SX-1250 - Distortion at low volume

ekooch said:

I took measurements on the 4 capacitors (all are new) and they were all the same: DC 70.2 - 70.4 volts
AC 154.6 - 154.8 volts

The 1250 was warmed up for about 30 minutes. Thanks again.

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Ok, that HAS to be 154 millivolts...

I checked it again and I had the meter set to 200V (AC) for the measurement. Am I reading it incorrectly? Is it possible I messed up the meter?

It's not possible to read that kind of AC voltage on the caps, for a variety of reasons (not the least of which is that they'd behave like hand grenades with 150V of AC on them, followed by the fact that you can't read more voltage on those caps than what is coming out of the wall .

EchoWars said:

It's not possible to read that kind of AC voltage on the caps, for a variety of reasons (not the least of which is that they'd behave like hand grenades with 150V of AC on them, followed by the fact that you can't read more voltage on those caps than what is coming out of the wall .

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Yup. Some of the cheap meters need a 0.1 uF capacitor in series with the lead if the ac is riding in upon a DC voltage. I have seen that requirement in the .pdf manuals I have downloaded when looking up meters. This one didn't mention it, but I was going to suggest trying it if I needed more AC on DC measurements.

This is a $20 craftsman that has currently been knocked down to $10

Well I stopped by Radio shack and picked up a package of .1 uF capacitors and tried the AC readings again. This time I got 0.00 to 0.03 volts on all 4. There was a bit of fluctuation for a few seconds before they settled down to these numbers.

Sounds good, what are your intentions as to parts?

Is the access to the adjustment pots straight down?

Are there any obstructions to the pot in that direction? They have to be accessed while the unit is running.

Regarding parts, I can order them later this week. The offset and bias pots are accessible - are these the ones you mean?

I found some time to get back to this. I pulled the board that has a different bias pot and took some pictures (well I tried to). Since I'm not sure what's been done to this unit I thought I'd post them to see if it would be helpful.

The pictures helped. The pots, however, are giving me fits, the triangular lead pattern usually has the "point" pointing in the direction of adjustment access. This is the opposite...

98% of the list is finished....

Driver board trimpots? Bourns Series 3386H. 500 ohm, for both the offset and bias.

Thank You....

I just gotta get a few samples, sometimes the drawings aren't revealing...

sx-1250 AWH-048 amp parts

sx-1250 amplifier , "driver cards" board : AWH-048 replacement parts:

Mouser parts numbers, a mix of Fairchild (512-) and ON-Semi (863-)

differential inputs, better off if the hfe is matched
2sa750 to-92 ecb 50v .05a .25w 100mhz hfe:450
twenty: 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

bias / vas
2sa858/2sc1439 to-92 ecb 150v 0.5a .5w 100mhz hfe:150
two: 512-KSA910YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.12 ea
two: 512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

pre-driver
2sa899/2sc1904 to-126 ecb 150v .05a 1w 100mhz hfe:150
two:x 512-KSA1142YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.21 ea
two: 512-KSC2682YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.31 ea

drivers
2sb630/2sd610 to-220 bce 200v 2a 25w 4mhz hfe:80
two: 512-KSA940TU_Q pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
two: 512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54

power outputs
2sb600/2sd555 to-3 200v 10a 200w 14mhz hfe:70
four: 863-MJ21194G to-3 NPN 250v 16A 250W 4mhz 8-75hfe $4.10
four: 863-MJ21193G to-3 PNP 250v 16A 250W 4mhz 8-75hfe $4.10

overcurrent detector
2sc869 to-92 npn bce 50v 30mA .2w 150mhz hfe:35
two: 512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

here's the insulators and thermal grease:
ten: 534-4662 to-3 mica insulators $0.16 each (two spares)
six: 534-4672 to-220 mica insulators $0.12 each
one: 532-249 1 oz thermal compound $4.71

here's the trimpots:
four: 652-3386H-1-501LF $1.29 ea bourns 3386H 500 ohms

as for the capacitors:
four: 470uF/80v 647-UPW2A471MHD @ 1.92 ea nichicon pw - power filter
two: 10/16 647-UVZ1V100MDD @ $0.06 ea nichicon vz - offset smoothing
two: 220/16 647-UVZ1E221MPH @ $0.35 ea nichicon vz - feedback integrator

at this point, there's not much left original....here's the diodes:

