EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: VK5RC on February 27, 2016, 10:52:28 am
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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!
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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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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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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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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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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.
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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.
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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/252131340125?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT)
http://www.ebay.com.au/itm/370327626910?_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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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.
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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.
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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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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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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?
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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
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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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It's APART, hopefully nothing too bad destroyed in the process .
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That certainly looks rather crispy! What temp is it supposed to operate at?
-Pat
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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. :-+
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@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.
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@ 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. :)
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Good work so far...
The good news is...
If the oven cannot be repaired you have lots of options.
It does look very repairable.
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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!
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@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.
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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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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 (http://www.ctscorp.com/wp-content/uploads/2015/12/OCXO196.pdf)
I love the old boat anchor stuff!
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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
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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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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
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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.
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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!
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You are making good progress.
I like the idea of a more modern oven controller.
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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!
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Nice job! The whole thing counting now?
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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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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.
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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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@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 (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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Nice job on the oven repair.
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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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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?
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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.
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-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 :-+)
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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
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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.
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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_ (http://www.shoppingsquare.com.au/p_400895_NonStick_Oven_Guard_For_Clean_Cooking_)
Ed
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"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?
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I wouldn't use it use fiberglass insulation or the mineral based insulation.
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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
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Those are typically known as sil-pats(not sure on the spelling). Just look up silicone baking sheets.
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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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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
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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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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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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
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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 (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.
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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 (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
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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.
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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
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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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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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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.
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Fingers crossed. This looks like quite a satisfying repair job so far. :-+
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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.
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Looking good!! :-+ Fingers crossed for you.
-Pat
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Nicely Done; looks good enough to get the job done, and that is all that counts.
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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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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.
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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.
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@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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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
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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).
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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.
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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.
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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.
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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
-
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.
Ugh!! Very frustrating, but it still sounds like you're making progress despite what seems like a roadblock at every turn. It would be interesting to hear the results if you give Sue's technique a try, but if not it sounds like you've come up with a workaround for the non-functioning heater control. Yay for thermal mass!
-Pat
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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
I was impressed when I first saw the fix demonstrated on a curve tracer.
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No doubt!! That's a potentially very useful tip for certain, as I'll doubtless encounter a leaky one at some point.
-Pat
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Also common on Ge transistors in a 4 lead can was tin whisker growth shorting out either the junctions or to the case. Shorted out to case simply solder all 3 junction leads together then pass 2A of current through the 3 leads and the case till the whisker blew apart. For shorted junctions you reverse biased it with a single C cell till it blew open. Had a 50% fix rate, and the whiskers would eventually come back, though you might get another decade out of the devices.
Still got quite a few NOS and used Ge devices around.
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Also common on Ge transistors in a 4 lead can was tin whisker growth shorting out either the junctions or to the case. Shorted out to case simply solder all 3 junction leads together then pass 2A of current through the 3 leads and the case till the whisker blew apart. For shorted junctions you reverse biased it with a single C cell till it blew open. Had a 50% fix rate, and the whiskers would eventually come back, though you might get another decade out of the devices.
Still got quite a few NOS and used Ge devices around.
Also good info to file away in the repository of knowledge. Thanks!
-Pat
-
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
I was impressed when I first saw the fix demonstrated on a curve tracer.
I would say stuff like this demands Dave have a "Vintage stuff" section. Because we need a repository for this information. I know there are lots of old enthusiasts on the internet, but when they shuffle off from this mortal coil the estate doesn't think keeping up old grandads "Bulletin Board" is conducive to the sale of all his cool stuff, and certainly not his lifetimes notes on his voltage standard for example.
Germaniums, PNPs, valves, Nixies, etc... (hell, even double sided PCBs with DIP and through hole components!) are all ancient but I am sure worthy of their own section?
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Yes, it would be great to have a vintage/restoration area. :-+
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Thanks for the great info SeanB and Sue , and offers of parts,(I think I am a OK in that regard).
I was not aware of the Germanium leakage issue, so didn't look/test for it. I think I have a NOS of Q1, the oscillator, and will try to have a look at the old Q1 for its leakage current, before I swap out the old heater controller. I might try the 'germanium cooking' if it is leaky for fun.
Re Vintage restoration, there is a sticky topic in repair that tautech encourage me to start, after some great work by robrenz, MartinM and Xrunner just to name a few but it tends to focus on the mechanical areas, https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/ (https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/)
This post has also shown some of the electronic knowledge of older components is also quite specialised and 'volatile'.
