10 7
Pin 11 5
Pin 12 -2.1
Pin 13 -2.2
Pin 14 0
Pin 15 0
pin Ground

If I use a ground reference from the other side of the chip, like pin 4, I get even lower reading sthan these, about 2 volts less on every pin. None of these readings make any sense to me. Am I using the wrong ground reference point?
 

Attachments

  • Technics_sa-400 FM Circuit.jpg
    Technics_sa-400 FM Circuit.jpg
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Pin 11 is the B+ power supply for this IC. If it is really only 5 volts, that will cause issues.

Measure this voltage on both ends of the 100 Ohm resistor that is connected to pin 11 of the AN377 IC. It should be about 13.8 volts DC on the end of the resistor away from the IC.

Make sure the battery in your meter is good and that your meter is working okay. Measure a new AA or 9 volt battery just to be sure.

You have to have good B+ voltage going the IC first, before anything else will make sense.
 
Curious what you find. As I Like Music has shown, you can accomplish quite a lot of alignment without special equipment, just using broadcast stations. You won't be able to mimimize distortion or optimize the channel separation, but getting the RF peaked up, the discriminator zeroed and the tuning meter working should be possible. Not being able to center the tuning meter on noise suggests something needs to be fixed before any sort of alignment is going to happen.
 
User error on the voltage capture! I was looking at the schematic cutout on that chip as a reference, not realizing that was the under side.
Correct readings are
1 2.1v
2 2.1
3 2.1
4 0
5 0
6 7.85 **
7 9.2 **
8 6.1
9 6.1
10 6.1
11 11.3
12 0
13 2.8
14 0
15 5.3

Pin 6&7 obviously look out of expected range. Pin 7 connects to that meter, so suspect the high voltage is what is making it swing left. Checked all the resistors on that line and all in tolerance and that 1uf cap was replaced (double checked and in tolerance). I wonder if the higher Pin 6 voltage is related and the source of the higher pin 7?
 
What constitutes a good 300 ohm antenna? Sounds like pro alignment is overdue. Without pricey test gear, expertise & experience, you might as well send it to the Lighthouse for alignment. There are no free or cheap lunches in alignment land.

This is the same advice you gave me concerning an SX-3700 doing the same exact thing. Bummer, but probably true.

^^^^^^^
Agree with above...

Sometimes it's hard to find a dead spot. If you spin the needle down the dial. Is should mostly be in the center with swings as you reach the stations. If while traveling it stays to the right or left the discriminator zero is off and depending how far off it is it will not receive stereo, it is usually tied into the muting circuit also. So if it isn't zeroed it will mute the output and/or disable the stereo.

I wonder if I could try this on my 3700 Pioneer. It gets most anything in mono, but doesn't pull them in when engaged in stereo, and fm stereo light NEVER lights up, even tho the FM "LOCK" is sometimes lit up.
 
I just aligned my Yamaha T-85 and they have you put it on automatic scan, then adjust the discriminator while it scans. Thus, turning the dial should work too. Almost any tuner will pick up enough signal buried in the noise, even with no antenna, to skew the discriminator. That seems to be the case even with the antenna input shorted and below ground in my underground lair. It helps if you can listen with headphones to see if you've found a truly signal-less spot. IMO, some times of day are better than others- you want bad reception!

Unfortunately, without an FM stereo signal generator, I don't know any tricks for you to do an MPX section properly.
 
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