Perplexing SX-838 Light Issue

ringading

ringading

Well-Known Member
I picked this unit up locally. They said that it didn't sound right. On testing I found out it just needs the controls cleaned.

Great looking unit with a nice cabinet.

The odd thing that I cannot figure out is the light issue. The dial lights only come on in aux/mic/phono mode and turn off in AM or FM mode - the opposite of what I would suspect. It also, at one point slightly flickered in AM mode. When I first powered it up in AM mode I thought the lamps were burned out but found this issue after fiddling around with it in other modes.

I haven't worked on one of these before - does anyone know if the dial lights are supposed to be on or off in aux/mic/phono mode?

To make things stranger the lights are different brightness depending on what speakers you have selected - A/B A+B. All lights also flicker ever so slightly.

I gave the power/speaker and function selection switch a good bath with contact cleaner and Deoxit but things have not improved much.

I am going to go back and look for bad solder connections or crimped wires for the AM/FM mode.

Any other suggestions are appreciated
 
No Dimmer Switch on the 838.
1.) Dial Lights Stay ON in ALL MODES as does the DIAL Pin.
2.) Function indicator Lamps (AM, FM, AUX, MIC, Phono1 or 2) come on and go off as they are engaged by individual switches.
3.) Speaker Indicator Lamps, go on and off as engaged by individual switches.
 
Doesn't appear to have been worked on so far. I haven't been able to poke around yet. I haven't had to trouble shoot lights yet. How do they get switched from the function switch? Obviously they are being switched somehow by the function switch. I am going to try and trace the wires from the switch to the lamp board to see if I can spot anything.
 
Cleaning the switches with contact cleaner does not always ensure that the contacts have low resistance afterwards. Sometimes the contacts are worn out, esp the ones used for lamps since they draw more current than the ones used to pass a low level signal. The way to check them is to use the ohmmeter to test the contact resistances after the cleaning. I have a qx-9900 with a similar issue, the contacts that drive the meter lamps, FM/AM lamps are switched through a worn out contact. Changing the switch wafer is a real PITA. One possible solution is to replace the lamps with LED's that draw significantly less current. Another solution is to use a relay to switch the lamp loads and use the sw contact to drive the relay coil which is a lot less current than the lamps.
 
Check the individual chassis grounds. There's a bunch of them. Visually verify them, AND do the tests with the fuses PULLED!!!
Otherwise you can get false readings.

The lamps are run on mixed power:
1.) Dial Lights Stay ON in ALL MODES as does the DIAL Pin. 300mA x 6 all run off of 7.5V AC with Fuse Fx 3 Amp (pin 23, blue wire)
2.) Function indicator Lamps (AM, FM, AUX, MIC, Phono1 or 2) come on and go off as they are engaged by individual switches, stereo lamp. 8.5v DC UNregulated (pin 24, violet wire)
3.) Speaker Indicator Lamps, go on and off as engaged by individual switches, dial pointer lamp all run off of 7.5V AC with Fuse Fx 3 Amp (pin 23, blue wire)
 
First off, I apologize. It is a SX-828, NOT 838.

This does have the dimmer switch and I have discovered that if I hold it halfway down then the lights come on in all modes and are nice and bright. I have sprayed the switch with contact cleaner and deoxit and it has not solved the problem. Not sure if I will have to take the switch apart and not sure I want to try - I have tried to take switches apart in the past and depending on the switch it can be impossible to get them back together properly.

I am going to try and get the switch out of the unit so I can get to it a little better to clean it.
 
have another go with d100 set the thing on its side use a syringe and flood the switch and leave it sit over night. cycle the hell out of it about 50 times. flush with d5. i had the similar probs on my 828. theres something missing on the schematic with regards to A ,B and c that feeds that circuit let me know where the source for "c" is if you find it. ill be damned if i could find it when my 828 lights played up. it was a deoxit fix thank god
 
I have flooded it pretty well and it only slightly improved - lights flickered on in AM but then went out after a few seconds.

I am wondering if I can swap switches between the dimmer switch and the FM Muting switch?
 
5% deoxit + 95% solvent for cleaning
100% deoxit

the switch is BURNED. No deoxit will cure that.

IIRC the switch does not come apart gracefully. I add a relay and use the original switch salvaged as much as possible to control the relay.

Yes, it may be possible to swap them. Then use the relay.

Then you will just have muting to contend with.
 
Yeah, seems like it is just burned out. Looks like the switch has a lot of very tiny moving parts so I don't think I will attempt disassembly.

I may swap them since I would rather have the dimmer working and am not picky whether my FM is muted or not.

Thanks for the link to the bypass instructions tsd71.
 
Some progress. I switched the two switches and now we have lights! I haven't tested the FM muting yet and have a replacement switch assembly from that auction site. Now I have intermittent issues with the right channel dropping out - I think it is a dirty volume pot problem as it was working previously and the left channel has been touchy as well. I get both channels when I switch to mono mode.

I have flushed it again with Deoxit and worked it back and forth and will test again in the morning.

Is it worth it to change transistors in the AF board while I have it out again?

From another thread(http://audiokarma.org/forums/index.php?threads/sx-828-main-amp-awh010-question.383775/):
Q3, Q4 SA725 -> KSA992FBU
Q5…Q8 SC1312 -> KSC1845 (original post calls for UBU but I only have FTA)
 
UBU and FTA are Gain (F & U) and packaging, Bulk (BU) and Tape(TA) loading for machine loading.
 
my 939 had this issue and, for now, a huge bath of CRC QD cleaner has it working correctly
I searched for a by-pass method and seems TSD's linked post may work well
 
Yup, changed all to LEDs already as I thought they were burned out when I first was testing it out. That is when I found that the bulbs werent the problem.

Now it seems that my missing channel is not a dirty pot but something else. I just get nothing from the right side - not even changing the stereo mix changes anything. Thinking it is an issue in the main amp? Somewhat new to troubleshooting but I believe if the controls on the AF (tone amp) board can't get sound from one side then it is likely on the main amplifier board.

Time to start taking some measurements off that board unless someone can chime in with a likely culprit.
 
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