Re-capping an SX 737

I got my Pioneer SX-737 home tonight and hooked it up . When I turned on the power switch i got the usual delay till the protection switch turns off . It turned off and then clicked right back on again . Now when you power the unit on , it never comes out of protection . I opened it up blew it out with canned air and then cleaned it with contact cleaner i got from radio shack . I then again blew it off with the canned air and let it sit with a fan on it for a few hours . It didnt help , still doing the same thing . I do notice , when you turn the volume up you can hear music coming from the board real faint and tinny ( i noticed this before and after the cleaning ) .

I plan on recapping the unit . Is it worth the money for high end caps ? Any tips on getting the boards out so i can access the bottom to de-solder ?

You have to trouble shoot it, do you have a DMM?

It is "in protection" meaning that the speakers are deliberately disconnected from the speakers to prevent possible fires. The protection circuit could be at fault or there could be a genuine problem in the amp. Not really a big deal to find and fix with the help available here, if you're handy, have soldering tools and a DMM. (since you ARE considering a re-capping)

Caps that are recommended for use in re-capping are :
(digikey.com) Use Panasonic FC or FM,
(mouser.cm) Nichicon uPW or uHE (the u is new & added to the designation)

Download (or buy a paper copy from rick at stereomanuals.com) the .pdf version of the manual from the database, study it for the board layouts and functions, and when you're ready I'll work up a list of points to take voltage readings at to diagnose it.

Thank you , I appreciate your help !! :yes:

I have a DMM and I have printed out the service manual and gone over it . Some of it is a little faint , from scanning perhaps . What do you recommend I check first ?

These are all ground referenced readings, the black lead of the meter goes to a bare metal chassis ground.

On the power supply board (AWR057), probe with the red meter lead:

pin 14 +35v regulated
pin 13 -13v regulated
pin 19 +13v regulated
pin 7 -38v unregulated (it can vary)
pin 9 +37v unregulated (it can vary)
pin 10 -37v unregulated (it can vary)

There are several fuses near there that need to be checked with power off.

Then we go over to the protection board (AWM025),

pin 1
pin 3

These are the amplifier outputs, and when good, will be read in millivolts, and when bad can be 30 or more volts, positive OR negative. If your meter doesn't autorange (automatically switch ranges up or down to get the most accurate reading) and the voltage you are trying to read is higher than the meter can handle at that setting, it will indicate an overload, which means you have to switch it to a higher range (or a lower range if the number is very close to zero). I say this because some folks get tripped up by this.


That 30 volts I mentioned, IF it's at the amplifier's outputs, would be a REAL reason why the speakers were not connected , If NOT, then the protection circuit probably is in error, and a few capacitors or transistors should cure it.

Thank you Mark !!

Yes , I have an auto ranging meter . Actually went out and bought my self a pretty nice one today along with a digital soldering station . Eh.... birthday present to myself ( don't tell my wife :nono: )

I will take the measurements and post the results . Thank you again , it's guys like you that make AK such a great place to belong to . :thmbsp:

Are these AC or DC voltages ?

Maron said:

Are these AC or DC voltages ?

Click to expand...

All measurements so far are for DC, and the volume is at zero.

Hi Mark , all measurements were triple checked .

(AWR057)
pin 14 +17.85v
pin 13 -13.32v
pin 19 +12.82v
pin 7 -37.71v
pin 9 +35.83v
pin 10 -35.50v

Fuses are good ..

(AWM025)
pin 1 -9.13v
pin 3 -35.05v

So , could it be a problem with the amp ? Pin 14 on the PS board looks out of wack , doesn't it ? Maybe a combo of the 2 ? Looking forward to hearing your diagnosis .

Thanks, Mike

sx737 AWR058 power supply rebuild

Oh, yeah, pin 14 IS out of whack

There's not enough voltage to pull in the protection relay even if it wanted to. The amp's power is way out of balance, and the output is too. Hopefully the amps will snap back with the correct voltages. If not, no sweat....