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Some testing results, using a Peak transistor tester (DCA Pro 75), the Q1 before and after heating (75C with 2V negative bias x 2) for 1Hr and some NOS x4 I have (2N652A)
Parameter Q1 Pre Q1 Post NOS1 NOS2 NOS3 NOS4
Hfe 153 186 639 188 339 129
Ic(mA) 5.01 5.04 5.01 5.02 5.00 5.00
Vbe (V) 0.359 0.356 0,332 0,316 0.457 0.314
Ib (mA) 5.00 5.00 5.00 5.00 5.00 5.00
Ic Leak (mA) 0.062 0.073 0.095 0.064 0.183 0.069
I will try replacing q1 with one of the better NOS (probably NOS2) to see if it works. :-\
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Some testing results, using a Peak transistor tester (DCA Pro 75), the Q1 before and after heating (75C with 2V negative bias x 2) for 1Hr and some NOS x4 I have (2N652A)
Parameter Q1 Pre Q1 Post NOS1 NOS2 NOS3 NOS4
Hfe 153 186 639 188 339 129
Ic(mA) 5.01 5.04 5.01 5.02 5.00 5.00
Vbe (V) 0.359 0.356 0,332 0,316 0.457 0.314
Ib (mA) 5.00 5.00 5.00 5.00 5.00 5.00
Ic Leak (mA) 0.062 0.073 0.095 0.064 0.183 0.069
I will try replacing q1 with one of the better NOS (probably NOS2) to see if it works. :-\
For the transistor that was repaired we used the rated max reverse bias voltage on all three (base to emitter, base to collector and collector to emitter. ) we used 100C for the temp and kept those conditions for the whole work day. Having access to burn-in ovens and power supplies was a plus. :) If you have replacements go for it.
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Sue thanks for passing on that experience / values. I can't easily get to 100C , I wasn't sure re the reverse voltage so went low to start with.
Thanks Rob.
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UNfortunately despite further germanium transistor 'cooking' and even one of the better NOS, the original heater oscillator controller circuit does NOT want to play ball.
I went with my heater oven control version, using the original thermistor as part of a voltage divider circuit comparing to a set value (10turn pot) via LM358 and tip33.
Oscillation of the main quartz crystal also proved to be a bit unstable and interestingly the crystal is slightly high in frequency, the tuning caps will only pull it down a modest amount before it too stops oscillating. The result is the readings using the internal oscillator are a bit low (5ppm or so), not too bad for something old enough to be my brother. Below is a photo of it measuring a 10MHz GPSDO in lock. And of it before cleaning etc
A bit more polishing of the metal surrounds and she is finished, a photo of the buggered bits!
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Did you make sure that the temperature is correct for the crystal? I've never seen any info on crystal aging and temperature shift. The correct temperature is the one that gives you the lowest or highest frequency, depending on what type of crystal it is. Yours will probably be 'lowest'. It's also usually possible to lower the frequency of an oscillator by adding a small capacitor in parallel with an existing capacitor in the circuit.
Ed
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Good to see you have it working, Good Deal :-+ :-+
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Thanks to all for the suggestions and help, re the crystal temperature, yes it is at the correct temperature, 66.5 C (I found belatedly printed faintly on the crystal glass envelope), The normal circuit has 3 variable capacitors attached for "frequency adjustment" these are what I was referring to. They can -'pull' it a hertz or so (1MHz crystal)
I have read an article about ageing of crystals, glass and to a lesser extent even welded metal envelopes can allow some contaminants to enter and then effect the quartz. Given the messy state of the oven insulation, and I suspect a lot of very hot volatile by-products of degenerated foam, I suspect that this may have happened.
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That's a fine looking anchor you've got there! Nice cleanup job - it looks almost brand new! (And it's obviously an older one since the frequency display is in kC rather than the kHz that my slightly younger one is marked in.)
It's nice to see it up and glowing in all its glory again.
-Pat
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Thanks Pat, I used a lot of the techniques for cleaning I learnt from robrenz and others,
General clean; magic sponge with IPA, top and bottom covers then a bit of Armourall
Tough Stickers; orange cleaner
Paint; light polish with car polish, avoid lettering. used a cotton bud.(Q tip)
Knobs; IPA then Mothers Back to Black
BNCs, switches; Simchrome , mostly hand polished then IPA wash esp on inside of BNC
Bit of fun, also a bit crazy, as I was testing it out, I switched on 'modern' gear, within 5mins had Thunderbolt GPSDO lock so ppb accuracy for the sig gen!!!!! but not nearly so 'cool' or fun. If the 5245L breaks in the future I will likely be able to repair it unlike my modern gear.
