Author Topic: Repair of HP 5245L Nixie Frequency Counter - V1 finished , V2 in progress  (Read 21064 times)

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Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #50 on: March 14, 2016, 03:34:07 am »
Those are typically known as sil-pats(not sure on the spelling). Just look up silicone baking sheets.

The sheet I'm talking about is similar to Sil-pads.  I'd say that Sil-pads have a bit more silicone than this sheet.  A good comparison!  Thanks!

Ed
 

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #51 on: March 14, 2016, 03:48:48 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
Those pads off gas chemicals that are toxic, which is why using them in a house that contains birds is not recommended.
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Offline Vgkid

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #52 on: March 14, 2016, 04:02:15 am »
You wouldnt want to use a silpat anyway,they have a fiber weave inside. And they arent cheap.
 The basic silicone sheets seem to be available down to 1mm thick. Though 1seller has a .25mm sheet,though those might have a adhesive on the underside.
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #53 on: March 14, 2016, 04:24:03 am »
I think I'd use kapton - it'll only be a few thousandths of an inch thick (should be less than a tenth of a millimeter), so you could easily go with a few layers and not have it be terribly thermally insulating.  It's good for high temperatures, and has an excellent dielectric strength (about 2kV/mil).

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

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #54 on: March 14, 2016, 09:38:11 am »
You do learn something every day, I had a budgie for years who used to fly around the kitchen , land on your shoulder, he LOVED gin and tonic! Quite happily go back into the cage at night.
I was planning on using part of one of these as insulation between the oven and the casing
http://www.ebay.com.au/itm/Heat-Resistant-Mat-Cloth-for-Brazing-Soldering-for-solder-circuit-board-633540-/400226351731?hash=item5d2f597a73:m:mnwpUu1g_ou3GMUCN6XRwmg
I have done some experiments re the Thermistor, wrapped in single layer fold of Kapton tape
Using my little MiniKits "oven-ette' see picture in earlier post, Fluke temp, Keysight DMM.
at 27.4C      5.0k
    32.3C      4.4k
    40.3C      3.5k
    50.8C      2.6k
    61.0C      1.9k
    66.5C      1.51k
    71.4C      1.3k
    74.9C      1.17k
The thermistor took about another 2mins or so for the resistance reading to stabilise  once the temperature had stabilised (which took a few mins itself)
Awaiting some parts from RS Aus.
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 #55 on: March 14, 2016, 05:28:14 pm »
I was planning on using part of one of these as insulation between the oven and the casing
http://www.ebay.com.au/itm/Heat-Resistant-Mat-Cloth-for-Brazing-Soldering-for-solder-circuit-board-633540-/400226351731?hash=item5d2f597a73:m:mnwpUu1g_ou3GMUCN6XRwmg

Is it just woven glass fiber with nothing to bind it together? How thick is it?

Ed
 

Offline Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #56 on: March 14, 2016, 06:36:04 pm »
That looks more heat resisting than thermal insulation. Several loose layers might work to trap air, but ideally I think you'd want to use something with a foam like or loose fiber fill.
Chris

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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #57 on: March 14, 2016, 07:30:17 pm »
What Gyro said - that's meant to keep a propane torch from igniting the house when you're sweating pipe fittings and the like.  You want something like a mineral wool or an expanding foam.  Foam would likely be the best, just check the specs to ensure it can handle the oven temperature.

