68k based computer in hp54502a oscilloscope

[ddavidebor]

Hi  guys

I'm trying to repair the computer inside an hp54502a oscilloscope

The clock is ok.

Sometimes all the bus are all stopped, nothing happens. Sometimes various data are on the data and address bus and the screen show horizontal lines.


Do you have any suggest? It's an awesome piece of equipment, it's a pity not repair it.

[dfmischler]

Do you have schematics?

[ddavidebor]

No unfortunately

[trackman44]

Maybe if you took pictures of the logic board and post them here we might be able to help. Also take pictures of the power supply.

Will

[Anks]

Check the reset halt lines maybe there is a watchdog firing.

[ddavidebor]

Check the reset halt lines maybe there is a watchdog firing.

Should it remain high or have some type of clock?

[Anks]

If I remember correctly the 68K wants to see at least a 100ms low pulse at start up on the halt and reset lines then these should be held high. The reset and halt lines have other functions but I doubt they are being used. One trick is to force the reset and halt high if the watchdog is barking. This will force the processor to run but there is generally a reason why its barking but should allow you to scope the lines for bus contention and stuck data/address lines.

Here is a good document on the 68K
http://patpend.net/technical/68000/68000faq.txt

Section 3.6 system control (Page 35) of the 68k user manual explains the rest and halt along with berr.
http://tict.ticalc.org/docs/68kUM.pdf

Hope this helps