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SX-680 both channels out

PatA

PatA

Super Member
Hey all, have an SX-680 that I'm working on for a friend, no sound from either channel. To this point I have Deoxited the controls, lubed pots and checked and fixed a few solder joints. I'm getting voltage readings today, I've checked at points that were mentioned in other SX-680 threads.
1. D309, 0vDC
2. Q402, 14vDC @ emitter
3. Q1, -.6vDC, pins 1, 10, -36vDC, pin 2, 36vDC, pin 9
4. Q2, -.8vDC, pins 1, 10, -36vDC, pin 2, 36vDC, pin 9
I haven't ordered STK0039's yet, I suspect they are the problem but I could use a little guidance, thanks.
 
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PatA said:
Hey all, have an SX-680 that I'm working on for a friend, no sound from either channel. To this point I have Deoxited the controls, lubed pots and checked and fixed a few solder joints. I'm getting voltage readings today, I've checked at points that were mentioned in other SX-680 threads.
1. D309, 0vDC
2. Q402, 14vDC @ emitter
3. Q1, -.6vDC, pins 1, 10, -36vDC, pin 2, 36vDC, pin 9
4. Q2, -.8vDC, pins 1, 10, -36vDC, pin 2, 36vDC, pin 9
I haven't ordered STK0039's yet, I suspect they are the problem but I could use a little guidance, thanks.
Click to expand...

I think your headed in the right direction. I had the same problem with a SX-580 a while back, ordered new STKs--took care of the problem for me.
 
Ummm, not enough checking yet. For the technically un-inclined, see if the tuner (Q401, 13v, 2sd712) works out through the tape monitor outputs, and if the phono amp (Q402, 45v, 2sc1384) also works out of the tape monitor outputs.

Then we have a fighting chance that the rest of the amplifier's circuits are getting juice.

Hmmm, D309?!!??!! ZERO VOLTS ?? On both (banded side +13, other side zero) sides? Not good - phono probably won't work either... check Q402, a little bitty 2sc1384 between the 2sd712 and the edge of the pc board. Also look for bad solder joints around there. Post all three of Q402's voltages.

So, don't spend $$ for the modules yet....
 
OK Mark, I think I'm starting to get my head around this thing. I desoldered and resoldered the joints at D309 and Q402, I had already done Q401, did this so that I can rule out solder joints as the problem. Also, in my first post, the voltages I reported for Q402 were actually for Q401. Here is what I have for Q401 and Q402 as of now.

Q401 - emitter, 14V, collector, 31V, base, 15V
Q402 - emitter, <>0V, collector, <>0V, base, <>0V
Still have <>0V at D309

"For the technically un-inclined, see if the tuner (Q401, 13v, 2sd712) works out through the tape monitor outputs, and if the phono amp (Q402, 45v, 2sc1384) also works out of the tape monitor outputs."

I'm not sure I'm tracing the voltage out right. I've traced voltage for Q401 along it's visual/logical path and it stays consistent, I just need to make sure I've traced it to the correct points. Do I need to work on that or try something else in light of problem at Q402? Thanks.

I'm stopping at this point.
 
Check DCv @ pins 1, 2, 3, on the lft. STK against pins 1, 2, 3, on the rt. STK. Do the same w/ pins 7, 8, 9, R + L. If they are the same thats a good sign. CC
 
Q402 isn't being fed. First check the voltages on both sides of R401, a 33 ohm resistor close to the fuse. There's a 220uf 63v capacitor connected to each side of R401. One (C404, on the destination side of R401) may have shorted out, popping the resistor.

You should see 50 volts AC (switch the meter to AC!!) at pins 11 and 12, with green wires coming from the transformer DIRECTLY - no fuses to interfere.

Then D401 and D402 have to both fail, or C403 has to short out to prevent the voltage from appearing at the feed (source) side of R401.

