HP 53131A PSU Mod and front panel repair - Page 1
Forum section: electronics / repair

author: Mrt12
author_rank: Regular Contributor
timestamp: September 28, 2025, 07:47:51 pm
content: Good day,I have a "new" HP 53131A acquired. It was cheap and the seller confirmed that the unit is working, and returns accepted, and the price was very cheap so I thought I give it a try.The unit arrived a couple days ago. Unfortunately was not the exact unit that was shown in the pictures, but it was working, "SELFTEST PASS" and the frequency counter works very well. But the Restoration will take some time - the whole unit is covered in sticky goo, the "noses" from the BNC connectors are filed off (perhaps for "quicker" connect/disconnect), some rust on the rear connectors, ...I started the restoration by replacing the BNC connectors. Unfortunately I have not the exact same ones on stock and for now, I try ones with a white plastic case. Probably with my next Mouser order will add the proper BNC connectors that have the metal case! (for this reason I did not solder them in.). Does it look acceptable? ;-)Also I notice I have this ugly hole in the front panel. Is there some technique to restore this to a nice cosmetic condition?Also the lens for the display has lost some of the printed text. Does somebody have an original HP branded lens and would sell it?Further I notice these units have a quite ugly power supply with the fan that runs all the time. As this particular one of mine has already some brown spots on the PCB, I assume due to heat generated in the power supply, I would not trust this to be connected to my mains socket all the time. So I wonder, has anyone ever did a modification of the PSU and replaced it with a modern one? it appears to me that there is only +/-12V. I am not even sure if the 9V are actually used, at least I didn't see it yet. For such a mod, I would use something like thishttps://eu.mouser.com/ProductDetail/TRACO-Power/TXLN-035-22M2?qs=eP2BKZSCXI6nXQ%252BpU8hqnQ%3D%3Dthat is a bit more modern and will also work nicely without a fan. Would it be possible?All in all not such a bad deal as the counter was super cheap compared to the other offers, and the VFD is in nice condition.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 29, 2025, 04:42:24 am
content: There is a youtube video where someone installed a hard power switch to turn the instrument off completely.I went and replaced all capacitors in mine, put a new fan in and installed the fan on rubber mounts, it's acceptibly quiet.The reason for the fan always being on is due to the OCXO options. They are kept warm at all times for stability, so the fan prevents things from cooking.The standard timebase can best be described as 'functional', the idea seems to have been that the customer would option in an OCXO if not using a house frequency standard.If you do keep the installed PSU, be sure to replace all electrolytic capacitors! They need replacement, and if they haven't leaked yet, they most certainly will (My score is 3 for 3 on these PSU's).... Use low ESR, High ripple current rated caps.As for the 9V output, IIRC, Ithinkthat is only used for the 12GHz Channel 3 option, maybe?Double check the service manual and schematics to be sure what voltage rails are present and actually needed.

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 29, 2025, 08:42:41 am
content: Search for the 'Gerry Sweeney' channel on YT if you are interested in the hard On / Off switch modification. I haven't done it myself, I use two Wi-Fi mains switches operated through Home Assistant to remotely turn my 53131A and 53132A on and off when needed. As a side note I've also set Home Assistant up so that it turns the main power to my bench off at 23:30 if the total load is below 20 watts. That way I know that my bench, test equipment, soldering iron etc (and anything I'm repairing) will be safe overnight.

author: DeepLink
author_rank: Regular Contributor
timestamp: September 29, 2025, 12:55:05 pm
content: The power supply runs all the time to provide power to the OCXO - if installedBy running 24/7 then electrolytics dries out, and you just need to replace them all + the fan.Then it is running fine againAs I use a house reference, I have modified my own counters with a "hard" switchAnd I did it in a way so I don't mess with the original boards/wires, just two small add on boardsOne board has the relay (and a tiny 230-12 volt SMPS) the other board sniffs the power switchWill take some pictures and I do have the Gerber files if anyone wants to make then/Hans

author: yngndrw
author_rank: Regular Contributor
timestamp: September 29, 2025, 11:43:31 pm
content: Quote from: TERRA Operative on September 29, 2025, 04:42:24 amThe -9V supply is used by the HP 10811 oscillator, used in options 10 and 12. I believe it's an extra cable which goes directly from the PSU to the oscillator board.There should also be a +5V rail.I've attached a schematic for the PSU, or at least I believe it's the correct schematic - It was posted elswhere on these forums and I squirreled it away.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 30, 2025, 08:49:27 am
content: Ah yep, I knew it was forANoption of some sort.... Didn't quite remember the exact one.