Wanted: Time interval PCB for HP 1726A scope - Page 1
Forum section: electronics / general

author: Farthings123
author_rank: Newbie
timestamp: March 07, 2012, 05:47:16 am
content: 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?ThanksRod

author: tekfan
author_rank: Frequent Contributor
timestamp: March 07, 2012, 03:49:04 pm
content: Hi and welcome to the forum!Congratulations on your new (old) scope.I am looking at this manual (page 217):http://cp.literature.agilent.com/litweb/pdf/01726-90903.pdfDo 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:http://www.ebay.com/sch/i.html?_sacat=0&LH_PrefLoc=2&_nkw=8048%20intel&_sop=15http://www.ebay.com/sch/i.html?_sacat=0&_nkw=ICM7218&_sop=15Is the display only partially not working (few segments not lighting up) or does it display just gibberish, similar to this?

author: Farthings123
author_rank: Newbie
timestamp: March 07, 2012, 05:24:54 pm
content: 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 mistakeRod

author: tekfan
author_rank: Frequent Contributor
timestamp: March 08, 2012, 03:48:01 pm
content: 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:http://www.intersil.com/data/fn/fn3159.pdfIf 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.

author: Farthings123
author_rank: Newbie
timestamp: March 10, 2012, 03:52:12 pm
content: 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?Rod

author: tekfan
author_rank: Frequent Contributor
timestamp: March 10, 2012, 05:31:46 pm
content: It could very well be the 8048 but first have a look at the manual on page 30 where it lists the error messages and make sure that the knobs aren't positioned so that it gives an error.Oh and if this is the only scope you've got, you can use it to probe itself.

author: Farthings123
author_rank: Newbie
timestamp: March 11, 2012, 07:16:26 pm
content: Thanks again for the input. I'm to the point of giving up on diagnosing the problem.I am using the scope to do tracing but I'm a little inexperienced in how the counter time interval works in conjunction with the scope's logic.The actual microcontroller on the A18 board is an NEC 8214H1. I cannot find any info on that chip anywhere. I presume its a clone of the 8048.So in reality I'm back to my search for a replacement board.

author: Farthings123
author_rank: Newbie
timestamp: March 13, 2012, 02:07:21 am
content: Thanks Tekfan for your advice.I have solved the problem. All that was needed was to re-seat the internal plug in PCB's. I should have done that in the first place. I'm happy now.Farthings123 (Rod)

author: tekfan
author_rank: Frequent Contributor
timestamp: March 13, 2012, 01:38:58 pm
content: Very nice. Now you've got a great scope for electronics and RF work.