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Sx-1280 power supply issues

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
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Working on an SX-1280 and I am suspicious I may have an issue in my PS, both my + and - 33v rails are about +-36v, everything seems to work except the DC offset is slow to settle (around 100mv and falling when the relay closes), max volume on phono causes a pop that causes the relay to close, the output of the power supply also seems to be fairly temperature sensitive, blowing on it causes the regulated voltage to shift +-5mv.

I am using the following transistors:
Q1 2SD712 MJE15032G
Q2 2SB682 MJE15033G
Q3 2SC1914A KSC2383YTA
Q4 2SA904A KSA1013YBU
Q5 2SB682 MJE15033G
Q6 2SC945 KSC2383YTA
Q7 2SC945 KSC2383YTA

Diodes:
D7 WZ-320 32V, 1/2 Watt BZX55B33 .5W, 33V, 2% Zener

I did notice a small amount of oscillation on the PS output, but I'm not sure if that's just my scope or an actual issue as is only 35mv on an 80v rail.
 

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Are the 68V rails as they should be? It's just a zener dropping the 33V rails, so something downstream is afoot if so.
 
raw, unloaded dc can be higher.

+/- 76 raw, +/- 65 loaded, 10v higher +/- 86 wouldn't faze me.


now, the regulated voltages +/- 68v have no excuse.
check their 32v zeners.
 
markthefixer said:
raw, unloaded dc can be higher.

+/- 76 raw, +/- 65 loaded, 10v higher +/- 86 wouldn't faze me.


now, the regulated voltages +/- 68v have no excuse.
check their 32v zeners.
Click to expand...

I've got 4 new zeners in there already unless somehow the new ones are bad
 
markthefixer said:
raw, unloaded dc can be higher.

+/- 76 raw, +/- 65 loaded, 10v higher +/- 86 wouldn't faze me.


now, the regulated voltages +/- 68v have no excuse.
check their 32v zeners.
Click to expand...

Whelp, I made an idiot mistake and am now out a PA1002, PA3001 and possibly a PA1001... Damn.
 
ouch....

If the rails are higher, the feedback transistor may have lower gain and is loading down the 15k / 15k voltage divider.

at 77v and 32.6v,it looks like a 15k and an 11k resistor due to the extra current.
about 800uA.

change the 15k for a 12k and it should be a lot closer. or parallel the 15k with a 68k.
 
markthefixer said:
ouch....

If the rails are higher, the feedback transistor may have lower gain and is loading down the 15k / 15k voltage divider.

at 77v and 32.6v,it looks like a 15k and an 11k resistor due to the extra current.
about 800uA.

change the 15k for a 12k and it should be a lot closer. or parallel the 15k with a 68k.
Click to expand...

So change R9 or R11 to 12k? (r9 and r11 are those voltage dividers, R9 comes off the rail, R15 comes off 32ish v)
 
Last edited:
R9 off the rail. either 12k or a 68k parallel...


R11 is already "affected, downwards" by the extra base current, we're just bringing the other half of the divider in line with it.

are you sure you used a ksc2383Yta part? i'd also try another c2383 in there, and just for giggles if you have one, a ksc1845 before changing the resistor.






0
 
Last edited:
markthefixer said:
ouch....

If the rails are higher, the feedback transistor may have lower gain and is loading down the 15k / 15k voltage divider.

at 77v and 32.6v,it looks like a 15k and an 11k resistor due to the extra current.
about 800uA.

change the 15k for a 12k and it should be a lot closer. or parallel the 15k with a 68k.
Click to expand...

It was a 2383YTA, for I also tried a 2690 to no avail. I feel like an 1845 might explode here, no? Using a 12k in series with a 470 I now have around 68vish v that seems to rise as the board gets hotter. The negative rail is 12k + 820 and giving me 67.5ish and it's the same deal, fluctuates with temp
 
markthefixer said:
R9 off the rail. either 12k or a 68k parallel...


R11 is already "affected, downwards" by the extra base current, we're just bringing the other half of the divider in line with it.

are you sure you used a ksc2383Yta part? i'd also try another c2383 in there, and just for giggles if you have one, a ksc1845 before changing the resistor.






0
Click to expand...

Just noticed that my negative 32v is at -33v and my positive 32v is at 36v...

Update, seems to be okay... Still popping when switching from Aux to Phono. Not sure why, phono doesn't have much if any DC on the output. Might try that delta Electronics power supply since it looks to be an improvement.

What baffles me is how long it takes the supply to stabilize, seems to be right around the time the relay closes. And affects the DC offset of the main amp.
 
Last edited:
goldswimmerb said:
Just noticed that my negative 32v is at -33v and my positive 32v is at 36v...

Update, seems to be okay... Still popping when switching from Aux to Phono. Not sure why, phono doesn't have much if any DC on the output. Might try that delta Electronics power supply since it looks to be an improvement.

What baffles me is how long it takes the supply to stabilize, seems to be right around the time the relay closes. And affects the DC offset of the main amp.
Click to expand...
The new power supply is well worth it.
 
I'll have to play around with my diff pairs, and recheck my bias, my R channel doesn't want to adjust lower than 50mv offset with the pot full CCW, weird that it adjusted down just fine with the higher Vin
ivandezande said:
The new power supply is well worth it.
Click to expand...

I went and ordered it, figured since I nuked the guys tuner I could at least upgrade the power supply. Fingers crossed that it's just the tuner ICs that popped from over voltage (the PA1002 literally exploded, the 3001 was pegging the meter and getting extremely hot), front end seemed to test okay but I have no way of really knowing till I get chips. Somehow the HA1197 survived, which is the one chip I would have been okay with exploding.
 
I do not see how you damaged the tuner ics when they run off the +13v supply. The 13v reg runs so hot it unsolder itself causing issues
the protection ic on awm -123 uses the 13v
 
rcs16 said:
I do not see how you damaged the tuner ics when they run off the +13v supply. The 13v reg runs so hot it unsolder itself causing issues
the protection ic on awm -123 uses the 13v
Click to expand...

Pretty Easy, The main filters still had some charge and the 13v Collector heatsink made contact with the metal casing of the AC in rectifiers on the protection board.
 
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