Another SX-850 in Protection Thread

[craigsc]

Hi, I am working on a SX-850 for a neighbor that had a snap, crackle, pop problem on one channel. That was the original problem. I fixed that problem by recapping and replacing the usual suspect transistors on the power supply. Brought it up and it sounded great except that the phono board had some noisy transistors as well (not the original problem and unnoticed because the first problem was so bad). So I replaced transistors and recapped the phono board and when I first brought it up, the protection relay started cycling every minute or so. Did some more research and thought maybe a dying Q5 on the power supply (should have done the first time). Replaced Q5 with a 2690 and thought I had it, but now its solidly in protection. Power supply voltage seem good, but protection not so good. Wanted to get some backup from the folks here before I head back in.

Power Supply:

1. 0
2. 5v
3. 5v
4. 13.9v
5. 13.9v
6. 51v
7. 51v
8. 36v
9. 36v
10. 36v
11. 28v
12. 0
13. -13v
14. -50v
15. -19v

Those look good

Protection Board:

1. .04
2. -13v
3. 15v
4. 0
5. -10v
6. 10v
7. 35v
8. 9v
9. 38v
10. 38v
11. 38v
12. -.4v

Those don't look so hot. I checked alignment on power amp board and everything measure good. Any ideas whats going on here?

Thanks,
Craig

 

[craigsc]

So, for fun I reflowed all the transistors on the protection board and she is up. Relay snaps quick and sounds perfect. I find it odd that this receiver had three different problems, power supply, phono board and then protection board, but that seems to be the case. All the problems were transistor related. Funny how we talk about recapping so much but not retraning!

 

[zebulon1]

Popping and crackle in one channel are signs of something other than a power supply issue.
Mostly a pre-amp, lesser probability, main amp, or like you found, in the EQ.
726's diff pair in the power amp.
2SC1312's in the tone amp.
725's in the EQ.
Your obligated to get them out if your capable.

 

[craigsc]

Zebulon,

Sorry for my incomplete description in my first email, I did replace the 726 differential pairs in the main amp as well. That is what cured the noise in one channel. Also did the power supply while I was in there. Thats the minimum I do on all amps/receivers I work on. Have done the EQ board and now the protection board as well. Everything seems good except now I have no stereo (light or sound). Seems like I should have done a complete recap/restore on this. Was trying to help a neighbor out with a relatively quick fix. Seem like either an IC chip or other issue. Did a simple adjustment on the FM front end to no effect. Any common FM problems on this series.

 

[zebulon1]

Your missing the 13.5 tuner feed at pin 4 of the power supply. It also powers the "Stereo" lamp.
Check fuse 4 and 5, should have a 1 amp fuse.
Check C12 and Q8 for correct installation. Missed solder connection?
KSC2073 or MJE15032 will work for Q8

 

[craigsc]

I have 13.9 at pin 4 on the power supply, 0 at pin 4 on the protection board. I’m guessing either the sanyos or the mpx ic.

 

[craigsc]

Another piece of data on my SX-850 tuner issue. When I depress the FM muting button, I get no sound. I feel like a read a post with similar issues but now I can't find. Love that AK search!

 

[zebulon1]

The 850 has a FM detect out on the back. Run a RCA patch to the Tape (left or right) in from the FM det jack. Select the tape monitor and listen to hear if you have any FM. Note: Its mono and composite at that point.
Other ideas: Not in any particular order:
Work the De-emphasis switch back and forth, clean the crust out of it.
Check for signals before the LP filter just after the IC HA1196 chip. Meter set for AC mV's. Should be jumping around when tuned in to an FM station.
Check for the 7.5 ACv at pin 35, 3 and 22 of the tuner
Check for the 13v DC at pin 34 of the tuner.
Check the function of the mute switch (Deox'ed?) and circuitry at pins 18 and 21 of the tuner.

 

[craigsc]

De-emphasis: no change
3, 22, and 34 are all 14.28 Dcv ( should be 13 plus or minus 1per the schematic)
35 at 0.004 Acv (should be 7.5)

Pins 18 and 21 react the same with mute, 1.3 with button out and .4 vDC with button in.

I did get same FM at det jack that I get from main connections.

Other thing I saw was Stereo light briefly engage when I first powered on receiver after it had been sitting. This leads me to thing it has something to do with the 14.28 volts? As it passed through 13 I got the stereo light? Maybe HA1196 doesn't like the 14.28 volts?

 

[zebulon1]

First; Explore the missing 7.5 AC volts? Feeds the Lamps, Protection and the Tuner.
14.28v Might be true?
What are the HA1196 pin voltages?
12.8v at pin 1 is the highest. They don't like to be over volt'ed for long.