The only fly in the ointment is to check if the voltage that supplies the 35v regulator's Q1 is correct. It should be 48v. It also supplies the 13v regulator (Q3), but could be too low to run the 35v while still running the 13v. Possible but not likely.

Q1 is the D313 transistor in the center of the board. The collector is the metal tab that forms part of the body. Measure that voltage, referenced to ground. It should be about 48 volts.

mouser.com parts numbers:
Q1: 512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
Q2: 512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
D9: 512-1N5244B Fairchild 14 V, 0.5W Zener $0.05
526-NTE303 1 gram thermal compound $1.77
534-4673 to-220 mica insulator $0.14 (IF you find one when you remove the D313 from the heat sink).

Hopefully all the capacitors are behaving themselves. One could have failed and brought the regulator to it's knees. Not as likely, but it's about a $7 gamble either on caps and one postage, or caps later on and two postages...
It all depends if you are game for renewing the caps on the board. If so I'll work up a list, if not get the transistors, we'll put them in and see.

Oh, the pinouts (lead arrangements) on the replacements are the same as the originals - ON PURPOSE...

edit, I added the cap list:
c7 1000uf (50v) 63v 647-UPW1J102MHD $1.87
c8, c9, c10 330uf (50v) 63v 647-UPW1J331MHD $0.66 (50v vs 63v was 0.01 more) ea
c11 220uf (16v) 50v 647-UPW1H221MPD $ 0.48
c12 220uf (50v) 63v 647-UPW1J221MPD6 $0.83
c13 100uf (50v) 63v 647-UPW1J101MPD6 $0.40
c16 220uf (16v) 50v 647-UPW1H221MPD $0.48
c17 22uf (25v) 50v 647-UPW1H220MDD $0.19
c18 100uf (16v) 50v 647-UPW1H101MPD $0.36
c19 100uf (35v) 50v 647-UPW1H101MPD $0.36 (total 6.59)

the (50v) in parens is the specified voltage of the old caps - for several reasons I went up in voltage and in most cases the difference in cost was only pennies for the safety margin - old caps could get 15% overvoltage no sweat - modern manufacturing holds the over voltage to a tight 10 %.

For completeness and posterity:
To get the rest of the parts for a completely new board, get two q1's (another's for q3), two Q2's (another's for q4) and :
D8: 512-1N5243B Fairchild 13 V, 0.5W Zener $0.05
d5, d6, d7 512-1n4004 conventional rectifier 1a 400v $0.06 ea
d1, d2, d3, d4 512-1N5404 conventional rectifier 3a 400v $0.19 ea

Q1 measures at 48.4v

I am going to go ahead and order all the parts . I was planing on replacing the caps anyways . I might as well do it while I have the board out .

Thanks Mark . Let me know if you think of anything else .

Mike

Hey Mark , the parts came in from Mouser tonight . I didn't end up ordering them till last week , with the holidays and all . Where do you suggest I start ? Any tips on how to go about doing this ? :scratch2:

Small easy steps, one at a time, check polarity, capacitance and voltage on each... assume nothing. Always have a graceful exit available.

DON'T watch the clock, or RUSH.

Write things down, take pictures... they're a MUCH more reliable memory.

If you start getting tired or making mistakes, don't push it, knock off for the day.

I generally work with the unit on it's side, secured to a lazy susan (turntable) type foot to eliminate tipping.

I rebuilt the board and everything checks out great on it :thmbsp: . But ..... it's still not coming out of protection . I checked board AWM025 and pin 1 reads -6.1v and pin 3 reads -33.25v .

What would you suggest be my next step ? :scratch2: Will the protection board need to be rebuilt ?

Maron said:

I rebuilt the board and everything checks out great on it :thmbsp: . But ..... it's still not coming out of protection . I checked board AWM025 and pin 1 reads -6.1v and pin 3 reads -33.25v .

What would you suggest be my next step ? :scratch2: Will the protection board need to be rebuilt ?

Click to expand...

