Completely dead SX-1250
- Thread starter EastPoint
- Start date
Reference the soft start circuit....
There is a thermal fuse tight to the big green resistor on the bottom middle board.
If it’s open it won’t fire up..
Some checks of other circuits should be done before replacing the Fuse.
Normally this is indicative of blown outputs.
There is a thermal fuse tight to the big green resistor on the bottom middle board.
If it’s open it won’t fire up..
Some checks of other circuits should be done before replacing the Fuse.
Normally this is indicative of blown outputs.
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OK, it measured open line, but then I turned on the receiver and turned it off, and now it measures around 1.2k ohms, which I'm guessing is other circuitry getting in the way. I also tried measuring the voltage from one of the fuses to each side of the thermal fuse, and I got around 90 VAC on one side and around 6 VAC on the other.
Different ways to approach this repair.
The thermal fuse is available, actually one of a very few NTE components we use in the Pioneers.
Depending on the condition of the set, if it needed a good service, I would check and repaste the outputs and see where that goes. Others might have another approach.
The thermal fuse is available, actually one of a very few NTE components we use in the Pioneers.
Depending on the condition of the set, if it needed a good service, I would check and repaste the outputs and see where that goes. Others might have another approach.
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I pulled all the outputs and they all seem to test fine. I used the diode test on my Fluke to test them, as per this thread:
http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/
I also checked them on the ohm test and none of them had shorts.
http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/
I also checked them on the ohm test and none of them had shorts.
That"s a good sign!
Parts Express:
Micro temp fuse
NTE 8108
https://www.parts-express.com/nte-8108-thermal-cut-off-109c-229f--nte8108
Visually check the soft start relay for signs of heat. If possible check the contacts for operation. Its possible to remove the clear cover and explore.
Replace the fuse carefully as the slightest heat will set it off. Use heat sinks clips or something similar.
Unplug both amp driver boards top and bottom Molex plugs.
Use the DBT to fire it up, 100 watt should be good without the amps plugged in.
The soft start relay is energized first and should be heard within a second or so from the flip of the power switch. Better to watch the power up process keeping an eye on the relay.
Check for a Dim bulb. If dim:
Check the Stabilizer board (AKA -Power Supply) for correct voltages.
If the DBT remains bright then a short exists and further troubleshooting is needed.
Note:
There is a ton of threads on this issue. I suggest you Google the Forum using the info you gained from the posts so far.If you haven't already.
If the soft start relay fails to engage there are other circuits to check.
My memory is fussy; I'll be doing research just like you. If I have time I'll check.
Note 2:
I don't know of a safe way to circumvent the micro temp fuse (So don't do it! Unless MTF chimes in) and continue troubleshooting the set.
Like I said, read up on past posts and discuss any findings with us before you apply power.
So far so good!
Parts Express:
Micro temp fuse
NTE 8108
https://www.parts-express.com/nte-8108-thermal-cut-off-109c-229f--nte8108
Visually check the soft start relay for signs of heat. If possible check the contacts for operation. Its possible to remove the clear cover and explore.
Replace the fuse carefully as the slightest heat will set it off. Use heat sinks clips or something similar.
Unplug both amp driver boards top and bottom Molex plugs.
Use the DBT to fire it up, 100 watt should be good without the amps plugged in.
The soft start relay is energized first and should be heard within a second or so from the flip of the power switch. Better to watch the power up process keeping an eye on the relay.
Check for a Dim bulb. If dim:
Check the Stabilizer board (AKA -Power Supply) for correct voltages.
If the DBT remains bright then a short exists and further troubleshooting is needed.
Note:
There is a ton of threads on this issue. I suggest you Google the Forum using the info you gained from the posts so far.If you haven't already.
If the soft start relay fails to engage there are other circuits to check.
My memory is fussy; I'll be doing research just like you. If I have time I'll check.
Note 2:
I don't know of a safe way to circumvent the micro temp fuse (So don't do it! Unless MTF chimes in) and continue troubleshooting the set.
Like I said, read up on past posts and discuss any findings with us before you apply power.
So far so good!
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Alright, I have the correct thermal fuse here. I removed the old one, and tested it, and it tested fine. Then I twisted it a little and it tested open again. That's interesting.
So I put in the new thermal fuse, being careful not to overheat it. I unplugged all the boards and fired it up on the dim-bulb, and it fired up fine. Then I plugged in one side of boards and it fired up fine as well. Then I plugged in the other set of boards and it fires up fine again. Both relays click, bulb isn't bright.
Is it possible that the thermal fuse just had some issues and that was what caused it? I haven't hooked up speakers yet, but it seems ok now.
So I put in the new thermal fuse, being careful not to overheat it. I unplugged all the boards and fired it up on the dim-bulb, and it fired up fine. Then I plugged in one side of boards and it fired up fine as well. Then I plugged in the other set of boards and it fires up fine again. Both relays click, bulb isn't bright.
Is it possible that the thermal fuse just had some issues and that was what caused it? I haven't hooked up speakers yet, but it seems ok now.
Hard to say but follow the below checks and adjustments to get the receiver back in a nominal condition.
Remain on the DBT.
Check and report the stabilizer +-65 volt feeds. The adjustment procedure is in the service manual. Read up on it before attempting the adjustments.
The amp adjustment procedure is in the manual as well.
