Can anyone check their HP 53131A Universal Counter for me? - Page 1
Forum section: electronics / repair

author: Generalquark
author_rank: Contributor
timestamp: August 07, 2020, 03:11:45 am
content: Hi,I have a 53131 Universal Counter that I have been working on. It failed self test for a couple things including a front end failure. I ended up finding that the -12V regulator was only putting out -4Vdc under load. I just replaced it today and it now passed self test, however I watched the -12V line start at -11.8 and continue to drop down the longer it was on. After leaving on for some time, and restarting the unit it failed self test as before. I did notice something odd while working on this power supply inside. When I plugged in the power supply immediately the fan would come on. This does not seem correct to me and I was wondering if someone could verify that this is not correct? When you have power going to the Universal Counter but have the power button off, is the fan running in the back?Thanks!Ben

author: bob91343
author_rank: Super Contributor
timestamp: August 07, 2020, 03:44:19 am
content: Get the maintenance manual. It should answer many of your questions. Asking them without doing your proper research won't get you where you want to go.

author: bdunham7
author_rank: Super Contributor
timestamp: August 07, 2020, 03:58:19 am
content: Your power button doesn't have an 'OFF'. Look closely!What is the regulator--an LM7912 variant? If so, check the input pin to make sure it has at least -15 volts, the regulator has a minimum drop voltage. If that's OK, then the regulator is likely self-limiting the current either because it is too hot or because the circuit is drawing too much current. How is it heat-sinked?

author: DeepLink
author_rank: Regular Contributor
timestamp: August 07, 2020, 06:49:28 am
content: Hi BenThese counters are getting quite old.Mine had seriously leaking electrolytic capacitors on the power supply boardThe power supply is always on, when power plug is insertedTherefor the fan is always runningThis is for keeping the OCXO running (if installed)You can find the service manual on-line2 parts - Assembly-Level Service Guide & Component Level InformationI guess that both can be found on the Keysight websiteNo information is given in these manuals regarding the power supply board (which is manufactured by Delta)I have attached the schematicAre you measuring the voltages on the Power Supply Board?If they are wrong here, you can take out the power supply board outWhile you have it out, do change all electrolytic capacitors (remember to use Low ESR, high ripple current types as this is a SMPS)I also did some other mods. to mine1. Changed all caps on Power Supply Board2. Changed one cap on main board3. Changed the fan to a Noctua NF-A4x20 FLX4. Installed a Power Controller (“Hard Power Switch”)I do run mine with an external 10MHz reference, and I don't have an OCXO installed. So do not need the counter to be warm 24/7

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2020, 06:56:50 am
content: The fan is completely normal, it runs whenever the counter is plugged in. This is because the power supply needs to be on always to keep the ovenized oscillator stable and ready to go, for those that have this option.

author: Generalquark
author_rank: Contributor
timestamp: August 09, 2020, 02:48:16 pm
content: Thank you for the tips! When the power plug is in, but the switch is not pressed in, the 7912CT Regulator reads VIN 12.54V and Vout -11.74. When the Switch is pressed in the VIN reads -12.1V and Vout reads -11.0V. The current draw on Vout reads right around .3 Amps. According to the datasheet, this IC can handle well over 1 amp, so it does not look to be loaded down. The VIN seems to be the problem. Could a leaking electrolytic like C7 be pulling the input voltage down?

author: aqibi2000
author_rank: Regular Contributor
timestamp: August 09, 2020, 03:58:41 pm
content: What is the voltage is at VIN with no regulator installed

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2020, 06:13:01 pm
content: Quote from: Generalquark on August 09, 2020, 02:48:16 pmYou can test C7 if you like, but at this point you need to check both sides of D251 with a scope. And thank DeepLink for posting a schematic!This PSU appears to depend on having enough current at the 5V tap to maintain the -12V Vin. So there could be some odd issue causing less +5V draw, too much -12V draw (perhaps 300mA is not normal) or a bad transformer.

author: james_s
author_rank: Super Contributor
timestamp: August 10, 2020, 11:56:16 pm
content: I would be very surprised if the transformer is bad. I've fixed a LOT of stuff and I don't remember ever encountering a bad transformer in a SMPS.

author: Generalquark
author_rank: Contributor
timestamp: August 11, 2020, 12:27:52 am
content: Ok, I checked the Regulator input with the Regulator out of circuit and the voltage is around -18V. When the IC in installed it drops down to around -12.1. I did change the output electrolytic capacitor, but i am unsure about the reliability of the schematic. Unless i am wrong, the capacitor ratings are incorrect on the schematic, and the reference designators do not seem to match up. Nothing seemed to change after swapping the electrolytic capacitor. I wanted to swap the diode, but could use a bit of advice. The diode i pulled off is a RGP10Bhttps://www.vishay.com/docs/88700/rgp10a.pdfI don't have one of those, but I do have a 1N4004. I usually just replace parts with what I pull out, but do you think it would be ok in this case to try replacing it to see if the diode is the issue? If it fixes the issue I could order the right part. Just looking for advice as the diodes look somewhat similar on the datasheets, but I am not sure about the fast switching part. Thanks Again for the help

