• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Help —SX-838 disaster

dustmaker

dustmaker

Active Member
Hopefully some one can Help me with a very problematic sx-838. I’ve had this unit for almost 20 years and have had some issues over in the past. IInitially, it would stop working when it got hot. I started a thread here around 2006 but before I did anything with it, I started traveling and since this was my garage unit, I didn’t run it enough for this to be a major problem. Eventually I moved it to storage and only recently brought it out. It would snap and pop like gunfire so I figured I would begin a rebuild, I started with the power supply board since it is simple and easy to access, which started all kinds of issues. Primarily, the receiver turns on but has zero sound and no change in voltage (at speaker terminals) when volume knob is rotated. This caused me to prod further.

1st - I was reading AC voltage where it should only be DC. Also, all DMM AC and DC readings were so far off everywhere, I suspected a bad transformer. After several weeks of poking, desoldering and measuring, I realized the plug was simply in backwards....I rotated the plug and the AC voltage went away and my dam reading at least got in the ballpark. The plugs of the 70s were not polarized and I never remember having an issue like this in the past so I suspect this is related to my other issues

2nd - the voltage on the pins at power supply are way off- low in all instances
AC
Pin 1. 36.5
Pin 2. 31.23
Pin 3. 31.22

DC Reading. Target
Pin 4. 41.2. s/b. 44
Pin 5. -41.2. S/b. -44
Pin 7. -40 s/b -45
Pin 10. 20.5. s/b. 22
Pin 11. 26. s/b 28
Pin 12. 29.8. s/b. 32
Pin 13. 32.8. s/b 35.5
Pin. 14. 12.4. s/b 13
Pin 17. 49.6. s/b 51

My household AC is measuring right around 120v, so that isn’t why the voltages are all lower.

3rd - from a previous post, I was looking at dc offset and I measured pins 5 and 20 on the -031 board. I am showing over 7vdc. VF 2 and VF1 have zero effect on this measurement.

4th - FM will not bring in any station. Signal strength will go up to 1/3 of the way but no higher and tuning meter does not move.

I suspect the last two issues are the result of the second item but I wanted to put all known issues in a single post.

An amateur looking for professionals guidance. Where do I start?
 
Register to hide this ad
What kind of DMM are you using? Plug-in benchtop or handheld battery powered?

Some test equipment that plugs into the wall may be connected to earth, which is essentially connected to neutral at your service panel bond, and you can get crazy readings if AC hot and AC neutral were swapped on UUT Unit Under Test

However, your 838 should otherwise not care that H/N are swapped. Doesn't matter until something touches something it's not supposed to.
 
dustmaker said:
Hopefully some one can Help me with a very problematic sx-838. I’ve had this unit for almost 20 years and have had some issues over in the past. IInitially, it would stop working when it got hot. I started a thread here around 2006 but before I did anything with it, I started traveling and since this was my garage unit, I didn’t run it enough for this to be a major problem. Eventually I moved it to storage and only recently brought it out. It would snap and pop like gunfire so I figured I would begin a rebuild, I started with the power supply board since it is simple and easy to access, which started all kinds of issues. Primarily, the receiver turns on but has zero sound and no change in voltage (at speaker terminals) when volume knob is rotated. This caused me to prod further.

1st - I was reading AC voltage where it should only be DC. Also, all DMM AC and DC readings were so far off everywhere, I suspected a bad transformer. After several weeks of poking, desoldering and measuring, I realized the plug was simply in backwards....I rotated the plug and the AC voltage went away and my dam reading at least got in the ballpark. The plugs of the 70s were not polarized and I never remember having an issue like this in the past so I suspect this is related to my other issues

2nd - the voltage on the pins at power supply are way off- low in all instances
AC
Pin 1. 36.5
Pin 2. 31.23
Pin 3. 31.22

DC Reading. Target
Pin 4. 41.2. s/b. 44
Pin 5. -41.2. S/b. -44
Pin 7. -40 s/b -45
Pin 10. 20.5. s/b. 22
Pin 11. 26. s/b 28
Pin 12. 29.8. s/b. 32
Pin 13. 32.8. s/b 35.5
Pin. 14. 12.4. s/b 13
Pin 17. 49.6. s/b 51

My household AC is measuring right around 120v, so that isn’t why the voltages are all lower.

3rd - from a previous post, I was looking at dc offset and I measured pins 5 and 20 on the -031 board. I am showing over 7vdc. VF 2 and VF1 have zero effect on this measurement.

4th - FM will not bring in any station. Signal strength will go up to 1/3 of the way but no higher and tuning meter does not move.

I suspect the last two issues are the result of the second item but I wanted to put all known issues in a single post.

An amateur looking for professionals guidance. Where do I start?
Click to expand...
Is your black meter lead clipped on the chassis for its 'ground reference' when measuring dc voltages with the red lead?
 
made in china said:
What kind of DMM are you using? Plug-in benchtop or handheld battery powered?

Some test equipment that plugs into the wall may be connected to earth, which is essentially connected to neutral at your service panel bond, and you can get crazy readings if AC hot and AC neutral were swapped on UUT Unit Under Test

However, your 838 should otherwise not care that H/N are swapped. Doesn't matter until something touches something it's not supposed to.
Click to expand...
 
