Step by Step work and rebuild on a SX-737, done by a newbie

[pa_200]

It was a long time, but now my SX-737 rebuild project comes to an end and I like to share my work and lessons learned with all other newbies, planning to start a similar project in the future.

But first I like to thank every single one, giving me hints and help with all the issues I had or produced by myself.
Especially @merlynski, @markthefixer, @larryderouin, @Watthour, @rcs16, and @zebulon1

These summary is meant to be a help to every non-professional and I will not only write what I have done, but as well point out all the mistakes I have made during the work, so you should be able to avoid doing the same.

The overall aim for me was to find out, if I can hear a difference between a SX-737 still original and a second 737 with new transistors and new electrolytic.
It is absolute amazing, but the difference is worth every penny, every minute and every sleepless moment.

Thanks to all AND please join in if you find a mistake and/or have more to add!

A big Thanks as well to the Pioneer engineers constructing that lovely receiver. None of them have ever though that this receiver will last for 50 years, nor that there will be all lot of maniacs around, keeping that excellent gear running and listening to the warm sound for another 50 years.

What you will find and what I will cover in this thread.
1. Basics, tools and how to start
2. The Power Supply Circuit AWR-057(058)
3. The Protection Board AWM-025
4. Equalizer Amplifier AWF-011, Control Amplifier AWG-030, Switch & Volume Circuit AWX-71 and the MIC Amp (AWG-066-0)
5. Power Amplifier Assembly AWH-033,Testing the STV-4H,6 Way to Test the output transistor,
6. Adjusting the DC OFFSET (Center voltage) and BIAS (idle Current)
7. Tuner Assembly AWE-043
8. Filter caps, Changing the light to LED, new power socket

 

[pa_200]

1. Basics, tools and how to start

Let’s start with the work, but always keep in mind, I had the second SX-737 sitting next, as a kind of master copy if I run into problems and needed a recheck.

Tools used beside the standard soldering stuff:
· DBT
· I use 4 DMM (very helpful when adjusting Offset and bias at the same time), a decent digital meter should have a diode test function
· Smartphone, to take hundreds of pictures, mainly before and during the work, MACRO function is quite handy
· LCR tester mega 328 (my every time saver if I need to identify the pin out)
· Notebook with the SX-737 Service Manual in color, so you can zoom in on the schematic and the board prints

Before I remove any part, there is always a test on DBT to find out if the unit has a short.
If DBT stays dim, I check every function, listen with head phone and speakers.

Next is a detailed documentary with the smartphone across every board, not only an overall picture, better you have close up picture from every part you plan to replace. That will be very important if the silkscreen on the board is blurred and you need to find the correct pin position with new transistors or caps.

NEVER expect the Service Manual foil print to be exact the same as your boards. There are always late production changes.

After getting the board out of the plastic holders, take some pictures from the foil side as well.
Make sure your pictures are focused, so better check with your PC before you start desoldering.

Take special care with the dial pointer, the little plastic pin likes to break. Same with the two STV-4H mounted to the big heatsink. If they are broken or bad, there is NO new replacement available worldwide!!

Last check for you, if you have never worked on an electronic board before and you are unsure what to expect, better give it to an experienced technician. Desoldering can become quite difficult on some of the old boards with lifting or burning the pats.

With this preparation you could start, but better you read everything first, before desoldering the first piece.
None of the wires has to be cut or unwrapped, just loosen the wires, get them out of the plastic retainers and get enough slack to move the board.

I recommend you start with the power supply, that is your first challenge to get to the foil side.
Next is the protection board, then Equalizer Amplifier, Control Amplifier, Switch and Microphone if you want, then Power Amplifier. Keep the Tuner board as last.

Don’t start like I did, with removing the STV-4H first. What will happen and led to confusion is documented here.
https://audiokarma.org/forums/index...roblem-with-short-circuit-help-needed.978277/
So you better have an idea and follow a certain path with little steps from board to board.

 

[pa_200]

2. The Power Supply Circuit AWR-057(058)
Coming to the singly boards.
Why to start with the Power Supply Circuit? You need a stable Power Supply and good voltages for every other board to be successful. It is the “heart and engine” of the whole unit and a has a major influence on nearly every board.