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2020, 01:04:56 am
content: Quote from: Generalquark on August 11, 2020, 12:27:52 amDepending on the SMPS switching frequency your 1N4004 will range from sub-optimal to not working. I wouldn't do it. You are probably barking up the wrong tree here--if you had a replacement diode handy, by all means swap it in--but I don't see what theory indicates that it is the problem. Do you have a scope?If you want to test the regulator, you need to make extensions for all three legs so that you can selectively disconnect them or insert an ammeter as you wish. First, connect it but leave the output pin disconnected and see what Vin does. If it does drop a lot, then put an ammeter on the ground pin and see how much current is being shunted through the regulator. My guess is that the regulator is not bad and the Vin will not drop until you connect the output. If the regulator is bad somehow, it's your lucky day!Assuming that's right--the regulator works and the voltage only drops with the output connected, you have only two possibilities--the Vin circuit is not supplying enough current or the device is drawing more current on the -12V rail than the designers anticipated. At that point you'd need to know what the normal draw is.

author: Generalquark
author_rank: Contributor
timestamp: August 11, 2020, 01:57:01 am
content: I do have a scope but have not anything. What am I looking for? I previously did make extension wires for the output and left them disconnected breaking the circuit. For some reason I did not write down the results, so I will have to go back and do it again. Something seemed odd when I did this. I believe the Vout pin read something really low and I don't think Vin was -18 any more and was down at -12vdc again. I will try to confirm that again and let you know what I find. I also checked current on Vout line and that is where I got the .3 amps of draw on the output. I also checked it with a current clamp around the extension wires that i ran with the circuit connected. The .3 amps was when the front switch was pressed in and the unit was on. I think it drew something like .008 amps with the switch out. I will attempt to confirm all this and thanks again for the pointers. This seems odd and that is why I was going for the Diode. I am now just working on the supply by itself without powering up the main board. I would like to see greater than -14 at the input as previously suggested.

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2020, 02:15:47 am
content: Quote from: Generalquark on August 11, 2020, 01:57:01 amThe scope would allow you to see the output of the transformer. But for now, if you have the PSU out, could you check R102 and R103? (in the 5V section) They're paralleled, so if the schematic is correct you should see ~25 ohms across them. Try both polarities of your ohmmeter leads just in case your DMM has enough test voltage to activate the blocking diodes.

author: DeepLink
author_rank: Regular Contributor
timestamp: August 11, 2020, 06:34:16 am
content: Regarding the schematic (which I didn’t draw – so I can’t take credit for that) is likely for a different versionDue to the long period these counters were produced, several versions of the power supply have pbeen used. But it gives a good look into the design principle.Easiest way to work on such a power supply, is to take it out and run it on the desk with a suitable loadAttached you’ll see the power supply spec. (again – not from mine)I order to maintain correct regulation of the output’s; a minimum load is needed.When I did work on mine it used a load of some power resistors (see attached)If you have stable output’s, with such a load. You can expect that the supply is all good.You wrote that you measured a current of 300mA on the -12V supply. This seems highOn the mainboard there are several tantalum capacitors, and these are prone to fail.But before you start de-soldering all these, do make sure that your power supply is 100% working

author: Generalquark
author_rank: Contributor
timestamp: August 15, 2020, 11:43:21 pm
content: Thanks for all the help! I tried replacing the Power Supply Voltage Regulator Input Electrolytic Capacitor and it seems to have worked. Everything seems to read correct now. Must have been leaking. I should always be suspect of those caps. Thank you for the schematic. Even though the values did not match my board, it was still helpful. Deeplink, where do you find schematics like that? Often times on the boards here people seem to have access to schematics I have not found. Is there a good place to look for them or do you have to just search around? Thanks Again!

author: DeepLink
author_rank: Regular Contributor
timestamp: August 16, 2020, 10:08:14 am
content: Found it here on EEVbloghttps://www.eevblog.com/forum/repair/agilent-53132a-repair-(and-possible-upgrade)/msg841547/#msg841547Jf2014 should have the creditBut google (and EEVblog) is your friendIIRC there is a few entries here on EEVblog were various manual sites are listedYes -https://www.eevblog.com/forum/testgear/test-equipment-manual-sites/msg2938048/#msg2938048