HP 9830A always turns on with blank display

I am glad you like them... There were also some theory-of-operation articles for the HP9800 series in the HPCC jorunal over the last couple of years. But I rather glossed over the power-on circuitry ans I felt it was not likely to be much of a problemn. Oh well...Yes, I actually picked up the HPCC DVD to get some of those. ?I have a subscription to Datafile, but for some reason never received the first 2012 issue. ?I suppose I have to wait until the next DVD update. ?I actually am much more comforted having the schematics, but I like the articles because there is frequently some subtlety that's not obvious without scoping, and I learn something by going through the circuit description while looking at the schematics.

power up and press any otehr key??Only the STOP key. ?I had pushed various other keys earlier with no effect. ?The only reason I went back an tried the STOP key is because it's in the operator manual. ?I was surprised by this directive, as I would expect only one behavior on power up. ?After than, the machine works as normal.

It is curiouse that pressing a key has this effect. The power-on circuit resets the microdoe program couter (OK, it actually sets all the address lines, the first address of the microcode [1] is 1 111 1 111 or 1717 in 'split octal'). The STOP key clears a few latchies on the I/O interface, not much more. [1] Let me know if you want a commented source to the 256 word CPU microcode.I would be most grateful for that.

Ah.. Init/ (power-on intialise signal) also clears the interrup flag latches on the I/O interface PCB. Perhps for soem reason the interruppt line (KeyDn/ on the schematics, as that's all it was usef for in the HP9810, and that's the machine I worked on first) is being set at power-on. That will cause interesting probkems with the machine tryign to exectue the interrupt service route far to oarly. A few quick things to try. Pull out the tape contoller boards. As others have mentioned there is a power-on init circuit there too, The tape controller boards are the 4 with one blue handle behind the display. You may have noticed that the board handles give the last 2 digits of the PCB part number using the obvious code. Anyway, the machien wil lrun without the tape controlelr boards. So try it now, does it give a prompt at power-up.I thought about that, and tried removing all the tape controller boards and, for good measure, all the option ROMs. ?No change.

leave the in-like AMP connetor pluged in, it conencts the mains switch. Power up again, do you get a dispal ynow. If so, suspect a problem on the keybaord enocder.I will try that when I get back home next week.

Most likely neither of these is the problem, the problem is the Init/ circuit on the PSU board. It's based roudn a 74121 monostabl IC. I would suspect the pair of 6.8uF capacitors associated with this IC. -tony