I find I learn electronics to a much deeper level by fixing some of this old stuff, have tended to stick with HP mainly because of their initial design, construction and manuals being so good.
Must confess I have just bought 5233L !! Have a 3456 awaiting a bit of TLC.
Hope your projects are going well.
Rob
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Thanks Pat, I used a lot of the techniques for cleaning I learnt from robrenz and others,
<snip>
Yep - I'm using a lot of their techniques, too - the magic eraser truly is magic - it's incredible what that gets off! A very light touch with it and some denatured alcohol (which is typically my next step up from Windex) took the discolored stipe off of the inside of the bottom of the HP3460B currently on the bench. And a light touch is key; it'll happily go into the finish if you scrub too enthusiastically as you've doubtless discovered yourself by now.
Bit of fun, also a bit crazy...
Definitely, but far more of the former than the latter in my eyes. Of course, in the eyes of others... :-// (Granted I look the same way at those who collect things like baseball cards, sports memorabilia, etc. etc, and then sit and watch TV all night so I suppose we all have our personal weirdnesses, right?)
Must confess I have just bought 5233L !! Have a 3456 awaiting a bit of TLC.
Hope your projects are going well.
Rob
:-DD :-DD :-DD And so it begins.
:-DD You've started down the road to 'collect the whole set'!! Your eBay search will soon start returning listings for 5327As and 5214Ls and 5325Bs and others that I haven't ferreted out of the catalogs yet... >:D I'm still on the lookout for a 524C, though I shudder to imagine how much it would cost to ship given that it weighs nearly 120 pounds by itself!
The 5245L is what started my addiction, and I too have a 5233L in my pile-o-stuff! That one I haven't photographed yet, and it's still on the need-a-manual list. Good luck with your new toys!
-Pat
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re the 5233L, I have a manual on the way (via slow carrier pigeon) if it is not too crazy, will scan and put it up on BAMA and let you know.
Rob
re Nixies (also have a soft spot for LEDs as well)
…….OK I 'll go quietly doctor………
Edit; typo
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LOL - I wound up ordering a copy of the manual immediately after making that previous post! (Though scanning it and uploading to BAMA would certainly not hurt.)
As nice as the scans are, IMO nothing beats having the real paper manual in hand, especially with the HP stuff with its seemingly 8 foot long fold out schematic pages! I've tried to print out and tape together a few of them, and decided that biting the bullet and spending the $10-$50 on the actual manual is well worth it in the end. The op & service manual I just got for the 8640B signal generator is literally twice as thick on the right side as it is on the left - the spine is 7/8" thick, the open side is 1-3/4"!!
My LED soft spot is the dot displays on some of the early HP LED stuff - the early 5300 series instruments and the 3490A DMM for instance:
(https://pmanning.smugmug.com/Electronics/Misc-test-equipment/i-t6qgDjx/0/L/IMG_2078-L.jpg)
(the nice Fluke 8845A belongs to work.)
-Pat
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Very handy to have access to nice gear at work (no such luck here now - I did when my father was working- a Lecturer in Civil Engineer at Univ of Adelaide )
Interesting re the 3490A, I was looking at one on eBay, it had conventional 7 segment LEDs, ?different production run?
A silly question, every month, I try to remember to switch on my gear for at least a few minutes or so, my theory is it may help electrolytics etc maintain some polarisation. Not sure if it does anything but quite a bit of my gear could sit unused for a year or two if I didn't. I definitely don't leave gear plugged into the mains as a lot have live fans and live transformers. I also have a central plug that I can switch off EVERYTHING at one point as I leave and if a bit of lightning around I even pull the plug out. Do you do anything similar with your old / good gear?
Rob
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Very handy to have access to nice gear at work (no such luck here now - I did when my father was working- a Lecturer in Civil Engineer at Univ of Adelaide )
Interesting re the 3490A, I was looking at one on eBay, it had conventional 7 segment LEDs, ?different production run?
Yes - the earlier ones had the dot type displays and at some point (I don't know when off the top of my head, but could probably make an estimate if I dig through the catalogs) they switched over to the 'normal' 7 segment displays. I'm sure it will come as no surprise to you that I specifically sought out one with the old displays. I don't exactly have acces to the nice gear at work, since the office with the gear is in California, and I work out of my house in Connecticut on the opposite side of the country. That photo was taken when I got that meter at a surplus place out in CA on one of my visits, and fired it up to check it out. All in all, given that it was made in 1972 and sat on the shelf at Apex for lord only knows how long, I think it held calibration pretty well.
