SX-1980 Slow to get out of protection

[jamescuda]

As the title says: I have a sx-1980 that takes about 3 to 4 minutes to come out of protection and engage the 2nd relay. I figure the PS board needs rebuilt, but would like to have an idea what is wrong before I just jump in there. Any help would be appreciated.

I ran some voltage checks on the board and this is what I've found so far:


Pin.....spec.....tested
#18 -80v.....-80v
#17 +80v.....+80v
# 9 +34v.......climbed slowly from 20v to 31v
# 8 -34v......-30.7v
#13 -13v......+12.7v
#14 +13.......+12.7v
#16 +8v.......+7.6v
#15 +5v +5v
# 6 @ 0v bounces between 0v and 1v
# 3 @ 0v bounces between 0v and 1v

 

[dr*audio]

You need to adjust the idle current and DC offset in the output stage before you do anything else. The problem could be the offset is too high, or noisy transistors in the output stage.

 

[markthefixer]

the amps run on +/- 80 so they shouldn't be affected by the drifting +34v
and the protection circuit runs on 13v that is zener regulated off of the +34v supply.

but yeah, 0 to 1v on 3 and 6 is not acceptable. watch the dc offset on both amp channels. see if the 0 to 1v tracks each other or are independent.

addressing the +34v problem:
c224 and c222 are across the +34v line and could be severely leaky
or R2 360 ohms 10w off board could be damaged. of course damaged q214 or q216 could do it. The +34 uses the same reference voltage as the +80, which is stable so the base voltage for q214 is not the problem. watch the pin 29 voltage (pin 31 feeds the R2 360 ohm resistor from the +80 regulator, pin 29 is the other side of the resistor) to see if it is stable at about 57 volts (80v - (360 ohms x 0.064mA) ) or if it rises along with pin 9, if so look to the transistors and caps.

 

[dr*audio]

On this model, the power supply board is on the bottom and doesn't get enough ventillation. The TO-220 case transistors, the ones with the metal tab at the top, usually mounted on a small heatsink) get hot and "desolder" themselves. Check the solder joints on the transistors. Look for a dark ring or void around the component lead. Resolder any that have the voids or look grainy.

 

[markthefixer]

On this model, the power supply board is on the bottom and doesn't get enough ventillation. The TO-220 case transistors, the ones with the metal tab at the top, usually mounted on a small heatsink) get hot and "desolder" themselves. Check the solder joints on the transistors. Look for a dark ring or void around the component lead. Resolder any that have the voids or look grainy.

It's worth checking, but these are the furthest from the heat on the far corner of the board, opposite the relays. The 5 and 13v supplies have the heat sinks under the 80v heat sinks and run incredibly hot. The only silver u-shaped ones on the board, the rest are anodized / painted black.