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

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #58 on: March 14, 2016, 10:24:37 pm »
The mat I think is just mineral / glass fibres threads woven into a cloth,  I suppose the more air there is, (ie like the open gathering of fibres in a ceiling batt)  the better heat insulation,  but the mat does resist heat transfer very well.
Once I get the bits I will do a bit of experimentation. ?
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #59 on: March 18, 2016, 09:26:42 am »
The bits arrived, mostly from RS, and most of the resistors and caps are Vishay, with reasonable heat tolerance. The original had a small copper strip over the thermistor, I presume to diffuse the heat of the heater wire which was placed over the top. I have repeated this design over the thermistor but also the thermal fuse. I went for a 90c fuse, which allows a little local heater overshoot but 90 isn't too hot for any of the components here. I used a small crimp also off RS for the heater wire and the thermistor joints, , boy do they cost a bit, I think I know what components I want to make for profit.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #60 on: March 18, 2016, 09:29:19 am »
Final Kapton wrap. I don't think I  will get any awards for even-ness of the wrapping of the nichrome heater wire or the Kapton, but electrically it checks out, no shorts etc.
Whoah! Watch where that landed we might need it later.
 

Offline Macbeth

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #61 on: March 18, 2016, 11:11:17 am »
Fingers crossed. This looks like quite a satisfying repair job so far.  :-+
 

Offline Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #62 on: March 18, 2016, 02:30:48 pm »
You didn't skimp on the Kapton then!  ;D

It looks good, a hell of a lot better than when it came out. It should hopefully work fine. I might have been tempted to bind the fuse to the outside of the winding to keep the wire in closest possible contact with the cylinder, but I doubt it would make much difference, it's very unlikely that the winding over the top would cause a hotspot high enough to trip the fuse, especially with the copper. I certainly wouldn't change it now!

It's probably a good idea to make the surrounding insulation as good as possible to compensate for the slightly higher thermal resistance between the coil and cylinder through the (inner layer of) Kapton.
« Last Edit: March 18, 2016, 02:57:44 pm by Gyro »
Chris

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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #63 on: March 18, 2016, 06:24:12 pm »
Looking good!!   :-+  Fingers crossed for you.

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

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #64 on: March 18, 2016, 07:10:04 pm »
Nicely Done; looks good enough to get the job done, and that is all that counts.
Sue AF6LJ
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #65 on: March 19, 2016, 08:15:19 am »
Coming back together, stuffing the mineral wool in, a bit of a problem, two of the very fine wires joining the Quartz Crystal to the feed throughs broke when I was desoldering, easy I thought, HA;  they won't tin under any circumstance! I have tried sanding them, used a 380C iron, 2%Silver/Lead/tin solder nothing works.

Any ideas what they are, why they are there (poss low thermal transfer) and how to tin them?? Just replace them?
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Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #66 on: March 19, 2016, 12:48:06 pm »
Don't know what kind of wire that is. perhaps in the maintenance section of the manual you will find an answer. HP was good about documenting such oddities.
Sue AF6LJ
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Offline SeanB

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #67 on: March 19, 2016, 01:17:54 pm »
Probably Invar or another low expansion stainless steel. You will need a hotter iron, some more aggressive flux ( like plumbing acid flux) to tin the wire first, then clean the acid residue off with a few washes with first laundry soap ( alkaline so it reacts with the acid) then baking soda in water  then finally water rinse then wipe with IPA. Once the wire is tinned then you can use regular solder to attach it.

I do have a small 500g pot of old stainless steel silver solder paste, which I have found will even tin Nichrome wire. Of course to activate the flux you need the Black and Decker 500W solder gun, which can get the iron tip to red heat if you do not do PWM using the switch to keep the flux burning off.
 

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #68 on: March 19, 2016, 08:08:54 pm »
@Sean B , thanks, bizarrely I also have some stainless steel flux and some 'solder' that goes with it (its a small kit from our local welding supply house), I got it from my late father who had a small farm, it's amazing the tools and 'stuff' I have now because of that. The thought of the plumbers flux crossed my mind.
I think I will take it out and replace it with a fine standard wire. I am not sure the mechanical +   characteristics of that wire is 'mission critical'.
Thanks for the comments/suggestions.
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #69 on: March 20, 2016, 12:22:36 am »
I gather that they go to the assembly of caps visible in the second image in reply 11?  I just looked through the manual I have for mine and saw nothing specific regarding any special wiring, so who knows.  Are they Teflon insulated, or sleeved?