1s1886 = 1amp 200v general purpose, being replaced by uf4004 = 400v 1 a fast recovery diode
eight: 512-UF4004 $0.12 d6, d7, d10 & d11
four: 1s2471 512-1N4454 $0.03 d8, d9
two: 1s2473 512-1N4148 $0.03

This is just so you can plan ahead.
There would be a few more parts for beefing up/renewing the protection circuits...
one: 522-ZTX1056A NPN TO-92E ebc 150v 3a 1w (Vce(sat)=0.3v @ 3a) 120mhz 30-1200 hfe relay driver transistor
one: 100uf/16v (100/35) 647-UPW1V101MPD $0.15 protection circuit timing capacitor
one: 4.7uf/25v (4.7/50) 647-UPW1H4R7MDD $0.23 protection circuit ac power on signal integrator
two: 100uf/50v (100/50) 647-UPW1H101MPD $0.50 protection circuit voltage regulator
two: 470uf/6v (470/16) 647-UPW1C471MPD $0.52 protection audio input caps

Thank you for the parts list. I just submitted the order and while I'm waiting I'll study the service manual and previous posts. Once the parts arrive I'll check in before attempting anything so I know where to start (output transistors first or amp boards?, etc.). I appreciate all the assistance.

The parts arrived from Mouser a couple of days ago. I am hoping to have some time this weekend to work on it. I have been reviewing the service manual to see where everything goes. I noticed in previous posts the concerns about the proper application of thermal grease on the output transistors and issues with wirewrap connections. Are there other potential problems I need to watch out for?

Should I start with the amp boards first or with the output transistors? Also, are there points where I should stop and test/measure the unit or should everything be done at once? Thank you.

Work on the distorted channel first, and we got these:
--------------
differential inputs, better off if the hfe is matched
2sa750 to-92 ecb 50v .05a .25w 100mhz hfe:450
twenty: 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

bias / vas
2sa858/2sc1439 to-92 ecb 150v 0.5a .5w 100mhz hfe:150
two: 512-KSA910YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.12 ea
two: 512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea
---------------

as just in case spares... they are matched pairs and are better left alone unless there are problems.... they were cheap insurance, the shipping would have cost more than the transistors if we needed them.

do all the rest of the parts we got at once without applying power. set the bias pots to zero ohms (minimum bias) and center the offset pots for now.

then double check everything including the lead layouts (only the overcurrent detector is different...) on all the transistors.

By doing one at a time, you have an original reference to consult.

when powering up, watch the bias voltage measurement like a hawk!!!

Have you tried cleaning the protection relay contacts?


My apologies, I have to plead guilty to markthefixer's charge. Won't let it happen again.

Pioneer79 said:

Have you tried cleaning the protection relay contacts?

Click to expand...

YOU haven't READ the thread...... We're Wayyyyy past that point......

The output transistors are in and I started on one of the amp boards. I noticed that the 2SA899A pre-driver was missing. I checked the other amp board, which is still in the 1250, and it appears that it's missing from that one also. From what I can see it looks like it was there at one time and someone took it out (?). I went ahead and installed the replacement.

ekooch said:

The output transistors are in and I started on one of the amp boards. I noticed that the 2SA899A pre-driver was missing. I checked the other amp board, which is still in the 1250, and it appears that it's missing from that one also. From what I can see it looks like it was there at one time and someone took it out (?). I went ahead and installed the replacement.

Click to expand...

Now I'm confused... did we ever power up the 1250 with the boards IN that had the 2sa899 MISSING??

I saw it missing in that last picture you posted....

I looked through the thread and refreshed my memory..... and somebody under your control pulled that transistor....

because we had both channels working, but not well. and if it were powered up without that transistor, the fireworks would have been impressive.

I had powered it up 2 weeks ago after installing the thermal fuse and it worked ok (still had the distortion issue). I just looked again at the other amp board which is still installed and I don't see the 2sa899. I'm pretty sure it didn't break off because it appears that the old one was desoldered. Since someone 'mickey moused' the thermal fuse by replacing it with a glass fuse and duct tape, is it possible that they messed around with the amp boards? I know it doesn't make sense but I'm not sure what to make of this. I might as well continue with the rest of the parts (?). Thank you.