PROBLEM: Technics SA-400 FM Stereo Alignment

There are a whole bunch of alignment adjustments and you need to go through the whole procedures. You'll need an RF signal generator, usually a sweep generator for most FM alignment procedures. This isn't something you can do at home with what you have.

You really need a service manual. I didn't find Technics SA-400 in the Sams Photofact , but I found this source that claims to have it:

I have a gut feeling that this problem is due to simple component drift, and not to the need for a complete alignment.

Am I correct in assuming that resetting the VCO control restores stereo reception? If so...

Try replacing the pot with a cermet trimmer. It should have better long-term stability.

When I owned the Heath "Most-Accurate Clock", I had stability problems that went away when I replaced the carbon calibration pots with cermet.

Just a thought.

I would echo what William posted. The fact that you were able to recover stereo ops by tweaking the VCO just about verifies that all the other adjustments are ok. And any good tech will tell you that indiscriminate tuning is not for the faint of heart: it usually creates more problems and is also hard to resist. I would go back, re-adjust the VCO to get the light back for a period of time to verify the fact and then look for something causing the VCO to drift. Caps, trimmers and xtals are always suspects. Good luck. Stuart.

And further echoing, he could try exercising the pot first, to see if the durabilty is improved. If it is, then a cermet replacement would be easily justified.

Bob Morein (310) 237-6511

It is *extremely* unlikely that the fault is due to any of the front end alignment being wrong. It just doesn't happen. The most likely cause is that the recovered pilot tone level is too low for the PLL to successfully lock to - hence the reason that when the VCO was readjusted to be spot on, it just about managed to 'hang in there' for a couple of weeks. Low pilot tone recovery is usually caused by the discriminator coil being slightly off alignment. It can be far enough off for the pilot tone to be low, without being bad enough for you to be able to hear the characteristic distortion of the audio when the misalignment is severe. As a first move, I would try just rocking the core of that coil, with an APPROPRIATE PLASTIC trimmer tool, plus / minus a half turn from its current position.

Do not at this stage attempt to alter the alignment of any other coils in the IF strip. Even with all of the appropriate test equipment, it is an art that is not easily learnt in any other way than by long experience. Most people who are experienced in IF alignment, can do it by 'feel' and watching the tuning meter. I seldom resort to generators and other fancy gear, unless someone has *really* been 'at' it, and thrown the alignment out so far that a proper starting point needs to be re-established.

What's the full model number ? SA - 400 doesn't seem long enough. It's usually SA - xx400. I might be able to find a manual if you still need one. You can always mail me direct, off-group, if you want.

Arfa

'SA-400 [M]' & 'SA-400 [MC]' Are US and Canadian model numbers. I have the manual that covers these models, but it's in storage right now.

OK...so...this is weird...

I haven't been listening to my SA-400 for a few days, because I just wasn't that enthused about listening to my garage sale find when I know that it isn't performing right.

ANYWAY...So I decide to listen to it today while I'm doing some cleaning up.

I switch it on, and lo and behold, the FM Stereo indicator is back on, and I verified that it is indeed putting out FM stereo again!

What gives?

Possibly temperature related ? Did you get the reply regarding discriminator alignment, that I sent you in response to the mail you sent me direct off-group ?

Arfa

Identical same antenna?

Nothing is perfectly stable.

This reversal is likely due to a temperature change.

In addition to the VCO, the discriminator adjustment may be marginal. Also, I've seen on some old Technics units that the mono/stereo switch may be scratchy; also the am/fm band switch.

Mark Z.

You don't have an antenna connected? Or you have an inadequate antenna? Unless I use an outdoor antenna, I don't get the stereo pilot detected on FM radios in my house.

OK I've got the latest mail you have sent me, so I've resent the earlier reply. Let me know if you don't get it.

Arfa

Sometimes the problem is signal strength, and sometimes the problem is multipath.

When FM stereo was first introduced, there was some consciousness-raising about antennas. It seems to have gone away.

I've seen stereo multiplex decoder IC's be thermally intermittent in these old receivers.

Perhaps try running it with the cover off and (gently) heat up this section with a hair dryer. If you can make it fail that way, could be the problem.

Why is it unlikely? I'm not familiar with this receiver, but if it is as old as it sounds, and has an RF tuner using the old finned air capacitors, these RF sections almost always benefit from tweaking. I can't remember a single one that didn't benefit when we fine-tuned the caps and coils in the RF section.

I'm not disagreeing with you, just interested in the details.

Also for the OP, I didn't see it mentioned in this thread, but be aware if your receiver has the old "can" tuning transformers, some of these actually have two slugs in them, upper and lower.

Cheers.

EeAeDdGgBbEx (darn, broke a string again)

I think you misunderstood the intention of what I was saying. I don't disagree that old receivers can often be 'improved' - at least on the face of it - by tweaking up the front end. I was just saying that this particular fault, which is a lack of stereo decoding irrespective of how strong the received station is, is unlikely to be as a result of anything prior to the demodulator being out of alignment. I suppose it is just conceivable that if the IFs were sufficiently wrongly stagger tuned, you might just be able to screw up the pilot tone recovery, but I think that if it was that badly misaligned, it would be unlikely that it would ever work, which it does sometimes, and that lack of stereo decoding would be the most noticable symptom. As the OP says that he is not skilled with receivers, and alignment was being suggested by other posters as being a possible cause, I did not want him to go in with random twiddling, which would quite possibly compound the problem.

I don't think that I can recall ever having a stereo decoding issue with one of these old receivers, that was not down to discriminator alignment, pilot tone PLL misalignment, or a genuine faulty component around the decoder chip. Would you agree with that analysis ? :-)

Arfa