The SX-1250 gold tier restoration is going to be posted here very soon, I have to finish processing the photos. In the meantime, I thought I would showcase this "straight forward" repair that turned out to have a few surprises.
This SA-9500 came in to my shop needed the right channel rebuilt, it suffered a nasty failure that left 3 of the 4 outputs and both driver transistors shorted out as well as a few burnt resistors and a bad emitter resistor.
Board before, note the burnt covers on the 150 ohm resistors.
New parts! I managed to find NOS drivers and outputs, for the resistors I used Vishay/Dale resistors and Bourns trim pots, 25 turn for the offset for easier offset adjustment and single turn for the bias.
All done! Replaced all the emitter resistors as well as the base resistors. I also replaced the 2SA726 differential input transistors with a 1% beta matched set of Fairchild KSA992 transistors.
All the old parts. The 150 ohm resistors got toasty!
The bias trimpot actually melted a little from the current overload and is burnt internally.
Got everything replaced, I also replaced the trimpots and the 2SA726 differential input pair in the left channel as they were flaky. Hooked it up to the DBT and fired her up, success!.....orrrr so I thought.
A loud hum greeted me through both channels when I hooked up speakers. Drat. A hum through both channels generally indicates a ground problem or a bad power supply, so I first looked at the input/phono amp board and checked the ground traces. Aha! A ground trace had completely lifted and appears to have had a current overload, as it was falling apart and appeared slightly burnt. I scrape away the old trace so it doesn't cause problems and solder a wire in place of it. Check the rest of the ground traces and ground points, they all look good. Fire it back up, hum is lower in volume but still there. Okay, that means it must be the power supply then. I first check the 470uF smoothing caps for the +60V and -60V regulated supplies. First one I check:
A loud hum greeted me through both channels when I hooked up speakers. Drat. A hum through both channels generally indicates a ground problem or a bad power supply, so I first looked at the input/phono amp board and checked the ground traces. Aha! A ground trace had completely lifted and appears to have had a current overload, as it was falling apart and appeared slightly burnt. I scrape away the old trace so it doesn't cause problems and solder a wire in place of it. Check the rest of the ground traces and ground points, they all look good. Fire it back up, hum is lower in volume but still there. Okay, that means it must be the power supply then. I first check the 470uF smoothing caps for the +60V and -60V regulated supplies. First one I check:
Yeahhhhhh, I think we may have found the problem. That's measuring 8.39nF, it should measure around 470uF, and the ESR is off the charts.
I replace both 470uF caps and just to make sure, I check the other caps.
Supposed to be a 47uF cap, it measures 966pF. 😲
Supposed to be a 100uF cap, measures 18.579uF. ESR for both is off the charts as well.
That is just a sample of what I found, in all only 2 caps still measured even remotely close to their original specs. Time for a power supply rebuild!
Power supply before, note I have replaced the 470uF caps already.
And after! I went ahead and replaced all the diodes and transistors as well, upgrading three of them to TO-126 case transistors and two (the positive and negative feedback transistors) to a TO-92L case transistors for better heat dissipation and longer life. The two current source diodes were replaced along with the bridge rectifier diodes.
All the old parts from the power supply.
Fired it up again and success! Dead quiet in both channels. Adjusted the +48V and -48V rails to exactly 48V via the PS trimpots and reset the bias and offset again to spec and we have a fixed SA-9500 and a very happy owner!
If you liked what you see, be sure to hit that +1 button down below. Comments and critiques are always welcome!
If you liked what you see, be sure to hit that +1 button down below. Comments and critiques are always welcome!
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