Project: Pioneer SX-750

[Super Noob]

Well was hoping my next project would be a SX-1250 found locally, but it looked to have fallen through. So last weekend I spotted, a smaller SX-750 really priced right. Based on my results with the SX-850 and how well it responded to my repairs and recap, I decided to go for SX-750.

When I saw it, I figured a new piece of glass would not be very expensive. The vinyl was pealing as well, but did I mention I picked it really cheap.

It was functioning for the most part and really dirty.

Turns out the glass was not necessary... someone just added some tint to the dial glass.

Pulled the vinyl and sunglasses. Here comes my rebuild.... may posts and not as well documented as I typically do.

Here comes the progress posts.

 

[Super Noob]

Did the LEDs while I waited on caps:

 

[Super Noob]

Main Power Supply Board. Noted that I added a heat sink on Q12 that I salvaged from a old computer power supply cut down.

Before:

After with New Caps and Relay:

Repasted all the transistors with heat sinks:

Old Caps:

 

[Super Noob]

Main Caps:

I ended up using 18,000 uF Nichicon Caps.... snap in type (was not the easiest). Thought I had height limitations, but I didn't account for the additional depth below or I would have bought screw terminals. Added 2.2uF bypass caps.

 

[Super Noob]

Amp Board:

Before:

After with New Trimmers:

Re-Pasted the Output Transistors:

 

[Super Noob]

Tone Control Board:

Before:

After:

Power Switch Working Fine = Went Proactive and Installed a Triac Mod (per K7sparky directions)

 

[Super Noob]

FM Board and Muting Assembly:

 

[Super Noob]

Finishing Up:

Setting Bias = God Bless 25 turn Trimmers (Dead Nutz) both Channels

Here show blows... on top my beloved SX-850

Have New Wood Veneer Here = May Tackle Today.
Sounds Great. FM Stereo Light Does Not Work (did not work before I started). Might post another thread on this. I get FM channels, tuning & signal strength. Have 13vDC on the pin 22 on the power supply, but the FM switch on pin 9 of the IC is not turning on. (am competent but don't want to make things worse).

Going to gift to a co-worker.

Did I say the SX-1250 fell through = got a call today and was able to secure. Can you say next project?

 

[paul_ral]

Well done!

 

[Jibe01]

Thanks for posting. Love the pics. I've got a 750 also. Needs some work. Maybe one day I'll get to it.

 

[Watthour]

Nice work, both in the save of the 750 and the documentation. In addition to the added heat sink, did you go with 63V or better caps in the C17/18 positions?

If you want to address the STEREO indicator you might be lucky. I just happen to have a 750 open on my bench right now, with a fully operating FM section.

 

[Super Noob]

Nice work, both in the save of the 750 and the documentation. In addition to the added heat sink, did you go with 63V or better caps in the C17/18 positions?

If you want to address the STEREO indicator you might be lucky. I just happen to have a 750 open on my bench right now, with a fully operating FM section.

Power Supply:
470uF 63v = C17 &C18 per a Leestereo build.
(In lieu of 330uF 50v)

I have done some reading about complicated FM adjustments. Like I said before I can get stations and signal up to level 3. Even tunes with FM mute engaged. Ready to try and mark original setting.

Don’t have a FM signal generator. Would like to try course adjustments with some oversight.

Garage refrigerator just died = compressor - so rolling with wife for a replacement for the next few hours.

Thanks = Watthour

 

[Watthour]

1. The C17/18 upgrade has been on MTF's recap list for almost a decade, although Mark lists 330µF and I bumped those to 470µF (UPM1J471MHD6TN) in about 2013;

2. The FM can be "peaked" without a siggen IF you are very meticulous and willing;

3. When you get back with your new Whirlpool and restore domestic bliss, we can walk you through the front end adjustments. The stereo won't keep your milk cold...

 

[Super Noob]

The Signal Location Is Very Close to Correct.

http://audiokarma.org/forums/index.php?threads/pioneer-fm-tuner-alignment.712855/

Let me know if I should be reading something else... thanks in advance Watthour.

