HP 5248M Nixie Tube Counter - Tubes issues - Page 1
Forum section: electronics / repair

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 16, 2018, 12:12:11 am
content: Got a HP 5248M Electronic Counter - given a massive clean out first as was kept outside for years in a shed or somewhere I believeJust managed to bodge myself a power supply to this thing with a homemade AC cordIssues - it has 8 Nixie tubes, but seems only 6 work, and they seem very blurred, most more than othersIf I press reset they seem clearer when reading zeroI'm totally new to HP Nixie tubes, so any advice on them or resources is greatly appreciated! Searching faded Nixie tubes online didnt return what I needed to knowThanks

author: PaulAm
author_rank: Frequent Contributor
timestamp: June 16, 2018, 02:20:51 am
content: You need to get the service manual on that.Those displays used cheesy optoisolators which have a tendency to go bad after 40 or 50 years. Don't go freaking about faded nixies until everything works. If you had to, you can find replacement nixies without too much trouble, but again that's the last thing to do.One of the first things would be to check the power supplies to make sure they're working correctly and the caps haven't dried out. HP used quality parts, but it's not uncommon to have to replace the filter caps on that vintage instrument. Check the riipple with a scope on all the power rails.You think they used decade counters and nixie driver ICs? Ha, wait till you look at the schematic.

author: particleman
author_rank: Regular Contributor
timestamp: June 16, 2018, 03:12:56 am
content: The nixies are actually pretty hard to find HP used 1970-0025 which are inverted. evil bay has 3 for $100 right now.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 16, 2018, 04:05:17 am
content: If the tubes are dim (and the drive transistors work), then they're just poisoned. Relatively easy fix.http://www.tube-tester.com/sites/nixie/different/cathode%20poisoning/cathode-poisoning.htm

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 16, 2018, 09:59:05 am
content: Quote from: Cyberdragon on June 16, 2018, 04:05:17 amSeems like every number is on at the same time, is that Nixie poisoning? And do I remove the Nixie tube from the board to perform this?Thanks

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 16, 2018, 10:25:23 am
content: Quote from: PaulAm on June 16, 2018, 02:20:51 amThanks for the advice, I am going to attempt this and will report back, can you advise at what point I should be placing the scope probe to look for ripple?

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 18, 2018, 01:58:17 am
content: Quote from: particleman on June 16, 2018, 03:12:56 amI dont think it uses the inverted, think its thesehttps://www.ebay.com/itm/Agilent-HP-Keysight-1970-0019-Nixie-Tube-Burroughs-5992/351486913716?hash=item51d640acb4:g:sioAAOSwu4BV1e~-

author: PaulAm
author_rank: Frequent Contributor
timestamp: June 18, 2018, 09:32:31 pm
content: I could be wrong about this, but I think those optoisolators use neon bulbs coupled to cds photocells and the neon bulbs go bad. You can replace them (the neon bults) and get the optoisolators back up to snuff. They're used for the Nixie decoding and with several numbers on at once, that's a good indication those things are bad.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 19, 2018, 09:26:42 pm
content: Quote from: Reprobyte on June 16, 2018, 09:59:05 amThe last two look poisoned. It can in extreme cases short the digits from metal deposits (and it looks like the last one might be getting there), but it's usually not all of them at once (they usually fry at that point). I've also had a nixie warp a digit and short somehow (ruined but barely readable), but that was only two digits.If they are inverted then it is probably the isolators. Just replace the neon bulbs in them if that's the case.EDIT: The usual method of removing shorts would be to blast them with low voltage high current. As for the last two you'll have to use a desoldering pump or wick and extreme caution not to break them.

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 20, 2018, 01:44:59 am
content: Quote from: Cyberdragon on June 19, 2018, 09:26:42 pmI have attached a screenshot of the tubes with and without the reset button held. The final tube appears to have worsened. Neither of the last 2 will set to 0 when reset is held. The 4 working ones seem clear with this held. I assume the 1s two are totally dead.Not sure what the issue is with the middle 4 as they seem to work with reset held. I am going to look for a faulty power rail as it was suggested power issues could cause this?

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 20, 2018, 04:08:32 am
content: That last tube has reached severe poisening. Since you have to remove it anyway, go ahead and try the recommended recommended rejuvination on a few of the digits and see of they come back. If it works go ahead and blast the rest of the tube for several hours.Looks like the second to last might need it too. Not sure if it's not displaying all digits due to more driver faults (it's not resetting so definately something wrong) or because most of them have poisoned. Rejuivenating can recover dead digits. For shorts, apply increasing high current low voltage across the shorted digits until the short blows clear (although if it's too bad the tube will just blow).Get the drivers going first, then see how many digits are dead and whether the tubes are worth recovering.EDIT: While rejuvinating might make the last tube readable again, the hazing is most likely permanent.

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2018, 04:08:56 am
content: The tubes are probably fine, much more likely to be a problem with the driver/decoder. Those HP opto isolator decoders were an ingenious bit of tech, they got the job done with the technology available at the time.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 20, 2018, 04:14:55 am
content: Quote from: james_s on June 20, 2018, 04:08:56 amUnless that's just the camera, that last tube looks bad. They should never look all hazy like that, that's ionized metal deposits glowing if it is. The digits also look fragmented and unrecognizable.The first four look fully functional, the second to last might be ok too but it's hard to tell.

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2018, 04:18:11 am
content: I can't really tell from the photo, but usually when nixies go bad the envelope is so silvered that the glow is hard to see. Either way I would make sure the drive circuit is working before worrying about the tubes.

