HP410B Vacuum Tube Voltmeter (VTVM) Repair

today on the bench i have a hewlett-packard 410b vacuum chip voltmeter or vtvm for short this i just recently picked up at a local auction and from the photo that was supplied because it's an online auction i could see that the probes here like this common one has no probe tip in it at all this has the collet nut missing from this particular probe and so i thought yeah well i'm probably going to have to make or find a suitable replacement because i want to try and keep the probes original if i can and start to go through this before we even get to a point of powering it on just want to check that all the controls actually function so just going through this switch feels okay the switch feels okay you can see how dusty and dirty grimy this is pots seem to rotate okay and we get to the ohms adjust and this is absolutely for oh it's it was frozen i've just managed to free it so it might need a little bit of lubrication there so okay so there's a little bit of work to do there now let's go and have a look at the the rear of the equipment and see what we find out about that okay so for those of you not familiar with this piece of equipment the rear panel comes off and the way it does this is there's a a button up here and this pool ring here you can put a finger in to actually give it a pull so you hold and pull and you can then remove the panel well we've got something inside here so before we get to that it actually gives you the positions of the valves or tubes in the unit and this is the ballast for the heater circuitry to maintain uh constant heater voltages and uh in here we've got a couple of containers a few things in them so let's just uh get them out and have a look says on here jig for making meter probes that's interesting and test testing probes so oh that looks good too all right let's just open this up and pour these out i'll just move the meter back a little bit so we can get an idea of what's going on here and oh look at that wow so i'll just move the camera so you can see a bit better and we'll zoom in a bit and we've got a whole series of probes probe tips so the missing ones i had and missing parts we've got enough to cover that we've got a couple of banana uh four four millimeter banana connectors we've got a couple of fixed probes as opposed to the the collet type which you can replace the probes we've got a couple of uh slotted type spade terminals so you can actually screw them down to something and a couple of alligator clips and they all screw in to they will screw into the uh to the probes so that's excellent and then in this one here they've got marked as a jig let me get that there i'll just make this move a bit so you can see and i'll open this up oh i've just dropped a nut but this looks like the the same thread and looks like a some sort of jig i'm not quite sure how to use it as a jig but before making up these uh these threaded sex brass sections that go screw into the into the probes so that's excellent that means i don't have to worry about trying to reproduce something for the missing pieces on the probes so that's excellent so that's a that's a good step forward so what we'll do now is just um pull the unit out of the enclosure and before we do that though i'll just put this on onto its side and spin this around so that you can see and we've got this connector here which has all the probes on it we shall just disconnect this from the thing i just hope i'm not getting in the way of the camera here just unscrew this and that and you just loosen this off and this whole section plugs into the base of the of the meter just a matter of carefully unscrewing it it's still connected so we're nearly out okay so that's out and that one's out so that just falls away and unplugs so that means we can get our our meat probes out of the way then we write this back up again so that we can get to with a phillips screwdriver and just undo these screws here which feels feels like this actually going to drop off the back so it seems like there might be something a bit loose there and we'll do this one and the end is actually moving forward as we speak nearly there okay that screws out now the whole front face should pop out and we can move unfortunately the power cable won't go through the hole here it's been it's a australian mains cable so the so we'll just get that so that we've got things better organized and we can now see we can see the back end here you can see it's quite dusty and dirty but it's not it might be a little bit of surface corrosion but nothing really major so that's quite good and we look around here you can see that the component actually some of the componentry looks like it might have been replaced we've got resistors here and here that don't look original and we'll go over this side and again we've got if you can see here these these are these aren't original resistors uh used in this and in fact this is tacked onto this one here so we're going to have to have a little bit of a look there also in the most of the units well as far as i know have a selenium rectifier here and this has already had a silicon rectifier to replace that selenium so there's obviously someone's been in here in the past and in fact the the line cord that's on here is moderately recent because it's using uh brown and blue for active and neutral lead wires and a green with yellow stripe for ground whereas if it was the original us cable it would have been black for active white for neutral and green for protective ground and if it was