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Help with my SX-3900

G

gagecalman

New Member
I have an SX-3900 that I bought new when I was in the Air Force. It has served me well. I cleaned it up and replaced all of the bulbs last year. This weekend I smelled something bad. I turned it off but now I don't have any output. When I first turn it on I hear a click and everything lights up. I do not hear the second click that I used to. I have the service manual and work at a calibration lab so I have access to some test equipment. I am a mechanical person but have someone who can help with the electronics. I don't know anyone in the area who repairs this good old gear. I could use some help with the repair or a recomendation for good service. Thanks in advance for any help. Jim
 
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Check the regulated voltages: +54v (pin 5) , -54v (pin 1, the outer pins, the center pin #3 is ground) at the power amp card 5 pin connector as well as the incoming protection voltage signal (it can be either + 25v or -25v depending upon variations in the model) on the 3 pin connector (pin 8) next to it. Zero volts won't cut it, nor will non symmetrical supplies.

Then find a point to probe for each amplifier output by the inductors on each side of the relay, probing the outputs before the relay interrupts the circuit. The DC voltage with no input (zero volume) should be well under 0.1v .

One of these 5 voltages will be wrong, causing it to go into protection, unless there is a problem IN the protection circuit (possible).

While the amp is difficult to troubleshoot, it may be preferable to a popped power supply regulator, due to the difficulty of getting to the power supply board where they shoehorned it in behind the front panel on the left side.
 
Hi Mark. Thank you for the reply. This is what I found. Voltage is +54.2(pin 5) and -54.6(pin1) but zero volts at pin 8. I didn't understand where to check the other voltage. I did find a transistor(I think) that has split apart. I want to insert a picture but can't figure out how. Thanks again for your help! Jim
 
Read the voltages on the other two of the three pins.

re: posting pictures, either use a photo hosting service and use the yellow icon to insert an image link
OR
below the reply editor box is "manage attachments"
your jpg has to be less than 900 x 900 and 125? Kb
use browse on the separate manage attachments page to find the jpg in your computer
and then upload to get it in.r
 
Hi Mark. Zero volts on pins 6&7. I'm sorry but I still don't understand where the manage attachments is. I don't see it on my reply screen. I wish I could attach the pictures. Jim
 
When you type in your reply, it is in a "reply to thread " box that fills a typical screen.

Look below the "submit reply" button in that box, and you will see another box just like the "reply to thread" box, titled "additional options".

Right bang in the middle of that "additional options" box is a button "Manage Attachments".

But back to the 3900...

On the power supply, pin 17 has ac coming in which is fed to a voltage divider (R1 6.8k, R2 27k) a Diode (D1) and a capacitor (C1 2.2uf ) and then the capacitor's voltage goes off the power supply board on pin 4 and comes into the power amp board on thew amp board's pin 8.

The polarity of D1 and C1 can be reversed from what is marked on the board !!

The older factory configuration where the polarities conform to the markings on the board, generates a negative voltage on pin 8, which drives an npn transistor (2sc or 2sd number) on Q139 in the protection circuit on the amplifier card.

The newer version generates a positive voltage by reversing D1 and C1, driving a PNP transistor (2sa or 2sb) in the Q139 position.

The reason I bring this up is usually C1 (2.2uf) shorts out, and the protection never connects the speakers to the amp.

Unfortunately that cap is a beyotch to get to on the power supply card.

It is right under the corner of the transformer mounted on the board, and the diode is right below the cap, with the band pointed away for the older negative voltage and towards it for the newer positive voltage.

I don't think it's possible to loosen the board mounts, and swing it down far enough to protrude below the bottom of the chassis.

As for the destroyed transistor, is it a LITTLE one, or is it about 1/2 inch square? Is it more towards the center inch or two of the board (protection) or the 4 inches or so to the left or right.

If you look at the manual carefully, on the pdf manual, page 27 (of 47) there is a layout of the amp board that should help identify the transistor's name.
 
Good morning Mark. I see the additional options box but there isn't a manage attachments option. There is an attach files "button" but it doesn't do anything. I emailed Grumpy for some help.
The transistor is shown on page 24 of the service manual. It is marked C2575 03E. Looking at the manual I think it is a 2SC2575. It is in the middle from top to bottom and about an inch to the right of the relay. There are 2 points marked on the board labeled 9 & 10. It is right in the middle of them. Thanks
 
That's Q126, part of the interface between the protection circuit and the amplifier. It is part of the over-current sensing circuit. I'm not sure how that thing could get enough energy through it to cause that much damage.

