• 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

Woe to Me! SX-750 Protetion Relay On Strike!

mvsjcl

mvsjcl

Member
Help, oh mighty electronic gurus! Just like that, no clicky from the protection relay of my favorite receiver. I get 0v across D9 on AWR-099 (Power Supply & Protection board). The collector of Q4 reads 19v, but the base of Q5 only reads 2.4v. That doesn't seem right, but I'm not sure of my next steps. Any help getting my beauty back on the air would be greatly appreciated!
 
Register to hide this ad
Oh. and by the way, since I don't usually get a chance to post on AK, let me say Hi to Dave. One of the nicest guys on AK. If I need a light bulb I can't get from anywhere else, I go to Dave. He's only a few miles away from me, so I drop in on him on occasion. A righteous dude. He knows the meaning of my handle, mvsjcl. That's one of the many reasons I like him.
 
I know. It's not as exciting as a SX-1980, or as sexy as SX-3900, but I like it plenty. It has everything I need, without having anything I don't, so it's perfect for me. I guess I'll start replacing stuff. I planned on eventually doing the caps. Perhaps this is as good a time as any to begin doing that. If there's something else I should look at, please let me know.
 
The low voltage on the base of Q5 indicates that there is either an abnormal amplifier condition (bias or offset) or a fault in the protection sensing circuits (AWR-099 Q1,2,3,4). For starters, check the cathode end of D5,6&7 on AWR-099. At least one of them will likely measure low. Then check your bias and offset. Also, verify that pin 23 is 34 VDC.
 
Idle current adjustment made to 30mv each side. Pin 23 reads 33.4vcd. Cathodes read: D5, 1.4v; D6,19.8v; D7, 18.1v. I take it that shows a problem in the over-current/over-load circuitry? I take it that my next step is to check my output transistors?
 
Yes, 1.4V on the cathode of D5 indicates an overcurrent detection. Since your bias/idle readings are good on both channels, it may be a false indication. I would check or replace Q1 & Q2 on AWR-099 next.
 
I think you might be correct. O-scope shows good signals left and right channel outputs. All output transistors (admittedly, in circuit) show no short between emitter and collector, and collector voltages are good. As you suggest, I'll give Q1&2 on the protection board a look-see. Your help is much appreciated.
 
Jumping pins 1 ----> 2 and 3 ----> 4 on the AWR-099 board, bypassing the relay contacts, gives me good output. I've ordered the transistors. Thanks again for your kind assistance. You just gotta love audiokarma!
 
Of course, bypassing the relay contacts will leave you without protection as well as the power-on muting function. You could have also lifted one end of D5 which should cause the relay to pull in and would verify the functionality of the remaining protection circuit functions while only disabling the over-current detection.
 
You are of course correct, which is why I won't be using this unit until the new transistors are installed. I merely bypassed the relay contacts temporarily to reassure myself that the upstream circuitry was operating as it was before the trouble occurred. Your assistance is exemplary!
 
Sounds good although what I was getting at is that lifting one end of D5 would test the remainder of the protection circuit operation leaving only Q1 & Q2 as the probable cause of the failure. Jumping pins as you have done does verify that your amplifier output is fine but you have bypassed the entire protection circuit in doing so.
 
Per your instruction, I lifted the cathode leg of D5, isolating Q1 and Q2 from the circuit. Instead of reading the output(s) of Q3 and Q4, 19.5v, at the base of Q5, I'm still reading only about 2.4v, with an emitter reading of only 3.5v. One side of R18 reads 34v, while the other side reads the 2.4v being applied at the base of Q5. Not sure where to go from here.
 
Perhaps you could measure the resistance of R18 (150K ohms) to see if it has gone open. Also, check the pcb trace from R18 to the base of Q5.
 
I put a jumper across D7, applying the 19v directly to the base of Q5. The reading there after the jumper was applied was around 3.7v instead of the 19 or so I expected to see.
 
Since the cathode of D6 & D7 are both high, I suspect that they are performing their function correctly. For some reason, Q5 is on but there is no obvious reason why the base is low with D5 lifted. It may be possible that Q5 is defective (B-C shorted?). I would remove Q5 next.
 
Yeah, somewhat puzzling. I measured no shorts, but replacing it is certainly worth a shot. I'll report back when completed. Thanks.
 
FYI, I used a simulation that I prepared of another Pioneer amplifier which uses a similar protection circuit design and by adding a 200uA leakage between Q5b-c, I'm getting results similar to the ones you are measuring. Q5b = 2.4V, Q5e = 3.2V, D5cathode = 2.2V, D6/7cathode = 16.2V
 
You must log in or register to reply here.
Back
Top Bottom