2270 - function indicators lights go on, then go out - SOLVED

[Dingman]

Hiyas peoples...
Here's something I haven't figured out on one of the 2270's that I'm working on.
On power up of the unit:

• Function light comes on, as expected.
• Function switch does change function lights, just as it should be.
• After about 5 minutes of on time, function light goes out. None of the lights work anymore, regardless of function switch position. Function switch does properly change inputs.
• Quick power cycle will not restore function lights.
• Power off, wait for... 10 mins? and lights work again.

I've tried tracing thru the schematic and also with voltages on the board, but I get lost at the function switch. I can't figure that out.

Any suggestions for testing and probing?

 

[Scotteboy]

I would try replacing the R017 (mine is 18 ohms) resistor from the incoming power lead (green on mine behind the dial lights panel) for the dial lights to the function light bank. It can test fine statically then heat up give a high resistance and limit current.

 

[avionic]

If you haven't already deoxit the wafer switch wafer that switches the function lights.

 

[avionic]

If its the rear wafer. Make sure it is actually rotating.And the shaft hasn't shifted forward.

 

[Dingman]

Problem solved.
None of us were right, but one of you led me in the right direction (meaning, I was on the wrong end of the circuit).
I'll get a pic, and post back so you can see...

 

[Dingman]

Well, I wasn't thinking the problem was up at the lighting board, I figured it was down in the function switch or farther upstream.
I did check the wafers on the function switch, that all looked good and it was already cleaned. So then, I replaced that 18ohm resistor, it was pretty crusty (looked like high heat over a period of time), but that didn't help either. It measures out at 24 ohms.
But when I powered back on, and tested voltages again across the resistor, I could see the problem existed on the lighting board side - so I gave it a good looking over.
I found that the board position is able to be slightly adjusted on the mounting screws. I found one end of the board forced the power trace up against the mounting screw and was shorting out to ground.
You can see where the screw wore thru to the trace. After the unit being on for 5 mins or so, that board must have heated up enough that it flexed against that mounting screw and shorted out the function lights. Cooled down, it was OK. I moved the board back from that mounting screw, all is good now.

 

[Scotteboy]

Glad you found it ! With these older units just about anything is possible and you just have to focus from one end and work your way through as you did.

Avionic could have just as easily been on the end that had the problem, these are complicated beasts ! I am trying to resolve a 2270 FM problem myself right now so it happens to all of us !!

Good detective work and thanks for showing all of us to help in the knowledge of resolving these problems !