Vexing C-4 problem

GasMan

Active Member
My C-4 is my main listening resource, right next to my desk. Avionic re-capped and changed lights to LED in late 2011 and it has been spectacular until recently. In November 2017 the right channel dropped out. Nothing I did externally made it recover so I removed it from my stack and put it on my workbench to look at later. Next day, I powered it up on my bench and all was well!! Worked perfectly every time I powered it up on my bench. Day before yesterday I brought it in and put it back in the stack. Beautiful music all day. Yesterday morning I powered up my stack and...right channel is gone!! Back to the bench (Sure glad I didn't sell the Kenwood Basic C-2!) where it sits now. The only observations I have made so far are visual. Solder points are all good and no plugs are loose. There is one difference between my office and my shop: temperature. My office environment is 65~70 Deg. constantly. Shop gets to the 40Deg range at night. Could that be an issue? When I acquired my C-4 the protect circuit was stuck on. Avionic had me replace several parts including D318, a VD1212 varistor. Could one of the remaining VD1212 in the flat amp board be temperature sensitive? Any suggestions would be appreciated.
 
Could one of the remaining VD1212 in the flat amp board be temperature sensitive?
Possible. But I doubt if thats the issue. I´m sure its either another solder connection issue or a dirty switch.
 
When the VD1212's act up. It usually is unwanted noise"popping/static" induced into the signal path.
 
I've had a nasty cold for the past week making me not interested in working in my shop. Looks like a warming trend is coming so I'll do some more troubleshooting during the week ahead. I'll give the solder connections a more thorough going over and check the record-out connections as well.
 
Had it open today and thoroughly inspected all the soldering throughout. Everything looked satisfactory so I then turned to the various plugs and sockets and disconnected/connected all of them. I'm convinced that connectors are not the cause of my problem. I did find something that may be the problem. While it was open on my bench I thought it wouldn't hurt to make sure that the circuit board mounting screws were secure. Surprised to find that several weren't tight. They weren't loose but the screws could turn about 1/8 to 1/4 turn. Maybe that was the issue. I've got it back in the stack and listening as I post this. Lets hope!
 
Well, after about 45minutes the right channel went off. No pops, no clicks, no drama. Just went silent. I took Clinic Audio's advice and checked the record outputs and they are both good. Whatever is happening is after the record out selector. I'll leave it off for now and see if it works tomorrow.
 
Well, after about 45minutes the right channel went off. No pops, no clicks, no drama. Just went silent. I took Clinic Audio's advice and checked the record outputs and they are both good. Whatever is happening is after the record out selector. I'll leave it off for now and see if it works tomorrow.
Try tapping on the reed relays when the channel fades out.
 
Does the tone by-pass button have any effect on the missing channel ?
 
I'll give it a try in the morning. Have to take it out of the stack to take the cover off. Re: RY301. Isn't that in the power good circuit? Doesn't it shut off all output?
 
I'll give it a try in the morning. Have to take it out of the stack to take the cover off. Re: RY301. Isn't that in the power good circuit? Doesn't it shut off all output?
If you have issues with contacts within the relay it can and will drop a single channel. Even though the relay is set.
 
Exact new replacements are hard to come by. Unless your clinic audio. Hundreds of reed relays available through mouser or digikey so we have the technology.
 
So just try the ratta tat tat on the relays when it acts up. Could even be a bad solder connection.
 
This is where and signal generator and and O'scope would be handy.
 
Ran all day without incident until about an hour ago when the right channel quietly dropped out. I'll pull it out of the stack and put it back on the bench and hope I can get it to fail there. I've got a scope and sig-gen and will see if I can trace the fault. Just for review, when the fault occurs audio is passed to the rec. outs, both 1 and 2. Also, balance control has NO audio when fully clockwise and full audio fully counterclockwise. If I read the schematic correctly, the balance pots are ahead of relay RY301. To me, it looks like I've got a component failure or a bad solder joint in between. Is a puzzlement. Glad I've got another preamp!
 
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