Pioneer SX-737 issues

[vortalexfan]

UPDATE:

OK So I monkeyed around with this stereo a little more and I realized that I had actually installed them correctly the first time and the second time I installed them incorrectly (which is why I was getting no response from the relay) so then I removed the improperly installed transistors and installed them correctly like how they were supposed to be.

So now I'm getting a response from the relay like I'm supposed to now, but when I go to hook up some speakers to the receiver the relay in there goes crazy it will click on and off continuously, but with no speakers hooked the the relay acts normal (it turns on once like its supposed to after a few seconds of turning on the receiver).

There aren't any other fried components in this stereo, I tested the finals and none of them tested bad, and there weren't any bad resistors on the board either they all tested within spec.

So I'm not sure what's going on here, maybe bad speakers?

[Electronic M]

Got an 8 ohm resistor or a pair of headphones you don't care about to try as a dummy load?

[vortalexfan]

Originally Posted by Electronic M

Got an 8 ohm resistor or a pair of headphones you don't care about to try as a dummy load?

I do have some old headphones I don't care about that I can use for a dummy load, and I know where you're heading with this, which is to rebias the amp, which is precisely what the service manual for this thing says to do whenever transistors in the amplifier are replaced.

But the thing is, I've tried hooking a dummy load to the receiver and tried rebiasing the amp and when I tried to rebias the amplifer section according to the service manual I can't get the bias down to zero, the bias measures around 35 VDC on both the right and left channels and turning the adjustments makes no difference in the bias voltage.

Is there something I'm doing wrong?

[quaddriver]

Originally Posted by vortalexfan

Is there something I'm doing wrong?

yes, you are forging ahead without a proper diagnosis. You need proper test equipment and read the pioneer section at AK - the 737 has long since been worked out, complete with modern subs and gotchas.

[vortalexfan]

Originally Posted by quaddriver

yes, you are forging ahead without a proper diagnosis. You need proper test equipment and read the pioneer section at AK - the 737 has long since been worked out, complete with modern subs and gotchas.

Ok. Well the problem is, I tried joining audiokarma and they wouldn't approve my membership, so unfortunately I can't use their services.

I guess I can just go on their website and just read through the old postings, I just wouldn't be able to comment on any of them.

[oldtvman]

Originally Posted by vortalexfan

Hello everyone, the other day I was given a Pioneer SX-737 Stereo Receiver that had the infamous static rumble that was coming out of the speakers that was an indicator of failed differential transistors in the amplifier stage (which in this stereo's case was a set of 4 matched 2SA726 Transitors, of which the modern replacements were KSA992 transistors).

Well I replaced the transistors the first time around and didn't realize that the emitter and base on the KSA992 transistors were reversed from what the original 2SA726 Transistors were so I ended up installing the replacements incorrectly and when I powered it on all I got was a loud hum out of the speakers so I quickly shut the stereo down and looked up the specs of the KSA992 transistors which was when I found a thread from AudioKarma that dealt with this issue and I figured out that I had the replacement transistors in backwards (the flat side of the transistor should of been facing the front of the stereo and not the back of the stereo like the originals did).

Anyways I took out the first replacements and installed another set of replacements (making sure they were in the correct orientation this time) and after I replaced the transistors the second time around I went to test my work and this time there was no audio period (no hum, or anything) and the protection relay wasn't even kicking in either (whereas before even with the original faulty transistors in the stereo the unit worked fine just too noisy to properly enjoy).

I did read in the service manual for this stereo receiver that the amplifier board has to be rebiased everytime a transistor is replaced on the board but I'm not sure if that's my problem or not.

Any ideas as to what could be causing my issues?

The 2sa726 are in the first stage of the final output, even if they failed unit wouldn't shut down. Check the output terminals and check for DC voltage, should be 0. A simple ohm meter can check the output transistor for front to back ratio, if you get a zero reading between them then you have shorted outputs, very rare on that model. The other common problem is the legs on the B+ regulator have cold solder joint problems, check that also.

[oldtvman]

Check the voltage between the base and emitter, if working properly you should have .6 voltage across them.

[vortalexfan]

Originally Posted by oldtvman

Check the voltage between the base and emitter, if working properly you should have .6 voltage across them.

I guess I forgot to mention that I did finally get this thing going.

Although for some reason on the AM Band the noise floor is high, (very loud and pronounced hissing and static) compared to my SX-1000 TD.

Is that normal?