Reviving dead SX-680

The black lead will clip to ground, one cap lead connects to the business end of the red lead, then the other lead of the cap is used as the probe. The AC signal passes through the cap and the DC is blocked by the cap. This is frequently required on cheaper meters, sometimes not required on better quality meters.
I got to run some errands this afternoon, will be back later. 20mV and 40mV may be too low, I suspect the signals will be between 100mVac and 2Vac ranges up to the power amp section.
The Aux input sensitivity is 150mV, which should drive the output levels to maximum rated out with the volume turned way up. We don't want to overdrive the amp into clipping. You don't need (or want) speakers connected for these tests. 30 Watts (rated max output) will be about 15.5Vrms into 8Ω. There are some approximate levels on the block diagram. More important than exact levels will be comparison between left and right channels at the same points along the signal path.
 
Q1 Reading
1 -1.3V
2 -35.3V
3 40mV
8 60mv
9 35.2V
0 1.3V
Left DC-Offset 40mv to 50mV

Q2 Reading
1 -1.26V
2 -35.3V
3 -2mV
8 42mV
9 35.2V
0 1.27V
Right DC-Offset 50mV to 55mV
 
Q1 Reading
1 -1.3V
2 -35.3V
3 40mV
8 60mv
9 35.2V
0 1.3V
Left DC-Offset 40mv to 50mV

Q2 Reading
1 -1.26V
2 -35.3V
3 -2mV
8 42mV
9 35.2V
0 1.27V
Right DC-Offset 50mV to 55mV
That looks about as good as it can get. DC offset is not adjustable on the 680 so the only way to improve it would be by selecting transistors. Not worth the effort. Signal tracing is the next step.
 
Thanks Merlynski, Both on mono (AM radio) and (Aux) Stereo. FM radio is another story for another day.. .
What I maybe did not ask clearly enough is that if you push the Mono/Stereo switch to Mono is the imbalance still present?
 
Yes Mon/Stereo switch on the Aux input, still the same, imbalance. I got the Signal and Function generator app, hook it up via the phone to Aux. I got with me a 564K (i.e 0.56uf) cap, an auto range DMM and setup. So we can trace vac.
If you can briefly describe all things I should trace and I will post all result at once rather one by one. I will try to trace, one thing at a time using hook. I don't trust my shaky hand to use probe, so I would be switch on/off in between.
 
Before I do this trace, let me check around tone control (treb/bass) section, compare left and right to see if I miss something.
 
If you can briefly describe all things I should trace and I will post all result at once rather one by one. I will try to trace, one thing at a time using hook. I don't trust my shaky hand to use probe, so I would be switch on/off in between.
The hook is a good idea!!
Make sure Balance is centered, Tone controls are Flat (off or centered), Loudness is off, volume about 1/3 of the way up. If signal levels are too low after the volume control turn it up a bit.
Feed into Aux a 1kHz sine wave at about 150mVac measured at the Aux input.
Measure with DMM on AC volts. Follow the signal path through the amp making comparisons Left and Right until you find a difference. First 3 are before the power amp
Left...........Right
+C507 +C508
+C511 +C512
+C301 +C302 These may be a very small signal as they are the inputs to the differential amp, thats ok.
STKp1 STKp1
-C311 -C302
L out R out
 
Gonna take a break, before everything became a blur... at one point there is big difference between -C311 (0.905vac) and -C312 (0.29vac) so out with old C312, new 47uf/50v in. No difference.
 
Gonna take a break, before everything became a blur... at one point there is big difference between -C311 (0.905vac) and -C312 (0.29vac) so out with old C312, new 47uf/50v in. No difference.
I am not expecting the cap to be bad or not, just picking test points to start isolating differences. The failure will be between the last test point that matches and the test points that don't match, but not necessarily at that point. We are just finding steps and then will narrow it down further and finer. Were the STK pin1 measurements the same? Enjoy the break, see you later.
 
