Replacing Transistors Pioneer SX-727

chrislee

New Member
Hi,

I love the old Pioneer SX-727 I bought, but I have the snap crackle pop syndrome, as described elsewhere by almost everyone. The more I learn on this forum, the more worried I am about replacing the transistors on the head amp myself.

Based on advice to previous newbies, I bought 512-KSA9992FBUs to replace the 2sa725 transistors. I understand that the outside legs need to be reversed. Some advice suggests I replace 4 of the 6 transistors on the board (Q1,Q2,Q3,Q4). Others say just replace two, but I’m unclear which two. Here’s a picture showing Q2, Q4, Q6.

How do I manipulate the white plastic legs on the corner of the board to expose the backside of the circuit board?

THanks, Chris
 

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Looks to me like the white pins have a flexible part that you pinch and then slide the board over. As to which ones to replace...since you have them on hand, save yourself the future hassle of having to do it again when the ones you don't replace fail. Do them all now.
 
DANGER WILL ROBINSON!

The KSA992 are NOT for the HEAD AMP(w21-002). Replace the ones on the headamp (2sc871 or 2sc458) with KSC1845's. You've got pnp's and need npn's.

The KSA992 replace the 2sa725's on the (AWK-010)AF BOARD (Q3, Q4).

The top of the Nylon pins squeeze together and while squeezing you lift up on the board. Don't get ham handed and pull too hard or you'll break the board in the corner. And yes the ksa992's get put in reversed from the 2sa725's. If the FLAT FACE of the transistor (with the lettering) is facing left, then the new transistor goes in facing right (or 180* out).

The Service Manual, Service manual ADDENDUM(HIGHLY RECOMMENDED TO GET AND READ!!!!) and schematics here. Also Owners Manual.
 
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Thanks. OK, so KSC1845s to replace 2SC458 on head amp board to fix snap crackle pop. (And for these, I don't swap the transistor placement.) They are labelled Q1-Q6 in the manual. I would prefer to touch as little as possible. Which do you suggest I change?
 
Chris; REPLACE ALL 6 on the EQ AMP. You don't change 1/2 or 2/3 of the spark plugs on your car do ya??

With the FACE of the transistor facing you (lettering on face) and the LEGS DOWN. The LEG ORIENTATION IS.........

2SC458 = B-C-E
KSC1845 = E-C-B

So the KSC1845 goes in 180* from the 2sc458. Again. If the 458 FACES LEFT, the 1845 will FACE RIGHT.

Same Deal for the KSA992's and the 2sa725 on the AF board.
 
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Larry,

Thanks for your time on this. In the interest of leaving a useful archive on this, here are the steps I plan to take. Please correct me where appropriate.

PROBLEM: Pioneer SX-727 or SX-x2x, crackling and pop on audio regardless of source selection or volume setting.

SOLUTION:

1) De-Oxit. See http://www.audiokarma.org/forums/showthread.php?t=207005

HEAD AMP BOARD

2a) On Head Amp board (W21-002), remove board by somehow manipulating the four plastic corner pins and gently lifting board off.

2b) Desolder the 6 transistors (old: 2sc458). Replace with KSC1845. Reverse the pin orientation: middle pin to same terminal, outside legs reversed.

2c) Replace board.

AF BOARD

3a) Somehow remove front faceplate of unit. Are there instructions somewhere?

3b) Access AF board (AWK-010). Replace Q3 and Q4 on this board. (Old, according to this forum, is 2sa775. My manual says they are 2sa572-5A or -5B.) Replace with KSA992. Reverse the pin orientation: middle pin to same terminal, outside legs reversed.

3c) Somehow reassemble front of unit.


I'm trying to avoid using alternate names for each board: EQ Amp, Control Amp, etc., to reduce the possibility of misunderstandings.

And if I had to rely on my decaying blob soldering skills to replace spark plugs, I might replace them one at a time!

Thanks again, Chris
 
Additions:
2D) Test unit to prove out work on Head Amp board.

3A) Exploded diagram in the Service manual. Pull the knobs, undo the 2-3 nuts, remove screws(2-3) on top flange, check bottom also. Take plenty of pictures for verification.

3B) With 2sa572's in there vs. 2sa725's, DO NOT ASSUME that the leg orientation is AUTOMATICALLY Reversed based on one transistor. See this from MARK THE FIXER. About 75% down is the part that is relevant to putting the right leg in the right hole.
http://www.audiokarma.org/forums/showpost.php?p=8086786&postcount=27

3D) TEST BEFORE PUTTING FRONT PANEL BACK ON.

3E) reassemble Front Panel to unit.
 
+1

It's very important to look at the transistor symbol that's used in schematics and printed on the circuit board. Once you know which part of the symbol represents which part of the transistor, your substitutes will be oriented properly. It might not always be a reversed look. Base, Collector, Emitter all have special symbols. All the current Qs you'll order will have a datasheet which will list the pin out of that specific transistor, which can but not always will be different from the original. There's six ways to insert a three legged transistor, only 1 is correct.
 
Hi,

Well, I wish I could claim I full success. Need a little further wisdom.

1) I was able to replace the six transistors on the head amp board.
2) I'm pretty sure I damaged the balance control when disassembling the unit. Can anyone help identify the replacement part #?
3) The diagram for the AF board is pretty poorly labelled. There are six transistors on the board. I can read the numbers on the transistors themselves. I can match Q1&2 to what is shown in the parts list in the manual. I can match Q5&Q6 too. The remaining ones should be the ones I want to replace: Q3 & Q4. However, the number I read on the transistor is 35 A763 P5. (The number in the manual is 2SA572-5A or 5B, the number this forum suggested to expect was 2sa775.) My replacement parts on hand are KSA992. Have I located Q3&Q4. Have I got the right replacement parts?

Appreciate the group's continuing wisdom...

Chris
 
The 2sa763 is a known substitute for the 2sa725 back then. If you look in the 737 manual (or any of that series) most transistors will have a primary transistor listed with 1 or 2 substitutes. Your's may have been made at the end of the series run and parts are overlapping from the new run. YES the 992 will work as a sub for the 763's. Go ahead and install them.

Damage the Balance control, HOW???? According to the schematic for that board VR1 is the Balance (see the notes on the schematic), then the parts list has it as a DUAL 100K pot, part #C82-049-a. Also used on the SX-828 and QX-9900.
 
Balance pot damage: Stupidity, of course. Inner core of the dial absolutely would not come off. Tried using two spoons, etc. For a brief moment i thought it would be a really bright idea to just keep turning the balance knob against one of its stops to magically break it free. Anyway, something clicked and from that point forward I had audio on one channel only and knob felt like it had sand inside.

To replace, do I need to scavenge a used part from another Pioneer, or is there a way to translate the spec. into a current, orderable Mouser part?

Thanks again...
 
My SX-1250 just started doing the snap crackle pop in the left channel yesterday. This thing has been recapped just months ago. Then I had the phono section redone as transistors failed there. Now this ... starting to look like the once skinny girl who's bloated beyond recognition to me.
 
Mission accomplished. I can't say I'd recommend having to replace the balance pot. Not an easy squeeze. Thank you Larry, et al.

Is it possible the unit has less power now than before? It doesn't seem to go as loud, but I have no data. Thanks again.
 
Not unless you have a transistor or capacitor in backwards. Also check for "COLD" solder joints on all the boards.
 
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