Electronics > Repair

HP 8903 won't complete power-on test


Hi, folks,

A friend of mine has an HP 8903 with a missing CPU board which he has just replaced.

At power-on, all the front panel LED segments light up.

The four status LEDs on the CPU board flash, although Charlie reckons that one goes off and stays off.

The instrument does not appear to become active.

I think that

(i) the various LEDs lighting shows that the microprocessor is beginning the Power On Self Test (POST), but

(ii) the micro is not completing the POST.

If this was a PC desktop, I'd expect to get a beep pattern when the POST halted as an indication of what was wrong.


1. Am I on the right track with my idea of an incompleted POST?


2. If I'm right, is there  any way of working out what is stopping the POST from completing? Is the non-lit LED on the CPU board a clue?

Any hints gratefully accepted from Down Under.


if CPU/ROM is ok it will talk to GPIB

The flash pattern of the test LEDs on the controller board is important during startup. According to the service manual the order, for a successful startup, should be:

* Indeterminate for about 1/8 second
* ( ),( ),( ),(1) for about 1 second.
* (8 ),(4),(2),(1) for about 2 seconds.
* (8 ),(4),(2),(1) with (1) blinking indefinitely until a key is pressed.

The actual checks begin at #2 in the list above.

During startup of the 8903B, all lights on the front of the panel should light up and stay that way till the unit is ready to run (this can take a surprisingly long time, not minutes long but more than long enough to seem like startup has hung).

If you don't get to #4 then the test LEDs should provide information where the startup failed. Again, according to the service manual the order of these tests are:

* Front-Panel Annunciator Check
* ROM Check
* RAM Check
* Instrument Bus Parity Check
* Keyboard Check
Grab the service manual and take a look at sections 8-27 & 8-28, from there you should look at the troubleshooting section in BD1 and follow up with the appropriate service sheets.


Did you check the power supplies for correct voltages and especially ripple?  The unit can hang if it's excessive.  I had to change the main filter caps in mine.  One was going bad, so I did the lot.
Unit then booted and works to spec.

ALWAYS check power first!


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