HP 3455A repair help - Page 1
Forum section: electronics / repair

author: Dragon88
author_rank: Regular Contributor
timestamp: February 13, 2015, 03:02:02 am
content: Hi,New user here, though I have been a long time fan of the EEVBlog. I was hoping to get some help with repairing an HP 3455A. I picked this up for $50 and knew it had issues. I have done some basic troubleshooting so far, but I ran into a roadblock due to the very poor quality scan of the service manual available online. I may have to pick up a real copy on eBay. Anyway, this is what I'm seeing. Any help would be most appreciated.-The unit powers on. The outguard section seems to work, the buttons and indicators all work. I can switch ranges etc. The display shows overload (OL) immediately after the autocal fails. It then sits on OL with the attenuation relays clicking.- Pushing the test button and cycling through the failures, it shows 11 through 8, 6, 5 through 0, occasionally a number such as 6.1846, then it does a lamp test.- After cycling through the failures at least once, the behavior seen on the display changes a little. The AC modes begin to show erroneous readings instead of just OL. The DC ranges still all show OL, EXCEPT for the 10V range. This shows a reading of about .6 Volts, which then slowly climbs toward a volt or so. If I short the inputs, it drops down toward .1 V.- Test points 7 and 8, which I believe are the +/- 10V references, are +/- 5 to 6V.- The large "pipe bomb" looking relay (K1?) does seem to work. I found this referenced in some google searching I did, so I checked it. In the lower ranges it passes the input through, higher ranges it does not.- The electrolytics all look fine. The other power rails measure fine.- The reference module in the back is present, though it powers on and behaves in the same way even if the module is removed. The upper 4 wire terminal on the front seems to have had something dropped on it, as it is bent downward a little. Otherwise the device is in good shape.- I re-seated the removable boards inside.I think I may have a shorted FET somewhere, especially if TP7 and TP8 really are the 10V references. Again, I don't have a readable schematic in hand, so I'm kind of shooting in the dark. Instead of starting to test every single component on the board, I was hoping someone with experience working on this model would have some ideas.Thanks!

author: Dragon88
author_rank: Regular Contributor
timestamp: February 13, 2015, 03:13:45 pm
content: A small update. I found a good pdf of the service manual online, and that has helped a lot. (http://bama.edebris.com/manuals/hp/3455a/)I am focusing on the area around test points 1 and 2. There is about 140 ohms between them, which seems very unusual based on the schematic. Also, TP2 is 38 ohms to ground. That node being shorted to ground would load down the 10V reference I believe.I got excited when I found 38 ohms across capacitor C24, so I lifted it, but of course that was not the short. I then lifted Q36 since that was the next likely fault to ground, but no dice. I may lift Q35 next. There are 140 ohms from drain to source. Either this is the culprit, or there's another path between TP1 and TP2 I'm not seeing, or I'm chasing a red herring...

author: Vgkid
author_rank: Super Contributor
timestamp: February 13, 2015, 04:48:03 pm
content: First thing I would do is check the PSU, on my 3456 it caused a large amount of the self test errors.

author: Dragon88
author_rank: Regular Contributor
timestamp: February 13, 2015, 05:29:05 pm
content: The +5, +9, +/- 15, +30, and -24 V rails all measure spot on. They also look clean on the oscilloscope.

author: Dragon88
author_rank: Regular Contributor
timestamp: February 14, 2015, 07:50:09 pm
content: I think I'll have to put this away for a while. I did remove Q35 and that was a bit of progress. The reference rails measured +/- 10.00V on all but the 100 mV range afterward. However, I don't think Q35 was the problem, it just cut one node off from another node that does have a problem.Attached is a picture where I'm working.I hope I can fix it one day. It's nice and clean inside and it deserves another chance.

author: pelule
author_rank: Frequent Contributor
timestamp: March 15, 2015, 07:32:32 pm
content: Did you get your problem solved?If not, please check ALL supply rails first (use Power Supply Schematic Figure 8-H-39 for reference, means +30V,~29.3V (J3 pin 11), -24V etc.). Only if ALL voltages are ok, continue your analysis.Next step is to note all error codes in "Test" mode (Cal# error constants).Next could be to use the table 8-B-7 (8-54) to select the possible cause of the error.Alternative you may use the Cal-Step procedure to step to the 1st (highes) Cal# error and to compare the values with the give ones in the entire tables.Or go step by step according with the Auto Cal troubleshooting diagrams and tables.Its the best method to detect the cause of the problem and not to be misleaded.The slow rising value at not-shorted inputs is caused by the leakage currents in the analog circuit. The rising time defines, if the leakage is in spec or out. See the special chapter for leakage tests.

author: Dragon88
author_rank: Regular Contributor
timestamp: March 22, 2015, 01:20:40 am
content: No, I have not fixed the problem. I also haven't worked on it in several days.The power rails all measured fine, except for the +/- 10V reference rails as I mentioned above. They were being loaded down and drooping to +/-5-6V. Lifting Q35 stabilized them to the correct values, but again I don't believe this transistor is the problem. It just isolated the reference rails from the node that does have the short, as far as I can tell.I'm fairly certain I have a shorted FET somewhere. I did record the error codes and consult the table in the Service manual. I checked every part mentioned in the table, but in-circuit I was not able to find anything out of the ordinary. I also tried shorting the multiplexing node to ground as mentioned in the manual to remove the OL, but that did nothing.My next step would be to continuing tracing back from Q35 in the schematic and checking/lifting parts. Past Q35 on one node I had something like 200 ohms to ground, which seemed suspicious. I wish I had a better plan than this, but I don't at the moment.Again, thanks for any suggestions.

author: Luddi
author_rank: Newbie
timestamp: March 22, 2015, 11:57:05 pm
content: my HP3455 had the driver ICs for the J-FETs in the inputpath blown. Check the driving signals for the J-FETs.Check page 8-51 Table 8-B-4.I think i replaced U4 and U5, LM339.

