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SX-1250 - Distortion at low volume

Looking at the schematic. With 2sa899 missing, 2sb630 does not appear to get enough negative feed to be active while 2sc610 is getting the feed. In essence, it does not appear to be generating a shorted path to have some fire show. Had any 1w resistor in between 2sc610 and 2sb630 instead of 2sa899 missing, then you'll probably have a set of trashed transistors. I would double check "WHY" that transistor is missing aside from whoever don't know what he's doing did to the amp board by making sure everything else is okay. My SX-1250 has that transistor in, so it should be there.
 
I can't get an accurate history on this because I bought it through an online auction. It's had several owners so it's nearly impossible to get the complete story. The last owner thought it sounded great, and when I got it I did too at first (even with the shipping damage it had) until I turned down the volume and noticed the distortion in the background. In fact, my family/friends thought I was hearing things until I hit the -20 db mute and demonstrated the problem. I'm fairly certain that the seller had no clue to this since he was using it in a garage/shop and probably had it playing loud. He responded to the shipping damage issue and made good on it. This is turning into a 'CSI - SX1250' episode....:scratch2:
 
I don't think I phrased it right. I meant "why" as in making sure the circuit is okay before you start replacing the 2sa899. My apology.
 
ccheath said:
Looking at the schematic. With 2sa899 missing, 2sb630 does not appear to get enough negative feed to be active while 2sc610 is getting the feed. In essence, it does not appear to be generating a shorted path to have some fire show. Had any 1w resistor in between 2sc610 and 2sb630 instead of 2sa899 missing, then you'll probably have a set of trashed transistors. I would double check "WHY" that transistor is missing aside from whoever don't know what he's doing did to the amp board by making sure everything else is okay. My SX-1250 has that transistor in, so it should be there.
Click to expand...

I was tired last night (still recuperating??) and didn't look close enough..... you are right, I thought the 899 was part of the bias generator circuit (which would have been very very bad), instead it is a "pre-driver" stage.
and with those 100 ohm emitter resistors, the outputs were saved from being biased on to a destructive level, in fact they were safely shut down.

WHICH could have been the problem.... that's the problem with remote troubleshooting, you can't document ALL the unexpected possibilities...

I am going to review the thread in light of this new information and post later.
 
markthefixer said:
I was tired last night (still recuperating??) and didn't look close enough..... you are right, I thought the 899 was part of the bias generator circuit (which would have been very very bad), instead it is a "pre-driver" stage.
and with those 100 ohm emitter resistors, the outputs were saved from being biased on to a destructive level, in fact they were safely shut down.

WHICH could have been the problem.... that's the problem with remote troubleshooting, you can't document ALL the unexpected possibilities...

I am going to review the thread in light of this new information and post later.
Click to expand...

OT: While we all appreciate your time helping several Ak'ers at the SAME time, you oughta take a vacation once in awhile. :yes: :D

Now back to project.
 
Well I'm reviewing what I've done so far and I'm pretty sure I have either the 2sa899 or the 2sc1904 installed the wrong way...or both wrong. I reviewed the transistor diagram in the service manual but I'm not certain which side is the 'front'. This is where my inexperience is showing....Thanks.
 

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GREAT picture, I can work from that.....

Could you take another picture JUST LIKE THAT from the other side of that heatsink, to show the right side transistor and the silkscreen. (IIAC the 899)

Then after tomorrow night (hopefully) I WILL give the pictures my FULL attention..

I have to upset my circadian rythm to answer a 9:30 am (I'm usually out cold then, my middle of the night) jury duty summons tomorrow and am running out of time tonight to prepare and then get to sleep, and hopefully I won't get stuck on a jury with a 3 to 7 day time investment...

You used the transistors from mouser, right...

If so then I have those exact ones here and will use them as examples to relate to the pictures, that coupled with the service manual will give me enough to work with to verify your work.

the to-126 package is an ECB layout and on the 2sa899 - ksa1142, the "a1142-y" is on the front (the side with three dimples) the E is the lead closest to the a of a1142-y and should be inserted into the hole by the angled line with the arrow . The C is the center lead and goes into the angled line without the arrow, and the B is the lead closest to the -y part of the marking and goes into the flat line that the other two intersect.

edit: the c1904 / c2682 looks correct, but the a899 / a1142 has the e and c reversed (outer leads), the a1142-y should be facing the OTHER way. The B is in the correct place..
 
Here's the picture of the other side. I hope it's helpful. All the parts are from Mouser so they should match ok. I am in no hurry on this...I'm lucky to get a couple of hours in over the course of a week lately. Once again, I am very grateful for the help.
 

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markthefixer said:
edit: the c1904 / c2682 looks correct, but the a899 / a1142 has the e and c reversed (outer leads), the a1142-y should be facing the OTHER way. The B is in the correct place..
Click to expand...

yup, I was right, the picture confirms it. I can also tell that the seven to-92 transistor in the foreground of the picture are original, they have undisturbed dust on them....:D
 
I reinstalled the a899 / a1142 the correct way. Hopefully I won't mess up the other parts this weekend. That dust is vintage SX-1250 crud...maybe I can put it up for auction when the project is done :yes:.
 
I noticed a difference in some of the capacitor voltages. I don't know if this will be a problem so I thought I would mention it.

four: 470uF/80v 647-UPW2A471MHD @ 1.92 ea nichicon pw - power filter
Click to expand...

They say 100v instead of 80v.


two: 10/16 647-UVZ1V100MDD @ $0.06 ea nichicon vz - offset smoothing
Click to expand...

These are 35v.

two: 220/16 647-UVZ1E221MPH @ $0.35 ea nichicon vz - feedback integrator
Click to expand...

These are 25v.

The part numbers matched up ok. I will hold off on these for now. Thanks.
 
ekooch said:
I noticed a difference in some of the capacitor voltages. I don't know if this will be a problem so I thought I would mention it.They say 100v instead of 80v.
These are 35v.
These are 25v.
The part numbers matched up ok. I will hold off on these for now. Thanks.
Click to expand...

I had to get higher voltages either to get the correct value, or because the margins on the newer caps are tighter than the older caps, and the changes made good engineering sense. In most cases when voltage was upped to accomodate the margins the price differential was pennies, if that much...

They are fine, get them...
 
Well I thought I did everything ok but something is wrong. After installing the components on both amp boards I decided to put everything back together and leave the protection board updates for another time (hopefully not a stupid decision). I checked all the parts and orientation etc and everything looked ok. As I was turning the unit over to install the connectors on the bottom of the boards it slipped and crashed onto the table - I'm not surprised really since things seemed to be going too well - no burned fingers or hot solder in my eyes, etc. When I tried to power it up nothing happened - no soft start click, no lights, no smoke, nothing. This was similar to when I messed up the thermal fuse so I checked it and the 20 watt resistor and they both appear to be fine. I did a search and reviewed some posts and tested that I can plug other things into the accessory outlets and they work fine. This includes the switched outlet so the power switch is ok. If I had messed something up on the amp boards, wouldn't the 1250 still power up? Does the missing - and now installed - 2sa899's have anything to do with this? I had also changed the output transistors and was careful to put them in the right places with the correction orientation and thermal grease. If there was a problem with them, would this no power situation result? Thanks for any help.
 
With the wall plug out, turn the power switch on, and grab an ohmmeter and do an ohms test between the two pins. There should be FAR less than 100 ohms...

If there is more resistance you need to find the break in the primary loop, using an ohmmeter.

Go from one plug pin and work your way around.

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I tested between pins 1 and 2 and the meter did not change - it displays '1' when I turn it on and when I tested nothing changed, it stayed on 1. I tested the thermal fuse and it showed '0' ohms. Am I doing the test wrong or does the '1' mean there's a problem? (I tried all the ohm settings on the meter and nothing changed other than the decimal point in the display).
 
You have to find points in the receiver that are part of the circuit marked in red, and verify that there are zero ohms between them ALL, which there SHOULD be with the power switch turned ON. The plug is NOT PLUGGED IN, it is one of the two test points you are starting at to find the break. There IS a break because not only doesn't the receiver turn on, but the "1" is displayed on you meter which means infinite ohms, as like the probes are not touching and NO current will flow.

test from one power plug blade to the white transformer wire, is it zero ohms? one of the two power plug blades WILL be.

Test from the white transformer wire to the brown transformer wire, what are the ohms?

