Sx-838 Rebuild

[torinogt]

I'm rebuilding a SX-838. The receiver had issues with crackling through the speakers. I deoxed all of the controls and switches and that did not help so. So I hooked up an MP3 player to the pre amp input and got the same results. I figured that if I'm going do any work on the board I m going to rebuild it. So I found a thread with a parts list and order all the parts. I replaced all of the ecaps, diodes and transistors on the power amp board. put all of the output transistors and put new mica and paste. my question is the trim pots, where should I have them set for the initial power on. I will have it hooked up to a DBT.

 

[tsd71]

The trimmers while on DBT should be set to the minimum. Idle current fully counter clockwise, dc offset set to half way. Use a 100 watt bulb when making the adjustments. It’s similar to full house power.

 

[larryderouin]

The 838 has the same style push switches along the bottom of the dial glass like the 939 and 1010 for spkr selection and Source selection. These are problematic for getting dirty FAST. Best thing to do with ALL of them is to remove them one at a time from it's board, dis-assemble it , clean to bright metal the slide leaves and the tabs in the switch housing. Then reassemble completely. This will last a few years. Ultimately it will need to be done again.

 

[superdog]

The push buttons are problematic.I was able to get to mine without disassembly if you can believe that.Enough to quiet the controls.Unless I used it pretty regularly the problem would be back.I really liked my 838.It was a downsize casualty.

 

[larryderouin]

To add to TSD71's post,
When you set them on the DBT, you are effectively "tickling the Dragon". When you go from the Wall to the DBT and don't return the trimmers to minimum and centered, you can blow out the amp by overpowering it as the lower voltage setting actually requires more trimmer to get spec levels (which is too much for wall voltages). Go ahead and use the DBT and test it up to spec levels, then RETURN THEM TO MINIMUMS. Power up on the wall and do the adjustments again.