had been done possibly more than 30 or 40 years ago here in australia that would have been red for active black for neutral and just green for the protective ground so so it's been probably i would say done in the last 30 years this this cable but um yes there's been a few mods done to this unit so we'll have to actually have a look as to why they've been done now also it looks like it's real relatively recently in here the uh it's only an 85 degree see a 10 microfarad 250 volt electrolytic has been placed in and a tag strip has been placed on the bolt holes of the original electrolytic capacitor that would have been sitting here so we've had the the electrolytic replaced already but i think we'll replace that anyway because um we just don't know it's providence but we can test it in any case so there's a a little bit to check out bit of cleaning to do but hopefully there should not be too much wrong with this unit all right that's the initial appraisal pretty much done after a bit of effort the the controls and the meter have come up quite well just cleaning everything up getting rid of some of the dirt and grime i've tested the tubes and they're fine i've checked out all of the resistor values particularly the replaced resistor values and they're all within spec the only thing was that the zero adjustment range had obviously been slightly increased with an extra resistor in parallel with the x6 k8 resistor that's in that part of the circuit to bring it down to about 5k4 so it'll give a slightly bigger adjustment range on the zero but that doesn't seem to really be an issue here i went then next to start verifying the calibration of the dc you first have to set the dc gain of the system up before any of the other modes like ac or ohms can be calibrated with any degree of confidence when checking the calibration of this meter for the dc ranges i found that it would set up okay at full scale and read pretty well spot on back to certainly about 60 percent of full scale but down below in the region say 20 to 40 percent but really extending from probably about 10 to 50 the error was starting to increase and that's to be expected but it was actually outside the specification given in the hp manual so i thought okay what have we got going on here is there an issue with the meter movement and after repeated tests that i performed i was starting to find that occasionally the meter needle particularly when you didn't have a lot of momentum say going from full zero to full scale but only going up the scale to say 20 30 40 percent or so the meter would tend to it looked like it was sticking before it actually got to the to the setting that you had intended and and you could actually tap on the the meter and it would then move a little bit and similarly coming back to zero intent was tending to stick but it wasn't always in the same place it was it was quite variable so i i initially was thinking possibly a bit of swarf or something in the meter movement bit of a little fragment of uh a ferrous metal or something uh that might have been sticking but but because of the intermittent nature i thought no it's got to be it's got to be something uh less aggressive than that so i then proceeded to take the meter apart and i'll i'll recreate this because i didn't have this on camera at the time four screws in the corners of the meter here are uh what holds the uh the whole meter into uh the panel and you can just remove those and the metal will come away just get these out and lift the meter away you'll see there's a capacitor across the meter here and just with a some adjustable adjustable spanner just loosen off the the nuts on the terminals of the meter take them off and we have the meter movement out okay with the meter out the next step is to undo three screws that hold the back of the meter into the face these have a spring roll washer underneath i just gotta be careful you don't lose that but you can just draw the the whole meter movement out like so and if we just carefully drop these out that's all the screws dropped out i'll just make sure that they're all put to one side out of the way there's a gasket around um around here as well this one's actually a little bit deformed so it probably doesn't do the best job but now what we have is the actual meter movement we'll just zoom in a little bit get that to focus and it was fairly difficult for me to actually see down here because i i had a fair bit of strong light trying to look down along the uh the meter magnet the curve section of the magnet where the coil goes around now if you just i'll just carefully do this you can see uh in there the coil as it moves and what i suspected was that and i could actually see a small amount of corrosion in the area and then little little spots of corrosion those i thought maybe were just intermittently failing on the meter coil as it moved and particularly with a low momentum coming up here that was causing it to stick so i thought that maybe what we should do is just use something like deoxit to try and remove that unfortunately the nature of this is you can't really get anything down in there without damaging the movement so you can't aggressively scrub the surface like you can on a pot or whatever but i thought just flushing it so the first thing i used was deoxit d5 did one spray of that and that didn't really do very much at all and i was also concerned that there might be some residue left that might also cause issues with that as i said it didn't seem to make much difference so next i tried some some fader