Technics SU-Z780 Clipping at low volume

mbz

Addicted Member
Got a Technics SU-Z780 on the bench, equipped with SVI3205 chip.
Appears to be BPC however at low volume it's better than some
other budget buys, sound is respectable in this price range.

Problem is the amp clips at moderate/low volume, about 2Vp-p out
into load resistors as per pix. Both channels clip the same.

Checked psu, no disturbance/wobble at clipping point. Main caps
test ok, rest of the unit is clean with no hot spots etc. Fan works
when vol cranked up. Signal from pre looks ok.

Looking for ideas before I take it apart and check IC solder connections.
Since both channels the same suspect +/-B power connections maybe.

The dismantle looks like a PITA so thought I'd ask first.Clip.JPG
 
What is the signal like at C501 and C502?
Apologies, been reading a great refurb post on a Yamaha CX-1, got one arriving in a few days.

I checked the pre out signal at C401,2 which should be the same as C501,2. No distortion at C401,2. Clipping as seen at spkr posts.
 
Since you are using a sound card based oscilloscope, what is the maximum voltage input for your sound card?
It's easy when you put it like that. Was wondering why it was exactly 2Vp-p and both channels. Will confirm with another amp...
Have got 2 techtronix cro's both dead, probably need to checkout the psu's.

The technics was distorting so decided to hook it up to the PC "oscilloscope"
 
If the +15V rail is iffy distortion can easily be there.... power ic requires the -15v rail to operate.

Seen input selector and front end ICs cause these issues
 
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If the +15V rail is iffy distortion can easily be there.... power ic requires the -15v rail to operate.

Seen input selector and front end ICs cause these issues
Up-Arrow-Transparent-PNG-279x279.png

Check all of the supply voltages going to the amp and preamp as well, first before going much further.
 
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Thanks guys. I think @ILIKEMUSIC has nailed it. Sometimes too close to the problem and hearing distortion and wanting to see it.
Needless to say I feel like a dill, leading candidate for a dumb dumb award. Will try another amp and confirm. Would like to save
the Technics, it does sound quite good even with original caps...
 
Heh who knew... I’ve never dreamed of using a pc as a CRO. I guess I’m too old school... have 2 of the cathode ray tube sets.

I’d be looking at getting your ones back to life...
 
Heh who knew... I’ve never dreamed of using a pc as a CRO. I guess I’m too old school... have 2 of the cathode ray tube sets.

I’d be looking at getting your ones back to life...
Yeah, I'm old skool also. Did some work on one of the units, it's better but still an issue with the time base circuit/power.
Thing is I'd rather work on vintage hifi and the pc scope was kind filling the void. Now it's cost me some time... bugga.
 
Know how that feels... I’d not like having my cros out of action... it would feel like a tow truck breaking down.

Hope all goes well your end.
 
Power amp output and pc sound cards are not a good match. .. I burned my sound card doing this. You should always use a 10x or 100x voltage divider to measure the speaker output on a PC.
 
Yep, just to confirm, hooked up another amp, it "clips" in the same way at 2Vp-p so it's "limited" by the sound card.
Will try and rig up the voltage divider/resistor. That or grab the hammer and chisel and sort out the cro...
Thanks for the help, save me a lot of time and some suggestions on where to look.
 
Good call on the sound card being the culprit.
The clue is the perfectly symmetrical clipping at such a low output. Either the unregulated +/- B would have to be equally really low, or the regulated part of the supply to the VAS, both +/- would have to be really low and equal.
 
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Good call on the sound card being the culprit.
The clue is the perfectly symmetrical clipping at such a low output. Either the unregulated +/- B would have to be equally really low, or the regulated part of the supply to the VAS, both +/- would have to be really low and equal.
Agree, well spotted by ILIKEMUSIC. There were a few clues for me, like exactly 2Vp-p and both channels equally affected, but wasn't the distortion only
on the left channel???, live and learn.

That said, there are numerous posts where a scope is needed but people seem very reluctant to use the PC version, which seems ok as long as a few basic
precautions are taken. In most cases just need to know that there is/is not audio at that point. Suppose an audio probe works just as well...
 
If you need to test the amp under a low ohm load, I'd at least wire a one ohm in series with a 10 ohm, that makes a 11 ohm dummy load. Or, a o.47 or 0.56 in series with a 8 ohm. Of course clip your probes across the smaller resistor. You'll need some math to know the real voltage you are having at the output.
 
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