I was back at it. Here are the finding
+C507 & +C508 (4.7uf) the sames
+C511 0.025VAC, +C512 0.030VAC
+C301 0.25VAC, +C302 0.48VAC

STK Left Pin1 0.35VAC, right 0.029VAC
-C311 0.935VAC, -C312 0.030VAC
L-out (Speaker post) 6.5VAC, R-out 4.7VAC
 
correction: didn't realize speaker B was depressed, while testing speaker A. Retest with speaker A depressed,
L-Out .0935VAC, right 0.03VAC.
 
The inequality starts right near the FET muting gate. Can you pull the FETs and put a jumper wire drain to source, leaving the gate open (no connection to diode). I think you said you had header sockets? remeasure +C511 & +C512, and +C301 & +C302.
 
I went back and reviewed the posts in this thread and you have found anomalies in this area (mute gate) before, so we need to concentrate on getting DC and AC levels to match around the FETs.
 
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Big thanks Merlynski for your help on diagnose this ailing patient. I have decided to put this receiver aside, before frustration get the better of me and destroy it. . No matter what I tried haven't make any progress. I have tried put the jumper in the FETs, new pair of FETs in, old pair 2SK117s in. BTW my component tester now report both 2SK117 as BJT instead.
 
Well, OK. If you decide to work on it some more post again. I think the problem is right in the area of the FETs. Good luck!
 
Hi all,
I am back at this receiver. During this time a have acquired a signal tracer (Eico 147A) and use its audio tracer to concentrate around around Q503/Q504 and Q301/Q302. Audio signal from Drain and Source for both Q503&Q504 are close to balance (by listening) but imbalance after Q301 and Q302. Back track a bit I see that C301/C302 (0.47uF/50v) in between them so I trace both -/+ side of the cap, sure enough the negative(-) sides are balance. Not so for the positive sides. I pull them out and carefully check and recheck the board layout (+ side of the caps are marked) against the schematic before put the new one in. Yes sir, it is the board positive marking of the C302 is wrong compare to the schematic (this is the KC model). After put new caps (0.47/63v) in sound are reasonable balance now. So I will continue to check the rest now.
 
Hi all,
I am back at this receiver. During this time a have acquired a signal tracer (Eico 147A) and use its audio tracer to concentrate around around Q503/Q504 and Q301/Q302. Audio signal from Drain and Source for both Q503&Q504 are close to balance (by listening) but imbalance after Q301 and Q302. Back track a bit I see that C301/C302 (0.47uF/50v) in between them so I trace both -/+ side of the cap, sure enough the negative(-) sides are balance. Not so for the positive sides. I pull them out and carefully check and recheck the board layout (+ side of the caps are marked) against the schematic before put the new one in. Yes sir, it is the board positive marking of the C302 is wrong compare to the schematic (this is the KC model). After put new caps (0.47/63v) in sound are reasonable balance now. So I will continue to check the rest now.
I am glad you figured it out, and that we were in the right area.:) Wrong silkscreen markings can be a witch to figure out:confused:. Thanks for posting the results:thumbsup:.
 
Last night I was at this receiver again,to tackle the problem with the LEDs indicator, none are working. Turn out the MZ-061 is at fault. Replaced it and LEDs are lit up (Except FM Stereo). While at it I realized that I had put 4.7uF for the C301/C302, thus pull them out and replace with 0.47uf/50v too. No harm done anyway.
Next up, back to the FM problem. When FM mute is on, no stations. When off, very very faint to no signal at all on normally strong stations. Also back then, I did replace the Q7/Q8 2SA726 with KSA992.
I am not sure how to use the RF trace on the Eico147A to trace for this FM problem yet, but my plan would trace/probe PA3001-A/Pin1(in), Pin6 (out) then PA1001-A Pin3(in) and pin4(L-Out) and pin5(R-Out). Any other points would be appreciated.
I did check confirm good supply voltage reading to the Tuner section and my AM is very good.
Can't believe how much problems this receiver has, but at the same time, I do enjoy doing this/learning this and the process gave me some sense of accomplishment.
 
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