A silly question, every month, I try to remember to switch on my gear for at least a few minutes or so, my theory is it may help electrolytics etc maintain some polarisation. Not sure if it does anything but quite a bit of my gear could sit unused for a year or two if I didn't. I definitely don't leave gear plugged into the mains as a lot have live fans and live transformers. I also have a central plug that I can switch off EVERYTHING at one point as I leave and if a bit of lightning around I even pull the plug out. Do you do anything similar with your old / good gear?
Rob
I don't really have a schedule that I follow, but I do try to at least power up the working stuff at least every 4-6 months or so, though despite the best of intentions some may go a year or more. I think it definitely helps to keep the electrolytics formed, but am not certain how critical it may be - some of my surplus things may well have been on shelves for years (the dust on the lid of the columnar counter I bought a few weeks ago on my last trip being a good example, as well as that on the 2401C, both of which came to life at least in a cursory fashion with what are likely the original filter caps). I'm beginning to think based on my limited sampling that keeping caps formed may be more critical in higher voltage tube gear with 300+volt B+ rails than in some of the lower voltage equipment.
I try to unplug things if electrical storms are about - that's definitely good practice. If a bolt of lightning can jump thousands of feet from the clouds to the ground, the surge it's capable of inducing won't even notice the millimeter or three gap in an open switch. One additional positive thing besides not keeping transformers/standby supplies hot and wasting power in unused gear by unplugging it is that in the event of lightning or any kind of mains surge, it's already safely unplugged without you needing to do anything. Win-win.
-Pat
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Pat , Thanks for those comments. Glad to hear I am not totally off the track. Yet.
73
Rob
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Sweet! It's manual day! The mailman just dropped off operating/service manuals for the 3460B DVM, 5233L counter and the 412A VTVM that I've had sitting around for six months or so. :-+
Now the REAL fun can begin.
-Pat
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Good to hear, take lots of photos, helpful when putting back together and for keeping us posted.
Regards Rob
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Will do! :-+
-Pat
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An interesting comment re glass enveloped crystals I received from a fellow ham whose technical knowledge I would rate very highly, the 'glass' crystals pick up contaminants through the glass and this eventually can make the crystal off frequency HOWEVER after a few MONTHS of being left ON and HOT the contaminants will often evaporate and return the crystal to frequency. I am not planning to try this as my lab is not close to the house and I leave it unpowered except for 1 or 2 very reliable components. I thought very interesting. it might also be why HP wanted the oven/crystal left powered ON.
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The glass is usually quite tight, unless it is somewhere cracked. So nothing serious coming in from the outside, except maybe small amounts of oxygen, hydrogen and helium. The bigger trouble is volatile stuff moving from the walls to the crystal and back - this can happen during heating and cooling.
Having the oven hot will also speed up aging - so long term drift will be higher with a running oven.
Turning the oven off and on may give some variable degree of hysteresis, but I am not sure this old counter will resolve / note it, as most of the time the turn on sequence will be rather similar.
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In buying a few plug ins for the 5245L I found an "As Is " unit for the price of the plug in.
The 5245L looked a bit rough - see photo 1 - but not much to lose.
Opened her up - no nasty surprises - all the bits there - no bulging caps. Ohms test of the mains plug gave about the right figures. Fired her up with an AC lab supply slowly over 5-8mins or so. Drawing about 0.6A (with 230V AC).
Looked at the back of the unit - switched the frequency source to its internal unit! A counter always works better with a frequency source!!!!
She works, I had a few close fit knobs, I don't have a "double" knob (I don't know the correct terminology) so put a cheapie on and drilled hole for the small central pot control to stick through. Gave it a 15min clean with IPA.
Looks Good (to me) :-+
Final photo is it counting a GPSDO, early after switch on. It got to 9.999 985 - no adjustment.
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Wow, Rob - that cleaned up quite nicely!
I'd be inclined to swap the sample rate knob with one of the double barred ones on the back panel to add a bit more uniformity to the front. I'm curious to see how accurate relative to the GPSDO it winds up after it's been on for a while. Congrats on getting the old beast back up and running - looks like it's a 1967-ish one.
