9830A Repair



#2

Hello,

I hope someone around here can help me.

I recently aquired a HP 9830A. After checking the the power supply I decided to turn the unit on. But for some reasons the display stays black. So I guess one of the boards is defective.

When I turn the calculator on, 2 random characters flash up for a fraction of a second, so I guess the Display and its controller is working.
I do have an osciloscope and a voltmeter.
Can someone help me to get this thing going?

Thanks in advance,

Sebastian


#3

The hp9800 series are bit-serial, which makes them 'interesting' to repair

You'll need the schematics, which are available from the Australian museum site. A logic analyser is very useful, but you can manage without one.

The display on the 9830 is partially scanned by the processor/firmware. So for the display to work, the processor, memory control, ROM, RAM and display all have to be working. Of course there are ways of tracking down the fault, but just about any problem in a 9830 causes a blank display

A little background info which will explain the 2 random characters you see at power-up. The CPU has a 16 bit I/O register. When talking to the display hardware, this register contains the 6 bit codes for 2 characters, and a 4 bit value saying where to put them. The character pattern generation is done in hardware. So at power-on you have a random pair of codes and location. There is a protection circuit that blanks the display if the processor isn't talking to it, hence these random characters disappear after a short time.


#4

Hello,
so thanks for this information.
Does this mean that every board has to work? So even the complete cassette drive contoller? I really don't know how to get started on this system. I got 2 ram cards so I ruled these two out. I also sensed the 8 Mhz at the clock.
But how do I find out wich components are working and wich are not? Do I have to check the boards by applying voltage and data signals to them outside the systeme, or is there a better solution?


#5

Sebastian,

there are some function critical boards, e.g. the 4 CPU boards, on which a failure of nearly any IC leads to a complete calculator malfunction.

The easiest way to find out the defective board is to do board swapping with a known functional machine. The boards of the malfunctioning machine are plugged into the working one until a failure occurs. In the next step one has to isolate the defectice ICs my means of a logic comparator (HP5011) and/or a logic analyzer. Of course, for use with the logic comparator you would need a working sample of every TTL IC. This may become difficult since some of them are no longer produced and hard to find.
Some defects can be found best by comparing the signals of the malfunctioning machine with those of a working one using the logic analyzer. Having a service manual handy is very helpfull.

Best regards,
Achim

#6

I don't normally post repair tips here any more due to the actions of one particular member, but I'll make an exception. If you contact me by e-mail, I can talk you through the entire machine.

Not all the boards have to be working, but most of them do :-(. And the ones that don't _have_ to work (like the cassette controller, expansion ROM backplane, etc) could fail in such a way that they prevent the machine from working.

The fact that you have 2 RAM boards (in the slots in front of the PSU) doesn't mean you don't have a RAM fault. The 2 boards are assigned fixed addresses (depending on which slot they're in) and
there has to be RAM at some particular addresses for things like
the system stack. So if that board has a problem, the machine won't work. Alas I am working from memory, and can't remember which slot has to contain a working RAM board.

So what I normally do is firstly remove all the 'unnecessary' boards. These are the 4 boards for the tape controller (Blue handles), the tape drive itself, the keyboard connector (in front of the display, but leave the mains cable to the on/off switch connected), the I/O backplane, the expansion ROM module backplane, and any internal expansion ROMS (but leave the black/red handled PCB that contains the last bit of BASIC in place).

Still no display? I then check the display strobe signal (see the scheamtics). It's almost always missing, but you'd feel a right idiot if you spent time debugging the processor section only to find the processor was sending data to the display and the fault was in the latter section.

Then there are several things to try. If you power up the CPU clock PCB (brown/red handles)on its own (either in the machine with all the other boards removed, or using a 5V bench supply), the BitClk and muClk signals will run. 16 bit clocks, then a muclock (microcode clock), and so on. Check this with a 'scope or logic analyser.

Since it's a bit serial machine, the M (memory address) and T (memory data) registers should have all bits changing. With the 'essential' boards back in the machine, check these with a 'scope or logic analyser.

Then it gets harder. I connect a logic analyser to the microcode address lines on the CPU control PCB (brown/orange handles) and grab the microcode listings (ask me nicely by e-mail :-)). Then I see just what the CPU is doing and go from there.


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