SX-737 questions

Syn_Error

New Member
This is my first post here on AK. However, I've been lurking and reading through many posts and messaged a couple other members and feel right at home with everyone. I'm planning on rebuilding an SX-737 I picked up a few months ago. I got the unit and used it with a turntable, a bluetooth receiver/transmitter, and FM radio. About a month ago, it started to act up such that after about thirty minutes or so, it would develop a crackle in both channels and then produce a loud pop and then silence. I would be able to turn it off for about five to ten minutes and turn it back on with no problems. But another fifteen or so minutes, it would crackle and then pop again.

I decided to go ahead and replace Q1-10 on the Power Amp Assembly (AWH-033) after reading 2SC1451 and 2SA726 are problematic. This has been done and everything checked out with the DBT. However, while looking at the diagram of this board, I noticed two resistors that are on the diagram but not in my board. Specifically R17 and R18. Both are marked as 15K on the diagram. There is an indication on the silk screen as to where they go. On the bottom/foil side, there is solder there, but no indication that these resistors were ever installed. AK member Motnick, theorized that as mine is a '75 model and his a '74, there may have been a revision and those were taken out of mine. I'm not sure if this is any way critical to this board. I asked Mark about it and he suggested I post this on the forum and ask everyone. It doesn't seem to be an issue, unless this is what the origin of my crackle/pop turns out to be.
index.php


After replacing the transistors mentioned above, I'm still getting out of spec voltage readings on this board. (AWH-033)
Pin 6: 32.7
Pin 10: -13
Pin 33: 26.3
Pin 28: -26.3
Pin 29: -26.3
Pin 25: -26.3
Pin 22: 26.3
I was able to get 3 and 17 to 0 and 1(2) and 18(19) to 20mV as per manual and corrected instructions from the forum.

I suspect my voltage issue will be originating from the Power Supply Circuit Assembly.
(AWR-057)
Pin 14: 32.6
Pin 15: 28.4
Pin 16: 28.4
Pin 17: 21.3
Pin 19: 13.6
Pin 20: 13.6
Pin 21: 13.6
Pin 12: -13
Pin 13: -13
Pin 7: -27.5
Pin 10: -26.3
Pin 9: 26.3

I'm thinking I'll have to recap AWR-057 and subsequently other boards. I've kept up with Motnick's rebuild and I'm taking that as some inspiration to get this thing working better than it is now. I just need some direction. Mark pointed me towards the power supply circuit first after I started to replace transistors followed by some voltage readings and I tend to agree. Get power sorted and hopefully things will fall into place. Guess all I need now are thoughts on those resistors and opinions on what to do next.

And if you're still with me after this long post, thank you!
 
I asked Mark about it and he suggested I post this on the forum and ask everyone. It doesn't seem to be an issue, unless this is what the origin of my crackle/pop turns out to be.
I doubt very much that the crackle/pop issue has anything to do with R17 and R18 not being installed. I would leave it as is and not install R17 and R18.
If you look at other similar designs, such as the QX-9900 (AWH-015), it does not use them but others like SX-838 and SX-950 use them. I do not know exactly why they are used, I'd have to do some simulations to figure out why they are required. To me it seems that they reduce AC gain.
 
So the previous voltage readings on those pins were taken when connected to the DBT. Connecting to mains power, the voltage readings have increased significantly.
(AWR-057)
Pin 14: 38.8
Pin 15: 33.9
Pin 16: 33.9
Pin 17: 25.4
Pin 19: 15
Pin 20: 14.5
Pin 21: 15
Pin 12: -13.4
Pin 13: -13.4
Pin 7: -38.9
Pin 10: -37.7
Pin 9: 37.7

(AWH-033)
Pin 6: 39
Pin 10: -13.4
Pin 33: 37.7
Pin 28: -37.7
Pin 29: -37.7
Pin 25: -37.7
Pin 22: 37.7
Pin 3: 0
Pin 17: 0
Pins 1(2): 19.9mV
Pins 18(19): 19.5mV

Closer to spec on some pins, I suppose. But are the readings that are over spec going to damage anything? If not, I may leave it as is unless you guys tend to think recapping may help making this receiver better than it already is.
 