IF C403 shorted, it could have destroyed D401 and D402. Replace them with 512-1n4004 diodes (mouser.com).
C403 and C404 are:
647-UPW1J221MPD6 220uf 63v from mouser.com too.

R401 is 33 ohms, 1/4 watt, and could be temporarily bridged to see if that is the problem. it is part # 660-MF1/4DC33R0Fat $0.06 ea - i had to get 1% instead of 5% or 10% to get stock.

If C404 is shorted, it can be removed to test, as long as C403 is good.
 
Only getting 39vAC at pins 11 and 12. I'm getting 55vDC on one side of R401 and 0vDC on the other, (see pic). Really appreciate the help.
 
Last edited:
Just wanted to verify, my picture is'nt very clear, I'm getting 55v at the supply side of R401.
 
PatA said:
Only getting 39vAC at pins 11 and 12. I'm getting 55vDC on one side of R401 and 0vDC on the other, (see pic). Really appreciate the help.
Click to expand...

Ac voltages are ok, your meter is reading a bit differently. 55vdc is what counts.

R401 is definitely shot - unless you find a break in the pattern. The usual thing that causes that to happen is the cap shorting out. You can pull the second cap (c404) like I said and short out the resistor (r401) temporarily to get it running as a test - it is slightly possible that something after the transistor in the unit shorted out - and you want to catch that before ordering parts.


You should order up those parts:
R401, both C403 & C404

also
Q402 is the 2sc1384, and since you're ordering parts anyway, get a fresh replacement that's a bit more robust.
original:
2sc1384 npn to-92 ecb 50v 1a 1w( 1 watt?? hah!!! BARELY!!!) 85-340hfe 200mhz
so get a:
512-KSC2682YSTU to-126 ecb 180v .1a 1.2/8w(8w with heatsink - you'll see) 200mhz 100-320hfe $0.31 ea

while the replacement is rated for only 0.1 amp versus the original is 1 amp, the circuit pulls less than that. There is a higher amperage (1 amp) replacement power transistor, but it has less gain, and they used the little guy originally because it would have more gain.(and it was cheaper)
 
Hey Mark, I pulled C404 and shorted R401 and got a positive test, so your right on the money, as if I expected anything different! I'll get my parts ordered ASAP, really appreciate the help, I'd have probably gone in circles on this one, gotta learn to see the forest for the trees on these things!
 
Finally got my parts in this past Saturday. Installed everything, checked voltages, all were right on spec. Buttoned the thing up, checked offset and hooked it up, it works great. This thread, even though it's about a lowly SX-680 was very eye opening for me. There's a good amount of insight as to how the bench tech thinks or should think. When all was said and done, I went back and further studied the schematic on the receiver. It was just like someone turned the key of understanding for me, it's all there, I have to learn to be more patient and really read those things! Anyhow, big thanks to Mark, he is an invaluable resource for this community.
 
AHart said:
Hi everyone,

Mark suggested replacing Q402 with 512-KSC2682YSTU which is no longer available on Mouser.

I'm curious if this is a proper replacement? 833-BC337-40-AP

Link: https://www.mouser.com/ProductDetail/Micro-Commercial-Components-MCC/BC337-40-AP?qs=sGAEpiMZZMshyDBzk1/WiyxEghVrdFZmHOhPL57U2Yw=

Thanks for any help!
Click to expand...

I have tried replace:
- 2SC1384 with KSC2690
- 2SD712 with KSC2073.

With (partial) success (i.e 1 channel is working). I just realized I post 'Revive SX-680' after you so I will monitor this thread for pointers.
 
Thanks. I've replaced the 2SC1384 and the proper voltages are now there, however, when I measure the DC offset, I get 49mv on the right channel but -33mv on the left.

I'm assuming the STK0039 output transistor is bad on the left channel but not sure if there is an easy way to test. I can access pins 0,9,8 and 3,2,1 on the right STK0039 - what's the easiest way to measure between the two to see if that left STK0039 is bad?

Thanks!
 
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