• 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

Pioneer SX-939 Circuit Protection Switch Issues

I

ignis-fatuus

Member
I picked up a SX-939 receiver recently and I've been plagued by the circuit protection relay going off and on again--or sometimes not on at all. The first thing I did was clean all of the pots and switches with Deoxit as its was popping and crackling all over the place.

I popped the case off and have been poking around the circuit protection board. I dug out my ESR meter from my CRT troubleshooting days and found that there are two caps which are metering out as bad. These are C1 and C2 and both are small blue 0.22 µF. Could these be possible/probable culprits? While I plan on recapping the receiver soon, I would rather not take a scatter-shot approach to fixing the current issue at hand. I am still familiarizing myself with the general circuit topography of vintage amps and receivers and was wondering if I'm headed down the right path or not.

A lot of the transistors look pretty old and I may also look for modern replacements where possible. If anyone has come across recent cap+transistor parts list, I'd happy to take a look as one as it would save me a whole lot of time in ordering parts.
 
Register to hide this ad
I just fixed an SX-737 that would drop into protection randomly. I replaced all of the caps and known problematic transistors on the protection board but it still clicked. Replacing the rest of the transistors on the protection board fixed it.
 
Aside from dirty pots and switches, when it was popping and crackling, was it coming from both speakers or just one?
 
cademan said:
Aside from dirty pots and switches, when it was popping and crackling, was it coming from both speakers or just one?
Click to expand...
OK, was playing with the receiver this afternoon and it seems that the loud popping is coming from the left channel. I am in the process of acquiring all of the caps and transistors that I may need for a rebuild; I'll start with the circuit protection board and then move onto there.

I am wondering if a dirty contact in the relay could be a possible culprit for the popping that I am hearing.
 
milo63 said:
I just fixed an SX-737 that would drop into protection randomly. I replaced all of the caps and known problematic transistors on the protection board but it still clicked. Replacing the rest of the transistors on the protection board fixed it.
Click to expand...
Yeah, I can definitely see a good many of the transistors being suspect. This restoration is certainly going to be a project.
 
ignis-fatuus said:
OK, was playing with the receiver this afternoon and it seems that the loud popping is coming from the left channel. I am in the process of acquiring all of the caps and transistors that I may need for a rebuild; I'll start with the circuit protection board and then move onto there.

I am wondering if a dirty contact in the relay could be a possible culprit for the popping that I am hearing.
Click to expand...

Dirty contacts usually don't pop or crackle. Dirty contacts will usually cause drop outs.

Popping and crackling in one channel is usually a noisy (bad) pre driver transistor in that channel. It is almost always one of those tiny black one's. Probably don't need to worry about the protection board as it kicked in when the transistor started acting up. It's doing its job.

I can never remember the number without researching threads but I want to say that it is a 2SA798 or one of those dual two in one transistors. I think that transistor number is mostly common in Pioneer equipment at least is was in my Pioneer SA-8500II.
 
cademan said:
I can never remember the number without researching threads but I want to say that it is a 2SA798 or one of those dual two in one transistors. I think that transistor number is mostly common in Pioneer equipment at least is was in my Pioneer SA-8500II.
Click to expand...
Would this be one of the transistors on the Power Amplifier Assembly board (AWH-030-0)? I'll have to take a closer look, but so far I am not seeing any 2SA798s.
 
ignis-fatuus said:
Would this be one of the transistors on the Power Amplifier Assembly board (AWH-030-0)? I'll have to take a closer look, but so far I am not seeing any 2SA798s.
Click to expand...

Like I said, I can never remember. :D

They wouldn't be on the power supply board. They would be on the main left or right amp board(s) depending on which channel is causing the problem. Can you post any pics?

it could be any of the small signal transistors but here is what the dual package 2SA798 looks like:

s-l1600.jpg


Here is what the 2SA726 looks like:

s-l1600.jpg
 
Last edited:
Yup, that's the one! It looks like this particular Pioneer model doesn't use the 798 so that rules that one out.

It looks like you do have the 2SA726(S) however you would also need to check all the other one's.

When they act up, a meter will show the voltage going all over the place. Also, read up on the use of freeze spray for locating bad transistors.
 
Last edited:
So it turns out there were a couple of 2SA726s and some failing caps on the amplifier board there were the cause of the intermittent cut-offs. thanks for everyone's help. I have recapped all of the boards and replaced all 2SA726s and 725s I could find. Now I just need to figure out how to align the tuner without a scope or signal generator.
 
If you didn't touch anything in the tuner section, why would you need to align it? If your dial pointer is off, the pointer itself just needs to be corrected.
 
cademan said:
If you didn't touch anything in the tuner section, why would you need to align it? If your dial pointer is off, the pointer itself just needs to be corrected.
Click to expand...
Yeah, I think the dial is just a bit off. I only just recently picked up this receiver and noticed that I was having a difficult time finding the stations that I normally listen to. I am guessing the easiest solution would be to loosen the set screw on the wheel on the side and match it up that way.
 
ignis-fatuus said:
I am guessing the easiest solution would be to loosen the set screw on the wheel on the side and match it up that way.
Click to expand...

Hi ignis, does the dial pointer stop at the correct points at the top and bottom of scale already? Also, is the pointer inaccurate on AM as well as FM or just the one?
 
You must log in or register to reply here.
Back
Top Bottom