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SX 3900 - Something blew up

JDW88

JDW88

New Member
Anyone have any issues with SX 3900 components getting fried?

I recently acquired a Pioneer SX 3900 from an estate sale for a song. With the exception of a couple push-button lights everything worked great.

I was listening to a record and maybe about 40% into the volume when a slight “pop” occurred follow by total silence and a wisp of smoke from about the center of the unit. The unmistakable smell of ozone is always a bad omen. All lights still lit but all channels are fried. Haven’t wanted to open it up until I have a better idea of what component burned out. More likely: will take to a professional for repair.

For a 125wpc unit I don't think I was pushing it's limits with the volume at 40% (though 40% with a unit like that going into Sansui SP X9000 speakers is definitely LOUD).

Not sure it it should affect anything but I run my PL 530 turntable into a Bellari VP 130 tube preamp. I keep the output on the preamp at about 90%. Obviously, I use the AUX input for the preamp and not the phono.

I've run the same set up into my Marantz 2235 with no issues for over a year.

If anyone has any suggestions on where to start or any insights I'd love to hear them. I'm pretty heartbroken as the five minutes it was working it was the best sounding receiver I've ever heard.

Thanks,
-Justin
 
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*shrug*

Always a chance of there being some SX 3900 buff who's had 12 of these and knows what their achilles' heels are.
 
The center is where the amps reside.
Power supply is on the left side.
NSA topography, parts are a chore to procure.
Not every tech is familiar with this type of amp.
 
Achilles's heel(s):
power supply ventilation / overheating - complete rebuild, PITA location behind left side of front panel, cabling must be bundled to allow airflow.
Also smart to put reinforcement eyelets in the power transistor mounting holes

power amp solder joints (wave solder went light and thermal cycling cracks them) to
onboard thermally HOT transistors - 4 especially under rated and changed to larger transistors.

power output transistor sockets crack - we have identified substitutes.

all correctable, and unit can be made really reliable.

WILL take some work, complicated by the power amp board blowing up.
THAT calls for 100% resistor (lifted leg) testing and ALL new silicon, including some upgraded transistors, KEEP the TO-220 driver transistors if not damaged.
 
this is gonna be an interesting thread...Mark: is there a BOM already posted or time for the OP to pour thru wisdom?
 
Found this from a past MTF post:
http://www.audiokarma.org/forums/in...0-powers-on-but-no-audio-output.190127/page-3

Quote from MTF:
"This is one of those innocent sounding posts that is going to eat up my entire evening......... sigh......

Also I have the sneaking suspicion that I have already posted much of this somewhere else in the Pioneer forum.

I have ALREADY done the shotgun thing with the 3900's amps. They can be finicky.

FIRST: Q101, Q102, Q103, Q104, Q105 & Q106 are NOT replaced willy nilly. They have to be PROVEN BAD.
Q101 and 102 are offset compensation transistors for the output offset compensation setup.
Q103 and Q104 are the differential stage transistors, are FIVE leads each, and are TWO transistors in a single case, and they share EMITTER leads.
IF you do INDIVIDUAL eplacements, you need to find the highest gain matched pair.
Q105 and Q106 make up the current mirror portion of the differential stage, they have FIVE leads each, are TWO transistors in a single case, and they share BASE leads.

I HAVE done what you propose - shotgunning the ENTIRE amp, with those exceptions above. Don't forget to do ALL the diodes too, including the zeners. When I used ON-Semi MJE15032G and MJE15033G transistors in the Q134 - Q136 positions ( the four heatsunk to-220 drivers across one edge of the board) I had severe bias control problems, I could NOT dial the bias down to zero, and I had NEW power output transistors in place as well, which when I had FIRST used KSA940 & KSC2073 transistors the bias adjusted properly. I used the mje's as an "upgrade" so that there could NEVER EVER be a situation where the drivers were loaded down. Needless to say the MJE's came out and the KSA/C's went back in.
The outputs were On-semi:
863-MJL1302AG pnp TO-3P/TO-264 bce 260V 15/30A 200W 45-150 30MHZ $3.50 ea
863-MJL3281AG npn TO-3P/TO-264 bce 260V 15/30A 200W 45-150 30MHZ $3.50 ea
These are the smaller brothers to the ones EW used in the sx-1980 restoration