author: pelule
author_rank: Frequent Contributor
timestamp: March 25, 2015, 09:39:21 pm
content: You mentioned, you have noted the cal# errors during test.Cal# errors are listed from the highest (#13) to the lowest number (#0), possibly followed by some dot digit values, if the calculation routine at the end of the test calcs wrong values.The first cal# error shown, is the first one detected by the test procedure (cal constant measurement value not as expected) and thus the one to solve/repair first.Lower/later errors may just a site effects of that first one.3455A MUST out of "test" mode and "AutoCal" MUST be OFF to do senseful measures at stable conditions. Otherwise the 3455A continuesly measure/switchs.Step to that "cal constant measurement" of that cal# err (see 8-B-11 Cal Constants Service Procedure, ~page 140).Now follow the trobleshooting diagram (chapter H) for the given cal# err.For example: for cal#13 it is Figure 8-H-3 (~page174)I have repaired severals of HP 3455A and this is by far the best way to detect a possible root cause.It also includes possible defects of comparators (LM339, U9[pin 1] driving Q35).Please note the LM339 has open drain outputs, it just shorts the gate of the FET to -24V (FET HiZ/OFF) or let the gate flow to the G.B. level (Gate Bias) supplied to the gate via a 100k (FET low/ON). Gate driver is U1 (TP3), the Gate Bias Amplifier.Just a short list of errors I had in the past (listed by occurence, highest first):- most often a bad/defekt electrolytics (1000µF) in the inguard supply- damaged LM339 (output shorted to -24V)- FET shorted to drain/gate- FET had too high leakage current (FET is highly temperature sensitive)- defekt 5V6 Z-Diode

author: cncjerry
author_rank: Unknown
timestamp: March 28, 2015, 01:11:26 am
content: I have 4 3455s and only one works. One I use as a parts box after blowing the CPU. I've spent hours on the others and the most recent has an FET problem I can't find. There are quite a few that tap the 10v reference and once it gets drug low it is very hard to find. Even with a working one sitting next to me I can't find the the shorted, or at the least leaking, FET.If your PS voltages are OK with acceptable ripple then don't touch them. Work from the highest error on down using the simplified switching diagrams. This worked for me in the past until I hit this one as an error 10 is the kiss of death for me.Leaking FETs plague these boxes and the other problem I found is that the switching is consolidated in a ROM that blows as well causing what looks like a multiple FFT shorting problem.They are great when working. I have four 55s, two 56s and two 57s. I love the red led display as my vision is bad. One of these days I'll get another 55 working.I'm going to go back and look at the recommendations in this note as well.Jerry

author: cncjerry
author_rank: Unknown
timestamp: March 28, 2015, 08:40:02 pm
content: After reading and posting yesterday I got motivated to pull another one out. This one has a more perplexing problem in that the Gate Bias, which goes quite a few places (every switching FET) is low. Probing to ground on a working 3455a is 3.5M; on this one it is 75k.Since the GB connections are all in parallel, I can't locate the problem. At first I assumed the most likely candidate would be an FET that had a GB resistor of less than 100k. As all but one of the GB resistors are 100k, if the FET was shorted, I was thinking that I should read the GB resistor resistance as a minimum. The only GB resistor that is less than 100k is R37 feeding Q1. Since Q1 accounts for about 1/2 the 3455a failures after the PS caps, I lifted the GB connection to no avail.The other place GB goes is to the cans of some of the metal cased FETs. Again, I looked for a metal FET with GB connection and a path to ground of 75k without luck.So the only thing I can think of now would be to lift each subsequent GB connection unless someone has run into this before?Any other 3455a engineers out there?thanks,Jerry

author: cncjerry
author_rank: Unknown
timestamp: April 01, 2015, 04:49:58 pm
content: Contributing further to the 3455a problems people have, I fixed my GB problem by replacing U1. Interesting though as I still have errors. Something is dragging the multiplex low. I suspect this is what blew U1 as a grounded multiplex or low value multiplex will heat U1. Using the switching diagram as well as the autocal tests, I've made a list of the possible switches that will drag the multiplex low. Sitting here in a meeting waiting to fly home and check the switches...The other issue is the expense of the parts. Using a donor parts box is the way to go but the shipping of same is high; purchasing original parts is crazy due to the cost. If anyone wants to part with another 3455a parts box, let me know as I would like to get three of my original four working. I have a pretty good handle on the inguard board now but it has taken a long time and lots of study.If anyone has a cheap FET switching alternative, please post. Need dual FETs as well.thanks,Jerry

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2015, 08:09:14 pm
content: Quote from: cncjerry on April 01, 2015, 04:49:58 pmHave you tried Sphere?Link to them in the first post in this board.

author: cncjerry
author_rank: Unknown
timestamp: April 01, 2015, 09:52:11 pm
content: Yes, tried Sphere; $25 for an FET. Overall cheaper to just purchase another parts box. Three of the 3455s I purchased were less than $50. There has to be a cheap FET that can be used as a replacement after all these years.Fixing this old gear is actually pretty fun and rewarding but I am trying to keep the costs down. i could probably replace the entire outguard with an STMF4 micro development board like a nucleo as well as most of the switching in the inguard. Then replace the ADC with a modern 24bit chip and get really carried away...

author: sync
author_rank: Frequent Contributor
timestamp: April 01, 2015, 09:59:52 pm
content: Quote from: cncjerry on April 01, 2015, 09:52:11 pmI saw 2N4117 FETs as switches in a bench DMM. But it looks like they were selected. The SMD version (MMBF4117) is cheap.edit: look at this thread:https://groups.yahoo.com/neo/groups/hp_agilent_equipment/conversations/topics/13073

author: Vgkid
author_rank: Super Contributor
timestamp: April 02, 2015, 03:41:03 am
content: Do you have a part number for the fet, In know the 439X are popular. Vishay/LIS(Liear integrated Systems) does sell low leakage variants

author: cncjerry
author_rank: Unknown
timestamp: April 02, 2015, 05:57:05 am
content: 1855-0308, 1855-0247, 5081-7047, are all available on eBay but I can't help but think there are later, lower noise variants that would fit the bill. All they are are somewhat higher voltage low noise switching FETs and while I am in there I would swap the entire multiplexer if I could find cheap enough parts.I love the meters as I can see them across the room. The challenge is that I only paid $50 for each meter, at most, so the thought of spending that much more to get them working doesn't make sense. They are quite a puzzle to fix but once you understand their switching logic they are pretty easy to fix. Like I said, more of a challenge than an economic value.

