Author Topic: Repair of HP 5245L Nixie Frequency Counter - V1 finished , V2 in progress  (Read 20700 times)

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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #25 on: March 11, 2016, 07:43:18 am »
Having a problem with most of my photos coming out upside down!
The Thermistor at ~22C is about 5.2kOhm, between fingers ~30C is 3.8kOhm which seems about right to me

Well, you ARE hanging inverted on the bottom of the world in Australia, aren't you?   >:D  <ducking>

Seriously, it looks like you're making good progress towards getting this sorted out!  That's good to see.

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 

Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #26 on: March 11, 2016, 07:50:35 am »
I agree the boat anchors are fun,  I do like the Nixie retro feel and look,  I also like red LEDs,
I think I will be able to fix her close to original,  I have a couple of fall back plans,  if I can't get the heater and heater control working,  but the crystal and oscillator are OK,  I will use a transistor based heater controller from MiniKits,  I have used them before.
Option 2, is a new OXCO (it needs to be 1MHz) I have one coming.... in case.
Regards Rob
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Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #27 on: March 11, 2016, 08:19:18 am »
Boy, that little board is really crispy isn't it?  Is C7 okay?  For some reason, I'd suspect a capacitor of going short before a resistor.  I see that HP specified a 200 volt rating for that capacitor.  I guess they wanted some extra margin to help it survive the high temperature.  In any case, that entire board and everything on it will have to be replaced.  It's good that the thermistor seems okay.

You might want to make a note of the setting on the pot so that you can set the new one approximately the same.  Although, the thermistor might not have the same characteristics as it used to.  You could be starting from scratch at setting the oven temperature.  What's written on the crystal?  Sometimes they mark the proper temperature.

It would be a good idea to include a thermal switch or fuse in the heater path so that this doesn't happen again!

What a mess!  :(

Ed
 

Offline Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #28 on: March 11, 2016, 09:30:08 am »
Yes, my bet would be on C7 shorted too. At least you have the option of plenty of 125'C rated capacitors these days. You might be at the point where you don't need to strip any more foam off. Just replace the capacitor, wrap it up and see what happens.

Yes, crimping an 80-100'C thermal fuse into one of the heater feed wires might be a good idea, put it in some glass fiber sleeving and tie it to the side of the cylinder.
Chris

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Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #29 on: March 11, 2016, 11:15:38 am »
D'Oh re the capacitor short, I haven't desoldered it yet but agree it would seem most likely, the other capacitor is fine believe it or not! I think the black covering is some type of heat shrink, not originally black in colour.
Thanks for the idea of the heat fuse, that is a definite change.
I hope none of those agents, e.g. old foam, laquer etc are not carcinogens etc gee stuff was going everywhere.

re Australia being upside down, reminds me of a good joke,
Q; How can you tell an Australian is 'on the level'  A; He drools out of both sides of the mouth at the same time!
Whoah! Watch where that landed we might need it later.
 

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #30 on: March 11, 2016, 03:43:05 pm »
You are making good progress.
I like the idea of a more modern oven controller.
Sue AF6LJ
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Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #31 on: March 12, 2016, 06:05:29 am »
Three cheers for wet and dry paper, Dremel tool and a fair bit of scraping under magnification.
It was the 1200 ohm resistor that is the short! Bizarrely both caps were in spec!!!!
The Thermistor was in the slot in the aluminium cylinder, covered by the nichrome wire.
I am thinking of wrapping the whole thing in Kaplon tape, putting the thermistor and the thermal fuse in the slot, covering them with section of Kaplon then the nichrome wire then another layer of Kaplon.  If I wind the nichrome centrally, i would have room for a 'back-up' thermal controller if needed. Overkill ?
Any other ideas?

PS Those photo gremlins are out again!
« Last Edit: March 12, 2016, 06:08:02 am by VK5RC »
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Offline FlyingHacker

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #32 on: March 12, 2016, 06:20:29 am »
Nice job! The whole thing counting now?
--73
 

Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #33 on: March 12, 2016, 06:25:32 am »
Not working yet, I have to wait for a 100C thermal fuse, some crimps for the nichrome, I am replacing all of the components on the PCB so have ordered some good Vishay resistors and caps, then will test before full reassembly, it a real PITA to get the oven in and out.
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #34 on: March 12, 2016, 07:00:33 am »
Wow, Rob -

That's an amazing clean up job - hard to believe that it's the same housing.  I'm surprised that it was the resistor rather than the cap that was shorted - fully expected it to be the other way 'round.