Now go to the OTHER power plug blade for the rest of the tests.
from that blade to BOTH sides of the (turned on) power switch should be zero ohms
from that blade to BOTH sides of the 12 amp fuse there should be zero ohms
from that blade to BOTH sides of the thermal fuse there should be zero ohms.
from that blade to ONE side of the 3.3 ohm 20w resistor here should be zero ohms.

from the OTHER side of the 3.3 ohm 20 watt resistor there should be zero ohms to the brown transformer wire.

The 3.3 ohm 20 watt resistor should be 3.3 ohms.

ONE of these portions of the circuit has TOO ("1") much resistance... if all were as they should be the receiver would turn on, so which is bad???

I don't know how to explain it any simpler..........
 
Here are the results of the measurements:

The resistor is a 4 ohm replacement.

Test from the white transformer wire to the brown transformer wire, what are the ohms?
Click to expand...
Zero ohms
Now go to the OTHER power plug blade for the rest of the tests.
from that blade to BOTH sides of the (turned on) power switch should be zero ohms
Click to expand...
Zero ohms
from that blade to BOTH sides of the 12 amp fuse there should be zero ohms
Click to expand...
Zero ohms
from that blade to BOTH sides of the thermal fuse there should be zero ohms.
Click to expand...
Problem: both sides were infinite ohms

from that blade to ONE side of the 3.3 ohm 20w resistor here should be zero ohms.
Click to expand...
Problem: both sides were infinite ohms

from the OTHER side of the 3.3 ohm 20 watt resistor there should be zero ohms to the brown transformer wire.
Click to expand...
Problem: Tried both sides - zero ohms, 4 ohms
The 3.3 ohm 20 watt resistor should be 3.3 ohms.
Click to expand...
4 ohms

Thanks again for all your patience as I crawl though this....
 
The wire between the 12 amp fuse and the thermal fuse is broken SOMEWHERE.

I don't have the unit in front of me.

Take out the 12 amp fuse from the fuse's holder, and redo the test between the power plug's blade and both sides of the 12 amp fuse's holder, one side of the 12 amp fuse's holder will now read infinite ohms while the other side of the 12 amp fuse's holder will read zero ohms.

Start physically tracing (following) the wire (or wires) connected to the side of the fuse holder that reads infinite ohms. I think it is the side of the fuseholder that is closest to the bottom of the soft start relay. There ALSO should be another wire connected either on the fuseholder (infinite ohms side) or on the (soft start) relay's connection. This second wire IS connected to PIN 2 of the board with all the fuses and the thermal fuse on the bottom of the receiver. IF you have to, follow it BACKWARDS, starting at PIN 2 and working back....

Put the 12 amp fuse back in and reconnect the ohmmeter to the power plug blade, verify that you get 0 ohms to both sides of the 12 amp fuse...

Then read the ohms at SEVERAL points in-between the 12 amp fuse and the thermal fuse by:
(KEEP the one ohmmeter lead connected to that power plug blade)

1. the (soft start) relay's connection to the 12 amp fuse holder , also physically examine the wire and move it a bit.
2. the wire connected to PIN 2 (ON pin 2) of the fuse board that has the thermal fuse on it, also physically examine the wire and move it a bit. Which component does it go back to, the (soft start) relay or the 12 amp fuse holder?
3. both of the actual leads of the thermal fuse.

by now you should have found a piece of wire with either a broken connection, or zero ohms on one side and infinite ohms on the other.

I hope by now that the light has dawned as to what we are trying to do by "tracing the circuit".
 
The interruption in the circuit was the solder connections for the thermal fuse. I reinstalled it and the circuit traced fine. But when I tried to power it up nothing happened. I shut it off and noticed that the 4 watt resistor was hot. I checked it and got zero ohms from the plug blade to one side while the other side of the resistor to the brown wire measured zero ohms. Also the resistor measured at infinite ohms. The thermal fuse did not blow because I hadn't yet tied them together. So either I messed up something (output transistors?) or it's related to the original condition this 1250 was in - the 3.3 watt resistor was burned and a glass fuse was in place of the thermal fuse and the 2sa899's were missing.
 
I'm sorry to say, this sounds like somebody else should pick up the ball with this project and run with it.

I think the unit was in worse shape that we originally thought at the start of this. It would be better than forging on ahead and damaging it more.
 
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