• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SX-838 won't come out of protect

nickhead

nickhead

Active Member
I'm "new" to the forums, although I've been a lurker for about a decade. I've absorbed a great deal of information from the threads here. I now have a problem I can't figure out. SX-838, relay not engaging. I'm not an electronics expert, but I have more than a basic understanding. I was trained by the Army in electronics 25+ years ago. Haven't done too much in depth electronics work since then. I do have multimeters, a scope, soldering iron, etc. I've taken some voltage readings from the protection board. Can anyone point me at where I should be looking?

Protection board voltages:
1: 0
2: -13.7
3: -44.7
4: 0
5: 0
6: 0
7: -44.7
8: -45
9: 37.1
10: 37.1
11: 37.2
12 7.9 AC

Thanks in advance,
Nick
 
Register to hide this ad
nickhead said:
7: -44.7
8: -45
Click to expand...
I'd be inclined to check Q11 and Q13 output transistors. For shorted junctions.

Q7 and Q8 are the drivers and may have an issue as well. But its usually the outputs that go south.
 
Last edited:
Thank you for the reply. Looks like Q8 is shorted, which seems odd to me. I'd have thought it was Q7 from the schematic. I just so happen to have some ksc2073's. I'll replace them both and see what happens.

Nick
 
I'm getting ready to remove it. I guess I don't understand this schematic at all then. I'm not seeing how Q13 will make Q8 look shorted.
 
nickhead said:
I'm getting ready to remove it. I guess I don't understand this schematic at all then. I'm not seeing how Q13 will make Q8 look shorted.
Click to expand...
Not totally shorted maybe 150Ω from emitter to collector.In both directions.In my book thats bad...
 
Removed from the circuit Q7 and Q8 look good, but so does Q13. I'm still going to replace Q7 and Q8 because I have them. See what it looks like then.
 
avionic said:
I'd be inclined to check Q11 and Q13 output transistors. For shorted junctions.

Q7 and Q8 are the drivers and may have an issue as well. But its usually the outputs that go south.
Click to expand...
 
Q7 and Q8 were replaced. Still won't come out of protect. Q13 tested good out of circuit, but shows shorted when back in. All of the DC voltages on the protect board are down 2V. Getting ready to pull all of the output transistors to test them. I don't have replacements for those on hand.
 
And one of the wires on D2 came off. I didn't catch that until it blew a fuse. Time to quit for the night. Just a nub left on that varistor to try to reconnect it. I'll start over again tomorrow evening. Thanks for your help.
 
Here's where I'm at now. Q11, Q13 and Q7 all test good. Q8 was testing bad, then tested good. I think that's irrelevant as Q8 is the left channel and my problem is right. Anyway Q7 and Q8 were replaced as I had them on hand. I now have a wire that broke on D2 and I'm trying to figure out how to reconnect it. There's not much wire left to solder on to. Hopefully I can get it soldered back on. Once that is fixed I would expect the relay to engage.
 
You may be able to use a fine file to remove some of the diode body material in order to expose enough of the wire to work with. The receiver should not be powered on without a working STV diode in place since it will result in a dangerously high output transistor bias current and a blown fuse.

If the output transistors are testing good, often the next most likely cause of high DC voltage appearing on the output is Q5 (2SC1451). When Q5 goes bad, it affects the input differential amplifier which results in a large DC offset. The 2SC1451 is at the top of the list of problem transistors and both Q5 and Q6 should be replaced with KSC3503. A problem in the input differential amplifier can also result in large DC offset. 2SA726 (Q1-4) is also known to be troublesome and should be replaced with KSA992.

It is strongly advised that a DBT (Dim Bulb Tester) be used upon power up after any work is done to prevent fuses from blowing and causing further damage.

Les.
 
Last edited:
Thanks for the tip, Les! A file worked fine. I was going to use a razor blade, I probably would have bled :-) D2 is fixed, but still voltage on 7 and 8. I think now the best thing to do is pull the board and rebuild it. It is all original (or was until I put the new KSC2073's on) so could use some new everythings. Continuing to work on it the way I am is only going to stress the wires more and put me in a worse position. I see the threads on here for what I need to replace. Looks like it's time to place an order. Thanks for your help guys. I'll report back my findings.
 
avionic said:
My bad. The schematic I was using was a poor scan. Q9 is the other driver for the right channel.
Click to expand...

Yeah. That's why I was a bit confused. I know I'm getting old and haven't done this in a while, but you had me thinking I couldn't read a schematic anymore :-)
 
I have the power amp board out and found Q5 shorted C<==>E. Ordered a bunch of components to rebuild that board.
 
You must log in or register to reply here.
Back
Top Bottom