author: pelule
author_rank: Frequent Contributor
timestamp: April 02, 2015, 08:03:43 pm
content: As far I could figure out, the single 40V switch JFETs are 2N4119A (ultra low gate-drain leakage less 100 nA) special for HP selected for extra ultra low leakage. I used standad 2N4119A from a good brand (Siliconix). They are still offered at the bay for acceptable price & shipment costs.http://www.ebay.de/itm/2N4119A-Ultra-High-Input-Impedance-N-Channel-JFET-Ampli-BY-MOTOROLA-/301445968609?pt=LH_DefaultDomain_0&hash=item462f9472e1

author: cncjerry
author_rank: Unknown
timestamp: April 04, 2015, 12:44:22 am
content: This box is a mess. So far U1, Q5 solved the gate bias problem, Q4, U6, Q2 which solved the first part of a code10, and now the driver for Q13, U17 if I remember, is bad. This last one with Q13 was dragging the multiplex down just like a Q2 failure. During autocal, Q2 switches on to ground the bus or you get an 11. If it doesn't switch off, it will also drag the bus down. In my case, both Q2 and Q13 because of the driver, caused the multiplex to be loaded down.I had to open my second parts box to fix this one, sort of sad, now I am down to two out of four.Thx

author: cncjerry
author_rank: Unknown
timestamp: April 04, 2015, 06:44:00 am
content: All things come to an end and I finally have this one up and running.The last problem was the driver for Q13, U8. Well, almost last as it had a bug in the ADC board. In default resolution it read low and wouldn't jump out of 10v in auto range. I don't know what provoked me to hit the high resolution mode but that fixed the problem. I swapped the ADC and all is well.I checked the calibration and had to dial it in a little and it is humming away. Hardly any flipping of the last digit at all, very stable for something that old.Sorry for hijacking the thread but there are a lot of these meters out there and it would have been great to have one spot for all things 3455a. I spent months fixing this one but had I read all the theory before diving in I would have saved a ton of time.I am going to miss working on it. There for a while it was my arch enemy and I almost gave up on it. The killer is that it had essentially two sets of unrelated problems (not including the ADC) and the boards hadn't been reworked from what I can tell. There were a lot of probe divots all over the place so maybe someone took a shot before me.My plans are to button this one up and start on the next. I'll need some FETs to replace those I took out to fix the next one. If I had a processor and ROM I would go back and restore my parts unit. It needs about $20 in opamps, FETs and drivers plus processor and ROM so maybe I'll find another broken one and go that route. There must be something wrong with me to spend so much time on a meter this old...Jerry

author: tautech
author_rank: Super Contributor
timestamp: April 04, 2015, 06:49:37 am
content: Quote from: cncjerry on April 04, 2015, 06:44:00 amYour determination is an admirable trait.

author: cncjerry
author_rank: Unknown
timestamp: April 04, 2015, 07:39:19 pm
content: Getting back to OP's problem, I suspect that the problem is in the Q2 driver U8. Keeping the unit in test and stopped at the 11 code, measure the voltage on A10U8 pin 13. It should be zero volts switching on A10Q2 which grounds the multiplex, U8 pin 14 -24v, Pin 1,2 also -24. This corresponds to A10U8 pins 4,7,9,11 being H,L,L,H respectively. If you need a U8 let me know.Additional troubleshooting:While stopped at 11, put AC 2v on the input and measure both sides of K1. You should see AC on one side and not the other as it shouldn't be picked. Continuing to the multiplex, you should not see AC as on code 11, Q1 is not switched on as well as K1. If you do see AC, this furthers support for U8 as that drives both Q1 and Q2 but this also means K1 is stuck which is not uncommon. When a driver fails, you will see either a short of -24v on the gate resistors switching them off, or a floating 0v switching them on. Any positive voltage on the gate means that, in my experience, the driver is bad.As someone mentioned, stopping on a code using test isn't proper in that the multiplex could float to the value being tested and move to the next code. If the failure is hard, though,then stopping during test is OK as you can be sure that the system test is at the right code. Your code 11 means that Q2 isn't switching on hard enough to ground the multiplex and I suspect it is floating due to the U8 failure.The next step if the previous doesn't work is to measure the multiplex during the code 11. There should be no AC (assuming you still have the 2v AC on the input) and the multiplex should be at zero. if it is and Q2 is switched on then the code 11 is false and there is a problem upstream in the ADC of the main amplifier which is rare. If the multiplex is floating, ground it with a lead to the A10 ground point or press the guard button and you can use the metal of the inguard section. Grounding the high impedance multiplex line is ok and recommended for OL testing. With the multiplex grounded, the code 11 should be bypassed and stop on code 10. Taking the machine out of test with the multiplex grounded, the OL condition might clear. If it does, then you have a leaking FET. You usually don't get an 11 with a leaky FET because the switched on Q2 should be strong enough to ground it out. That's why I am pointing at U8; it isn't switching Q2 to ground the multiplex. It has to stop on code 10 because it switches 10v vref onto the multiplex and measures it. With the multiplex grounded, the 10v will be measured as 0v. You can see that solving a code 11 followed by code 10 will clear most OL errors.Post the results of the tests above and we can go from there. The 3455a is quite a bench meter for the money. Simple to calibrate and I think you can actually swap calibrated modules from others. The simple math functions allow you to measure current with an external sense resistor.Jerry

author: cncjerry
author_rank: Unknown
timestamp: April 05, 2015, 12:06:33 am
content: I think I set a record fixing two in two days. I worked on these things for months prior to committing myself to reading the book! I threw some parts in my third meter and shot the bugs one at a time. It had a blow U6 driver which manifested as a code 10,5,0. Once I fixed that, the 10v ref was low and that turned into a calibration module problem which I swapped out. Once I got the 10v ref up, I still had the 10,5,0 and I traced that to U12. At first I thought it was U8 but I lightly touched the input to the driver and the suspect driving voltages were correct which pointed me to the latch. I swapped in an FET for Q1 and up it came, not as stable as the other two but getting better as it warms.So my inguard skills are growing and I wish I had parts for the fourth meter but this one has been butchered.Anyone needs help with a 3455a, pls post here or send me a PM. I have a fair amount of parts including a complete outguard.Jerry