lubes and deoxit f5 and that actually cleared up most of the issue we weren't getting any sticking with the movement in this 20 to 40 percent of full scale range but what happened then was when it was returning to zero it got rather intermittent as to coming back to the zero point and what i suspected then was the fact that rather than it be a surface corrosion that the the actual coil was fouling on that it might actually be dirt and grime that had got into the jeweled bearing that's in these or certainly i don't know if these have actually got a dual bearing i presume they have but certainly into the bearing for the meter movement so the next step was to take off the meter scale so that it wouldn't suffer any damage potentially from isopropyl alcohol and the idea then was to upend this so i lift this up i'll just move this out so we had this vertical in my ultrasonic bath filled up to about here so that it covered the the whole meter movement section and ran the ultrasonic bath with isopropyl alcohol for about 10 minutes after that time the movement had freed up enormously and was coming back to zero so i then shook off a lot of the excess isopropyl alcohol and then place this in toaster oven for about half an hour at 60 degrees c just to bake out any of the remaining fluid and that seems to have actually resulted in the meter functioning correctly so quite possibly it was more likely the bearings in the meter had accumulated some dust and dirt over the years and the spraying of deoxit d5 i think actually moved the position of that dirt and grime closer to the zero point and got rid of it in the areas where the meter was actually sticking at the 20 to 40 percent range so that's now fixed and i was able then to actually do a proper dc calibration and i'll talk more about that in a moment after letting the hp 14b warm up for about 15 minutes or so so that it can thermally stabilize you should be able to adjust the zero setting to give zero on the display and then actually be able to switch between positive dc and negative dc without any significant change to the meter needle and that's what we've got here next we should be able to go and check the calibration and for that i'll just show you we're just using a keithley 230 programmer voltage source and while keithley is not up to the sort of level or this particular unit anyway it's not up to the level of a flute calibrator with say seven or eight digit precision it's way better than i need to actually calibrate the hp 14b at least on dc volts now with the kethley 230 supplying one volt into the hp 410b you can see that it's slightly off of one volt so we'll just get around the back here and adjust until we get close on one fold and you can see that that's pretty right now now if i go to zero again we've got zero showing okay now what we'll do is uh try some part the way up the range so let's go for 200 millivolts and it's just a hair over 400 millivolts it's just slightly higher again 600 millivolts again slightly high 800 millivolts so we could actually slightly reduce our gain to try and improve over the whole range so now if i go back to 200 it's still a fraction high there if i go to one volt that's reading just a hair under not even that but it's close so we go say 100 millivolts and that's where most of our error is and we're talking it's reading 110 instead of 100 and if you do the quick math on that that is one at that point it's one percent high so that's quite good now if we go say to a different range let's say this to zero to start with we'll change ranges let's go to three volts and we'll put three volts in here and it's reading three volts on the scale below if i go to ah i'll go back it'll go back to zero just redo that so if we say two volts it's reading just a hair high and one volt and we're reading certainly better than one percent at that so now if we go to say our 10 volt range and it appears that down at ten percent of the range we are about one percent in error which uh that's one percent of full-scale deflection i might add and hewlett-packard give a specification of plus and minus three percent of full scale with selection so uh it's quite a bit better than the uh the absolute worst spec that they offer so now if we go to 10 volts and that's reading pretty well spot on it's just a hair high but that's okay and then if we go to 30 volts range we're reading just a little bit high so generally it's reading slightly high but that's fine so now we go to 30 volts and considering the considering the technology here it's not doing too bad a job because most analog meters i'd never say they're much better than about two percent so that's that so we go to 100 volts and that's readings just slightly low on the 100 volt range so we go to 100 volts and again it is just slightly above but that is fine that is more than adequate for a meter of this caliber so that's the dc range done i mean we could go higher i can go up to 300 volts and we see the 100 volts on that range is reading quite well and the keithley doesn't have capability to go beyond plus or minus 100 volts but that's fine after further checks of the ac ranges and the ohms ranges this hp 410b has just come up and within spec and seems to be quite stable so i'm quite happy with it and just to conclude i'll put up a still shot of the sorry looking condition it was in at the auction where i found it

Channel: Peter O'Neill

Views: 3,985

Keywords: VTVM, Vacuum, Tube, Volt, Meter, analogue

Id: bzC3vM8fqJg

Length: 29min 9sec (1749 seconds)

Published: Mon Jun 20 2022