These are the six transistor types I used across the amp:

512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea Q107, Q108, Q111, Q112, Q117, Q118, , Q103, Q104
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea Q113, Q114, Q119, Q120 , Q101, Q102, Q105, Q106


************************************
start edit 9/12/2010 replacements
512-KSA910YSHTA to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.07 ea Q115, Q116, Q121, Q122 9/12/2010 obsoleted
512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea Q109, Q110, Q123, Q124 9/12/2010 large to-92 discontinued
these (ksa910/ksc2310) ran hot...

edit 9/12/2010 possible ksa910/ksc2310 replacements
512-KSA1013YBU to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17 (R=60-120, O=100/200, Y=160/320)
512-KSC2383YBU to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17
---or---
512-KSA1220AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.39 (R=60-120, O=100/200, Y=160/320)
512-KSC2690AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.40
---or---
512-KSA1381es x to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.27 ea
512-KSC3503estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.28 ea


***********************
original transistors:
2sa985/a pnp to-220 bce 120v/150v 1.5a 25w 180mhz 35-320hfe orig 180mhz nsa driver
2sc2275/a npn to-220 bce 120v/150v 1.5a 25w 200mhz 35-320hfe orig 200mhz nsa driver

these:
***************!!!!!!!!!!!!!!!!!!!!
757-2SA1930(Q,M) pnp to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver
757-2SC5171(Q,M) npn to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver
***************!!!!!!!!!!!!!!!!!!!!
old and slow:
512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54 Q135, Q136
512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54 Q133, Q134

***************************
9/12/2010 end possible replacements

misc diodes:
512-1N5234B 6.2v zener D101 - D106
512-1N4148 D113 - D116
512-UF4004 ultra fast rectifier 1amp 400v $0.13 ea D111, D112, D117, D118
512-1N4454 D129, D130, D132, D133
512-1N5245B 15v zener D131

and two transistor types I used in the protection circuit:

512-KSA1013YBU to-92 ecb 160v 1a .9w 50mhz 160-320hfe $0.17 ??Q139 (if 2sa1100)??
512-KSC2383YBU to-92 ecb 160v 1a .9w 50mhz 160-320hfe $0.17 Q125, Q126, Q137, Q138, ??Q139(if 2sc2575)??,
512-KSC2690AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.40 Q140

edit, Here's the caps:
two 4.7uf 25v amplifier input caps 647-UPW1H4R7MDD 0.14 ea
one 100uf 25v protection cap 647-UPW1H101MPD 0.36 ea
one 470uf 16v protection cap 647-UPW1E471MPD 0.68 ea
two 0.22uf 50v protection caps- changed to poly metallized film 598-DSF050J224 0.50 ea


I have decided to NOT include the original transistor specifications in this post, nor detail (argue) the reasons for my choices of either which transistor replaced what, but even why there MAY be two different replacement numbers used for the same original number - it has to do with what is doing what, and where in the amp. (hint - a few get WARM, and I'm glad I did those substitutions)

The results speak for themselves, I have about the same distortion numbers that original versions of working amps produce.

PAY VERY CLOSE ATTENTION TO THE LEAD ARRANGEMENTS. The emitters are marked on the board.

You have a choice for Q139, depending upon it's type and the variant of the protection circuit on this unit. If it's the 2SA1100, it is the newer circuit.

I also did NOT have positive results using a zetex ztx1056a in the Q140 position, it immediately pulled the relay in on powerup. It could have been a leaky D131, but I just returned the original 2sc1384, after which it operated normally".
 
They all run hotter than a two dollar pistol on a Saturday Night.
They are beautiful and I loves me some Blue Fluoroscan any place Pioneer used it.
But that series is my LEAST favorite of the Silver Era. Those wiring harnesses did not age well.

Good luck with it. If you don't work on them yourself, then you'll need to seek out the help of a good shop technician.
 
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