author: cncjerry
author_rank: Unknown
timestamp: April 05, 2015, 10:35:11 pm
content: Quote from: Dragon88 on February 13, 2015, 03:02:02 amOne other point on this one. I mentioned that I had the same problem with TP 7 and TP 8 reading low, approximately the same as yours. That problem is in the calibration module. When I swapped the failing one with a good one, it all worked fine. If you trace the circuit, +VREF goes a lot of places but feed at least a 100k gate resistor, for instance, so the fact that it is low means a failing module. I have one that failed the same way and will open it today to see if I can isolate the problem. There is an oven baked zener diode (or the like) inside and don't know if they are available.I am in the SF bay area and if you are as well and you would like me to take a look at it, send a PM.Jerry

author: Dragon88
author_rank: Regular Contributor
timestamp: April 06, 2015, 09:09:32 pm
content: Jerry, thanks for all of your detailed replies. When I have time, hopefully in the next few days, I will pull this beast out and report on the measurements you suggested. I did consider a failed calibration module, and that would be quite unfortunate. I did open it up and check visually for anything grossly wrong when I first started troubleshooting. If I can't find any likely issues in the inguard section, I'll have to pull that out again and start measuring.Out of curiosity, have you gotten any of your repaired units calibrated? I have nothing remotely capable of calibrating a 6 1/2 digit multimeter. If I fix it I will considering taking it to the local cal lab, but the cost would probably be 3x what I paid for the unit...

author: cncjerry
author_rank: Unknown
timestamp: April 07, 2015, 03:44:10 pm
content: I calibrate mine to a calibrated 3457a meter. I can get it close enough for basement work and that is good enough for me. The ultimate value means less to me than relative anyway. The challenge with the 3455a meter is that they are old and many have run continuously for years, maybe decades, in lab environments. I usually find more than one bug repairing them. The one I just finished calibrating it to my dc standard while also measuring the same voltage with one of my 3457a meters is rock solid. Ohms as well. AC no matter what I try they are a little flakey and it could be my source ground loop. This one had a weak U5, a 7406, on the A/C RMS board to boot. It manifested as a value of 1/10th while in 1 V range.By the way I reseated my calibration module hard and my vref problem went away on that unit. Make sure yours is all the way in before measuring TP7 and TP8.We will get yours running ok. Make the checks and post here. I would stop pulling FETs as the drivers and latches in my experience are 2:1 over FETs.Jerry

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 12:56:13 pm
content: This morning I replaced the two FETs I had lifted, and re-seated the cal module hard a few times. Then I took some measurements.After replacing those FETs, the reference rails returned to +/- 5.6V in code 11 in Test mode.Sitting at code 11, U8 pin 13 reads 1.334V. The multiplex reads 1.347V. Pins 1, 2, and 14 reads -24.14V.I switched on guard and grounded the multiplex to the inguard metal enclosure. The unit immediately bypassed code 11 and went to 10.Is U8 just an LM339? I have an LM339N in my parts bin.Thanks.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 02:42:54 pm
content: So I replaced U8 with the LM339 I had, and it didn't fix the problem. New chip is functional, but with the same problems/output as before.I looked at the inputs to the comparator in question and traced the non inverting input back to a flip flop, where Q5 was floating. Data input 5 to that flip flop was also floating, so I traced backward to an inverter, which is acting as an inverting buffer for the data lines. All of the data lines seem to be floating, on the inputs and outputs of the buffer.Tracing back from there, perhaps the inguard processor is dead....Actual I don't know if the data lines are floating, I haven't put it on a scope yet. Does the inguard processor continue to use the data bus while sitting on an error code?

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 06:17:28 pm
content: Nope, the flip flop and all that was a red hearing. Checking around with the scope and Group A procedures from the manual, the digital stuff is working fine.So it's back to Q2 not getting turned on to ground the multiplex during code 11. But replacing U8 didn't fix it.

author: cncjerry
author_rank: Unknown
timestamp: April 11, 2015, 08:26:57 pm
content: U8 controls Q2. Tracing back, U12 Pin 15 as a latch should be nailed up and not pulsing to switch on Q2. So put a scope on U12 Pin 15 and it could be pulsing not at full TTL levels. I had this problem where U12 pin 15 was pulsing so I went back and put a scope on pins 9 and 14. U12 pin 9 is the enable pin. So if you sync your scope on U12 pin 9 and look at 14 during the 11 code, you'll probably see they coincide setting the latch so pin 15 should be nailed up.So it looks like the problem could be in U12. When you see U8 pin 13 at anything over zero, usually it is because the input pin 11 is pulsing from the latch. A way to test this is to stop at the 11 code and ground carefully and quickly the input pin and and also try touching it to +5. If that gives you the correct FET control thru the U8 driver then the latch is bad, in this case, U12. You can also check other latches to see the correct operation of the proceeding latch.Whenever you see the driver inputs at anything other than 5 or 0, the prior latch is not latching. You could have a bad U22 but unlikely.I'll use my test box to stop on code 11 in an hour or so and give you the correct levels going back to U22.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 08:58:34 pm
content: Ok, after I get those levels from you I will measure and compare with the levels on my U12 and U22.When I investigated U22 I assumed the data bus would be quiet while sitting on an error code, but this was quite wrong. There was a lot of activity from the processor actually. The average DC levels on the multimeter made me think several pins were floating, but the scope revealed the data signals.I re-seated the processor just in case, and I snapped a picture while it was out. It's quite a nice IC. "They don't make em' like they used to"...