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 20, 2018, 04:25:13 am
content: Quote from: james_s on June 20, 2018, 04:18:11 amIt is super dim. But the main red flags are the hazy glow (the coating itself possibly glowing) and unrecognizable digits.Get a close up of the last digit.The second to last tube might also be dead. It appears to have the exact same 0 to 2 short from digit warping as one of my tubes. Rule out the driver first though.

author: andy2000
author_rank: Frequent Contributor
timestamp: June 20, 2018, 05:18:49 am
content: You can swap the tubes around to see if they are bad, or if it's a driver issue.

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2018, 04:25:09 pm
content: If you try to power multiple cathodes at the same time you get the same sort of hazy glow though, but yes, swapping tubes is a fairly simple way to check.This may be excessively obvious, but did you check all the voltages from the power supply?

author: Cyberdragon
author_rank: Super Contributor
timestamp: June 20, 2018, 10:06:38 pm
content: There's definitely a driver problem as all the digits are lighting, so that definitely needs to be checked, and probably first. Swapping or even checking the tubes will require desoldering and resoldering all those pins very carefully.

author: Cubdriver
author_rank: Unknown
timestamp: June 21, 2018, 06:59:15 am
content: Make sure that all the power supply rails are present and within spec ripple-wise before going too far down a rabbit hole troubleshooting the nixies and their drivers. If the power supplies are too far out of whack, you'll never get it working.That being said, the tubes are socketed, and can be swapped around between the decade/driver boards for fault isolation. If the fault follows the tube, it's likely the tube; if not it's in the driver. I believe that the lowest order decade counter (the one on the extreme right, least significant digit) is slightly different than the others as it must operate at a higher frequency than all the rest, but am not 100% certain if this applies to your counter; I seem to recall reading it with regard to the 5245L. Other than that, the remaining decade counter/drivers should be interchangeable with one another.But to reiterate, start with the power supply section.-Pat

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 22, 2018, 05:33:38 am
content: Quote from: andy2000 on June 20, 2018, 05:18:49 amSwap the tubes or the cards around?

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 22, 2018, 05:37:03 am
content: Quote from: Cyberdragon on June 20, 2018, 04:14:55 amThe last one looks terrible now but looked better when it was first switched on the other day after years of storage by the ownerHow do the first 4 look fully functional, can you elaborate as I'm a nixie tube noob, they seem to be blurred unless I press reset then they show a clear 0 - is that expected?

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 22, 2018, 05:40:33 am
content: Quote from: Cubdriver on June 21, 2018, 06:59:15 amThe far right one has a different board it appears,Thanks, I will start with the power, any good guides on troubleshooting ripple?Im also measuring ~4volts from the case's metal at ~0.05 amps, is that normal?

author: Cubdriver
author_rank: Unknown
timestamp: June 22, 2018, 06:09:43 am
content: Quote from: Reprobyte on June 22, 2018, 05:40:33 amIf you remove the cards, the tubes may easily be swapped from card to card as they are socketed. The photo below shows a card from a 3440A DVM; the construction is very similar to those in the 524xL nixie counters. The large rectangular black plastic part that the tube plugs into is the decoder section others mentioned earlier in this thread. It contains the neon lamps and photo sensors used to drive the nixie tube. The tube is plugged into a socket on the front face; it may be removed by rocking it back and forth slightly whilst pulling forward on it.As far as troubleshooting, what test gear do you have? A scope? That can be very helpful for looking at supply ripple.I normally use an AC coupled scope probe across the supply rails to look at ripple. Take note that in this older HP stuff, there are often positive grounded supplies (meaning that the positive side is connected to chassis ground and the rail is negative). Be sure of how things are connected and don't just clip a scope ground lead to the negative end of the supply - you might wind up shorting it to ground through the scope and chassis ground if it's a negative supply. Best to get a schematic in hand before troubleshooting to see what's what.How are you taking the measurement of voltage and current at the case? What sort of meter, and how is it connected? Is the mains cable well grounded?If there's a lot of noise on the supply rails causing the decade counters to run at high speed, it can potentially make the tubes look bad as it will appear that multiple numerals are lit. I believe that pressing and holding the reset sends a signal that forces all of them to zero and makes them ignore any other input.-Pat

author: Reprobyte
author_rank: Regular Contributor
timestamp: June 22, 2018, 10:07:09 am
content: Quote from: Cubdriver on June 22, 2018, 06:09:43 amIve got a Tektronix 2225 and Keysight DSOX1102 100Mhz, both fairly new to me as I'm teaching myself slowly!I am going to follow your method as instructed with the scope, probably use my older one first to be carefulI am lucky to have the original printed schematics, giving them a good look over before I start so I understand betterthe case measurement was on a Fluke 17b+ - I made the mains cable myself, that's why I was double checking - its crimped insulated around each pin and earthed to the earth UK pin. has a lesser voltage reading when plugged in but not switched on at socket but still has the reading til unplugged, unsure if thats normal, but its very low reading compared to the 230v ACah thanks that makes sense with the zeroesthanks for the help!

author: factory
author_rank: Super Contributor
timestamp: June 23, 2018, 08:05:01 pm
content: I'm guessing it the one that was on ebay recently that looked to have been stored somewhere damp (that put me off bidding).I have had few failed nixie tubes in my HP stuff due to corrosion of the pins, leading to the glass cracking and the neon gas escaping.As for the wire ended neon lamps, the steel leads can also rust & break or snap off.Having said that the gold platedsteelrust leaded transistors in 1960's stuff can fail in exactly the same way, had one like that recently in the trigger circuit of a Tek scope.David