In prep for replacing caps and such on AWR-057, I have put together the following with the lists from mark in other threads:

AWR057,8 c7 1000 50 647-UPW1H102MHD
AWR057,8 c8 330 50 647-upw1h331mpd
AWR057,8 c9 330 50 647-upw1h331mpd
AWR057,8 c10 330 50 647-upw1h331mpd
AWR057,8 c11 220 16 647-UPW1V221MPD
AWR057,8 c16 220 16 647-UPW1V221MPD
AWR057,8 c12 220 50 647-upw1h221mpd
AWR057,8 c13 100 50 647-UPW1H101MPD
AWR057,8 c19 100 35 647-UPW1H101MPD
AWR057,8 c17 22 25 UPW1H220MDD1TD
AWR057,8 c18 100 16 647-UPW1E101MED
AWR057,8 q1 2sd313 512-KSC2073TU
AWR057,8 q2 2sc945 512-KSC2383YTA
AWR057,8 q3 2sd313 512-KSC2073TU
AWR057,8 q4 2sc945 512-KSC2383YTA
AWR057,8 d1 sr3am-4 512-1N5404 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d2 sr3am-4 512-1N5404 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d2 sr3am-4 512-1N5404 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d4 sr3am-4 512-1N5404 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d5 1s1886 512-1n4004 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d6 1s1886 512-1n4004 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d7 1s1886 512-1n4004 Any alternate part options? Mouser marked as obsolete.
AWR057,8 d8 wz-130 512-1N5243B
AWR057,8 d9 wz-140 512-1N5244B
AWR057,8 r3 280-CR5-180-RC

For the 512-1N5404, Mouser has 1N5404RLG as their current part. Would this be alright as a replacement?
For the 512-1n4004, Mouser has 1N4004RLG as their current part. Would this be alright as a replacement?

Thanks for the help and encouragement everyone!
 
My 737 is a '75 model and with the "0" revision of the board. R17 & R18 are blank spots. It's not the problem with your unit. Leave them be. If the crackle-pop was intermittent and random I'd suspect the 2sa726/2sa725's thru out the unit along with the 2sc1451's, and or a POWER SUPPLY PROBLEM.

Service Manuals were usually printed from the pre-production prototype units. Production line changes were made all the time on these units and DID NOT make the manual. They may have made a Service Bulletin that was sent to authorized Service Dept's, which we'll probably never see. The only PIONEER receiver I know of that had 2 Service manuals is the SX-626 which was due to a complete redesign of the unit 1/2way thru the 2 year model life. Also revisions to the boards didn't make the manuals, which is why some SX-1010 Amp boards don't totally match the schematic or the board drawing OR the parts list.
 
Last edited:
I replaced all transistors except the Q11-14 on the AWH-033. The crackle/pop does not seem to be an issue anymore. I've gone ahead and ordered some more components following Motnick's thread. I may go ahead and do the recap as this has been a fun project so far.
 
I replaced all transistors except the Q11-14 on the AWH-033. The crackle/pop does not seem to be an issue anymore. I've gone ahead and ordered some more components following Motnick's thread. I may go ahead and do the recap as this has been a fun project so far.

At this point you might as well go ahead and do the complete recap. It will be worth it in the end. I am almost finished with mine and I am very happy with the results. It is a great feeling knowing that, with the help of this forum, I was able to do all of the work myself. I am very greatful for all of the help that I have received here!
 
Last edited:
I've completed out AWH-033 and AWR-057. After all these were replaced, unit fires up and the circuit protection relay switch clicks into gear. I forged on with AWM-025, the protection circuit assembly. I used the parts list Motnick's thread listed.

AWM025, C1 .22 10 505-MKS2C032201BKI00
AWM025, C2 .22 10 505-MKS2C032201BKI00
AWM025, C3 330 6 647-upw1c331mpd
AWM025, C4 330 6 647-upw1c331mpd
AWM025, C5 4.7 25 647-UPW1H4R7MDD
AWM025, C6 100 16 647-UPW1E101MED
AWM025, Q1 2sc869 512-KSC1845FTA
AWM025, Q2 2sc869 512-KSC1845FTA
AWM025, Q3 2sc945 512-KSC1845FTA
AWM025, Q4 2sc945 512-KSC1845FTA
AWM025, Q5 2sa733 512-KSA992FBU
AWM025, Q6 2sc945 512-KSC2383YTA
AWM025, Q7 2sc1384 512-KSC2690AYS
Flyback diode, 583-1N4004-T

After replacing these parts, the relay switch does not click as it used to.
Checking voltages, they read:
Pin 1: 0v
Pin 2: -13.1v
Pin 3: 0v
Pin 4: 0v
Pin 5: 0v
Pin 6: 0v
Pin 7: 0v
Pin 8: 0v
Pin 9: 35.4v
Pin 10: 35.4v
Pin 11: 35.4v
Pin 12: 7.6v AC

Any clue as to why the relay switch isn't kicking in?
 