author: cncjerry
author_rank: Unknown
timestamp: April 11, 2015, 09:23:54 pm
content: I often suspected those pins which are very, very soft, are like pure gold. Careful when reseating it.U8 Pin 11 should be a solid high, > 2.2V and usually at 3v or higher. If you use the table on page 8-51, or page 141 of the PDF reader (which I have memorized all of the appropriate pages), you can see the levels for U8 when stopped on code 11.If your U8 Pin 11 is lower than 2.2V then your U12 is most likely the culprit. I've had to replace U12 on one and U16 on another so those latches are common failures. I think what really happens though, is the driver as in your case U8, goes bad and feeds voltage back to the the latch. So maybe replacing U8 was worth your time anyway.Look at U8 11 with a scope. If it is pulsing the U12 is probably bad. You can tie U8 Pin 11 to +5 briefly without causing a problem in U12 since it is probably bad anyway. When you touch U8 pin 11 to +5, U13 should go to 0 causing Q2 to turn on.I have a feeling the problem with your ref voltage is that U12 also switches in the calibration resistors in the ref assembly. So what happened in my case, was that U12 was bad causing the code 11. Grounding the multiplex gets you to a code 10 because Q13 and Q14 are dragging down the multiplex and possibly your Vref as well.When ordering, I would pickup a a couple of 74LS04, 74LS174 and some LM339s to keep on hand. They are cheap.I'm kicking around restoring my parts box as well but I would need a ROM which is $25 from that guy on ebay. The other thing I've been thinking about is taking out all that logic on my parts box and just switching it with a Nucleo micro board. Half the logic is for autocal and the other for switching. All it does is switch cycle the autocal in / out of the measuring circuits about twice a second and feed the gain adjusted value to the A/D converter. I'll bet I can take all that 74LS out keeping the LM339 drivers in a couple of hours.Once you have it working, you'll have a pretty stable DC voltage and resistance meter capable of some pretty accurate measurements. The A/C RMS side of the meter is tough to calibrate. I have to make some shielded cables because I get nothing but noise on the lowest two digits.hurry up and test U12, dieing to see the results.

author: Vgkid
author_rank: Super Contributor
timestamp: April 11, 2015, 09:34:07 pm
content: It looks like the die cover was set on fire.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 10:25:24 pm
content: Well, U8 pin 11 is sitting steady at 3.58V.I checked out Q2. Indeed the gate pin is sitting at the 1.3 odd V from U8. I checked the ganged resistor R48 and it seems fine. I tried touching the gate of Q2 to ground quickly, while watching the multiplexer voltage. It jumped slightly from 1.32V to 1.45V while I was doing that, then back to 1.32. Strange.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 11:11:52 pm
content: However, when I dropped down to code 10, I noticed the -24 volts on the gate of Q2 was not steady. I checked back to the latch, and the Q5 pin seems to be pulsing. I'll have to put it on the scope to be sure.

author: cncjerry
author_rank: Unknown
timestamp: April 11, 2015, 11:51:58 pm
content: so Q2 isn't capable of dragging the multiplex to ground. I would go back to the code 11, print out page 8-51 and probe the table for code 11. Start with the -24 pins and then the zero levels. Once you find the wrong level look at the corresponding input pin for a pulse from the corresponding latch. That should id the failing driver or latch in a minute or two. If any of the latch pins are pulsing then I suspect the latch. Alternatively, you can probe U6 down to U4 pins to see if any are pulsing. That way you don't have to keep going back to page 51.The gate bias floats the gate pins up to the multiplex level and that must be why you are seeing the positive voltage on Q2. The gate bias can also cause the -24 to jump around a little.

author: cncjerry
author_rank: Unknown
timestamp: April 11, 2015, 11:55:26 pm
content: Also, just for grins, pull the ref unit and see if it still stops on code 11 or goes to 10. If it goes to 10, then check the gates of Q13 and Q14 for correct levels. There are some really funky switching loops that happen when a latch or driver fails.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 12, 2015, 12:37:55 am
content: On code 11, I found several odd comparator outputs.U4, pin 13, 5.5V instead of 9.5V.U5, pin 13, pulsingU9, pin 1, .5 V instead of -24U9, pin 2, 1.3 V instead of -24U9, pin 14, 5.5 V instead of -24But I believe this traces back to more than one latch.Yup, latches U12, U13, and U16.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 12, 2015, 01:04:38 am
content: On the scope, I found a pulsing output on U13, and one on U16. While sitting on code 11 again.

author: cncjerry
author_rank: Unknown
timestamp: April 12, 2015, 02:28:27 am
content: If you are stopped on code 11:U4, pin 13, 5.5V instead of 9.5V. <- ignore, it is wrong but because the multiplex is most likely wrong.U5, pin 13, pulsing <- problem with U16U9, pin 1, .5 V instead of -24 <- problem with U13U9, pin 2, 1.3 V instead of -24 <- maybe ignore because of gate bias float otherwise U12.U9, pin 14, 5.5 V instead of -24 <- problem with U13.the most obvious error is the last one at 5.5V. I had the same thing when I had a bad U13. Look at U9 pin 9. Try the ground and +5 trick with a jumper and see if you get the correct output. If you do, then one problem is U13.By the way, does it look like the caps were replaced on the digital supply? Could have been enough leakage to take out a chip.When the unit stops on a code, it should halt the switching. I haven't tried it, but when stopped on 11, hit the hold manual button and see if the pulse goes away.If it was my unit, I would swap out U13 first and see if symptoms change. The boards can take a lot of abuse. I use one of those spring solder suckers on the bottom followed by braid on the top. I also have an ice-pick like tool in a screw driver handle with the tip bent 90 degrees about 5/8ths down. Once I suck the solder and push align the pins in the holes, I pry with that tool like a lever. No problems so far.U5 Pin 13 is strange. That is driven by Pin 11 and you can also test it by grounding and jumpering to +5.It is not unusual to have two bad LM339 or latches. Part of the problem is they all go around the same time. The other problem is that you don't know who was in there before you. Two of the boxed I had came with probe dimples all over the place.That table on 8-51 should be your guide when it is stopped. You can ground the multiplex, skip the code 11 getting to code 10 and then try looking at the pins from table 8-B-4 again to see if U13 or U16 are wrong.This is developing into one of those very satisfying experiences when you fix it.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 12, 2015, 03:00:04 am
content: I have to put in a Digikey order this week so I will order some of those 74LS174s. I will replace U13 and U16 and check for improvements.I do believe someone poked around in here before me. No signs of damage or replacement of parts, but the eBay seller indicated that they tried to fix it, and hoped someone else would have better luck. I'm out $50 if I never fix it so no biggie. But I would like to give it a second chance.

author: cncjerry
author_rank: Unknown
timestamp: April 12, 2015, 04:17:24 am
content: Don't give up. I spent a ton of time on mine and ultimately fixed three. I have a feeling it will be u13.

author: Dragon88
author_rank: Regular Contributor
timestamp: April 16, 2015, 06:01:52 pm
content: I replaced U13 and U16 today, but unfortunately there was no improvement. Same error messages and behavior.

