witchy at binarydinosaurs.co.uk
Sat Oct 10 19:57:02 CDT 2015
On 05/10/2015 16:55, "tony duell" <ard at p850ug1.demon.co.uk> wrote:
> And I would be surprised if you got nothing on the screen either. Odd bit
> rot would mean the wrong patterns for some characters. But still something.
The heater is working in the tube and now that I've got a proper DMM I'm
getting the correct AC voltage across pins 9&11 on the transformer.
Also the blue and brown pins look OK.
> This is presumably a large-screen machine with a 6845 on the mainboard.
> Is it initialising that (look at the vertical and horizontal sync outputs with
> your logic probe, are they toggling)?
No. Only HSYNC is toggling, VSYNC is low.
> Is it accessing the kernal ROM? Is the CS/ pin pulsing low?
Yes. The others are high.
> What about the RAS/ and CAS/ signals on the DRAMs?
RAS/ is toggling, CAS/ is high on all chips. VBB is -4.98, VCC is 5.08, VDD
>> A lot of the repair pages use a piggyback 6502/ROM/RAM add-in called a
>> PETvet but all I have is a DMM, logic tester and other working PETs as
>> sources of chips I can borrow.
> I assume the logic tester is a simple logic probe and not a logic analyser,
I should receive an 8 channel logic analyser on Monday but for now I'm
getting results from the DMM and probe. Socket (empty) for UD11 looks faulty
in that it's VCC and GND pins show nothing. Continuity on the others looks
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