Quick things to check:

-verify polarity on the electrolytic caps you replaced (the Wima films are non polar and fine either way)
-verify you installed Q1 & Q2 replacements properly. The original 2SC869 has a BCE pin out while the replacement KSC1845 has a ECB pin out.

Any more than that I'll leave you to the experts, but those are fairly common issues.
Mike
 
I verified the polarity of the caps and researched that wima cap as that threw me off for a sec before i headed back home for the weekend. I'll have to take a closer look at those transistors. I may have been looking at the wrong datasheet for one of them while installing those. This will have to wait until next week unfortunately. At least it's protection circuit assembly. If it was the power supply board I'd have to consider harakiri. That thing is just a pain to work with.
 
Aw come-on, the 737 is a breeze to work on compared to the 1010 and a few tube units I've done, 1st heading the list is the Sansui 1000A.
If this is your 1st receiver, it's not a bad unit to rehab, and the work WILL bring a lot of satisfaction. I would suggest that you DOUBLE CHECK the attachment of the wires to their pins. The solid core wire can and does break off at the pins with flexing while replacing parts. My big PITA was Pin 5 on the protection board. Seems every time I looked at the unit sideways, the SOB would break at the pin causing a NO START. Ended up replacing the whole lead all the way back to it's source. That solved the problem. Yours is most likely transistor(s) or a backwards cap. As Wima's are bipolar,(or non-polar) they swing both ways. Either way is good.

One thing to bring up that isn't brought up often enough is the fact that the PIONEER BOARD LAYOUTS in the Service manuals are FROM THE SOLDER SIDE, NOT THE COMPONENT SIDE. This messes up a lot of people when they go to install transistors.

 
Larry, I agree about the satisfaction. It's been a fun learning experience. I don't think I saw any of the solid core wires broken off but I'll double check. I'm leaning towards a transistor at this point. I was looking at the silk screen on the component side when installing them. I think I was looking at the wrong datasheet if it is a transistor as I had a couple of them open at the time of installation. The unit lights up and all that. Just no satisfying click of the relay switch.
 
relay isnt clicking because q7 is turned off ..or some other problem ..i guess you installed transistors backwards or poor solder joint .. check orientation of all replacements . they can be different to originals . this catches lots of folk out .
 
I can admit this now that my SX-737 is up and running. Maybe it will help you also.

After I had completed the first couple of boards on my 737, I ran into an issue where the unit would not come out of protection anymore and the offset was way high. I went over everything a hundred times and could not find anything broken or installed incorrectly. I was losing sleep for a few nights as I could not find my mistake. Out of desperation I decided to clean up the solder side of each PCB that I had done work on with 99% alcohol. After letting it dry up really good, I powered the unit back up on the DBT and the relay clicked! Both bias and offset were off by a little, but dialed right in.

I guess I had just not been thorough enough about cleaning up the flux after soldering, and it had caused all of my issues. I still don't understand why it was causing the offset issues, but what a relief it was for me to stumble onto the solution. It was a good lesson for me as well! Since then I have been very good about cleaning up the PCB after each work session.
 
Thanks pete and motnick! I'll check all that out and let you know how it turns out in a couple of days. Really interesting situation with the flux, motnick.
 
Thanks pete and motnick! I'll check all that out and let you know how it turns out in a couple of days. Really interesting situation with the flux, motnick.

Yeah, I was pulling my hair out for a couple of days. I figured I would throw it out there. Maybe it will help you.
 
Last edited:
So I found the problem. It was Q3 not being installed the right way. I installed it such that it was EBC but it is really ECB. This was quite literally the last transistor I installed before going home for the weekend. Lesson, don't rush. Also, it's great to hear that relay click in to place. Thanks guys!

Edit: This was my first foray into the world of vintage audio repair. This has been such a fun project. A friend of mine who gave me a Technics SL-D33 also gave me suggestions as to which receiver to get. After seeing his SX-737 I was hooked. Needed one. Now said friend is wanting to dig into his unit. This stuff is addictive, man!
 
Last edited:
Back
Top Bottom