Check the voltages first. You should have on the AWR-057(058) these values:
First AC with black lead to the chassis
Pin 1, +7.5 VAC
Pin 2, from fuse F1 to pin 12 Protection Board +7.5 VAC out and to the Lamp Assy
Pin 3, to F2, +37 VAC
Pin 4, to F3, +37 VAC
Pin 5, = GND AC
Pin 6, to F4 +35 VAC
Pin 8, +27 VAC
Pin 11, to D3+D4 +27 VAC

Next DC with black lead to the chassis
Pin 7, unregulated (it can vary) -38 vdc
Pin 9, + to C1, unregulated (it can vary) +35 vdc
Pin 10, - to C2, unregulated (it can vary) -35 vdc
Pin 12, regulated -13 vdc
Pin 13, regulated -13 vdc
Pin 14, regulated + 36 vdc
Pin 15, + 31.5 vdc
Pin 16, + 31.5 vdc
Pin 17, + 24 vdc
Pin 18, = GND DC
Pin 19, regulated + 13.5 vdc
Pin 20, regulated + 13.5 vdc
Pin 21, regulated + 13.5 vdc

BEFORE you start unsoldering, ALLWAYS and EVERTIME discharge both big Filter caps, if not, at least the C3 will still have a high load and gives you a nice spark, sometimes with a few unwanted extra issues!!

Use one of the actual 737-BOM List for your replacement and change at least the caps.
Q1, Q3 and R3 will get really hot and only the Q3 has a factory mount heatsink, so I added a heatsink to Q1 as well to keep it cool.

R3 is the hottest resistor in the whole 737 and can get up to over 120°F, leaving burn marks on the pcb. During the later production they mounted that big 5 watt resistor a bit higher to get a better airflow for cooling.
I did the same on my board as you can see on the picture.
You will find other thread with a complete rebuild of the power supply with caps, transistors, zeners and all big diodes.

I personally test every old part, write down the numbers, especially the transistor pin out. The same with the new ones, just to avoid a typical mix with the pin out on the new transistors. Be careful with the pins and the marking on the silkprint before you solder them back.

Having finished the power supply, don’t plug it in, take a coffee/tea break first, then recheck every part you have changed again with the foil print for correct pin out.

You will be surprised, how often you find yourself with a part installed wrong!! You will not be the first to install parts incorrectly, and will not be the last, everyone has done it and it happened to me too often in the past.

Last step, ALWAYS power up first on DBT. If the lamp stays dim, you can go to full power, measure AC and DC voltage to check your work was successful.

 

[pa_200]

3. The Protection Board AWM-025

Coffee break, then Protection Board comes next!

With the experience made so far, this board will be much easier.
Start again with the voltages on the AWM-025 with black lead to the chassis

Pin 2, -13.3 vdc
Pin 9, relay pulled in, +9 vdc
relay not pulled in +31.5 vdc
Pin 10, +35 vdc
Pin 11, +35 vdc
Pin 12, + 7.5 VAC

Change the capacitors and transistors. Don’t forget a 1N4004 as fly-back diode added between pins 9 (anode +) and 10 (cathode -)

Take the same precautionary measure like on the power supply before and after your work. At least most of the new TO-92 Package transistors may have a different pinout as the old ones.

KSA992s and KSC1845s will face the opposite way compared to old ones 2SC1312 and 2SC1313

4. Moving on to the Equalizer Amplifier AWF-011
Replace Q1 – Q6 with KSA992FBU and KSC1845FTA and the 8 electrolytics. For C1 and C2 (1 µF 25v) I used 1µf 63v film caps.

This board appears to be very easy, BUT it can happen as well, that at least one transistor is backwards installed after the job. So better recheck one more time.

The Control Amplifier AWG-030
This one was a bit tricky for me, and needed special care while desoldering in the area around pin 5 - 8 without hurting or breaking the signal wires.

Replace Q1 – Q6 with KSA992FBU and KSC1845FTA and 15 electrolytics. For C1 and C2 (1 µF 25v) as well use 1µf 63v film caps.

If you want, to the Switch & Volume Circuit AWX-71 (only one cap) and the MIC Amp (AWG-066-0)

 

[pa_200]

5. Power Amplifier Assembly AWH-033

With the experience you got in the meantime, you can go now to the Power Amplifier Assembly AWH-033.
This will be a longer part, but I think it’s necessary.

