HP488, MCLR pull up not present

[Mark]

Hi,

I have had intermittent problems with programs hanging on the HP-488.
I have traced it to MCLR apparently floating, the Reset switch clearly pulls
it to 0v but the pull up is not present. Conneting MCLR to 5v solves the problem.
Putting a meter on MCLR gives no detectable resistance.
The ICD2/USB jumper is in the correct place and I have reseated it.

I cannot find R12 10k pull up to check if it is OK but no reason to believe it isn't.
No precusor problems/accidents etc prior to this and resistors are fairly robust.
Programming still works fine.

How can I fix/diagnose further this problem?

Thanks,

Mark

[petesmart]

Hi Mark,

Attached is an extract from the circuit diagram for a HP 488

I note that R12 (10 K) is attached to U9 which is the on-board Micro which manages the programming. Not sure if this is the resistor that you're looking for.

If you move across to the right of the diagram (red circle) you will see that pushbutton 1 is connected to R34 (10 K resistor) via J20: C, pin 8,9 - which eventually finds its way to pin 13 U3 - in idle mode I believe the output of U3 should be 5 V
-
What you may do is remove the target chip from the HP 488 board and connect a multimeter to the MCLR pin (pin 4 on U8/14) and check for 5 V. If you do not have 5 V at this point then I would look at pin 9 on J20: C. This should have 5 V, if not next step is to find R 34 (10 K), check each end of this resistor, one end should be 5 V.

If you cannot find 5 V on either end of R34, next step is to go to pin 13 of U3, hopefully there will be 5 V this point. If not a quick check of pin 11 on U3, checking for VCC (5 V) if there is 5 V at this point and not on pin 13 it is possible that U3 is faulty. From this point I will have to hand you over to Ben from the MM team to assist further.

mclr 1.JPG (83.44 KiB) Viewed 3153 times

hope this helps – let me know how you go

all the best,

Pete