If the transistor is physically damaged, cut the leads (with the power OFF) on the remains to remove it and hopefully isolate it from the circuit.

Between the point marked 10 and the coil of wire is a GREEN resistor (a hot dog shaped thing), it is 10 ohms. It is the ONLY part between the blown up transistor and that coil. Take a voltage reading on the lead closest to the center of the board and the relay.

ON the other side of the board, as a MIRROR image there is ANOTHER 10 ohm resistor, ALSO measure the voltage on it's lead closest to the center of the board and the relay.

Both of these points are the outputs of the amplifier before they go into the relay, where it is keeping them disconnected from the speakers etc...

The thing is, this is looking more and more like a professional job. That is the hardest amp type (NSA) of the Pioneers to troubleshoot, and it is absolutely unforgiving of errors or faults, erupting with spectacular results when only slightly provoked.
 
If the voltage is between the resistor and ground there is no voltage on either side of the board. Q126 is actually split in half. 2 leads are on one half and 1 is on the other half. Does Q126 have to be removed to check voltage? I didn't want to remove yet in case I do have to take it to the shop. I still can't attach. They are still troubleshooting. Thanks again for the help.
 
No voltage on the resistors is good, they should be in the low millivolt range each. try measuring them on the millivolts range.

Cut one of the two leads to break any short circuits on the transistor fragment first.
 
Good morning Mark. I cut one of the 2 leads and retested after it warmed up for 5 minutes with no imput. On the good side of the board I have 10 mV and the bad side I have 15mV. They were a little bit higher upon initial power up.
 
That's GOOD news, the low millivolts at the outputs means that the fault lies in the protection circuits and that the amplifiers are probably working OK.

Do you feel up to replacing several transistors (and diodes)in the protection circuit on the amplifier card?

If so we aren't going to try to troubleshoot which of several parts are bad, but rather, due to the minor costs involved, replace them all.
 
edit: whoops - mouser.com parts numbers
there is an option during checkout that includes USPS First Class Mail, that is less expensive than UPS.

Q125 & Q126 & Q137 & Q138 & Q139 2sc2575 to-92 BCE 50v 0.2a 0.3w 90-800hfe 200mhz
512-KSC2383YBU to-92 ecb 160v 1a .9w 50mhz 160-320hfe $0.18

D129 & D130 & D132 1s2471 small signal diodes 75v .2a
512-1N4454 $0.03 ea

D131 mz-150 15v 0.5w zener diode
512-1N5245B Fairchild 15 V, 0.5W Zener $0.05 ea

C140 100uf 25v
647-UPW1E101MED 100uf 25v $0.26 ea

C139 470uf 16v
647-UPW1C471MPD 470uf 16v $0.38 ea

C129 & C130 0.22uf 50v non-polar (electrolytic)
598-DSF050J224 0.22uf 50v non polar Metallized Polypropylene Film Capacitors Cornell Dubilier $0.50 ea

That's the whole circuit,
 
Hi Mark. Sorry to hear about your basement. I ordered the parts today. I will let you know how it turns out. Thanks again for your help.
Jim
 
gagecalman said:
Hi Mark. Sorry to hear about your basement. I ordered the parts today. I will let you know how it turns out. Thanks again for your help.
Jim
Click to expand...

When you replace the parts, check the resistors for proper value while the parts shunting them are out.

Plus, before pulling the board, you can clip out Q139 and see how that affects the voltage on pin 8. (other than disconnecting the wire TO pin 8 and measuring the wire) We still need to see if the power supply is sending the correct signalk to the protection circuit.

We are replacing the parts in the circuit due to the physically damaged transistor we found.
 
Good morning Mark. I received my parts from Mouser. I removed Q139 and voltage is 28 on pin 8. I also heard the relay click. I don't know if thats good or bad. I am taking it to work tomorrow to replace the parts. Thanks for your help.
 
gagecalman said:
Good morning Mark. I received my parts from Mouser. I removed Q139] and voltage is 28 on pin 8. I also heard the relay click. I don't know if thats good or bad. I am taking it to work tomorrow to replace the parts. Thanks for your help.
Click to expand...

That's GOOD news, getting plus 28v on pin 8, this means that we don't have to mess with the (deeply buried) power supply.

Check the values of: R171, R173, R167, R169, R170, R168, R172, R174, R197, R193, R192, R196, R191 & R199 while replacing the caps, diodes and transistors

REMEMBER, the lead arrangement on the replacement transistors is the OPPOSITE of the originals, meaning that the flat faces the other way. The board has a little "E" by the emitter lead's hole in the board.
 
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