Yet another SX950 protection issue that has escalated .....

[kc99]

Hi there, im new here , recently retired as a 2 way tech for 30 odd years, decided to tear into my sx950 that ive had a couple years.. (ebay purchase).. the protection would kick out after 30 min of playing, and got steadily worse.. I recapped the protection board, no change, checked the off sets and bias, it was close, but something has happened, now it will not come out of protection, and the offset will not adjust down, it will vary about .450 to .650 volts .. thats POINT 450 to 650.. So, I checked the power supply board, with these results..

1) 0 volts ground
2) 5.170 VDC\
3) 5.170 VDC
4) 13.20 VDC
5) 13.20 VDC
6) 50.20 VDC
7) 50.20 VDC
8) 36.20 VDC
9) 36.20 VDC
10) 36.20 VDC
11) 27.57 VDC
12) 0 volts ground
13) - .143 volts
14) -4.59 volts
15) - .570 volts

I realise 13 to 15 is not correct, any ideas where to start? I just ordered all the caps, semi, and trimmers to go thru this beast.. mouser should ship all out with the exception of 1 transistor...

 

[markthefixer]

AWM-101 board, these are regulated voltages, unless otherwise indicated. the voltage readings are ground referenced:
pins 2 & 3 +5.4v dc
pins 4 & 5 +13.9v dc
pins 6 & 7 +51.5 v dc
pins 8, 9 & 10 +36.5v dc
pin 11 +28v dc
pin 13 -13.5v dc - .143 volts
pin 14 -51.5v dc -4.59 volts
pin 15 -19v dc - .570 volts
pin 17 - raw unregulated dc to the outputs,
pin 18 + raw unregulated dc to the outputs

Looks like ZD2 popped. Why?? Be aware that Q2 and Q4 are highly suspect.
Pin 13 does go directly to the zener diode, but the connection of the protection's awm-062 pin 2 to it has R17 at 5.6k and R29 at 47k to isolate it from adverse effects.

That doesn't address any "oops"'s to it

 

[kc99]

Ok, Thank you sir, Ill check Q2 and Q4 and the Zener and see if I can figure out what took the zener out...

 

[kc99]

Ok i replaced the zener it was shorted, got my voltages back, going to proceed with recapping and transistor replacement, I was wondering if something else is wrong, If i try to adjust the bias and idle pots its a very unsteady reading, it will not settle down on a digital fluke meter..it is constantly moving and will not let you get a decent adjustment....

 

[kc99]

and. thats with new multi turn pots

 

[kc99]

should i bre concerned about this? i dont want to fry something else,, its not coming out of protection, id like to get it semi operational to check my progress, instead of changing all these parts out and hoping for a return to normal... any thoughts on how to proceed?

 

[petehall347]

it does appear to me that reporting back with the protect board voltages would be a good move so long as supply correct and stable .

 

[kc99]

ok will do, cause somethings not right,, those pot settings are way too unstable

 

[petehall347]

somehow missed the flaky pots . have you replaced them yet ?

 

[kc99]

yes,, put bourns multi turn pots in.... get the same unsteady readings,,, almost like AC voltage thats scaring me a bit, i dont want to fry new components as im putting them in, and i hate to install all the new caps and transisters and have something fry everything, and id like to check things as i go... as far as operation.

 

[petehall347]

i take it its both channels behaving badly ?

 

[kc99]

yes.. same story

 

[petehall347]

back to supply .. do you have a scope or at least a dc blocking cap to add in series with your + meter lead ?
mark will have better ideas . one thing is do you have a DBT ?

 

[kc99]

yes i can put a cap, but, arent we looking for a DC voltage? i dont have a DBT, Guess i can put one together,, I do have a variac, but i know thats not the same,, test equipment wise, i have everything u can think of lol

 

[petehall347]

i suggest dbt in use and look for suspect ac voltages riding on dc supply .
going in the way of caution i would disconnect all output transistors first as they are the most expensive hard to replace if original ones .

 

[kc99]

ok will do i can throw a scope on the supply to see if i can see whats going on.. wonder if theres anything special i should know about a DBT? I mean its juat a 100 watt bulb in series with the ac line to the unit, correct?

 

[petehall347]

yes dbt lowers the current plus voltage drops at same time . should save the outputs at minimum . i disconnect them anyway for strange faults like this .

 

[petehall347]

you said bias and idle pots its a very unsteady . these are one and the same thing .
bias is idle current . dc offset is what goes to the speakers .
are both these unstable ?

 

[kc99]

yes and heres some fresh readings somethings way bad wrong on the protection board,,,,, heres the voltages....

1) cant measure constantly fluctuating
2) -30,34 volts
3) fluctuating cant measure
4) 0
5) 0
6) 0
7) 0
8) 0
9) 35.2
10) 35.2
11) 35,2
12) 0

power supply....

1) 0
2) 5.255
3) 5.255
4) 13.2
5) 13.2
6) 50.2
7) 50.2
8) 35,9
9)35,9
10) 35.9
11) 27.1
12) 0
13) 33.9
14) 65,6
15) 23,5

 

[petehall347]

i cant believe for one second the supply voltages at 13 14 15 are positive . even if negative there is a fault needs looking at . zd12 appears to be open circuit .