Not from those readings, those are inputs for the protection circuit, and it is justified in rejecting them. They are the DC offset voltages from the amplifiers and should be in the millivolt range.

First step: always check the power supply voltages after doing anything, this time we cover them all on the AWR-057 board because one of the secondarily derived voltages could be doing things to us:
pin 14 +35v
pins 15 & 16 +30v
pin 17 +23v
pins 19 & 20 +13.8v
pin 21 +13.5v
pins 12 & 13 -13v
pin 7 -38v
pin 10 -37v
pin 9 +37v

If the power supply voltages are reasonably close, then I would try to adjust the dc offset of each amplifier channel to minimum (zero).

We at this point can hopefully trust the power supply and protection board.

I'm kicking myself for not including the amplifier's replacement transistors in the list
primarily because I didn't look, and both the 2sa726's and the 2sc1451's could be trouble.
But since both channels are doing it, I have to find what's in common besides the power supply board.

2sa726 (ecb) > 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
2sc1451 (ebc) > 512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea
2sb527 (bce) > 512-KSA1220AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.39
2sd357 (bce) > 512-KSC2690AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.40
------- output transistors (unlikely but for completeness) -----------
2sb530 > 863-MJ21193G pnp to-3 250v 16A 250W 4mhz 8-75hfe $3.83
2sd370 > 863-MJ21194G npn to-3 250v 16A 250W 4mhz 8-75hfe $3.83

Hey Mark , here are the readings that I just pulled .
pin 14 +34.9
pins 15 & 16 +29.5 / +29.5
pin 17 +23v +17.5
pins 19 & 20 +12.9 / +12.9
pin 21 +12.9
pins 12 & 13 -13.2 / -13.2
pin 7 -36.4
pin 10 -35.3
pin 9 +35.4

I do not have a "dummy" load unit , so I used an old pair of 8ohm speakers . I tried to adjust the offset voltages on the amplifier board , as per the service manual . I am not really getting any adjustment from VR1,VR2,VR3, and VR4 . Could this be because I am using speakers for the load ? or something else ?

Also , I missed the cutoff for shipping today with Mouser , so the order will not ship until Monday . If there is anything else you may think of that we might need , or that we should do while I have the unit apart , let me know .

Once again , thank you Oh Pioneer Guru . :thmbsp:

So you tweaked the bias pots in the hopes of lowering offset? Not good...

EchoWars said:

So you tweaked the bias pots in the hopes of lowering offset? Not good...

Click to expand...

Just being thorough. I just wanted to see if there would be any effect at all, one channel was at -6v instead of at the supply rail. There could have been a failure of the offset voltage divider network, there is a diode in it.

No speakers, don't know why Pioneer persisted in that method, PLUS with the protection circuit unhappy, the speakers are NOT connected.

Here's a neat bit of troubleshooting you could do, read the base to emitter voltages (red dmm probe on base, black dmm probe on emitter) on all of the transistors on the power amp board, if one isn't somewhere around 0.6v, that could be one of the ones that are bad. It's easy, just go by the symbol silk screened onto the board under the component. When you find a bad one, then look up what it is by the number on it and the position on the board.

BE SURE that all but the very tips of the probes are insulated so when they slip, nothing is shorted.

Before you do that, you can take ground referenced voltage readings of the output transistors, to be sure they are good, if they look suspicious then we'll pull them for a closer look with out of circuit testing.

read the ground referenced voltages on pins 20 through 33 inclusive and post them. one of pins 32 or 33 may or may not be there.

20= -31.0
21= -30.4
22= +33.2
23= -31.5
24= -32.5
25= -33.2
26= -6.0
27= -6.4
28= -33.2
29= -33.2
30= -5.5
31= -5.0
32= +33.3
33= +33.3

For the transistors on the power amp board , I'm trying to find a schematic I can read a little better for that board .

Maron said:

For the transistors on the power amp board , I'm trying to find a schematic I can read a little better for that board .

Click to expand...

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