SX1050 power amp section goes silent

do the test without ANY speakers, listen for the click of the protect relay, do you have headphones?

The idea is to not load the outputs, to see if it's DC or over current that is triggering it.

The FACT that the sound comes back FASTER than the time out makes me think the protect driver circuit is at fault, probably the relay driver transistor.

If you don't do the non-speaker test, we have to start electronic troubleshooting, eliminating protection input connections until the problem stops - which then shows what's up. The alternative is sitting there staring at a DMM for hours...

best advice - rebuild the protect circuit (it's very easy to get to). new electrolytic caps also q6 and q7 and maybe q1, q2, q3, q4 (all can be ksc1845 or equivalent). the specs for q6 and q7 are elsewhere now, and i need to get some sleep, the sun came up before I got to bed last night / this morning / NOW....

edit - FIRST, set the DC offset as close to zero as you can. If it is jumping around on one channel - that's your smoking gun...
 
Hi Mark,

Will check the DC offset tom, my meter died, needs new batteries.

Thanks a bunch !!!
Dont be tempted read this until tom, get some sleep :)
 
Hi Mark,

DC offset measurement taken from speaker terminal after ten minutes of warmup

Right Channel: Keeps jumping From negative 6.7 mv to negative 8.5 mv
Left Channel: Keeps jumping From negative 6.4 mv to negative 8.2 mv

Did not try to adjust DC offset at this point will wait for further instructions

As for the transistors in the protection board, you posted this during my recap of my SX1050. You posted it as "Just in case" parts

awm-090 q6 2sc945 512-KSC2383YTA
awm-090 q7 2sc1384 512-KSC2690AYS
awm-090 d5 eqa01-33r 512-1N5257B

FYI. I buy my parts locally (Manila, Philippines) as customs/taxes for parts from mouser cost a ton

FYI The protection circuit was recapped (as well as all the other board except tuner and phono) on August 2012. Now, I dont remember if I replaced any of the transistors in the protection circuit, I remember replacing the transistors on the power supply board. I did have a record of all the parts replaced on this unit, sadly my laptop crashed and took all the files with it:)

Also why does my board have an "A" after the 090 ?

IMG_4122.jpg
 
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Well, the dc offset is not the problem. There could be a noisy transistor on the driver board, or a bad transistor on the protection board. Let it run for awhile with no speakers and no source and volume at min. No subwoofer connected. See if it goes into protect.
 
I am looking at the manual and can not seem to find the driver board

Turned the unit on for 25 minutes, it was quiet as mouse, no click on relay after initial relay click
 
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ok, WAS that 25 minutes with any sound signals (even though the speakers are not connected?) going through it. IF NOT, send audio through at moderate levels and then turn off the speakers and repeat the "25 minute test"...

we have established the amplifiers are somewhat stable, now we warm them up a bit and feed a signal through, to the protect circuit. This checks the DC offset under audio conditions, as well as the protect circuit.

the protect has new caps, but old transistors. The mouser numbers are only intended as a guide, in this circuit, when some local replacement is close, use it, they and the originals are pretty common. C945, A733, C945 in a pinch for q6, and a medium gain npn transistor (with enough voltage rating (60+v) and collector current drive 200 mA+) to drive the relay current.


edit - those two blue caps by pins 2 and 9, I cannot see the decimal point, I REALLY HOPE those aren't 22uf... the positions call for 0.22uf
 
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Hi Mark,

Tested with signal, no speakers for 25 minutes.
Volume at 9:00 oclock, no relay click. After 25 minutes, I adjusted the volume to 11:00 oclock and the relay clicked. After that I could not make it click again, left it at the 9:00 clock position and the relay clicked once.

The capacitors you mentioned are 0.22, 50volts, service manual requires 25volts. Should I replace these with stacked film capacitors ? BTW you gave me a scare there :)

Just in case I need to replace the transistors in the protect circuit (AWM-090)
q1,q2,q3,q4 I can use ksc1845 or equivalent
q6 I can use C945, A733, C945
q7 I can use 2sc1384

Thanks !!!
 
markthefixer said:
ok, WAS that 25 minutes with any sound signals (even though the speakers are not connected?) going through it. IF NOT, send audio through at moderate levels and then turn off the speakers and repeat the "25 minute test"...

we have established the amplifiers are somewhat stable, now we warm them up a bit and feed a signal through, to the protect circuit. This checks the DC offset under audio conditions, as well as the protect circuit.

the protect has new caps, but old transistors. The mouser numbers are only intended as a guide, in this circuit, when some local replacement is close, use it, they and the originals are pretty common. C945, A733, C945 in a pinch for q6, and a medium gain npn transistor (with enough voltage rating (60+v) and collector current drive 200 mA+) to drive the relay current.


edit - those two blue caps by pins 2 and 9, I cannot see the decimal point, I REALLY HOPE those aren't 22uf... the positions call for 0.22uf
Click to expand...

