Boompsie wrote:

> I have a friend's 707 that constantly lights up all lights but
> functions fine otherwise...I took the switchboard out of mine and put
> it in his 707 and all works fine so I know the problem lies in the
> switchboard.
> I'm kind of guessing the problem might be in the transistors (115 67F)
> q301-q304, though it's a guess. Does anyone have any experience with
> a problem like this

Ok, there are a couple of things it could be. Let's dig out the service
manual, and scoot over to page 10 (707-svc10.tif).

Firstly, I don't think it's the four transistors. This would cause one
group of LEDs to be either on all the time, or off all the time, and
would probably not all fail at once.

Now I know you know how multiplexing works, but for the people on the
list who don't here's a quick run-down. You've got a grid of LEDs, with
all the anodes (positive) connected together in rows and all the
cathodes (negative) connected together in columns. When you want to
switch on a particular LED, you connect the column to positive and the
row to negative, and the LED comes on. Great. Now to make it useful
what we do is cycle between which row is on very quickly, and change
which columns are connected to ground when we do so. Persistence of
vision takes over, and we have a stable display.

Ok, so what drives the columns, then Well, there's a driver chip
(M54517) which appears to be just a bunch of Darlingon transistors in a
chip, and a latch. I'd be slightly surprised if all the trannies in the
driver chip failed at once too (although not *that* surprised...), but
I'd be paying close attention to the 40H174 latch. Check that the clock
pin is being clocked, and that data is making it all the way to the data
in pins and appearing on the output.

It may well be that it's a dry joint at one of the connectors, so check
those carefully too.

Gordon