Technics SA-310 Intermittent Weak Left Channel

Hello all...

I thought that I'd clean the badly pitted contacts on the protection relay in my Technics SA-560 receiver and got a little carried away. Turns out I broke some of the fine turns of magnet wire on the coil while I was trying to open the relay's case. So while I wait for my replacement relay, I pulled a little SA-310 out of storage.

The SA-310 basically works fine, but I've noticed that if I turn up the volume up high, the left channel starts to become weak and distorted. Turning the volume back down restores perfect sound. The controls are clean. I'm thinking it's a power supply problem, maybe weak filter caps not being able to keep up with the increased power demands. It dates from 1983, so maybe it's due for some maintenance...

Any thoughts on this? Am I on the right track?

William

Reply to
William R. Walsh
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Measure the B+ at the output stage on the suspect channel as you apply a

1khz sine wave from min to max output and report back.
--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

Hi!

I don't have any way of doing that. Any other ideas for a test that would produce a similar result?

William

Reply to
William R. Walsh

Could be dirty controls, but it really sounds like maybe a bad output IC to me. Do try pushing turning wiggling etc the various controls and switches and see if any of them affect your problem. Maybe not much else you can do.

Mark Z.

Reply to
Mark Zacharias

None I can think of other than using an alternative audio source like an FM tuner or tape playback to generate signal and monitor the rail voltages with an analog meter or digital with a logarithmic display. If it falls more than say 10% at full volume you may have an issue with the rail supply. That's about it, can't help more if you don't have a minimal amount of test gear like an audio generator and volt meter.

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

As Mark says, but I would also feel inclined to check the soldering on the output devices, as Pans and Technics are buggers for bad joints on any heatsink-attached devices, on virtually any models.

Arfa

Reply to
Arfa Daily

Hi!

I did try playing through all the sources and they're all equally affected. Output from the Tape REC connection is clean and undistorted. The controls on the unit are clean and in good shape, push buttons and sliders alike.

I've noticed bad soldering in these units. I was in the SA-560 at first to fix the nonworking indicator LEDs in the front panel. What little solder there was on the ones that didn't work just vaporized when I touched it. My SA-929 had bad soldering on its protection relay coil connections. I also have a Technics equalizer where many of the indicator lights above the pushbuttons didn't work...and it was the same thing.

So I'll take a look. It would be nice to get the unit running like it should again, and I would hope that I don't have to replace the STK2038 II module. There's a replacement on eBay but it's listed as a "generic" part and I'm not sure how I feel about that. I would have reservations about its quality or ability to meet specifications.

William

Reply to
William R. Walsh

Hi!

I do have an analog multimeter (if I can find it!) that would probably do. What I don't have is any sort of an audio signal generator. It did occur to me that I have some various audio frequency sweeps and the like stored on my computer, but I don't know if there are any fixed frequency samples in there.

My intention is to perform some checks on the power supply. I did notice what looked like dimming or blanking from the tuning display during loud passages in the music. It was so brief that I could not be sure.

William

Reply to
William R. Walsh

Basically you want to put a load on the power supply. Any way you can get the volume up so you can observe the drop, if any, in voltage on the rail could help you. Personally I haven't seen a lot of standard pass-filtered supplies dip in voltage drastically enough to weaken the output of the channel significantly and intermittently as in your case. I'd be looking for a lose connection first.

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

Hi!

Eventually the left channel "stuck" in its broken state. Now it's distorted no matter the volume or source. I've also noticed a "thump" sound from the left speaker at startup where there was none before. (The right channel continues to function normally.) Knowing these receivers' tendency to put a power supply rail on a speaker without warning, I've taken the good speakers away and put crappy ones in their place.

About an hour ago, I powered the unit up and played the drums on it to see if anything was obviously loose. That failed to produce a result, so I took Arfa's advice and examined the soldering on the STK 2038 II module. I found only two joints that looked suspect, so I redid them and found no improvement in the units' behavior. I looked all over the main board and didn't find any other problems.

I suspect the power supply--especially the filter caps--mainly because this unit runs *very* hot and always has. It'll get hot enough that you don't want to keep your hand on the top of the cover. The filter capacitors sit in the worst of this heat, right between the power transformer and power amplifier module. What kind of shape could those and other capacitors in the vicinity be after 27 years? They're 85 degree C units, so I wonder...and replacements are cheap enough...