author: cncjerry
author_rank: Unknown
timestamp: April 16, 2015, 10:43:12 pm
content: Damn. I went back thru your data a couple of times since the last posts. I have two on my bench right now and was trying to simulate your code 11 and as we know, your pins posted above were wrong.let's take another approach. stop on code 11 and ground the multiplex. that should take you to code 10. Look at the table again for code 10 and tell me what pins are wrong. Also make sure to check the enable line on u16 and u13 pin 9. Check the voltage on both sides of resistor R4 as well as TP 7 and 8.lastly, check u14 pin 4 with a scope for the clock pulse as well as pins 1,2,3,5 which are the device select lines. They should look alike and not be floating.On one meter I had to isolate the multiplex by clipping the solid wire on each side of R38. You can touch solder the wire back together. On the code 11, something is holding the multiplex up; On code 10, something is keeping it from getting to 10v.Since those two large 100k resistors, r59 and r61 aren't used at this point in autocal, you can lift the blue and brown wires to isolate q13 and q14. Q1 isn't used so you can lift that off the multiplex as well.If we can get past code 11 and 10 to get the gate bias up on Tp3 we will know more.

author: cncjerry
author_rank: Unknown
timestamp: April 17, 2015, 05:21:35 am
content: Here's another one for the list of 3455a bugs:This one seemed to work ok until I tried to calibrate it. 10v and 1v were ok; AC RMS as well; 1K ohm standard came right in; When I tried to calibrate the 1M with which you use a 100k resistor, I ran out of range. Another strange problem was that the lower ranges all read about the same but dropped about 700 ohms or so when switching higher. I tried a couple of different calibration modules without luck. I spent four hours on the ohms converter and got nowhere, maybe added about 200 to 300 ohms to the low reading. Swapped the ohms converter boards, etc.I went back and recal'ed the DC volts and I noticed it ran away on the 10v leakage test. This points to Q5 or Q17. I've had Q5 failures before but not this time, of course, it was Q17. Once I swapped it out, everything came into calibration. Go figure.These meters can get the best of you sometimes. They use about 30 or so FET switches which are prone to leakage and all kinds of intermittent failures. It is not unusual to have more than one driver or preceding latch fail at the same time.

author: cncjerry
author_rank: Unknown
timestamp: June 19, 2015, 01:06:55 am
content: Adding to the 3455a repairs:Found a black face 3455a on eBay, with shipping I paid $72 for it. It was listed as defective - blows fuses.The first thing I did was check the various voltage test points to ground finding the +5 to be nearly a short. I used a slow-blow fuse to power on/off quickly and the +5 wasn't coming up. The 4200uF cap on the inguard +5 was shorted so that fix went pretty quickly.Once I got the unit up and powered, the meter was reading 3x the known value in all cases. This pointed to the reference. Inspecting the reference I found the 29.3v was low, sitting around 3.5V. Somebody had replace a few glass diodes with the wrong type installing one backwards to boot.I brought the unit up and found I wasn't able to get the range I needed to calibrate it. I swapped in another A/D converter which solved that problem so I guess I have to dig thru that to have as a backup.Anyway, once I got it sorted out, it calibrated really easily. This particular meter must have blown the +5V cap early on and then sat. It had some salt stains on the aluminum frame, a couple of buttons needed cleaning, but it is very, very stable with a nice bright display.Hope this helps someone in the future.Jerry

author: stevopedia
author_rank: Contributor
timestamp: September 17, 2018, 04:51:55 pm
content: I know this thread is old, but since I've got another HP 3455A with problems it seemed more sensible to me to post in this thread than to start a new one.So I've got a 3455A that's gradually developed an odd fault. A few years ago I replaced all the power supply electrolytics and the original fan with a modern 12 V one, since the original's bearings had failed and squealed horribly. It ran beautifully well afterwards, until very recently when it would seem to lose its mind: wildly varying measurement results, even with the input shorted; unsteady trigger rate when set to internal trigger, etc. Power cycling would reliably restore the DVM to proper operation.I started to investigate that behavior. My first thought was that the communications between inguard and outguard were faulty, so to that end I scoped A3TP1 and TP4 and the corresponding pins on A10U26, being careful of grounding. When I first set that up, the meter was working properly and the waveforms matched, indicating the communications were working. I decided to leave it with the scope running over the weekend. On Monday I found the instrument completely halted: the "sample" LED was lit continuously, and there was a continuous stream of data from the outguard to the inguard while the inguard-to-outguard line was held low. After power cycling the instrument, it behaved exactly as if I'd unplugged A1J7, the cable that carries the data between the inguard and outguard: blank display, all the front panel LEDs lit. There was no traffic at all on the lines between inguard and outguard.I followed through the flowcharts in Service Group H, and finally landed at the flowchart labeled "B" on pages 8-115/8-116. It appears that the device select process isn't working properly--U26 only makes device select pulses on two of the four pins specified. The 1-of-8 mux appears to be working correctly, so that leaves me with unfortunately the very worst of two choices: either the inguard CPU or ROM is bad. How do I tell which it is? The PA lines look OK, and the Data lines look okay-ish,though D2 looks particularly ugly to me. I'm very much hoping that it's the ROM, since that at least appears to have a modern replacement available from D'Asaro Designshttp://www.dasarodesigns.com/product/replacement-ingaurd-rom-for-the-hp-3455a-voltmeter-hp-pn-1818-2270/.Has anyone here had a similar fault with the 3455A before?

author: pelule
author_rank: Frequent Contributor
timestamp: September 18, 2018, 10:48:18 am
content: I had similar error (caused by socket of the ROM). Cleaned it, and failure was gone.Is your ROM soldered or is it on socket - you have to careful desolder it to check the ROM.Are you able to read out the ROM content (for plausibility)?/PeLuLe

author: stevopedia
author_rank: Contributor
timestamp: September 18, 2018, 12:55:57 pm
content: It's soldered in, so I can't imagine that would be the problem. The CPU is socketed, though... Hmm... I'll try cleaning its socket and see what happens. Good thought, thank you!And, as a footnote to add to the weirdness, I tried the instrument again this morning and started right up, like nothing was ever wrong. Thermal expansion or whatever causing contact weirdness? I'd believe it.