I'm not quite following what you mean by 'winding the nichrome centrally'.  Are you suggesting winding it more tightly and covering a smaller area of the housing?  FWIW, I'd be inclined to duplicate the original winding arrangement as closely as possible, but perhaps I'm misunderstanding your intent.

Excellent progress!

-Pat

Edit to fix typos and tablet autocorrect errors I missed earlier.
« Last Edit: March 12, 2016, 08:15:03 am by Cubdriver »
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Offline bitseeker

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #35 on: March 12, 2016, 07:04:22 am »
Excellent progress, indeed. It's always a treat to see these vintage instruments coming back to their former glory. Of course, the Nixies are a bonus.
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Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #36 on: March 12, 2016, 07:09:48 am »
@cubdriver, the original had the nichrome wire wound evenly along the length of the cylinder even though the crystal real only took up about the centre half. The aluminium is quite thick about 2-3mm so even if I just wound the nichrome around that 'central half',  the ends I could use to attach a heater unit or two  (MiniKits make a nice little unit http://www.minikits.com.au/Oscillator-Heater ). The thickness of the aluminium should conduct the heat pretty evenly throughout the oven if using either heater- I think!
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Offline Vgkid

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #37 on: March 12, 2016, 07:24:20 am »
Nice job on the oven repair.
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #38 on: March 12, 2016, 07:57:32 am »
Ahh, ok, gotcha now.  I still lean towards replicating the original setup.  It worked for a very long time before failing, and after your repair will likely keep the crystal happy and toasty for another forty years.  I do like the idea of adding a thermal fuse, though. 

I took a quick look at the Minikits heater you linked to, and don't think it would help much in this case.  It appears to be designed for heating a small metal cased crystal that is soldered to the back side of the board behind what I'm guessing is a d-pak (or similarly) cased power semiconductor or resistor that acts as a heating element.  The 'heater' is thermally tied to the copper pour on the back side through a bunch of vias, probably to decouple it somewhat and act to smooth out the temp variations seen by the crystal can as the heater is cycled on and off and its temperature oscillates as a result. 

I doubt it would put out enough thermal energy to make much difference in the temp of that big honking alumin(i)um ;) can that houses the crystal - it appears to me to simply be too small.  Perhaps if you replaced the surface mount part on the board with something like a TO-220 packaged one wired remotely from the PCB and mounted directly on the Al crystal housing it might do a bit of good, but you'd probably also need to mount whatever senses the temp for the control circuitry with it as well, and even then it would still be for all intents and purposes a point source of heat on the can rather than the nice evenly distributed heat source that the full nichrome winding originally used was.

My thoughts, for what they're worth.   :)

-Pat
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Offline Macbeth

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #39 on: March 12, 2016, 10:45:18 am »
Nice work!

I'm surprised a resistor shorted. High or O/C, burnt or blown apart are common enough. I wonder how it's constructed to go short?
 

Offline Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #40 on: March 12, 2016, 11:02:03 am »
Wow, you really went for it since yesterday didn't you!  ;D I wasn't sure if you'd go for a full rebuild.

Yes I'm as surprised as the others that it was the resistor rather than the cap.

With regard to the winding, yes to the Kapton, and I'd also go for the original winding form. Remember that the wire is working across a thermal as well as an electrical insulation. if you wind over a shorter length near the thermistor, then the thermistor is going to pick up more heat directly from the winding (conduction and radiation) relative to what it's sensing from the cylinder. Also you have a uniform leakage to the outside world across the full length - not forgetting the end-caps and leadout wires! If you localize the heating then you will get some thermal differential along its length.

Glad you've included the thermal fuse too, don't forget to crimp its leads too, they don't like soldering irons. You'd probably be ok insulating it an strapping it well to the outside of the winding. If you put it in the recess then make sure that its wires are in close contact with the cylinder too so they don't draw heat away from the nearby thermistor.
« Last Edit: March 12, 2016, 11:06:53 am by Gyro »
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Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #41 on: March 12, 2016, 11:27:03 am »
-Pat, I agree with you and think I may go with the original layout, those guys at HP really did know there stuff, I think I would be kidding myself if I could improve it. the miniKits heater can pump out about 3 watts or so quite well, the SMD chip is roughly like a 3/4 TO-220 and takes quite a bit of soldering to get it to flow properly, I made a little "resistor oven" for some crude resistor temperature co-efficient measurements on some low temp co vishays, it worked well there. The photo below shows a resistor (leads sticking out with Kelvin clips)  held in a brass U shaped 'oven' soldered onto the back of the MiniKIts. The resistor is held in place with a bit of foam and a plastic clothes peg (very technical!) The brown cable is a temperature probe.