William

Reply to
William R. Walsh

You could always swap the caps since each channel has its own pair. Or just shotgun them all and cross your fingers.

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

I reckon about a 70% chance that you will find several 'cracked right round' joints on that STK, which may or may not be the source of your current problem, but which will need attending to anyway ...

Arfa

Reply to
Arfa Daily

Do you have any kind of iPod Touch or iPhone ? There are a number of rather good free signal generator apps available. I have a couple on my Touch.

Arfa

Reply to
Arfa Daily

The ones I've seen with the notoriously bad solder connections are much later models using the SVI series outputs - not STK.

Mark Z.

Reply to
Mark Zacharias

Hi!

I swapped the caps at the kitchen table and held my breath. There was no improvement in the unit's behavior at all.

Thinking that I'd rather not smell burnt speaker coils, I put a speaker protection device between the receiver and the crappy speakers. All of a sudden, the protection device kicked in and muted the left channel. I haven't checked to be sure, but my guess is that the hybrid module's left channel finally went completely bad. (At the moment, I can't find my multimeter.) Interestingly, the headphone output still seems to be OK. I'm not sure where its signal comes from.

I found some replacement modules on eBay and ordered two of them. They're said to be generic replacements, so I hope they come close to meeting the specifications of the original. I also intend to find a suitable power source in the unit for a small fan that I will mount to the heatsink. And I sure won't crank it up that high ever again!

Replacing the amplifier module doesn't seem to be so bad (16 pins) but I am wondering about what work might be required to finish the job. Would there be any need for bias or other adjustments? I have the service manual but it does not mention anything about this. Only removing the module and its heatsink are discussed.

William

Reply to
William R. Walsh

Hi!

Yes, I have a first generation iPod Touch. I should have thought of that..."there's an app for that" too I guess.

Out of the applications you have, is there any one that stands out above the rest, whether free or not?

William

Reply to
William R. Walsh

I don't think you need to worry about biasing a hybrid module. Those things are STK-something right?

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

I gotta get me one of those. Still listening (and watching) my 30 gig Ipod. Looked for the new 64 gig I Touch, pricey! But not out of my price range even with the 3G option.

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

Hi!

The solder on this unit looked bombproof. I could only find one joint--and this was only if I squinted at it the right way--that looked bad in any way. So I redid it, made it look "perfect" and found no improvement. I guess I'll get to practice sixteen times when my new module arrives.

I've found plenty of other bad solder connections (but I think I mentioned that already)...some of the panel indicators on my SA-560 and an equalizer of some kind (don't recall the model #) needed to be redone before they'd work. The EQ was easy, the SA-560 not so much.

Was there any difference to speak of between an STK and the SVI modules? The "SVI" designation struck me as nothing more than a house numbering scheme from Matsushita Electric, and some of my service manuals show an SVI numbered part when the actual receiver had an STK-XXXX II of some type in it. I also saw similar numbering with the tuning microcontroller...SVIxxxxx in the book, and the actual part number/manufacturer name in the receiver.

William

Reply to
William R. Walsh

Hi!

In this case, it's an STK-2038 II module that is used.

This receiver has something called "Computer Drive". It's not clear what the "Computer" in use (a 4-bit Matsushita microcontroller) actually *does*, but I really think it is only watching the input signal level, temperature (via a tripping-type sensor set to 60C! Ouch!) and triggering the "preheat" operation, operating some LEDs and generally looking pretty. (My only other "Computer Drive" receiver, an SA-560, doesn't seem to *have* the "Computer Drive" IC. It has only a small circuit that switches on for a time at startup to "preheat" the power transistors.) I smell marketing and maybe some "audiophoolery" here...?

There's also "New Class A Synchro Bias" which I take to mean (after having done some reading) that the transistors in the amplifier are prevented from ever switching off completely when the unit is in operation. If I'm not mistaken, wouldn't that result in a lot of needless heat dissipation and a stressful operating environment for the STK-2038 module?

There are what appear to be bias adjustment pots on the main board in what little outboard supporting circuitry the amplifier uses. So far as I know, they're not mentioned in the service manual, so my guess is "no adjustments intended".

William

Reply to
William R. Walsh

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