Technics SA-310 Intermittent Weak Left Channel

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.

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

Hi!

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.

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

William

Hi!

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.

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

William

"William R. Walsh" <newsgroups1_at_idontwantjunqueemail.walshcomptech.com
wrote in message news:Tb-dnXWkarbnCfXRnZ2dnUVZ_qSdnZ2d_at_mchsi.com...
Hi!

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.

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

William


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.

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.

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.

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.

Hi!

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.

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

Hi!

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.

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

Hi!

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.

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

Hi!

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.

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

"William R. Walsh" <newsgroups1_at_idontwantjunqueemail.walshcomptech.com
wrote in message news:ZbqdnWGcB-688fTRnZ2dnUVZ_q2dnZ2d_at_mchsi.com...
Hi!

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.

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



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

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