You beat me to it, Mark!:D
 
I started changing the transistors for the protection circuit board. I was able to replace Q1 to Q6 without a hitch

The Q7 (2SC1384) which I got failed upon testing. I got 2 and both were defective on the meter. Argh !!!

I have the following transistors on hand
a92351
a628a
d525
2sb507
2sb507
b683
c1318
a628a
2n 5551
c1318
2n 5551
1381
c945 k83c
c945 k83c
c733
sa992fbu
ksc1845
d1616

Any of these applicable for the 2SC1384 ?
 
Tested the unit with new Q1 to Q6, after 12 minutes, protection engages. I really need to get that Q7.

What are the chances that the only transistor left out of 7, is the one that is defective :(

Darn, now the protection will not click .. will let it cool for a while and see if it clicks after twenty minutes

Relay clicks again after half an hour of rest
 
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(2SC1384 or 2SC1166): 512-KSC2690AYS
Matt and MTF have this listed as a equivalent on the protection board for the 1250.
 
when the relay should be on, check the Q7 base voltage - it should be at or over 0.7 volts.

If so, that says q7 isn't functioning, now, once the relay pulls in, the base voltage will rise to about 9v, because the emitter voltage rises due to the action of the relay current through R25.
 
Hi Mark

After relay clicks. Black lead to chassis and Red lead to Base of Q7 is 5.99 to 6.02 volts

I tested it while its off and its reading 0.55volts at the base of Q7 (Black lead to chassis)

Thanks !!!
 
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Hi Mark,

Installed a new Q7, relay clicks, tested it, protection engages after 2 minutes with low volume music with speakers connected. Relay will not engage again :(

EDIT : tried again after a few hours and the now both relay clicks

Protection engages after 2 minutes without any source connected or speakers.

Thanks for all your help
 
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Now we need to find out why the protection is coming on. With no signal and no speakers, turn the receiver on and wait for it to go into protect. Now put your meter on Volts, and read the voltages on pin4 and pin17 of the amp board, with the negative meter lead on ground. Note, if you are using needle tip probes, tape the tip of the positive probe so only the very tip is exposed, so that when you slip with the probe there is less chance of shorting things out.
 
Turned on the unit. protection relay clicks, then disengages after 20 seconds, then clicks again. Turned off the unit. Turned the unit on, no more second relay click. Measurements then follows

Pin 4 : -5.5 TO 7.5 mv
pin 17 : -6.0 to -9.5 mv

thanks !!!
 
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Turned the unit on, I hear the protection relay clik, I was able to measure the voltage on pin 4, -11.somethig mv, then the protection relay clicks again after a few seconds.
 
When the relay is supposed to be on, yet isn't pulled in:

read and post the voltages on EVERY pin of the protection board, all will be DC voltages EXCEPT pin 3 which will be an AC voltage.
 
Hi Mark,

Turned the unit on, protection relay clicks and then after 15 or so seconds goes into protect. Measurements below. The protection circuit click once during the process and then instantly goes into protect. After about 5 minutes, the unit shuts down. I turn off the unit and turn on again and protection relay does not click.

1 0.1 mv
2 -13.00 v
3 7.65 v (ac)
4 -6.00mv
5 -10.00 mv
6 -4.5 mv
7 -11.00 mv
8 -6.5 mv
9 7.00 mv
10 6.6 mv
11 -8.5 mv
12 55.4 v

Thanks !!!
 
The fault lies ON the board, all inputs say things are OK, pull in the relay.

read (CAREFULLY) and post the three voltages on the pins of Q7.

NOTE: there are TWO relays in the SX-1050. The HUGE round toroidal transformer needs a special starting circuit when the main capacitors need charging, the huge current will blow fuses, so a 3.3 ohm 20 watt resistor (and a "thermal fuse" for when the relay fails) is added is added to slow down the charging for the first second or so, then it "gets out of the way" with a relay shorting the resistor out with a click.

You MIGHT be hearing that click and thinking it's the protection relay engaging.

The protection relay is a SECOND click 4-7 seconds later.

Since the voltages on pins 4,5,6,7,8,11 are fine, it is safe to hook up a pair of speakers (not the sub yet) to hear if the protect is really allowing sound through.
 
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