Lessons learned first!!
Don’t make the same mistake like I did, and power up with the STV-4H removed!! Otherwise the four resistors R23 – R26 will produce some smoke.

Quote: “If the output transistors are removed and the STV4s removed the voltage across those resistors can increase excessively, because the STV4 is not present to limit the voltage across the bias spreader transistor's (drivers) bases: Q7, Q9 and Q8, Q10 will saturate, current will flow and those resistors will overheat.”

https://audiokarma.org/forums/index...ort-circuit-help-needed.978277/#post-15205368

Take special care with the STV-4H, the leads can brake very easy AND there is now direct replacement available, only a workaround.

Parts you should replace at least.
All four trim pots and eletrolytics, will come back to that a bit later

Very important, Q1 – Q4 (2SA725/726) replace with KSA992 and Q5 Q6 (2SC1451), here you will find mainly the KSC3503 as a replacement but I used a different one.

During the last month, I tried to find more information about the 2SC1451 and some other parts used with the SX-737. So I found a very interesting thread some years back with this recommendation for the 2SC1451.

“The absolute best part for the job was some Panasonic transistors that were discontinued a few years ago (2SA0914 and 2SC1953). I should have bought hundreds of them before they disappeared. Damn... Anyway, the best replacement now for the C1451/52 and A809/810 are the

Sanyo 2SA1209 and 2SC2911”.

https://audiokarma.org/forums/index.php?threads/2sc1451-transistors.550878/#post-7536976

I know, every BOM-List recommends the SC3503 as replacement for the SC1451 but comparing the values of all three, I ordered as well the 2SC2911 and will give it a try.

But compare the transistors by yourself:
The original SC1451 hat a hFE of 221 and 223.
The new SC3503 I got are between hFE 145 and 153. The SC2911 are all spot an hFE 176 so I used theme instead of the SC3503, mounted my preferred heatsink with a new 3mm nylon screw to isolate the heatsink from the collector, and I am very happy with the result.

Some words on the trim pots.
These old pots are super-touchy so you better replace them. With 1-turn pots, adjusting can be tricky, while 10 or 25-turn pots are easier to adjust, but they are more expensive.

What I found out when I tested the 50-year-old trim pots with the help of the 4 DMM. After powering up BIAS started above 400mV and went down but not below 60mV. DC Offset started with 25mV, after 10 minutes went up to 30mV. Turned both down to 20mV and it maintained stable around 20mV.

Next I turned down the Bias, no stable value at all, what a difference! The voltage jumped between 0mV and up to 100mV or more for a peak, coming down again. BIAS was complete unstable and always moving in a range of 50mV up and down. Did never remain stable below 20mV.

Keep this in mind if you plan to keep the old trim pots on that board! There is a good chance not recognizing that if you have only one DMM available and can measure and adjust only one voltage at a time.

Electrolytic
No big deal with the bigger ones, only C1 and C2 the 0.33µf 25v. On later models one can find 4.7µF tantalum caps instead of the 0.33µF.

Quote: “One word: RUMBLE.
Turntable rumble. 4.7uf will pass much more of it, as the -3db point is drastically lowered.”

Lots of information what could be the reason, I had as well the 4.7µF and replaced them with 0.33µF 63V Film for a turntable test with my Technics. Everything was fine, so the 0.33 film caps will stay.

If you like to go deeper into that subject,
https://audiokarma.org/forums/index...wer-amp-board-need-help.745914/#post-10112346

Testing the STV-4H
Only if necessary I would touch the two Varistor D1 and D2 since the leads will break off very easy and there is no NOS replacement available.

If you need to test it, use Merlynski test jig from here
https://audiokarma.org/forums/index...80-smoker-project.980474/page-3#post-15331656

My two good varistor tested with 2.2 and 2.15 vdc to give you an idea what to expect.

If you have removed the STV-4H and you have afterwards no clue which leg is anode or cathode. The white point on the side marks the cathode!! And don’t forget, the D1 and D2 cathode on the Power Amplifier Assembly AWH-033, are both facing inwards.

Next to last job on the Power amp, write down each position and/or make some pictures, then remove the output transistors for testing and cleaning.

