“These things take time.”
And Patience.
I know this thread just stopped back in early July and anyone doing a search for info on a SX-939 or SX-9930 might come across this thread and think, “Oh Great, another thread where the OP just vanishes and we don’t know at what point he gave up on, or blew up, the receivers.”
Happy to report that I did neither. Both amps are up and running. The 939 was always the one of the two that was in the worst shape and gave me the hardest time. Thanks to Larry’s suggestion, I did the receivers one at a time. After grappling with all the problems I had with the 939, the 9930 was a breeze.
I did not touch the tuner boards because I felt that the trouble was not worth the gain. Both tuners work but are weak. I counted the radios I have in the house the other day and came up with 14 which includes a Pioneer TX-7800 tuner in the rack. I decided to move on.
The amps idle current is set at 25mV on all channels and I was able to get the DC offset below 5mV or better on all the speakers. The crackling on the 939 is gone and the muddy sound of the 9930 has disappeared. Both units have their quirks, but I can live with them for now. An example is the lights on the 9930 wink at me every once and a while. I cleaned the hell out off all the switches, yet out of nowhere the lights will wink and the Phono 1 switch on the 939 has to be wiggled slightly if rarely a channel drops out when playing. Wiggling and winking, I wonder if my amps are mocking me or trying to be sexy.
I replaced the lights and the protection relays. There are minor differences between the 939 and the 9930 but they are basically the same. The amp board was where most of the difference in caps were found. Both units sound great, very little hum which only comes through on headphones with the volume at 12 o’clock or more. Music is pleasant, yet precise. Does not have that laboring behind feeling that used to be there. I’m very pleased with the results. My SX-737 will benefit sometime down the road.
Things I learned:
Have Patience. These things will not get done in a day.
Don’t Get Cute. If the unit looks unmolested and you come across a cap uF value that is different than what you see on the schematic, don’t think adhering to the schematic will be the right path.
Don’t Be Stubborn. If your vetting failed you and you come across caps that you ordered the wrong part for, don’t put in the wrong part because you feel you must be in charge. Leave the correct original cap in place and add the right replacement part to your order list. The original cap is 40 years old, it can wait a couple days or a week to be replaced.
Take The Time To Learn. I imagine that some of the people here on this board shake their heads when thinking of people with little to no knowledge grabbing a solder iron and jumping into a project. I am guilty of knowing less than I should. But I am also grateful for all the posters over the years that have shared their experience and knowledge so we can keep the music playing. If a problem crops up, which it will, the answer can be found by using the search engine. In the searching you will find as much or as little of the information as you need. Finding the solution to a problem is better than being told the solution. Thanks guys for all the help you have posted over the years that has helped me without you even knowing.
Double Checking Is Not Enough. You think you have a grip on the orientation of that transistor you just installed. You have checked the ECB against the data sheet and even went so far as double checking with your transistor meter. You have bent the base pin before installing so you get it right. Maybe its just me, but that is not enough. Check it, install it, check it again, walk away for a minute and come back to check it again. Only then can you solder it but check it again after the solder cools.
Staring Is The Answer. Practice your icy stare. I believe that if you stare hard enough and long enough at something that is giving you fits with your recapping that the problem will eventually go away. Ok, maybe it is just a theory that is yet to be proven. But like I said, “These things take time”
Testing
Center Stage
Right at Home
. I wouldn't worry too much about matching the KSA992's having an adjustable offset circuit, also l have a bag of them here and they all test VERY close for hFE straight out of the bag, l was going through them all the other night just for curiosity.