I am using two different ones. I have a 20 year old Craftsman hand held for volts/ohms/capacitance/transistors. Because the voltages seemed so off, I also checked with a new Klein fork meter that also has leads to check voltage/resistance/etc. I obtained similar readings. I don’t trust the Craftsman for transistors and capacitors, but the dc and ac voltage seems to read consistent with the Klein.
 
merlynski said:
Is your black meter lead clipped on the chassis for its 'ground reference' when measuring dc voltages with the red lead?
Click to expand...
I have the black lead clipped to the chassis. There are three or four black wires on the one large capacitor. I attached my alligator clip to that connection.
 
Thank you, that clears it up!
There is no better place than directly to the star ground on the chassis.
Was that where you were connected when you got weird voltages before flipping the power cord?
 
dustmaker said:
Is there a way to check those parts without pulling out the entire board?
Click to expand...
I don't think you need to check them until we are sure the power supplies are good. They are part of the adjustment circuit for dc offset. If there was something wrong with them both dc offset measurements would be off.
If you can reach them from the top you can check them in circuit. That would probably work OK for the diode test setting on your meter, but the resistor is a high value buried in with a bunch of other components.
 
merlynski said:
Thank you, that clears it up!
There is no better place than directly to the star ground on the chassis.
Was that where you were connected when you got weird voltages before flipping the power cord?
Click to expand...
No - I had my ground lead on different spots. I couldn’t decide on the best one. Unfortunately, for the last few weeks, I was using the soldered power cord connection that has an insulated resistor attached to the chassis. I assumed this would be good, but obviously it isn’t. I just checked my voltages on pins 4&5 of -055 using the chassis ground in the picture and now I am NOT getting any significant AC voltage. I switched the polarity of the cord and am getting the same results. Problem 1 is now solved. It was how I had my test lead grounded. Thank you for that quick zero in on that first issue.
 
I will recheck my voltages using this chassis ground. Right now, I’m not 100% sure where I grounded yesterday when I took my readings.
 
Item 2: Now that you have a good chassis ground reference, could you post the power supply voltages again?
 
Readings are very similar to what I have posted above. Below are updated readings

AC
Pin 1. 37.76
Pin 2. 31.42
Pin 3. 31.43

DC Reading. Target
Pin 4. 41.9. s/b. 44
Pin 5. -41.8 S/b. -44
Pin 7. -41.2 s/b -45
Pin 10. 20.5. s/b. 22
Pin 11. 26. s/b 28
Pin 12. 29.7 s/b. 32
Pin 13. 32.64 If s/b 35.5
Pin. 14. 12.13 s/b 13
Pin 17. 49.7 s/b 51
Pin 21. 7.1. s/b 8.5

AC
Pin 22. 7.55. 7.5
Pin 23. 7.79. 7.5
Pin 24. 6.09. ?
 
dustmaker said:
AC
Pin 1. 37.76
Pin 2. 31.42
Pin 3. 31.43

DC Reading. Target
Pin 4. 41.9. s/b. 44
Pin 5. -41.8 S/b. -44
Pin 7. -41.2 s/b -45
Pin 10. 20.5. s/b. 22
Pin 11. 26. s/b 28
Pin 12. 29.7 s/b. 32
Pin 13. 32.64 If s/b 35.5
Pin. 14. 12.13 s/b 13
Pin 17. 49.7 s/b 51
Pin 21. 7.1. s/b 8.5

AC
Pin 22. 7.55. 7.5
Pin 23. 7.79. 7.5
Pin 24. 6.09. ?
Click to expand...

I don't see anything there to be a problem, the DC voltages are within 10% of nominal values, except pin 21, and that is a half wave rectified lamp circuit feeding 7.5v lamps. Pin 22 is the 'AC present' feed to the Protection pcb.

Let's move on to the Protection pcb AWM-025.
Measure DCV on
Pin 2, -13vdc:
Pin 11, +35vdc:
Pin 1, 0vdc:
Pin 3, 0vdc:
Pin 10, +35vdc:
Pin 9, about +9vdc:
Cathode D5, about +20vdc or more:
Cathode D6, about +20vdc or more:
Cathode D7, about +20vdc or more:

Measure ACV on pin 12, 7.5vac:
Those will tell us a lot about the health of the unit.
 
merlynski said:
I don't see anything there to be a problem, the DC voltages are within 10% of nominal values, except pin 21, and that is a half wave rectified lamp circuit feeding 7.5v lamps. Pin 22 is the 'AC present' feed to the Protection pcb.

Let's move on to the Protection pcb AWM-025.
Measure DCV on
Pin 2, -13vdc: -13.07vdc
Pin 11, +35vdc: +32.47vdc
Pin 1, 0vdc: +9.27vdc
Pin 3, 0vdc: -8.38vdc
Pin 10, +35vdc: +32.47
Pin 9, about +9vdc: 8.18vdc
Cathode D5, about +20vdc or more: +16.5vdc
Cathode D6, about +20vdc or more: +16.71vdc
Cathode D7, about +20vdc or more: +17.01vdc

Pin 5 +0.02vdc
Pin 6 - 0.17vdc
Pin 7 -8.36vdc
Pin 8 -8.51vdc

My readings are in blue. Please note that D3 cathode is reading 0.018; D4 cathode -0.588 - - - yes that is negative
Measure ACV on pin 12, 7.5vac: 7.54vac. -0.21vdc
Those will tell us a lot about the health of the unit.
Click to expand...
 
I think I have all the components to rebuild the AWM-025 if needed. I did adjust vr1, vr2, vr3, vr4 on the 031 when I was trying to duplicate the other 838 recent repair
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom