Pioneer SX-737 issues
 

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?

DC on the outputs detected by the protection circuit doing its job preventing DC to the speaker connectors. Probably fried a final or more and\or drivers, maybe a diode or more. Resistors need checking too. No telling with no checking for shorted transistors etc., but that's where you need to start.

So basically you're saying that I fried the output transistors and some other components in that path when I accidentally installed the replacement differential transistors in the wrong way.

If that's the case then I may just have to junk this receiver out then because there's no way I'm going to find some replacement output transistors for this receiver because they're the old socketed Germanium output transistors which are no longer made and they don't have any modern replacement's for them either.

I've been told there's a good stock of Russian germanium transistors on IIRC ebay.

Would they be direct replacements for a 2SD370 and a 2SB530?

I'm wondering because that's what this receiver uses for its output transistors.

Also how do I test to see if the output transistors are truly fried on this receiver?

Beats me I don't like SS amps enough to service them...IIRC the folks I heard that from said you have to do your own research to find subs.

It's not an amplifier, it's a receiver.

And if I have to research the substitutes for the output transistors for this receiver myself then that basically just puts me back to square one, basically I might as well junk out the stereo because it's not worth my time and money to have to scour every dark corner of the internet just to try and find a NOS Russian Germanium output transistor substitute for my receiver that may or not even exist!.

2SD370 and 2SB530 are silicon transistors.

jr

It makes no difference to me whether it is a amp or reciever or a transistor am portable...SS is SS, and I barely care about it.

Compare datasheets between what what you have and what you think might work.
Not everything is going to be handed to anyone trying to repair vintage gear. Sometimes you gotta do the research to fix something....may as well get good at research... Folks who know are dying off. And sometimes even if there is someone who will hand you the answer if you know how to pull it from documents you find it faster yourself.

It ain't my problem if your not motivated enough to follow up on some potentially helpful advice...
Some categories of things you probably should not buy broken in the first place if you are not interested in putting in some effort to fix...I walk away from stuff for that reason regularly.

Hi-fi amplifiers are usually direct coupled - which means shorting out one transistor has a strong chance to short out every other transistor on the amplifier board.

You will need to check them all now. Actually, I suggest to join Audiokarma and post this question in the Pioneer sub-forum, you will get good suggestions for replacement parts. Most people use On-Semi transistors which you can get from Mouser or Digikey for 70s Japanese stuff.

Simple E-B-C testing of any of the transistors with an Ohm meter would tell you if they are shorted. E to B and C to B should test like a diode, conduct one way, not the other. I'm sure there's a Youtube video somewhere explaining how to do that.

Better yet would be the diode check function of a multimeter, so you can read the junction voltage. This is really basic stuff, and every transistor on that channel will need to be checked now.

An inexpensive China tester is another alternative.

https://live.staticflickr.com/65535/...a8b906_z_d.jpg

Shown testing a common TO-3 packaged 2n3055 npn silicon power transistor.

jr

The Pioneer SX-737 is known for weak electrolytic caps and output transisters. Perhaps the differential transistors just brought these down early.

Start by checking the emitter resistors (very low value resistors by the output transistors. They open in case of a short in the output transistors. If they are good, you need to look up "dim bulb tester". It's a lightbulb in series with the hot lead of an extension cord. If you have a variac, much better. Like mentioned earlier, a simple DVM in diode check mode can find gross faults in transistors. I have repaired many Pioneer receivers and I have no problem with that model.
Look for the output inductor (one per channel) and check from ground to the inductor with a DVM for voltage. A few millivolts is OK, volts are going to keep the speaker relay from closing. Look also for blown fuses.
A schematic can be very helpful.
Do not use output transistors from EBAY. You will get counterfeit devices that will fail and you will go in circles. As mentioned earlier, ON semiconductors work in that receiver.
Go slow and be careful and you will get it going. Be careful reading about "rebuilding" the power supply. Output failure rarely damages the power supply.

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?

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?

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.

Worked on hundreds of those beasts
 

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.

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. :D

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?