Author Topic: Repair of HP 5245L Nixie Frequency Counter - V1 finished , V2 in progress  (Read 21036 times)

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Offline VK5RC

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An as-is fleaBay bargain has got a little more complex. Initially sold as doesn't power up but at a good price so I took the chance.
A dead fuse and when replaced and the power switch turned on (not plugged in) gave a hundred ohms or so, so it was looking good.
When powered up, all the Nixies lit. See photo below.
But it wouldn't count signals put into the input socket when on "frequency" but it would count if you manually 'started' and 'stopped' the counter. It would count properly if a reference frequency was put in the Frequency reference port at the back.
The counting circuits and stop start circuits were obviously working.
This device  powers the crystal oven if the unit is plugged into the mains, and a sign on the front of the unit encourages you to do so, I think someone had taken this a little too seriously and left it plugged in for a decade or so.
The power is supplied by its own transformer and a small PCB A25 found underneath (most of the PCBs are accessible from the top). This rectifies the AC and has a few components to vary the heater current.
Both the transformer and many of the components on A25 did not look happy!
« Last Edit: September 18, 2017, 09:42:53 am by VK5RC »
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #1 on: February 27, 2016, 10:54:40 am »
It is hard to see but the transformer is quite swollen, a brown colour as is one of the transistors on A25 (Q4)  and the filter capacitor C4.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #2 on: February 27, 2016, 11:06:54 am »
The Heater transformer secondary windings were ok but both of the primary windings were open, (it is a dual voltage model) and used in 115V country so they were in parallel. Not looking good.
Following the power path I found on A25 two of the rectifying diodes had also failed as a short.
C4 was open, I lie actually it was about 50pF !
Q4 was a short Emitter to Collector. In the above photo it does look quite heat discoloured. It is a Germanium 2N383.
The main power controlling transistor for the heater is mounted off A25 on the chassis nearby, is also a Germanium, 2N1183. It was also a Emitter to Collector short.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #3 on: February 27, 2016, 11:17:06 am »
A bit of transformer hunting I think has a replacement that fits,  a toroid, also with dual primary windings, I like to try and maintain the equipment to be functionally as close to the original as is reasonable.
DigiKey part 1295-1055-ND certainly fits and the voltages are reasonably close. Phasing the primaries so the dual voltage switch still does its job.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #4 on: February 27, 2016, 11:24:38 am »
A few NOS of replacement Germaniums are coming via post, in the meantime a bit of chassis cleaning. The top and bottom panels come off easily but adhesive foam on the inner surface of the panels (to stop rubbing against chassis etc) had perished and was littering the inside of the unit with a brown dust.
Quite a bit could be pealed off but the adhesive was very reluctant. Orange cleaner and my alternative to fingernail ( not too hard not too soft) tongue depressor.
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Offline gadget73

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #5 on: March 02, 2016, 04:39:40 am »
Nice job.  Not too many folks bother with repairing old test gear like this, but its still plenty usable.  Also, that bank of nixie tubes is just way too cool.  Looks better than the bubble LED display in my HP 8640b at any rate.

Where did you manage to come up with Ge transistors?  I've heard quite a surplus of old Soviet ones can be had but you don't run across much of US production ones these days.
 

Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #6 on: March 02, 2016, 05:04:47 am »
Good progress.
As for not counting If you have the external STD input in the rear; look for a switch that changes the divide ratio, that switch gets intermittent.
I have two of these...
One is a spare parts unit that had a shorted electrolytic in the heater supply.
I saved the transformer from self destruction.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #7 on: March 02, 2016, 07:15:20 am »
Hi, thanks for those comments, I think it is a lovely looking bit of gear but showing its age.
@gadget73 re Ge transistors I used fleaBay but they weren't cheap e.g.
http://www.ebay.com.au/itm/252131340125?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
http://www.ebay.com.au/itm/370327626910?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

@AF6LJ  thanks for those comments, I have had to put this 'job' on the back burner for a while as my work Fax a newer "HP" 1536dnf MFP , the Autofeed has just stopped working so and have found it is a common fault of the Flat cable, I haven't re-powered the 5245L up again to see if the oscillator is actually oscillating, but I definitely will check at the switch, it may be a lot easier to get to than the actual board when powered up.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #8 on: March 08, 2016, 10:30:49 am »
More trouble, I should have looked at this before getting in too deep. the resistance of the heater in the quartz crystal's oven should be about 1100 *  Ohms or so, mine measures 5, so a short in there somewhere.
I am of two minds, have a real go at trying to get it back to original or put in a good double OXCO, there are some 1MHz 12VDC versions about for not too crazy money. Trying to go 'all original' could  be a long haul. I looked at some videos of people opening up OXCO and it didn't look pretty!
Mmmmmmmmm
EDIT * Heater resistance should be 110 Ohms not 1100.
« Last Edit: March 10, 2016, 11:12:40 am by VK5RC »
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Online Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #9 on: March 08, 2016, 07:53:39 pm »
I think I'd be tempted to delve into the existing OCXO, it sounds as if you'd prefer to keep it as original as possible and there's going to be some cost involved in finding an alternative.

