• 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-850 in protection mode

J

jescher356

New Member
Hi all,
I was listening to a correctly functioning SX-850 at 40% volume and it went into protection mode.
I’ve looked at many threads but do not know where to go next. Here are the voltages:

Protection Board:
1 .023v
2. -13.2v
3. -.014
4. .01 v
5. -.035v
6. -.013v
7. -.002v
8. -.0023v
9. 34.58v
10. 34.59v
11. 34.59v
12. 7.49vac

Power Supply Board
1. 0v
2. 5v
3. 5v
4. 13v
5. 13v
6. 47.92v
7. 47.93v
8. 34.5v
9. 34.5v
10. 34.5v
11. 26.3v
12. 0v
13. -13.9v
14. -51v
15. -18v

I looked closely on both boards for cold solder joints and found one - on Q8 (it had been replaced) but didn’t affect the protection mode. I will rebuild both the boards once I determine the issue, but want to understand what has gone wrong.

Any suggestions or help is appreciated.

Thanks
Justin
 
Register to hide this ad
perfect spot to post for help! My sx850 was in the same boat last year and the folks on here walked me through the board rebuilds. If by chance nobody replies I can dig up the paperwork with the readings I was getting and where that led me. The rebuilds worked great,but understanding exactly what happened, I never found out.I,ll be following your post.
 
With AWM-062 (Prot) pin 9 at 34.58v the protection relay has dropped, but none of the prot board input pins show a failure.
I am suspecting a failure on the Prot board.

What are the dc voltages at:
Cathode D5:
Cathode D6:
Cathode D7:
They should be about or above +20vdc.

Base of Q6:
SB about -1 vdc.

Base of Q7:
or 6-way test Q7 (power off),
Rec. replacement for Q7 is 512-KSC2690AY (Mouser)

Power Supply voltages are a bit lower than listed on the schematic, but I see none to cause concern.
 
This is a receiver where I'd say troubleshooting is a waste of time until the power supply, protection, and amp boards are rebuilt. All have MAJOR issues waiting to crop up, and I've had 850/950's come in with each one of those three being the root cause, and sometimes two of the three.
 
Shotgun those boards! Hit the transistors and diodes too. Hit the protection first if you want.
 
Hi Merlynski,
Here are the voltage measurements - note: diodes were not stable but the number reported is where it was mostly.

Cathode D5: 5.8v
Cathode D6: 4v
Cathode D7: 33v

Base of Q6: 4.2v

Base of Q7: .008v
not sure what a 6-way check is...

Looks like there are some significant variations from what you said they should be. What do you think?
 
ivandezande said:
This is a receiver where I'd say troubleshooting is a waste of time until the power supply, protection, and amp boards are rebuilt. All have MAJOR issues waiting to crop up, and I've had 850/950's come in with each one of those three being the root cause, and sometimes two of the three.
Click to expand...

So you're saying that there may be several components that failed at the time I lost the sound? I do plan to rebuild the protection and supply, but I should to the amp board too?
 
jescher356 said:
So you're saying that there may be several components that failed at the time I lost the sound? I do plan to rebuild the protection and supply, but I should to the amp board too?
Click to expand...
Nada - just saying the problem could be on any of those three areas, and even if you fix one, the other two are ticking time bombs to make your amp stop working again.

Make your own component lists, there’s plenty of restore threads telling you what to replace on AK.

What some people miss is the Sony drivers in the amp board. Use KSC3503/KSA1381.

Oh, and not to mention the phono amp and preamp are full of noisy transistors.
 
jescher356 said:
Hi Merlynski,
Here are the voltage measurements - note: diodes were not stable but the number reported is where it was mostly.

Cathode D5: 5.8v
Cathode D6: 4v
Cathode D7: 33v

Base of Q6: 4.2v

Base of Q7: .008v
not sure what a 6-way check is...

Looks like there are some significant variations from what you said they should be. What do you think?
Click to expand...

6-way transistor check: https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ from Echowars on AK. (Avionic beat me to it)

On the prot board:
Cathodes of D6 and D7 show imbalance in the DC offset detector so it thinks there is a dc offset problem.
Measure DC voltage on the + side of C3, and + side of C4, should be close to 0 vdc.
Those are the voltages on the bases of Q3 and Q4. If those dc voltages are 0 test/replace Q3 and Q4.

Test Q1 and Q2, they get leaky with age. They are the overload detectors. D5 being pulled down to 5.8v indicate a problem there. Measure the dc voltage drop across D5. You can unsolder and lift one end to disable that circuit.

The circuits with Q6 and Q7 appear to be functioning correctly.

I like to troubleshoot problems first, then rebuild / refurbish. There are a bunch of components on the power amp, power supply and protection boards that do frequently go bad in various ways, I agree they need refreshed with modern components. If you place one large order for parts it saves money for shipping.
 
Here are the results from the 6 way test on Q7 (removed from circuit):

1. OL
2. 3.3m
3. OL
4. OL
5. OL
6. 3.5m

Based upon what I read it sounds as though this transistor is bad. is that correct?
 
jescher356 said:
Here are the results from the 6 way test on Q7 (removed from circuit):

1. OL
2. 3.3m
3. OL
4. OL
5. OL
6. 3.5m

Based upon what I read it sounds as though this transistor is bad. is that correct?
Click to expand...

I did not indicate I thought Q7 was bad, but rather that it was functioning correctly (post 12).
What range/setting on your meter did you use?

The circuit after D5, D6 and D7 is working correctly according to the measurements you posted previously, the problem appears to be in the overload and/or dc offset detection circuits, before those diodes.
 
Last edited:
It was from your first post... took me a while to get the 6 way done for Q7. Subsequently you indicated that Q7 looked OK.

The meter was on auto range.

Development: once I soldered Q7 back in, I turned the unit back on... and now the relay is clicking back on! Should I go back and do voltage measurements on the protection posts?

Base Voltage for Q3: .2v
Base Voltage for Q4: .01v
 
jescher356 said:
It was from your first post... took me a while to get the 6 way done for Q7. Subsequently you indicated that Q7 looked OK.

The meter was on auto range.

Development: once I soldered Q7 back in, I turned the unit back on... and now the relay is clicking back on! Should I go back and do voltage measurements on the protection posts?

Base Voltage for Q3: .2v
Base Voltage for Q4: .01v
Click to expand...

I looked back over the posts and I realize that by indicating a replacement I inadvertantly implied a failed component. Sorry about that.
If it is working now it is only a matter of time before it flakes out again. The unit may warm up and go back into protection. I would make a parts list, order the parts and rebuild it. I would start with the protection board.

If your meter has a diode test position use that for testing transistors, ohms or volts ranges are not very useful for transistors, imho.

If it is working there is nothing to find by taking measurements.
 
You must log in or register to reply here.
Back
Top Bottom