author: GregDunn
author_rank: Frequent Contributor
timestamp: September 18, 2018, 04:24:09 pm
content: I have seen so many computers and even analog gear develop intermittents over the years due to socket issues. It's gotten to the point that if a piece of equipment starts acting up, I will always at least partially remove and replace socketed items (depending on accessibility) to wipe any oxidation off the contacts before moving on to other tests. It works more often than not. These days, a tiny drop of Deoxit on socketed pins after cleaning and before re-insertion is not a bad idea.

author: pelule
author_rank: Frequent Contributor
timestamp: September 18, 2018, 08:29:11 pm
content: QuoteI had the same at two of my 3455.root cause of 1st = bad/dirty connections of the CPU plug-in boardroot cause of 2nd = thermal problem of a TTL IC (can't remember which, but found with coolant spray)/PeLuLe

author: bob91343
author_rank: Super Contributor
timestamp: April 08, 2019, 11:32:05 pm
content: A new reply to an old thread.I just acquired a 3455A that works very well with one exception. It will not go into 4-wire Ohms mode. Sometimes the button will flash as I push it but it ends up off and back to 2-wire.I have the cover off and have begun printing parts of the manual but I am unsure where to start. It may be a control issue, as the active circuitry seems fine, all ranges, all modes, except 4-wire. It even agrees with my other instruments, including a 3456A.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 09, 2019, 12:13:00 am
content: maybe the tact switch is bad

author: bob91343
author_rank: Super Contributor
timestamp: April 12, 2019, 09:46:12 pm
content: I am sure the switch is okay. I think the signal to actuate the relay is missing. Perhaps a bad component or bad solder or something like that. I am hoping it's not unobtainium. But first I need to know where the switching is coming from. The unit is so complex that simple things aren't simple.

author: 0culus
author_rank: Super Contributor
timestamp: April 12, 2019, 09:50:50 pm
content: Be sure you re-seat everything and clean contacts if needed. I have two 3455As. The older of the two (now a parts mule because it's always been flaky) had a bad connection in the ribbon cable that goes from the digital board to the display board, which caused one of the display sections to not work. It was very subtle; I spent a lot of time suspecting one of the Darlington pairs that switch the display modules before finding it while probing the display signals all the way to the display modules themselves.A lot of the time issues in old HP gear are purely mechanical and not electrical I've found.  not saying it isn't electrical, but it's worth checking out mechanical problems first.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 12:47:59 am
content: I will do just that but it would be nice to know which cables and boards are involved in this situation. This was obtained from another engineer who certainly was competent enough to solve this kind of problem. But he did tell me what was going on, implying that he had attempted to repair it.Bob

author: 0culus
author_rank: Super Contributor
timestamp: April 13, 2019, 01:13:11 am
content: Quote from: bob91343 on April 13, 2019, 12:47:59 amI would start at Service Group D (Section VIII, page 8-65) in the service manual. It mentions that the ohms mode depends on some dc mode circuitry, and recommends ensuring correct operation there before troubleshooting ohms mode.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 05:04:04 am
content: Well the picture has changed. Now I turn on the unit and it goes straight to a row of zeros. Then I push Test and it doesn't test as it should. However, now the 4-wire Ohms button works.The machine ignores all inputs. I tried reseating all the connectors (there aren't many), tapping on the boards and components, wiggling wires, switching power on and off, all to no avail.If I switch to a very low range it will read OL.At least before, it worked for the most part. Now it's worthless.

author: 0culus
author_rank: Super Contributor
timestamp: April 13, 2019, 05:17:55 am
content: Check to make sure there are no sticking key switches on the front panel. A stuck switch will cause most HP gear to go haywire until you unstick it.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 05:52:37 pm
content: I tried working all the buttons many times. They all respond to some extent, causing the button to illuminate but there is no meaningful response from the meter.I tried the usual plugging and unplugging and tapping and flexing approaches.I don't want to give up, as this is a very nice machine in spite of its large size. I just don't know what to try next.

author: 0culus
author_rank: Super Contributor
timestamp: April 13, 2019, 06:31:39 pm
content: Go to page 8-79 in the Operating and Service Manual. Follow the flowchart and see where it takes you (if you're failing to enter self test, you should go down the right side).

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 08:28:42 pm
content: One of the instructions says to go to U10. I cannot find U10.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 08:30:55 pm
content: I also cannot find 'inguard ground'

author: 0culus
author_rank: Super Contributor
timestamp: April 13, 2019, 08:57:36 pm
content: I'm sorry, I'm not near either of my 3455As right now or I'd be in there helping you figure it out. All I have to go off of is the service manual. My suggestion is you get the version from Artek Manuals if you don't have it, as it has better quality scans of the diagrams and schematics. Each schematic should be accompanied by a component locator floorplan. Study the floorplans and try to find it. The Artek version is also OCRed so easier to search for key words.

author: 0culus
author_rank: Super Contributor
timestamp: April 13, 2019, 09:08:59 pm
content: U10 should be on the A1 digital board, towards the rear, to the right of the mains transformer as you look from the front of the unit. It's a SN74LS04N inverter (still easily sourced).Quote from: bob91343 on April 13, 2019, 08:30:55 pmI *think* inguard is referring to the A10 board, which is inside the large metal shielded box.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 10:58:35 pm
content: I did a seat of the pants search for inguard ground. With no power applied I put the scope probe where I was supposed to and, of course, with no ground I saw noisy hum on the screen. Then I probed with the ground lead and discovered only two places that eliminated the hum. One was the holddown bracket for the little ferrite magnetic component near the rear of A10. The other was the shield bracket on the long board at the side (A14?). I connected to the latter. But the instructions are terrible.I will continue. At this point I am starting to think there is a failed semiconductor component.This is supposed to be a fun hobby but there are times when I wonder. In between I am constructing shelving to augment my storage in the lab (table saw, feet or casters on bottom). And trying to obtain an unonbtainium gear for a chart recorder. And repairing a TS-700A transceiver. And constructing a power supply to run some 1 GHz FET probes from Tektronix. And putting the finishing touches on an antenna rotator repair.I need more projects.

