RX02 / DSD440 / SA801 problem

From: Tom Leffingwell <tom_at_sba.miami.edu>
Date: Tue Mar 12 13:50:01 2002

On Sat, 9 Mar 2002, Tony Duell wrote:

> It's virtually all in th ASIC. There are some relevant inputs :
> Pin 33 (Step pulse input)
> Pin 29 (Step direction input)
> Pin 34 (Step Enable /)
> Pin 27 (Output Enable) ?
>
> You might want to compare the states of the 2 enable pins between the 2
> drives. Just in case there are problems there.

They appeared to be the same on both drives, except for a slightly lower
voltage on pin 34 on the good unit (3.7 on the good one, 4.9 on the bad),
while trying to move.

Also, it appears I have an intermittent problem. After awhile, I noticed
the drive was actually positioning correctly. I went to connect it back
to the PDP, and it worked for about 10 power cycles. Then it got to where
it wouldn't work when connected to the PDP, and would work without
it. After a few more times, it got back to where we are now--where it
doesn't work at all. All the connections seem pretty good. The unit was
physically moved during the above events.
 
> 2 other areas to investigate. Firstly the clock for the ASIC. This is the
> Xtal (Y1) and associated bits connected to pins 19 and 20. Look for a
> 5MHz oscillation on at least one of those pins with a 'scope.

I have 5Mhz on both pins.

> And POR (Power On Reset), on pin 14. This should be low briefly after
> power-on and then go high. Check the capacitor connected to this pin ( on
> the 25229 board, this is C21, .47uF) and the power fail comparator
> circuit (on the 25229 board, it's pins 10, 11, 13 of the LM339).

Pin 14 does as you indicate. C21 seems okay. The LM339 is fine, although
I only checked the section you mentioned.

-Tom
Received on Tue Mar 12 2002 - 13:50:01 GMT