Project: SX-7730 (SX-737)

[larryderouin]

2 of each. MJ21193G & MJ21194G.

The amp is a full complimentary type, with Dual voltage rails (+ and - sides). Analogous to a Push-Pull Tube Amp where each output tube of a pair alternates power output on the sine wave.

DON'T Mix them up. You'll release the Magic Smoke and cause massive death and destruction of the electron populace in transistor town, and capacitor city.

 

[soundguy9]

Protection Assembly (AWH-025-C)
Capacitors:
C1 0.22 uF 10v Electro (CSSA) 0.22 uF 100v MKS2D032201B00KI00
C2 0.22 uF 10v Electro (CSSA) 0.22 uF 100v MKS2D032201B00KI00
C3 330 uF 6.3v Electro (CEA) 330 uF 16v UPW1C331MPD
C4 330 uF 6.3v Electro (CEA) 330 uF 16v UPW1C331MPD
C5 4.7 uF 25v Electro (CEA) 4.7 uF 50v UPW1H4R7MDD
C6 100 uF 16v Electro (CEA) 100 uF 25v UPW1E101MED

Transistors:
Q1 2SC869 KSC1845
Q2 2SC869 KSC1845
Q3 2SC945 KSD1616A
Q4 2SC945 KSD1616A
Q5 2SC733 KSA733CLTA
Q6 2SC945 KSC2383YTA
Q7 2SC1384 KSC2690AYS

D8 1S2473 New 1N4004
1N4004 fly-back diode is added between pins 9 (anode +) and 10 (cathode -)

I am getting a low tump sound when I turn off the power switch. I did a rebuild of the protection circuit as per the above except that I replaced all transistors regardless if they were good or bad. Also, I did not have the KSA733 so I used KSA992 at Q5. The relay goes on about 5 seconds after the unit is powered on. I have ordered some KSA733Cs but I was wondering if I should put the original transistors back on the board? What are your thoughts?

 

[Greg64]

Double check installation directions of all components.
…speaking from experience…but cannot guarantee it resolves the issue.

Next thing might be to look for open/failing resistors?

 

[soundguy9]

Double check installation directions of all components.
…speaking from experience…but cannot guarantee it resolves the issue.

Next thing might be to look for open/failing resistors?

You are correct as I have had the same experience many times in the past. I made the error on this unit but then the relay did not engage at all. The relay is engaging now but it is slow to disconnect so I hear a tump sound when the unit is powered down. Anyhow, I will go through each part and verify my workmanship later this week.

 

[Greg64]

I guess if once you’ve confirmed everything you could swap the old 733 back in. Another thought might be to disconnect the flyback and see if it goes away.

 

[soundguy9]

I guess if once you’ve confirmed everything you could swap the old 733 back in. Another thought might be to disconnect the flyback and see if it goes away.

Good suggestions, I should be able to get to it later this week.

 

[soundguy9]

I was able to spend some time on it today. I found that both KSC1616As had the wrong orientation on the board! Initially I had installed Q3 with the wrong orientation and the relay would not engage. Then I removed and installed Q4 with the same wrong orientation as Q3 and the relay started working! Anyhow, I found my mistake (Kudoos to Greg64) and placed two new KSC1616As with the correct orientation and the relay is engaging. Then I did a short victory lap and when I turned off the unit I heard the same tump sound

Then I replaced the new KSA992 with the original 2S733. This made it to respond slower (engaging the relay) but did not solve the tump sound when turning it off. Then I removed the flyback diode and the tump sound got slightly louder. I replaced both snubber caps on the back and on the power switch and it did not help either.

Anyhow, this is where it is at today. I don't know if replacing the relay would help it or not. I have all the old parts that I could place back on the board.

 

[Greg64]

Any updates?

 

[soundguy9]

Any updates?

Yes, I replaced the 4.7uF capacitor. The power cycling response time can be adjusted by the value of the cap.
I started a new thread and there is significantly more information related to this topic. For more details, search for thump sound when powering down and should be able to find it. Let me know if you need more information or can't find the thread.

 

[Greg64]

Indexed here for a fast follow >

Pioneer sx-737 thump sound when turning the unit off!

Hello Pioneer experts, I have been working on an sx-737 for one of my friends. I followed Larry's suggestions and posts in the past and the units sound good and operate reliably (thanks Larry and others). I started a second unit for a friend, the same resutls, it is now a very good sounding...

audiokarma.org