HP5326B 50MHz Counter/Timer/DVM Problem. - Page 1
Forum section: electronics / testgear

author: simonmc
author_rank: Contributor
timestamp: May 13, 2014, 08:45:22 pm
content: I have the above counter that is not working. I have been going through the service manual and have found my first problem. The /10 counter U3 on the A5 time base card has stopped working, which means that i cannot select any time base above 10ms. The chip is marked Motorola 1820-0413. Apart from being an 8 pin gated divide by 10 counter, i cannot find anything else about this chip, yet alone a supplier. Do any of you good people have any suggestions as to how i can repair this board??

author: edavid
author_rank: Super Contributor
timestamp: May 13, 2014, 10:05:56 pm
content: Sounds like an HP custom part. I would try to patch in an ordinary counter. Is the gating used? Do you know the logic levels?

author: simonmc
author_rank: Contributor
timestamp: May 13, 2014, 10:26:38 pm
content: The 8 divide by 10 counters are daisy chained together, counting from the main 10MHz oscillator. The front panel timebase switch will pull down one of the select lines to one of these counters. Depending on the position of this switch, the output of the particular counter is outputted to a common gated line for use further on in the system. The logic levels look TTL, but there is some ECL used as well on the board. Attached is a pic of the board. The counters are the eight 8 pin chips.

author: Fank1
author_rank: Regular Contributor
timestamp: May 13, 2014, 10:31:18 pm
content: This may help they have a lot of test equipment parts. HP & Tek.www.sphere.bc.ca

author: simonmc
author_rank: Contributor
timestamp: May 13, 2014, 10:39:18 pm
content: QuoteThanks i will have a look.I had another look at the service manual and the reauired part is listed as a divide by 10 counter, TTL, 12.5Mhz Min. The drawing is on pages 60,61 and 62 of the the PDF of the service manual. I would post the drawing but dont know how to pull it off the PDF.

author: simonmc
author_rank: Contributor
timestamp: May 13, 2014, 10:54:48 pm
content: Quote from: simonmc on May 13, 2014, 10:39:18 pm

author: edavid
author_rank: Super Contributor
timestamp: May 13, 2014, 11:06:55 pm
content: It appears that you could use something like a 74HCT160 for the dividing, and part of a 74HCT03 for the gating.

author: simonmc
author_rank: Contributor
timestamp: May 14, 2014, 06:19:24 pm
content: I think what i may do is build a small board with a 4 74HCT109's (dual jk flip flops) and a couple of 74hc03 (quad nand gate) for the gating, building in effect a 8 stage gated ripple counter. Using SMT parts and headers to fit in the original holes for the defective chip/s, It would not take up much space and hopefully get this part of the counter working, and using as much of the original parts a posible. If i can remember how to use eagle (I have been using altium at work) i will post a drawing of what i have in mind.

author: edavid
author_rank: Super Contributor
timestamp: May 14, 2014, 06:36:32 pm
content: Quote from: simonmc on May 14, 2014, 06:19:24 pmThat's not going to divide by 10.

author: simonmc
author_rank: Contributor
timestamp: May 14, 2014, 07:11:57 pm
content: QuoteYou know what. Your correct, and also why you said to use the '160's. I think I got more caught up in trying to remember how to use eagle...Ok then a bit more thinking required..EDITAfter a bit more digging doesnt a divide by 10 counter like the 160 only give a pluse out at the clock frequency when it counts to 10? Therefore to get the output i require, i have to feed this into a jk to get the required actual divide by 10 signal then use that to clock the next counter?EDIT2OK, After some even deeper digging into datasheets i think i will use 4 74hc390's in place of the JK mentioned in my last post with the 74hc03's for the gating. Drawing to come.

author: simonmc
author_rank: Contributor
timestamp: May 14, 2014, 09:47:25 pm
content: Ok, I knocked up this drawing of what i think may work. Its only a quick sketch and i have probably made a few mistakes as it getting late. Wire names denote where they would connect to the original board on to headers that replace the original chips.

author: edavid
author_rank: Super Contributor
timestamp: May 15, 2014, 12:10:42 am
content: Quote from: simonmc on May 14, 2014, 07:11:57 pmIt depends on what the downstream circuitry requires... if it needs a square wave, yes, the '160 would not be a good choice, but you could use half of a 74HCT390.QuoteI thought you said only one of the divider chips was broken. Why do you want to replace the ones that still work?BTW your diagram is too small to read.

author: simonmc
author_rank: Contributor
timestamp: May 15, 2014, 06:40:10 am
content: QuoteThe rest of the circuitry requires a square wave, at least that's what the original circuit outputted.QuoteI did think about just replacing the one broken chip, but as this is in the time base circuit, i was worried that the replacement circuit would have a different propagation delay or some other timing difference to the original. If i replace them all, with this circuit then all time bases will be 'matched' if you see what i mean. It probably wont make much difference, but as i have to make a board anyway and the extra parts are only a couple of pounds then why not? I will make the board with the LPKF machine at work.Attached is a bigger version of the drawing. I still have some checks to make on the original cct, such as if the reset/clear line is active high or low.