HP 3455A Digital Voltmeter Repair Attempt  - Page 1
Forum section: electronics / repair

author: JFPhy
author_rank: Contributor
timestamp: February 09, 2025, 09:17:43 am
content: Hello All,This is my first time posting on this forum. I've received an HP 3455A and have had no success in reviving it. I've included some images of what I get when turning it on. First, it lights up and displays a decimal in the far right. Next I can click the "Test" button, and the screen goes blank indefinitely. If I press any button on the panel a few times, I can get numbers to pop up, and all 8's if I press once more. Then, the cycle repeats as I keep pressing random buttons with no set amount in between. Keep in mind these numbers that appear are static, they do not change. The only time I can get them to change is when I hit "Enter Y/Z" I can punch numbers in using the "keypad". It will not recognize an input (short or non-short). Here are the following test that I have done so far:Main Outboard and Inboard TP voltages: GoodCleaned front panel switches and reset all ribbon cables: Seem to be goodI realized that I was not getting any voltage on A12. According to the manual I should expect square wave inputs (both on A10 and A12). When measuring T1 on A10, I do not see any square wave, only +5V on all three "pins". Due to my lack of understanding, the circuit diagrams do not label what input I should have going across from A10 T1 to A12 T1 on the two white jumper wires. Should this be a square wave? I have nothing going across as far as I can tell as I get almost 0V on the entire A12 board no matter where I probe.My biggest issues is that I cannot get the instrument to run a "Test" to give error values which would point to a troubleshoot tree.Any help/recommendations would be greatly appreciated.Cheers,Josh F

author: AVGresponding
author_rank: Super Contributor
timestamp: February 09, 2025, 10:14:30 am
content: If you're not getting a 31MHz square wave on A10 T1 between pins 1 and 2 or pins 3 and 2, you need to check A10 U33, which is the source of the drive signal for T1. You should have the square wave between U33 pins 12, 8, and 7, and between pins 9 and 7.If not then check for a square wave at pin 3

author: JFPhy
author_rank: Contributor
timestamp: February 09, 2025, 10:35:03 am
content: I may have been measuring T1 incorrectly. I do know that I get no signal at pin 9 but something on pin 12 of U33. I’ll update tomorrow with the readouts of U33. I did check this but was unsure of what I was seeing. I am not familiar with flip flops so this is all new for me. Thanks for the information confirming the square wave.-Josh

author: JFPhy
author_rank: Contributor
timestamp: February 09, 2025, 10:47:45 am
content: Can you guys explain what these numbers mean around U33 and U31? They aren’t pins, and the aren’t voltages from what I can tell. To clarify im referring to the ones where the pin numbers should be. I’m sure it’s simple but it’s not clicking for me.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 09, 2025, 11:07:12 am
content: They are the pin numbers. That's a very clean image, the HP scans aren't that good!The 7474 (U33) is connected to divide the clock signal on pin 3 by two in each f/f, by connecting the inverted Q output to the D input. This makes the output toggle between 1 and 0 on each clock pulse, effectively dividing the input frequency by two. The output of the first f/f drives the clock of the second. So you get a symmetrical square wave output that is 1/4 of the input frequency.The Q and not-Q outputs of the second f/f in U33 are driving the two transistors that drive the primary of the switching transformer.It's worth noting the frequencies mentioned in the manual may well be way different to what you see; this isn't necessarily a fault in your DUT, but could easily be an error in the manual. It happens quite a lot.

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 09, 2025, 01:19:10 pm
content: If you haven't already, check all of the voltages (inguard and outguard) with a scope for excessive ripple. I had a 3455A that was misbehaving because there was so much ripple on one of the 5V lines that the thing was bouncing in and out of functioning correctly. If you are just measuring voltages with a voltmeter, you can get an average reading that will be correct, despite the ripple.Also, if the ripple checks out OK, when the 3455 locks up upon start, it sometimes means that it is waiting for its autocal routine to finish, but it never does. A lot of the times, this can be traced to a fault on the little A/D board on the inguard section or faulty LM339 comparators on the inguard main board or damaged switching FETs on the front end signal acquisition section. These faults can be tough to track down and the service manual's troubleshooting trees are generally useless.

