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SX 1250 Tone Switch Makes Loud Pop Which Cycles the Protection Relay

Hugh Jorgan

Hugh Jorgan

New Member
Unit plays in all modes. Great sound quality.


Problem doesn’t happen with the two preamp jumpers unplugged.

Does happen regardless of source or speakers selected.

The only unusual thing I’ve found so far is the two supply voltages on AWG 041 (+25 and -25) are showing +28.71 and -18.85.

The Power Amp and Stabilizer voltages are dead on at +/-65, 0V and 100mv per the manual.


Where should I start looking next?
 
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common cause to both channels. Power probably.

does it matter if tone is on or off (bypassed)?

Does it matter if balance is centered, or full left or full right?

Does it matter if volume is at zero or full?

Has any recapping or re transistoring been done on it?




btw: Your avatar, is that the Wanamaker Grand Court Organ Console in Philadelphia?
 
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Turning tone on and off is what causes the pop sound.

Balance has no effect.

Volume at normal levels or at zero has no effect (haven't tried it at full volume though)

I replaced all the electrolytics as a shotgun approach (problem was happening prior to the re-capping though).

Not sure on the avatar.

Your power comment sounds like it's the likely cause. I checked the voltages on the little power supply board underneath (AWR 107). They're way off during normal operation. Is that how you check them, or are you supposed to disconnect the loads?

The voltages I measured are:

Pin 9 - Supposed to be 85V - Measured 66.8V
Pin 10 - Supposed to be -87.5V - Measured -68.5V
Pin 8 - Supposed to be 23V - Measured 18.58V
 
DIm bulb - yes. Haven't let it off the leash yet. I'm also on a variable AC power supply. Should I turn it up so I get 120V at the input to the 1250?
 
yes, plus I would get the +/- 25v a LOT closer, then reevaluate.

BOTH of those voltages are directly related to the zener diode voltages, if you have to replace them and purchase them, go with 2% tolerance at 24v

They did the power supply like that, to get the +/- 25v dialed in, then the +/- 65v can be adjusted to spec.
Symmetry on the 25's is more important then the actual value(s).
 
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