Sansui AU717 dropping left channel after rebuild

[daviddi922]

Hi All,

I restored my all original Au717 last year following Leestereo's guide and parts list.

Before doing this, the channel would drop and would only come back on if I quickly turned the volume knob higher.

I thought cleaning all the switches and pots along with the rebuild would have fixed it.

Well, the last few months the channel will drop randomly. SO I removed the preamp-poweramp selector switch off the board, disassembled it competely, and reassembled it and it is now shiny and brand new looking.

I notice that right before the channel cuts out, it starts to get static and then will go in and out and then completely out. If I move the volume knob sometimes it will come back in.

Been trying to diagnose this this week so I took all the panels off the amp and had it playing trying to get the left channel to go out. Last night it finally did.

Things I noticed this time which was interesting: First time it went out, the protection relay kicked in and started flashing. After turning it off and back on. I was able to get the left channel to go out again, but this time I tapped on the speaker relay and it came back on while tapping and eventually stayed.

If I continued to tap on the relay, it would go out momentarily like the contacts were jumping.

So my question, could it be the relay thats bad even though I replaced it?
Should the contacts be that sensitive to the tapping that they lose contact?
Week voltage leading up to the relay?

I really just want to get this buttoned up and enjoying it. It was my dad's since he was in high school.

Thanks in advance!

 

[Aud10file]

It could be a bad relay but unlikely if it’s a new Omron relay. These relays are pretty rugged in my experience. I would first inspect for cold or broken solder joints on the relay and surrounding components related to the left channel. That may fix it if you luck out.

The root cause of the problem could still be elsewhere on the board. Tap testing is a useful tool but not 100% dependable to identify the actual problem as the tapping could be affecting some other component or connection on the board.

 

[TupolevFan]

Most likely reason dirty switch on the back. The best course of action is remove small board F2670. Desolder the switch using solder wig carefully disassemble it. Carefully remove the sliders and soak it into DeOxit D5 while you clean the contacts on the switch. to clean the switch use eraser ( yes the one you attach to the top of the pencil and i use the one that look like a sloped roof or pyramid.) As an addition to it you can rinse it in a warm water and use "Scrubbing bubbles" you may do couple iteration until it will shine like Tomcat's balls ))). Then put back sliders and carefully assemble it. Make sure you do not use any excess force (not to bend sliders) Tools you will need : Eraser, "Scrubbing bubbles" (Rite Aid usually have it). Instead of scrubbing bubbles EchoWars used "Brasso" (home Depot has it) but in this case you must rinse it really really well as it is abrasive. Also you will need long nose plies, Solder wig, Soldering Iron of cause. ))

 

[daviddi922]

I will try reflowing the relay and check surrounding components first.

As for the switch on the back, I've already removed it and taken it completely apart and cleaned it. It is completely shiny now and reassembled it and soldered it back to the board.

When using the switch separated. The preamp section works perfect to another power amp of mine, both channels flawless.

If I use the power amp in with aux cord to my phone, the right channel works and not left. So I know it's in the left power amp section.

 

[TupolevFan]

Aha.. Sorry i did not read your message carefully sorry. in this case i tend to agree with the
Aud10file. You will need to inspect power supply board on a subject of cold solder or may be even corresponding driver board for the same.

 

[PF Sound]

Double check all the components/jumpers in the glue-affected areas. Something that may not look bad, could be. I've found some........both jumpers & components. Since this was happening before & after restore.......

 

[daviddi922]

I reflowed the left channel board, as well as the power supply relay and a couple of the wire pins that I removed to pull the board out.

Had it playing for 2 hours yesterday and couldn't get it to cut out. I still have it all apart and it is intermittent so I will have to keep testing.

Other suggestions will be greatly appreciated!

 

[TupolevFan]

Well i guess it's an oscilloscope time. If you suspect it is around relay and you have dual channel scope then put one probe before relay and one after and wait for the event of dropped channel and see if you have signal at the out put of the driver board before relay and after relay. May be it is the speaker selector which comes after relay.