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

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #70 on: March 20, 2016, 03:42:30 am »
Definitely sleeved,  problem was they were wrapped around the standoffs before soldering,  I don't know how they tinned them first time,  the two that I didn't break are the only ones I can solder,  (quite easily). 
Whoah! Watch where that landed we might need it later.
 

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #71 on: March 25, 2016, 11:55:31 am »
Lots of cursing etc.
Rebuilt the oven using some stripped wire wrap wire in place of the fine 'not tin-able' wire, joining the crystal to the outside world. All back in the chassis.

The heater control circuit is a most unusual design in that it is not a DC sensing voltage but a 300kHz signal, derived from Q1 on A25. The wheatstone bridge type sensing circuit has resistive and capacitive elements. :--
I have tested all of the components around Q1 ( the oscillator) and Q2 the buffer/driver and can find no fault (pulling out both Q1 and Q2 and all of the caps as well). It is NOT oscillating. This absence of oscillation allows the heater circuit to switch on fully and stay switched on, certainly getting over 70C.

When I assembled the oven I buried a Fluke temp probe, it is on the outside surface of the heating coil, and did some measurements of the thermistor again. As the crystal should be at 65C or so, the thermistor (NTC) measures about 900 Ohms at 65C. I decided to try my hand at building a new heater controller. A LM358 op amp circuit with a voltage divider, 10turn trim pot, driving a TIP33C, has the oven heater up to 65C and cycling on/off about every 20 secs or so when up to temperature after 15mins or so. About 0.5C temp variation is seen on the outside of the heater coil; inside the oven (3mm think aluminium and then more mineral fibre insulation before the crystal) should result in a pretty stable crystal temp.
Once the crystal got up to a modest temp, I experienced the "Joy of Oscillation" (- apologies to the Solder Smoke guys) of the main reference crystal oscillator circuit. :-+
Will build the new heater controller into the 5245L in an unobtrusive manner, and do some final testing / adjustment.
Whoah! Watch where that landed we might need it later.
 

Offline SeanB

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #72 on: March 25, 2016, 12:16:13 pm »
Q1 is leaky, not surprising on a 50 year old Ge transistor, or has alloyed itself to too high a gain. The curses of alloyed transistors, the gain gets better with heat, along with the leakage getting worse, till they join in the middle with infinite gain. As well those carbon composition resistors have drifted to drive the oscillator out of the sweet spot where it oscillates, it just now is an amplifier.

If you want I can send you some used crystal heaters, a small PCB that fits on a block with the crystal inside, and hich is a self contained single power transistor as both sensor and heater.  Came out of some old PMR base stations.
 

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #73 on: March 25, 2016, 12:31:33 pm »
Germanium transistors get leaky over time. If that is all that is wrong you can reverse bias all the junctions and heat the transistor to just over 100C for an hour or so...
Allow it to cool on its own until it reaches room temp and then remove the reverse bias.

Contamination was the number one cause of failure of those old GE transistors.
I had this demonstrated to me once when I needed to replace a leaky transistor in an old HP scope. This technique repaired the transistor and the scope lived to function for a few more years or so.
Sue AF6LJ
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #74 on: March 25, 2016, 02:20:50 pm »
Germanium transistors get leaky over time. If that is all that is wrong you can reverse bias all the junctions and heat the transistor to just over 100C for an hour or so...
Allow it to cool on its own until it reaches room temp and then remove the reverse bias.

Contamination was the number one cause of failure of those old GE transistors.
I had this demonstrated to me once when I needed to replace a leaky transistor in an old HP scope. This technique repaired the transistor and the scope lived to function for a few more years or so.

Wow!  I'd never heard of doing that!  Now that I've gotten involved in playing with/repairing equipment of this vintage, that's definitely a tidbit of knowledge to sock away for possible future use.

Thanks, Sue!!

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


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