1 Aug
2014
1 Aug
'14
1:43 p.m.
Thanks Tony! I believe its a 9836A (would be sorta overjoyed;) if it were
I would certainly check. I've wasted far too much time based on false
'obvious' assumptions :-)
You should open the machine up anyway to check for obvious falts (loose
parts, signs of burnt components, etc. Testign the PSU on a dummy load is
normalyl somethign I recoemnd doign on old machines, but it's not totally
trivial for this amchine. The power supply consists of an unregulated DC
supply digivng 20-30V (the mains trasnfoemr and smoothing capacitor are
obvious, the rectifier is the TO3 can on the little PCB at the back that
also holds the votlage selector switches -- this is not a power
tranistor, its' a double diode). The output of this feeds the regualtor
board giving +5V, +12V, -12V. This board plugs itno a conenctor on the
case which arries the DC input and a connectr on the motherboard which
carries the DC outputs to the rest of the machine. And of course the
motherboard has electroncis on it, inclduign the programmed 8042 keyboard
controller.
With 3 machiens ot maintain (a 9826, 9836A and 9836CU) I found it worth
making a test box for the PSU board. This takes in 20V from my bench
supply. It has a pair of edge connectors to take the PSU board, dummy
load resistors, montior lamps and sockets to conenct a votlmeter to check
each output. But that is rather too much work for just one machien I
think, and you may decide to risk it.
Anyway, gettign back to the video side, in all cases the video is
cotnroleld by a 6845 chip, so the syncs are only present after the CPU
has intialised the video system .If you have a relatively late CPU board
there's a row of 8 LEDs on it which give the self-test codes (And indeed
failures). it's worth watching these as the machine starts up, The
official service manual gives the codes, etc.
a C model). Yes, it was the Aussie site (I've
been scouring it for my
21mx). That was my next step, drag out my o-scope and determine the scan
rates but with the pin outs you provided that should be a simple step now.
THe vertical sync is a fairly noarrow pulse at 60Hz, and is quite
dififuclt to see on most 'scopes. It's almost impossible to see 2 of them
and measure the time ebtween them. You may find a frequency counter more use.
I need to see if any of my monitors will sync at that
freq. Thinking about
it, I've got a nice monitor for my amiga that might sync.
Let's find out what hte sync frequnecy is first, and which machine you
ahve. If it's a -A, the video is 2 TTL signals. 'My' schematics for the
machine include the various monitors, the video circuitry of the mono
monitor would eb a startign point for combining these signals to drive a
more noraml monitor.
-tony