6 Way to Test a Transistor
https://audiokarma.org/forums/index...-transistor-testing-basics.43186/#post-432830

First you need a decent digital meter with a diode test function. Forget about using the ohmmeter part of your meter, it is way too unreliable for this. Almost all digital meters nowadays have a diode test included.

After a good test clean up the old grease on the transistor and on the big black heatsink, that removes as well the unpleasant odor caused by the old thermal paste. Use new TO-3 glimmer blades together with a tiny layer new thermal paste before the output transistor can go back to the heatsink.
Don’t use force, they should go in with some light finger pressure and sit flush on the heatsink.

Nearly done, take a long break before you recheck all changed parts for the correct pins.
Make a test run on DBT, before going on.

 

[pa_200]

6. Adjusting the DC OFFSET (Center voltage) and BIAS (idle Current)

This part will cover adjusting the Center voltage (Pioneer's name for OFFSET Voltage) and Idle Current (Pioneer's term for BIAS)

Use larryderouin procedure
https://audiokarma.org/forums/index...37-offset-and-bias-help.979262/#post-15222581

and Merlynskis corrected manual
https://audiokarma.org/forums/index...-turns-on-and-off.956522/page-2#post-15276469

You can have more Information here as well

With the use of 4 DMM, I was able to measure both cannels at the same time. All DMM were set to mVDC and after powering up, the DC Offset voltage was always around 20mVDC and the bias voltage around 15 mVDC. Turning the trim pots should be done careful and it requires only very small movements! We are talking millivolts!!

With my first Pioneer I was unsure and always turned down DC Offset voltage immediately. But with more experience I accept the higher DC Offset and wait until the power regulator had the time to start working. You can see the bias voltage rising and the DC Offset going down. Since no speakers are connected there can be now harm to them.

Now I only check at the beginning that the DC Offset is below 50mVDC and decreasing, then wait for a few minutes to adjust both, bias to 20mVDC and DC Offset close to 0 mVDC. Let it warm-up for at least 10 minutes and recheck/adjust all 4 values again to have them correct and stable.

If you don’t wait, you will find yourself adjusting the VR3 and 4 back and forth all the time.

 

[pa_200]

7 - Tuner Assembly AWE-043
Nearly done, last board is the Tuner Assembly AWE-043

My start with the tuner was first very disappointing, no signal at all. You can read the story here.
https://audiokarma.org/forums/index.php?threads/sx-737-tuner-awe-043.975508/

The culprit was a failed Q5 HA1137, after removing it, putting in a DIP 16 socket and a new HA1137, the tuner worked again.

These are the other parts I have changed on that board.
Q8, Q9 (2SA725) – Replaced with SA992
C28, C33, C38, C39, C40 – Replaced all with film, the light blue Sanyos where really bad with up to 99 ohm ESR
C25, C27, C50 – Replaced with new electrolytic

We don’t use AM anymore, so all AM leads and the antenna have been removed as well. Just in case you are missing the wires.

 

[pa_200]

8 - Filter caps, Changing the light to LED, new power socket

Main Filter Caps,
I like the idea with Bleed Resistors and Bypass Caps. The new Caps have only 35mm diameter and the smaller cap holder need some work to fit with the mounting screws.

I hope that nobody will complain about the way I have attached the resistor and cap and the flexible connection with all wires. But this gives me the chance to change the Filter Caps very easy if needed.

New Filter Caps are 10.000µF 100V, Bypass Caps 1uF 400v, Bleed Resistors Metall Oxid 6.8K Ohm (3W)
https://audiokarma.org/forums/index.php?threads/project-sx-7730-sx-737.979234/#post-15215369

You are strongly advised to write down and make picture of each filter cap and the wire going to which pin. On my first 737 I removed the wiring without a note because it looks so simple with the 8 leads for rewiring, BUT I got lost and nearly mixed up the wiring.

To give you a better clue, here is how it was connected on my SX-737
C2 +pin: one red wire to pin 33 power amp and second red wire to pin 9 power supply
C2 –pin: one black wire the chassis ground, second black wire to +pin of C3

C3 +pin: one black wire to – pin C2 and the white wire to the transformer
C3 –pin: one orange wire to pin 29 power amp and second orange wire to pin 10 power supply

----------

Changing the Pointer, Meter and Dial Scale lamps with LED

This time I used Light Blue LED for the Signal and Tuning Meter, BUT WARM White for the 5 Dial Scale sockets. That gives a nice and smooth blue color on both meters, and a nice blue/green on the dials scale.

