"Engineer" bravely wrote to "All" (16 Jun 04 23:04:54)
--- on the heady topic of "Technics SA5160 distortion on one channel"

En From: "Engineer"

En Greetings.
En I wonder if anyone has some insight into this problem.

En This receiver uses two STK0177 power amplifier "blocks", each with 10
En in-line connectors. The LH channel is highly distorted, the RH channel
En is perfect. The distortion is present on FM, AM and AUX (with sig,
En gen. input) so it's not the FM demodulator - it's definitely in the
En audio stages.

En The DC voltages on each of the two STK0177's power amplifier pins are
En as follows (with 115 VAC supply):

En Pin # RH channel LH channel (volts)
En 1 +0.251 +0.229
En 2 +0.002 +0.002
En 3 +0.255 +0.229
En 4 -26.90 -26.77
En 5 -1.272 -1.288
En 6 -28.29 -28.28
En 7 +0.060 +0.20
En 8 +28.23 +28.22
En 9 +1.309 +1.257
En 0 +26.95 +26.77

En (I hope the columns hold together!)

En I have not yet put a 'scope on each STK0177 line to watch for clipping
En or drive distortion (pin-out not known.) Nothing gets too hot and
En there is no perceptible difference in temperature between the two heat
En sinks (but they are thermally connected do some degree, so that's not
En conclusive.).

En Do people think this is a failed STK0177? Or is there another
En candidate component? Unfortunately, I do not have the schematic.
En Also, does anyone have a pointer or URL to the pinout and/or spec
En sheet for the STK0177? I did a quick Google but with no success.

En BTW, this unit is very difficult to service since all the electronics
En are on one large PCB and you can't access the bottom of this PCB
En without disconnecting just about everything and lifting it up (unlike
En other Technics models where you can take out the metal floor plate.)

En Thanks for all comments.

En Cheers,

En Roger

With a lot of obvious distortion a scope will just confirm it but it
could be useful to locate where it starts. If Pin 7 is the output to
the speakers then 0.20 volts on the left channel is certainly cause
enough for distortion. The +Vcc is also a little low compared to -Vcc.
This unbalance indicates the IC is sourcing significant current from
the positive supply into the speaker. Try and measure the same voltage
without the speakers. If it goes way up then the IC could be bad.
However, a leaky coupling cap can also unbalance the IC or high ESR
can cause a parasitic oscillation though the latter isn't as likely.
Good luck and be careful!

A*s*i*m*o*v

.... KPLA: Klingon Radio: All Klingon Opera, All The Time.