HP 545A Logic Probe - Page 1
Forum section: electronics / repair

author: WA4AOS
author_rank: Newbie
timestamp: February 24, 2020, 10:13:35 pm
content: Greetings to the group.I have an HP 545A Logic Probe with a defective U1 IC. AKA:A1U1 With an HP part # of 1820-1610.I can find no information on this part and it was a HP Proprietary part.HOWEVER, it's just the input part to a logic probe. Has anyone found a suitable sub or work around?If I knew more about this IC, I could have some headers made with SMT devices and mount to the existing location.This may be one of the best designed probes ever and a fix would be GREATLY appreciated.Any thoughts or suggestions??Regards,Glenn

author: duak
author_rank: Super Contributor
timestamp: February 25, 2020, 12:14:57 am
content: hp's logic probes and pulsers have a write up in the Dec 1976 issue of the hp Journal:http://hparchive.com/Journals/HPJ-1976-12.pdfThe article on the 545A doesn't have much detailed info but does show the input circuitry and the power regulator circuit. Should be possible to realize the rest with SMD logic.I've got a few of these things and they were and are handy. I replaced the light bulb in one with an LED. I can't remember if I needed a series resistor or not.

author: WA4AOS
author_rank: Newbie
timestamp: February 25, 2020, 12:22:52 am
content: Thanks, that is a start... I thought about a new board with off the shelf stuff but the pcb in the probe is nice and a header with SMT stuff might be the trick.I have the 546A pulser and 547A current tracer as well. I have found a number of really strange problems with the 547A.. I wish someone would come out with a new version of it.Again thanks,Glenn

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 25, 2020, 01:09:06 am
content: You'd think there would be a market for that. Prices on 547a probes are just crazy.

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2020, 01:34:59 am
content: The 547a looks like a neat tool, I'd never heard of it before. I had a look and the schematic is in the manual but I don't know what the two linear ICs are. Certainly it should be possible to design something modern that does the same thing though.Also the probe tip looks to be part of the secret sauce, I don't know whether it's just a coil or something else but I'll take another look when I'm home and have a proper computer in front of me.

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 25, 2020, 02:52:47 am
content: Looks like someone made their own version using a (now obsolete) analog IC:https://forum.allaboutcircuits.com/threads/current-tracer-similar-to-hp547a.146322/Registration is required to download the attachments. Not much in the way of details for the sensor, but it might be fun to make one up.

author: bitseeker
author_rank: Super Contributor
timestamp: February 25, 2020, 03:23:03 am
content: Thanks, Paul. I hadn't see that project before.

author: WA4AOS
author_rank: Newbie
timestamp: February 25, 2020, 02:54:26 pm
content: Hi Guys,My question was regarding the HP 545A probe not the 547A. Can we PLEASE stay on topic as I am trying to find possible info regarding my topic of the HP 545A.Thanks,Glenn

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2020, 08:47:50 pm
content: The first reply is probably about as much info as you're likely to get. It's a proprietary HP part, and given that logic probes are still readily available I doubt anybody has taken the effort to design a modern replacement for that particular model. From that pdf you can get a pretty good idea of what's in that IC though.The 547 is a lot more interesting IMHO.

author: WA4AOS
author_rank: Newbie
timestamp: February 26, 2020, 01:03:05 am
content: I breadboarded a solution using SMT parts. 2 current comparators and the usual support circuitry. Wonder if there is any interest in having some boards made. Still need to work out the real estate issues but plan to use a 0603 header, At least I have something working now.

author: Shock
author_rank: Super Contributor
timestamp: February 26, 2020, 02:28:51 pm
content: It may be useful if someone wants to DIY one, not sure how many broken ones there are out there. Do a quick schematic and parts list and attach it to a post, it will be used by someone eventually.I have the complete set of 3 with logic clip and comparator, nice tools.A few playlists of videeos I found on them:https://www.youtube.com/playlist?list=PL7K-x1E-x5j_XhmMCJg99_BhLusGDCUgThttps://www.youtube.com/playlist?list=PLI-IyFhSMr13mx2syl1g4vpEmtnTJZjZJ

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 26, 2020, 03:13:35 pm
content: I still have a logic probe I made years ago that used a 7 segment LED display. Depending on the logic level it would display L, H or O. It was a fun project that still sees occasional use.

author: WA4AOS
author_rank: Newbie
timestamp: February 26, 2020, 04:07:45 pm
content: I too have multiple sets of the 3 probes. HP sold them in the $300 price range EACH back in the day. Compared to anything else on the market now, they are superb. Lot's of TTL and CMOS stuff out there and not sure why HP stopped making themI am considering a current sensing outside box where the probe could be plugged into and give it a voice with tones like some of the newer probes. And unlike some of the audible units out there, you could custom define tones and level to suit the operators taste. With a outside box, other possibilities are possible such as a second set of LED's and more storage than just one toggle.I still need to do a few teaks to my replacement of U1 but I'll post something soon.Now days , scopes are very affordable and even the Asian $10 ebay special scope would probably work very well in place of a probe and tell a lot more about the state of an address, control or data line.