If it's a heater short then you're not as likely to have to delve deep into the oscillator compartment itself. It's most likely to be something easily repairable like plastic insulation that has punched through as a result of compression, heat and age. It sounds worth a try at least.
Chris

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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #10 on: March 08, 2016, 07:55:57 pm »
Thanks Gyro, I think I am going to have a go at repairing it, when work and family stop getting in the way!
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #11 on: March 10, 2016, 04:14:40 am »
A few brave pills later I am into the OXCO, check out the size of the crystal, in a glass envelope, not seen that before. The oven itself is looking tricky, I think the inner metal tube has been "cast' in with expanding foam, which has oxidised, possibly with excess heat as the oven heater died/shorted/age.
Edit; re the crystal size, the lines on the cutting matt behind are 5cm apart.
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Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #12 on: March 10, 2016, 05:16:12 am »
Did you confirm that the short is actually in the heater winding?  Could one or both of the oven terminals have shorted to the outer case?

 

Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #13 on: March 10, 2016, 06:55:07 am »
Wow!  That is quite a crystal!  Nice restoration work.  I've been lucky so far - mine worked when I got it, so other than popping the top briefly to look at the innards, I haven't spent any time under the hood.

I'll be watching now to see your progress - I love to see an old piece of gear like this repaired/restored and kept in operation.

 :-+

-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 #14 on: March 10, 2016, 08:25:24 am »
Hopefully +++ I can get it back together,  the heater is very low resistance 5ohm, should be much more. No short to the case or earth.  I am in the process of digging out the foam,  aaaarrrggghhhh.
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #15 on: March 10, 2016, 09:01:16 am »
It's APART, hopefully nothing too bad destroyed in the process .
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Offline Cubdriver

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #16 on: March 10, 2016, 09:35:31 am »
That certainly looks rather crispy!  What temp is it supposed to operate at?

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

Online Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #17 on: March 10, 2016, 10:53:42 am »
Nice to see that work and family didn't get in the way for too long then.  :)

Nice oven, it's always nice when things have been built, with screws etc rather than just solder filleted. Nice crystal -

Are you sure that heater resistance is supposed to be 1.1k? There don't look to be very many turns on that element winding and there's quite a thermal mass to get up to temperature. The first thing to check might be heater resistance to the inner can, that will give an indication of whether it has burned through the insulation between them. Looking at the type of insulation I would guess at the oven temperature being in the 50-60'C range.

One good thing is that it should be reasonably easy to re-build the heater from scratch if necessary. Take the length of the original wire and (if not re-usable) source some resistance wire of the right resistance at that length and re-apply using modern electrical insulation, ie. liberal application of Kapton tape. You could maybe use expanding foam as insulation - or probably better, some sort of wrapped insulation so you can easily get it out again, maybe even mineral fiber wrap. It looks totally salvageable anyway.  :-+
Chris

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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #18 on: March 10, 2016, 11:08:02 am »
@Cubdriver; I can't find any reference to the oven temp in the manual. I can only guess it is about 60C, as the device can be operated up to 55C supposedly in calibration.
@ Gyro, it is a typo, the heater should be 110 ohms, rather bizarrely is hasn't shorted to the inner tube structure. trying to get the foam remnants off the circuit components ( an AC wheatstone bridge type circuit) is proving very tricky, The heater wire looks like relatively conventional nichrome type wire (so far)! I haven't resorted to industrial solvents yet! I like the idea of the mineral fibre, so less messy.
I will keep updating.
Thanks for the comments.
« Last Edit: March 10, 2016, 11:12:17 am by VK5RC »
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Online Gyro

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #19 on: March 10, 2016, 01:28:44 pm »
Quote
@ Gyro, it is a typo, the heater should be 110 ohms, rather bizarrely is hasn't shorted to the inner tube structure. trying to get the foam remnants off the circuit components

It sounds as if it's worth concentrating your foam removal efforts on the 'lead-in wires meet coil' area in that case.  :)
Chris

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Offline AF6LJ

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #20 on: March 10, 2016, 02:10:50 pm »
Good work so far...
The good news is...
If the oven cannot be repaired you have lots of options.
It does look very repairable.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline edpalmer42

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #21 on: March 10, 2016, 03:35:12 pm »
A quick google search suggests that this problem is not unusual for that model.  Be sure to check the oven controller for failed components that would have caused the heater to turn full on!
 

Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #22 on: March 11, 2016, 06:15:58 am »
@EP42, I keep finding more issues, but I think I am getting close.
Below is a photo of the bottom of the Oven unit and the small PCB attached there, found after much digging through old foam.
A picture of the schematic of the heater unit.
The PCB and a crude reconstruction identifying the components, R2 should be 1200 ohms but measures a dead short!
The other resistors and C6 still measure OK. The heater wire had to come off and it measures OK, but old foam and laquer everywhere.
« Last Edit: March 11, 2016, 06:26:09 am by VK5RC »
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Offline VK5RC

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #23 on: March 11, 2016, 06:20:45 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
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Offline FlyingHacker

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Re: Repair of HP 5245L Nixie Frequency Counter
« Reply #24 on: March 11, 2016, 07:17:28 am »
Looks like fun! I have an older 5512A Nixie counter. Mine does not have an OCXO.

If you can't get that rebuilt, and it is 10MHz (I assume) maybe you could shoehorn one of these guys in the can and do a modern re-stuffing http://www.ctscorp.com/wp-content/uploads/2015/12/OCXO196.pdf

I love the old boat anchor stuff!
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