author: JFPhy
author_rank: Contributor
timestamp: February 09, 2025, 08:25:16 pm
content: UpdateI re-measured the pins on U33. Here are the results:1: 5.09V2: 61kHz square wave (looks good)3: 123kHz square wave4: 5.09V5: 61kHz square wave6: 61kHz square wave7: ground (21mV)8: 4.1V9: 150mV10: 5.09V11: 61kHz square wave12: 4.1V13: 213mV14: 5.09VWith this being said, the manual states that if no signals are found at pins 9 and 12, check U33 and U31. As far as I can tell, the clock signal from U31 to U33 pin 3 (123kHz square wave) is good. The following check is to check U33 pin 13 for HI06 coming from Latch U12. With pin 13 on U33 being, 213mV, I believe this is an issue. My next task is to check U12. If you guys see any flaws with this observation or see a flaw in U33 please let me know.-Josh

author: JFPhy
author_rank: Contributor
timestamp: February 09, 2025, 11:51:00 pm
content: Update 2The output on pin 2 (Q1) of A10U12 was low. So, I injected 5.14V into U33 pin13 to bring it high. Low and behold, there is now the 32kHz signal at both pins 9 and 12. I also checked if A12 had any life. Indeed it does. I tried cycling the power while holding the injected voltage to U33 with no luck. I suppose now it's time to figure out why U12 is not outputting high on Q1.

author: garrettm
author_rank: Frequent Contributor
timestamp: February 10, 2025, 01:36:15 am
content: Have you checked that the calibration module on the rear of the unit is installed? Sometimes people remove them to take the zener diode and matched resistors. You will need to pull the module out to see if the board is still inside it. I would also check the main board for the precision 10 Megohm resistor and its matching 100kohm counterpart. If those parts are missing you might as well toss this guy in the trashcan.

author: JFPhy
author_rank: Contributor
timestamp: February 11, 2025, 07:36:11 pm
content: Update 3This will consist of two parts. After realizing the issue with the Latch signal on Pin13 of U33 from U12, I wanted to know why this was the case. U12 is dependent upon both U14 and U22(data lines). My knowledge of digital logic is very limited. But, as you’ll see in the image attached, pins 18-25 of U26 (main processor) look terrible. I’m not entirely sure what they should look like but this just doesn’t seem right.So, I slept on it and figured if it were the processor then all of those digital lines would not look the same. It must be something else. I did a sanity check and measured all of the test points on A10. All seemed good except for TP1. This pointed me to the comparators U4,U5,U6,U8,U9. I remember watching a great video on YouTube (don’t remember the gentleman’s channel) who solved his issue by checking these comparators and having a bad U9 ic. My U4 is 100% not working properly with U5 being a bit odd but still logically fine (gives -12V instead of -24V with shorted output). So, I will order new LM339’s to replace these and see if that fixes the issues. Other than that, I cannot directly pinpoint another issue with the system.To answer previous recommendations, the 10V reference is fine. This was a university instrument that I snagged before going to recycling. While it’s possible for parts to be missing, I don’t thinkit was tampered with.-Josh

author: JFPhy
author_rank: Contributor
timestamp: February 16, 2025, 07:45:08 am
content: Replacing the 339 did nothing as it shouldn’t have. I miss read the voltages in the manual. Any idea what could cause the weird data signals above? Both nano-processors give the same garbled signal. I’ve also determined that the opo-couplers seem to be doing their job. I find it strange that both nano-processors exhibit the same odd signals.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 16, 2025, 10:38:15 am
content: If the ripple on the power rails is within spec, and the signal path from the (ROM, EPROM, EAROM?) to the processors is good, you'd have to suspect a corrupted FW

author: JFPhy
author_rank: Contributor
timestamp: March 15, 2025, 05:40:16 am
content: Hello all,After much trouble shooting,I think I have narrowed it down to A10U27. It is an SN74S03N quad channel NAND IC. According to the schematic down below, I should have a data signal resulting from the DATA TRANSFER IN out of U28(which is present). I see this on pin 2 of U27, and pin 1 is high. The output on pin 3 (TP9) does not resemble a working signal. I will try to put the images in order.So, I took it out of the A10 board and setup the exact situation on a breadboard. Same negative going 3kHz pulse on pin 2, same 3V input on pin 1. The result on pin 3 is what I get. While it is "working" in that it is acting as a NAND gate, the output is not "HIGH" as it's only 550mV (below the .8 of the 74LS03N spec sheet). The spec sheet calls for a pull up resistor. I placed a 10k ohm resistor between the output and VCC with no luck. Is it fair to say this may indeed be the culprit? It would be good to know what you guys with working 3455A's get on TP9.Thanks again for all the help. Troubleshooting this thing has taught me a lot about logic IC's.Cheers,JoshImages are as follows:In HP 3455A A10U27:Pin1Pin2Pin3Breadboard:Pin3