author: bob91343
author_rank: Super Contributor
timestamp: April 13, 2019, 11:10:18 pm
content: Yes there is a U10 near the mains transformer but that's on the wrong board.I skipped that step and measured the appropriate voltage on the big 40 pin IC, 5V.Took another break. But the manual skipped me to a much later point so maybe there is a light at the end of the tunnel diode or something.

author: 0culus
author_rank: Super Contributor
timestamp: April 14, 2019, 01:03:21 am
content: If that's not the U10 you're looking for, where is it? That's the only U10 I've found in the service manual. Perhaps there's a typo or the service manual doesn't have your revision.You'll find it.If you're like me, you end up using interleaved timesharing for your projects. Work on something else then come back to the 3455A. The good news is, most of the logic in it is bog standard TTL, so finding parts for those is easy from digikey or your favorite vendor.

author: bob91343
author_rank: Super Contributor
timestamp: April 14, 2019, 06:29:46 pm
content: Yeah I can't find U10 even though it's referenced in the procedure. I need the one on the A10 board. It has U numbers up into the 20s at least but no 10.Meanwhile I got the power supply set up for the FET probe. First quick test showed bandwidth of 250 - 275 MHz, not too shabby but not up to specification. Will measure more carefully.The workbench is so cluttered it's hard to proceed without screwing up something.I need a little gear for my chart recorder and can't find an original part. I wonder if there is some kind of hobby store where I can pick through such things to find what I need.

author: 0culus
author_rank: Super Contributor
timestamp: April 14, 2019, 11:36:19 pm
content: I won't be back to my units for a couple more weeks, but if you're still having issues then I'll crack one of mine open we can try to compare notes.

author: bob91343
author_rank: Super Contributor
timestamp: April 14, 2019, 11:58:26 pm
content: Unless I did something wrong, it appears that the big ceramic IC needs to be replaced.I am sure it's nothing I can buy from the local parts store.Bob

author: 0culus
author_rank: Super Contributor
timestamp: April 15, 2019, 12:04:06 am
content: Might be able to grab a parts unit cheap on fleabay.

author: bob91343
author_rank: Super Contributor
timestamp: April 15, 2019, 12:10:43 am
content: It's U26. I need a part number. It's socketed so that's one good thing. I pulled it and replaced it, no change.It's marked with 5.0 Volts and I get something near that at the test point but not exactly.I am highly suspicious that replacing it will not get me home. But that's just a gut feeling and I have been wrong before.

author: 0culus
author_rank: Super Contributor
timestamp: April 15, 2019, 12:30:50 am
content: A1U26?HP p/n 1820-1112, manufacturer p/n SN74LS74N. It's a dual D-type flip-flop

author: bob91343
author_rank: Super Contributor
timestamp: April 15, 2019, 12:59:04 am
content: No. It's A10U26 and the part number is 03455-62501.It's a nanoprocessor.

author: 0culus
author_rank: Super Contributor
timestamp: April 15, 2019, 01:18:46 am
content: Hmmm, now that might be hard to find. I do have a parts mule 3455A...so if you are unable to find another one let me know and I'll see if I can't get you the one out of that unit.

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 01:43:57 am
content: Quote from: bob91343 on April 15, 2019, 12:59:04 amHP cross ref info on this page:https://www.sphere.bc.ca/test/hpparts.html#cross

author: bob91343
author_rank: Super Contributor
timestamp: April 15, 2019, 02:02:03 am
content: Thanks guys! Stealing the part from the defunct voltmeter seems the most efficient method. And the link to HP parts seems not to include that number.I did request quotes online but am pessimistic.

author: pelule
author_rank: Frequent Contributor
timestamp: April 15, 2019, 01:44:31 pm
content: The A10/U26 "Inguard Controller" is a HP specific processor.(manual description: NANOPROCESSOR ASSEMBLY", Manufactored by HP, Palo Alto).Thus you will not find an alternative part in the cross-reference lists.You may lucky if it is not the processor, but one of the logic chips around or the ROM chip.Otherwise you can just try to get it from a "parts only 3455A" as you mentioned already./PeLuLe

author: bob91343
author_rank: Super Contributor
timestamp: April 17, 2019, 07:47:01 pm
content: What are the chances of you locating that IC I need? I can't seem to find any other source, other than buying a complete unit for parts. That's a bit of overkill, especially in light of this already overcrowded place.I am willing to pay for it.Bob

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2019, 09:33:15 pm
content: Quote from: bob91343 on April 17, 2019, 07:47:01 pmQuote from: bob91343 on April 15, 2019, 12:59:04 amSeems a weird part #You might want to double check it.Nothing here ?https://www.sphere.bc.ca/test/hp-parts/hpparts2.html

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2019, 01:42:33 am
content: Yes it's a weird part number. But that's what it appears to be.I suspect that HP decided to build these different models and when it came down to brass tacks, they couldn't get the right parts. So they locked up a group of engineers and told them to come up with something that works. And they did, but it's really uncommon technology.I presume they let the engineers out afterwards.

author: tautech
author_rank: Super Contributor
timestamp: April 18, 2019, 01:54:39 am
content: Quote from: bob91343 on April 18, 2019, 01:42:33 amStill, as Sphere specialize in breaking gear why not drop them mail seeking advise ?

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2019, 01:57:47 am
content: Thanks. I will consider looking Sphere up and contacting them. Or maybe I can buy a broken unit. Still, that would cost plenty what with shipping and all. I will look on ebay. If I bid on a broken unit I could specify that just the IC be shipped but that's getting a bit difficult I think. And I'm unsure whether that will fix it.

author: intabits
author_rank: Frequent Contributor
timestamp: November 12, 2022, 03:36:55 am
content: As well as P/N 03455-62501, the manual's parts list also shows 1820-1691 and 1820-1696 with Qty=2In my 3455A, the 2 pieces (A3U9 & A10U26) are both are marked 1820-1691.I believe this is the correct part number. It's called a "Nanoprocessor"Some interesting articles:-https://www.righto.com/2020/09/inside-hp-nanoprocessor-high-speed.htmlhttps://www.righto.com/2020/09/hp-nanoprocessor-part-ii-reverse.htmlhttps://www.cpushack.com/2020/08/09/the-forgotten-ones-hp-nanoprocessorAlso, be aware that the voltage hand written on the part is the "bias" voltage required (determined at the factory), and a resistor needs to be changed to set the bias voltage to match that required by any replacement part.