Follow this procedure:
If anything seems weird stop and report.
Unplug both Amplifier boards. There are plugs on the top and bottom of the boards. remove all the plugs.
Operate the set on the DBT.
Check the regulated +-65v's on the Stabilizer board. The procedure is in the Service Manual. Be very careful as one wrong slip will make an already difficult repair worse. Use micro clip probe grabbers.
If the regulated +-65.0 voltages are good, follow the procedure below:
The start-up procedure for the sx-1250 after repair/restoration of the amp driver boards and stabilizer board “Tickling the Dragon's tail”:
1) Both amp driver boards (AWH-048) unplugged. Volume to zero or min.
2) Stabilizer board (AWR-106) plugged in.
3) Set connected to the DBT with a 100 watt bulb.
4) Flip the power switch and check/adjust Stabilizer board (AWR-106) rails to +65 - -65 volts. Make sure voltages respond to adjustments and hold steady.
5) Power down.
6) Plug in one output board.
7) Turn VR2 on amp driver boards (AWH-048) fully CCW.
8) Power-up and check/adjust the amp driver board (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv. Make sure voltages respond to adjustments and hold steady.
9) Back off VR2 CCW
10) Power down and connect the (second) amp driver board (AWH-048).
11) Power-up and check/adjust the (second) amp driver board (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv. Make sure voltages respond to adjustments and hold steady.
12) Back off VR2 CCW
13) Replace the 100 watt bulb with a 150 watt bulb. This step is not really required.
14) Repeat steps 7 and 10, checking for the Soft Start and Protect relays to energize.
15) Back off both VR2 pots CCW on both amp driver board’s (AWH-048).
16) Power down.
17) Remove DBT and operate set at full line voltage.
18) Make final adjustments to the Stabilizer board (AWR-106) set rails to +65 - -65 volts.
19) Make final adjustments to both amp driver board’s (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv.
20) Listen to Rush 2112.
Remain on the DBT.
Check and report the stabilizer +-65 volt feeds. The adjustment procedure is in the service manual. Read up on it before attempting the adjustments.
The amp adjustment procedure is in the manual as well.
Follow this procedure:
If anything seems weird stop and report.
Unplug both Amplifier boards. There are plugs on the top and bottom of the boards. remove all the plugs.
Operate the set on the DBT.
Check the regulated +-65v's on the Stabilizer board. The procedure is in the Service Manual. Be very careful as one wrong slip will make an already difficult repair worse. Use micro clip probe grabbers.
If the regulated +-65.0 voltages are good, follow the procedure below:
The start-up procedure for the sx-1250 after repair/restoration of the amp driver boards and stabilizer board “Tickling the Dragon's tail”:
1) Both amp driver boards (AWH-048) unplugged. Volume to zero or min.
2) Stabilizer board (AWR-106) plugged in.
3) Set connected to the DBT with a 100 watt bulb.
4) Flip the power switch and check/adjust Stabilizer board (AWR-106) rails to +65 - -65 volts. Make sure voltages respond to adjustments and hold steady.
5) Power down.
6) Plug in one output board.
7) Turn VR2 on amp driver boards (AWH-048) fully CCW.
8) Power-up and check/adjust the amp driver board (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv. Make sure voltages respond to adjustments and hold steady.
9) Back off VR2 CCW
10) Power down and connect the (second) amp driver board (AWH-048).
11) Power-up and check/adjust the (second) amp driver board (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv. Make sure voltages respond to adjustments and hold steady.
12) Back off VR2 CCW
13) Replace the 100 watt bulb with a 150 watt bulb. This step is not really required.
14) Repeat steps 7 and 10, checking for the Soft Start and Protect relays to energize.
15) Back off both VR2 pots CCW on both amp driver board’s (AWH-048).
16) Power down.
17) Remove DBT and operate set at full line voltage.
18) Make final adjustments to the Stabilizer board (AWR-106) set rails to +65 - -65 volts.
19) Make final adjustments to both amp driver board’s (AWH-048) trim pots VR1 for 0 volts and VR2 for 100mv.
20) Listen to Rush 2112.
Alright, I set the stabilizer board rails to 65 and -65. They were a few volts off but nothing serious. I was able to set both driver boards' VR1 for 0 volts, but on both of them the range for VR2 was about 0-5 mv, testing between terminals 7 and 19 as per the manual. Any idea what I'm doing wrong?
Also, the stabilizer board had no problem getting to the correct voltages, but all the radial capacitors on the stabilizer board look pretty well-used--the plastic coverings have shrunk considerably. Should I just leave well enough alone, or should I recap the whole board?
Also, the stabilizer board had no problem getting to the correct voltages, but all the radial capacitors on the stabilizer board look pretty well-used--the plastic coverings have shrunk considerably. Should I just leave well enough alone, or should I recap the whole board?
Fredrik 1977
Super Member
id replace them while in there.
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You mean the Idles only go from zero to 5m's? On both driver boards?
The DBT will limit the range of millivolts to about 35 - 50m's.
Maybe there is some damage?
Double check your connections making sure you don't get hooked up by trimming the wrong trimmer like most of us have done.
The manual is correct.
The DBT will limit the range of millivolts to about 35 - 50m's.
Maybe there is some damage?
Double check your connections making sure you don't get hooked up by trimming the wrong trimmer like most of us have done.
The manual is correct.