author: Kean
author_rank: Unknown
timestamp: March 15, 2025, 06:00:25 am
content: Quote from: JFPhy on March 15, 2025, 05:40:16 amAre you sure about that part number?I think it would be SN7403N or SN74S03N. See attached datasheet.The minimum VIHis 2V, so the 3.5V input on pin 2 should be OK.The outputs are open collector, so they do indeed need a pull-up resistor.I presume you checked for 5V power on pins 7 & 14.As these parts have 4 logic gates, you could see if any of the other gates are working.

author: JFPhy
author_rank: Contributor
timestamp: March 15, 2025, 07:20:59 am
content: Late night spelling error. Yes, it’s a SN74S03N according to the data sheet. The other three channels exhibited similar but not exactly the same output behavior (one may have been slightly closer to ground at all times). And yes, in the meter itself I had 5V at pin 14 and ground at pin 7 as expected.I expect the output to have a full swing from 0-5V. Is this a fair assumption?

author: JFPhy
author_rank: Contributor
timestamp: March 15, 2025, 07:38:00 am
content: Also I’m just noticing this thread should be in the “Repair” category not test equipment.

author: tautech
author_rank: Super Contributor
timestamp: March 15, 2025, 07:44:50 am
content: Quote from: JFPhy on March 15, 2025, 07:38:00 amAs the OP that started this thread, moving it to a better place in the forum is easy.Use the Move Topic feature at the bottom of your page.A Moved link will remain here for others to follow and those that have replied won't lose links to this thread.

author: Kean
author_rank: Unknown
timestamp: March 15, 2025, 08:01:40 am
content: Quote from: JFPhy on March 15, 2025, 07:20:59 amIn general yes, but the output swing will depend on the pull-up resistor value and any other connected circuitry (loads or paralleled O/C outputs).Per page 5 of the above datasheet showing the SN74S03 electrical characteristics, VOL(max voltage for output low) is specified as 0.5V max at IOLmax of 20mA.And you can see the high and low input threshold voltages: VIHis 2V min and VILis 0.8 max.With a 10k pull-up to 5V it should swing pretty close to 0V and 5V. You might want to trace that signal as it is possible there are other open collector outputs driving the same signal low, but if the part isn't doing that on the breadboard then it seems likely it is bad.For testing, I'd temporarily ignore the 3kHz pulse, and just start by verifying basic operation per the truth table. If it can't do that, then it is stuffed.

author: JFPhy
author_rank: Contributor
timestamp: March 15, 2025, 08:10:06 am
content: Good call on tossing the 3kHz signal. I’ll give that a shot and see what happens. Thanks for the help.

author: JFPhy
author_rank: Contributor
timestamp: March 15, 2025, 07:08:47 pm
content: Quote from: Kean on March 15, 2025, 08:01:40 amWell, after testing the component with normal DC to check the truth table, Ch1 is not functioning properly. It remains high no matter the inputs. Ch2-Ch4 work perfectly. I'll go ahead and order a new one. Lets hope this is all that's need to get this thing up and running!-Josh

author: Kean
author_rank: Unknown
timestamp: March 15, 2025, 07:49:32 pm
content: With U27 removed, maybe check the HAZ signal coming out of U15 pin 10.In your earlier traces it was only about 3.5V where it should be reaching 5V. It was maybe being loaded by the faulty U27, or it could also be faulty.If it looks OK with U27 removed, then that is a good sign - else maybe source a replacement SN74LS174N for U15.The signal also goes to J2(P) then to A14 board Q1 via 10k resistor, so with the series resistor I don't think it will be a problem there.

author: JFPhy
author_rank: Contributor
timestamp: August 06, 2025, 01:56:31 am
content: To add a conclusion to this thread. I am convinced the issue is that there is no F1 data transfer clock signal on TP2 (A3) from the Main controller (Line DC1). Why this is, I am unsure. Because of this, no data can be sent too and from the Inguard/Outguard via the opto couplers. I found this very strange as the main U9 Nano Processor on board A3 does have an input clock signal going into it. The data stream F0 (Line DC0) is present. Unless you guys have an idea why there would be no clock signal on pin 36 of U9, I'm calling this one a loss.Cheers,Josh