- I haven't cleaned the gunk off and retested the resistor yet, I think it is a carbon type, I wonder if the 'short' may be the foam plus also some capacitor 'goo', the PCB had some corrosion in that vicinity. I am going to replace the lot with I believe equivalent parts.

-Gyro, thanks for the comments re the thermistor, I have not used them before , so the comments appreciated.

I was handling the crystal very carefully so only recently turned it over to find the label , the crystal / oven needs to be at 65.5C (written on the crystal as was previously suggested  :-+)
Whoah! Watch where that landed we might need it later.
 

Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #42 on: March 12, 2016, 05:00:50 pm »
Great job on the cleanup!  Looks brand new!  :-+ :-+

Are you going to reuse the old resistance wire?  Was it enamelled?  If so, that enamel will be very brittle after being cooked.  Handle it carefully.  Also, have you tried to solder it?  Some types of resistance wire solder very nicely.

Was the original heater just wound around the oven in a simple coil?  I know that later HP crystal ovens wound the heater in a manner that cancels out any magnetic field from the coil.

I'm building a crystal oven from scratch.  I was thinking about covering the form with kapton tape as you mentioned.  Then I remembered that I had something perfect in my junkbox.  A few years ago, my sister gave me a special sheet that you put in the bottom of the oven to catch drips and splatters.  Wash it off and it's good as new.  When I looked closely, I saw that it was a woven glass fiber mat with something like silicone to hold everything together.  I thought that this might be handy to have in the workshop so I bought a couple of extra sheets.  If it can handle the heat of a food oven, it won't have any trouble with the much lower heat of a crystal oven.

Ed
 

Online VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #43 on: March 12, 2016, 10:49:52 pm »
Ed- the nichrome wire is still in reasonable shape so I was planning to reuse it.  It had crimps,  I have not been able to solder that type of wire before,  I am also planning to put in a thermal fuse in series,  a comment was these don't like soldering either.
Interesting idea re the thermal 'mat'  I have one I use for hot air rework.  I might wait until I have it all together,  and see what space I have left.
Whoah! Watch where that landed we might need it later.
 

Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #44 on: March 13, 2016, 10:47:27 pm »
I was thinking of wrapping the oven sheet around the metal core and then wrapping the heater around that.  This sheet that I'm talking about is really thin - about 0.004" or 0.1 mm.

This is the one I'm talking about:  http://www.shoppingsquare.com.au/p_400895_NonStick_Oven_Guard_For_Clean_Cooking_

Ed
 

Offline Macbeth

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #45 on: March 13, 2016, 11:42:57 pm »
"It is not recommended to use oven guard to those who keep pet birds in their house; misuse may cause damage to the health of the bird."

WTF? erm... yeah, I don't have any birds in the house, but I think maybe I'll pass on that out of stock product  :wtf:

ETA: Do not use in conjunction with aluminum foil ... what about an aluminium tube?
« Last Edit: March 13, 2016, 11:46:10 pm by Macbeth »
 

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #46 on: March 14, 2016, 01:23:04 am »
I wouldn't use it use fiberglass insulation or the mineral based insulation.
Sue AF6LJ
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Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #47 on: March 14, 2016, 02:22:12 am »
Yeah, I don't get the birds or the foil.  I've had one in my oven for a few years.  It has a few marks on it, but it hasn't degraded at all.  So if it shrugs off oven temperatures over a period of years, it won't have any trouble with temperatures below the boiling point of water.

@AF6LJ, I was suggesting that this be used to provide electrical insulation between the aluminum core and the heater winding.  I agree with you that fiberglass insulation or something similar should be used around the winding to provide thermal insulation to replace the original foam.

Ed
 

Offline Vgkid

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #48 on: March 14, 2016, 03:30:05 am »
Those are typically known as sil-pats(not sure on the spelling). Just look up silicone baking sheets.
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Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #49 on: March 14, 2016, 03:32:15 am »
I just googled on birds and silicone.  Turns out that birds are really sensitive to strange fumes.  An overheated non-stick pan can be lethal.  Same with properly-used silicone bakeware.  Even hot oil, sauteed butter, or ordinary pan-frying can kill them.  Who knew?

Ed
 


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