Re: 7704A Mainframe Panel Lights No-Op

Albert Otten

On Mon, Jan 1, 2018 at 10:26 pm, William wrote:

(seems like this guy is rebelling against me or is deteriorating.)

The panel light for the controls in the front middle of the mainframe are
dead. That is the button lights for the Vertical and Horizontal Mode buttons
have gone out. (Scope still works.) Additionally, the A and B trigger Source
lights no longer work as well. Before some worked and some did not. Now the
whole thing is dark.

In looking at the service manual, I stumbled onto page 6-3 and note that there
are four boards ("board 11", "board 12", "board 13", "board 14" which are
Vertical Mode, Horizontal Mode, A Trigger Source, and B Trigger Source
respectively.) and each of these is supplied with a "+5V Lights" source from
the Calibrator Board 10 but the schematic shows no originating source and I
have not been inside the uni yet to see what's what.

Anyone troubleshoot this problem and can offer advise and information?

Thanks in advance.
William, k6whp
"A friend told me to cheer up, things could be worse.
So I cheered up and, sure enough, things got worse!"
This can't be a big problem. Must be a bad contact somewhere, with just a very few places to check.The 5V(lights) is generated bij the SMPS diagram <8> but is also used there in the balance input to the SMPS control IC . So the source should be OK.
From there the 5V(lights) goes to the rear panel illumination on/off switch. Did you exercise that switch a few times?
Next via board 32 <9> and P32G to the Main Interface Board at <2>. The plugins get their 5V(lights) from the main interface board. So please check P32G for good contact.


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