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Wanted: Time interval PCB for HP 1726A scope

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I just acquired a HP 1726A scope which I presumed was working properly.
Though the scope works OK, I can't do Delta T time measurements as the digital display is no good.  I need either the A21 PCB (sits on top of the main case) preferably,  or the LCD 8 digit display (common Cathode) and/or the Driver chip (an ICM7218BIPI+ which is a 28 lead plastic DIP).

It could be the 8 Bit processor (a 8048) but I think they are almost impossible to get.
Can anyone help?


Hi and welcome to the forum!

Congratulations on your new (old) scope.

I am looking at this manual (page 217):

Do you mean the A18 delta time board?
It looks like the LED display is on a connector so it shouldn't be difficult to figure out if all the segments are working. 5 volts and a few 100 ohm resistor should do it.
If you find them all light up, it may very well be the ICM7218 driver or the 8048 processor. Either way you can get them very cheaply off ebay:

Is the display only partially not working (few segments not lighting up) or does it display just gibberish, similar to this?

Thanks for the reply. The display is dark so I'll do as you suggest.  If its the display itself then I am clueless as to which are the V supply pins.

The manual I have is not explicit on the layout or pin designations and the diagrams are so small and fuzzy. Your photo is exactly the units I'm asking about and yes its the A18 board. My mistake

On second thought the LED display is almost certainly still good. It has no active parts on it so it is very reliable, especially if it's made by HP.

If the ICM7218 is in a socket then you could pull it out and build a circuit that tests the IC.
For example in the datasheet on page 8 there is a very simpe circuit for a common cathode display:

If the IC tests out good then it may be the 8048 processor. Be sure that you check the +5 volt supply rail on the delta time board. It's probably good since it's derived from the scope but still make sure it's in tolerance.

I spent a bit more time in diagnosis. After disconnecting and reconnecting the capacitor C63 on the reset circuit to measure it, I got a error 2 display. The count light went out (this led stay on all the time in the fault condition). So I suspect its in the 8048. I could not reproduce the error display again even going through the performance tests the manual described.

I guess I'm stuck. I presume the 8048 has a pre-programmed ROM embedded so another "generic" processor might not work.
Maybe I need to take this thread elsewhere in this forum. Any suggestions as to where?


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