Before I used only COOL BLUE on my SX-828, for my wife it’s a bit too blue in a dark environment, but we will leave it for now and compare it with the new mix on the SX-737.

The old fuse-type-lamp-ends have been a bit wider (6.3mm old vs 6.0 new) so I gave each lamp socket a little squeeze to hold the new LED firm in place with good contact. Test the LED if you can easily move them with your fingers or if they sit firm in the socket clamp.

The Dial pointer is now a 3mm red LED together with a 330 ohm resistor.

The last change was made with the power cable. Replaced it with a IEC-60320 C14 socket, so we can detach the cable during transport.

Thanks again to all who took the time to contribute.

 

[Mikerodrig27]

Thank you for posting this! I appreciate the amount of detail in your writeup.

 

[emsrph]

Yes, appreciate your documentation and tips. I don't have a 737 but the read is applicable to most receiver work.

 

[thismansaboy]

This is an amazing help! My 737 keeps popping into protection, but I never bothered to go for it. A newbie as well, I had great success with my SX 626 and just decided to let the 737 gather dust. Now I need it, and started doing my research, only to find this post, which wasn't there a year ago! Such a wonderful contribution. Thank you for inspiring and providing the information!

 

[mel clifty]

This is very good thank you

 

[Jimbo40]

5 years ago I had never worked on electronics, and understood next to none of it.
Now I do, and rebuild with skill and knowledge.
Not to the level of the experts in here, but am well informed, and have no fear of moving into a big project.
Your tutorial was well done, and just what is needed for a hobbyist starting or afraid to start.
And for the beginner; TAKE PICTURES, PICTURES, PICTURES! Closeups.
My pictures have helped a hundred times.
Your camera is one of your most useful tools.

 

[fromnowon]

Makes me want to find time to finish my SX-737 project.
Thanks @pa_200 for all the effort and research you did, it will help a lot! Interesting points about 2SC1451s.

 

[larryderouin]

I think pa200's points on the 2sc1451 were from a logistic's and availability standpoint as he's in Germany. The KSC3503 as used by AK'ers is predominately a U.S. thing. Fairchild/Onsemi components may or may not be as accessible or as inexpensive in Europe. Another sub that fits or exceeds original parameters is always a good thing to have on call.

 

[Mr_Mustrd]

Thanks for posting this @pa_200! Pretty soon I’ll be embarking on restoring a SX-636 and while the 737 and 636 are different there will be similarities between the two and your post will come in handy! It was your post I learned how to set the bias and offset for the SX-727!

 

[thismansaboy]

Hey, could you describe the process you used to convert the power cord to the IC type? It looks like you cut a hole on the back panel and drilled some fastener holes. Is this all it took? Are there any watch-outs in the rewiring you'd care to mention? Has anyone posted a thread detailing this procedure? Thanks!

 

[pa_200]

It looks easier as it really is.

Before you cut the old cord, make sure you have enough slack left. Next is the position for the hole. You need to keep the position of the transformer in mind, to have some working space.
I drilled four holes and used some kind of a Dremel tool and Mini Files for the final work. Get all the metal filings out and recheck the area, that you have not produced a short with some small metal parts left.
The wiring colors just copy from my picture and make sure, the yellow/green wire is in the middle, if you have a ground wire. I you have only life and neutral, take the left and right pin.
First power up on DBT isn’t a bad idea!

 

[thismansaboy]

Howdy, looking for a little guidance: C14 and C15 on my PS board (057) have developed some white fuzz on their surface. I scraped at it trying to get numbers for replacement, but it appears even that has dissolved. The schematic says they're .001/50v. Is this a 1000pf? Sorry for asking basic stuff, but I'm not sure. I have some 1500pf/50v on hand. Will there be any adverse effect using them as replacements.

 

[larryderouin]

Bookmark this page. uF to nF to pF conversion. Yes 0.001uf = 1000pf. Not sure on repercussions on a 1500pf as replacement, but I wouldn't do it. Stay with values on the board/original part.