 

[Super Noob]

Have peaked the FM signal by adjusting TC3, TC1, & TC5 to maximum points (in that order) ever slight adjustments from the original position.
(using not metal instruments)

FM signal and sound improved. Dialing in the stations now between 4 to 5 on signal range with FM muting engaged.

Improved = Time To Keep Reading (heading to the deep end of the pool).

 

[Watthour]

You should probably go back to the manual and review sections 4 through 18. Instead of a signal generator, you can use a moderately strong broadcast signal near the lower end of the scale, between 88 and 92 MHz. Tune T4 with a non-metallic slotted trimmer tool for best signal on the signal strength meter. Tune to a place on the dial where there is no apparent broadcast, ground the FM antenna input to a chassis ground, and adjust the core of T6 with your non-metallic screwdriver to obtain a dead-center indication on the tuning meter. Remove the FM antenna jumper.

Tune a moderately strong station in the 88-92 MHz range, and adjust the cores of T1, T2, and T3 with a non-metallic hex tool for peak indication on the signal strength meter.

Tune the receiver to a modertately strong station between 104-106 MHz and adjust the trimmer caps TC1, TC3, and TC5 on the tuning cap as you did before to obtain best signal strength.

Go back to the station near the lower end of the range and recheck/adjust T1, T2, and T3 for peak indication. Then got back to the station at the upper end of the scale and adjust TC1, TC3, and TC5 for best signal.

After several trips back and forth on the dial you should have it about as good as it will get.

When that is complete, once again tune to dead air, ground the FM antenna input, and readjust T6 for dead-center on the tuning meter.

Finally, tune a moderately strong station near the center of the scale and adjust the upper core of T7 for highest/best audio output level.

Beyond that, you would need a MPX generator to effectively adjust the separation, pilot signal, and distortion. However, presuming there has not been substantial drift in those areas, your front-end peaking should get a solid signal to the MPX detector so it can perform at its best.

 

[Super Noob]

You should probably go back to the manual and review sections 4 through 18. Instead of a signal generator, you can use a moderately strong broadcast signal near the lower end of the scale, between 88 and 92 MHz. Tune T4 with a non-metallic slotted trimmer tool for best signal on the signal strength meter. Tune to a place on the dial where there is no apparent broadcast, ground the FM antenna input to a chassis ground, and adjust the core of T6 with your non-metallic screwdriver to obtain a dead-center indication on the tuning meter. Remove the FM antenna jumper.

Tune a moderately strong station in the 88-92 MHz range, and adjust the cores of T1, T2, and T3 with a non-metallic hex tool for peak indication on the signal strength meter.

Tune the receiver to a modertately strong station between 104-106 MHz and adjust the trimmer caps TC1, TC3, and TC5 on the tuning cap as you did before to obtain best signal strength.

Go back to the station near the lower end of the range and recheck/adjust T1, T2, and T3 for peak indication. Then got back to the station at the upper end of the scale and adjust TC1, TC3, and TC5 for best signal.

After several trips back and forth on the dial you should have it about as good as it will get.

When that is complete, once again tune to dead air, ground the FM antenna input, and readjust T6 for dead-center on the tuning meter.

Finally, tune a moderately strong station near the center of the scale and adjust the upper core of T7 for highest/best audio output level.

Beyond that, you would need a MPX generator to effectively adjust the separation, pilot signal, and distortion. However, presuming there has not been substantial drift in those areas, your front-end peaking should get a solid signal to the MPX detector so it can perform at its best.

Ok... will do this after work tomorrow. Thanks so much for your help and interest.

 

[Super Noob]

Watthour =

Having you in my corner made a difference. The FM tuner is working great with these minor tweaks.

Thanks so much for your help - very appreciated.

 

[Watthour]

The SX-750 is a bit of a "sleeper." With a little upgrade here and there it belies its 50WPC rating. As you discovered, the tuners are pretty good. The weakness of the power switch is easily remedied, as you found out. With the rest of your upgrades and refreshing done, it should make for a solid unit.

 

[kydog]

Very nice work!! I have a SX 850 that could use your touch. Lol