-Pat
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Wow, amazing what a frequency reference can do. ;D All the Nixies look good too. :-+
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Thanks guys, I am amazed some eBay sellers put so little into selling the item, the clean up took me 20mins, he/she could have asked for another $100-150.
I have some original HP knobs coming but they are a little bigger, I think I will put them on the back, bring the back ones to the front. For the dual knob, I have a spare little Tek red knob and have tried (successfully) drilling a hole in the sensitivity knob to make a hybrid dual knob.
Rob
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Knob hacking! I like it.
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I like the term "Knob hacking" , first coined by bitseeker :-+
I used a cheap knob first, seen here on the sensitivity control, I used a 6mm drill down the collett to find centre, then drilled a 3mm, then a 6mm holes from behind. The red little Tek knob is 10.3mm in diameter, so I drilled a 10.5mm hole from the front but used a drill press at quite high speed, with a low feed rate. I used a 6mm drill to hold the knob (held on by its own grub screw)
I think I will try one of the bigger spare HP knobs next.
Below is what the counter settled down to after about 1 1/2 hrs with a GPSDO input.
I have been battling which plug ins can be used in which 'serial numbers' 5245Ls, as there was several changes throughout production , once I think I have it correct I will reproduce it here.
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Rob, if you hadn't said anything, I'd have thought that concentric knob hack was possibly original. Well done! :-+
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I am reasonably happy with the hack. See below.
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Yep, good stuff. Such clean knobs, too!
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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.
snip...
Looking at the manual it states the frequency of the oven controller oscillator is actually 3Kc (kHz) not 300kHz.
I'm having similar problems with a 5243L of similar age to the 5245L that started this thread, it has failed in a different way as it doesn't turn the oven heater on at all. I've found the oscillator based on Q1 is not running at all, I've tried changing both Q1 & Q2 with alternates but they don't work either. I found this out after fitting a fuse to protect the transformer in case the heater element fails S/C in the future. |O
I also have an incomplete 5245L (missing all removable boards) that still has all the oven part intact, the 5243A-65T board has some component differences as it's slightly newer, testing that one the oven controller is running at 2.7kHz. I've now pulled both of the controller boards and tested them separately using some resistors & capacitors in place of the bridge circuit in the oven assembly and both the Q1 oscillator sections work fine, this makes me think the problem is actually within the oven assembly itself, either a component has failed in the bridge circuit or it needs adjustment, trouble is the adjustment resistor isn't accessible as far as I can tell.
David
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The yellowed electrolytic in the first board image certainly looks to be a bit past its prime...
-Pat
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It was knackered, the yellow stuff is just a sleeve, I've noticed the Sprague capacitors with the white rubber seal seem more prone to drying out, the blue Philips capacitor in the next picture replaced it.
There were two more on the regulator board that also needed replacing, the third had already been changed at some point in it's working life & was fine.
David
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Yeah, that particular series from Sprague does seem to dry out pretty much as a matter of course. I suppose in their defense they probably didn't expect people to still be using whatever they were in 55+ years later (I think I see a 1963 date code there), so perhaps there should be some slack given in that regard.
-Pat
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To be fair those capacitors didn't stop the counter from functioning, just something I've noticed from working on other vintage HP TE, that those are usually bad and the others are mostly OK.
Yes this counter is from 1963 (serial prefix 328), the 5243L (20MHz) was introduced in 1962 and quickly became obsolete with the introduction of the 5245L (50MHz).
David
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Well it turns out the adjustment resistor (R4) for the oven temperature sensor oscillator is accessible, it's hidden under a cap on the side of the oven assembly, however this adjustment is completely undocumented, could I have the victim of the phantom twiddler?
Before I had a look at that I checked the cold resistance of the leads for the bridge components and compared these with the working one.
Resistance of wires in faulty 5243L;
Red to Blue = 9.83k
Blue to Grey = 862Ω
Grey to Orange= 1.215k
Resistance of wires in working 5245L
Red to Blue = 10.40k
Blue to Grey = 1.323Ω
Grey to Orange = 1.214k
Ambient temperature was approx 15°C, the red/blue wires are the Thermistor (RT1), the blue/grey wires are the 1k pot + 442Ω and the grey/orange wires are the fixed 1.2k resistor.
After this I set the pot to match the working unit, reconnected the four bridge wires only and used an external -22V supply for testing.
Success it now oscillates, I noted that the adjustment will eventually stop the oscillator and it won't restart until the pot is turned back & the power supply cycled. I need to see if the other one does the same.
Also the amplitude differs between the two units, this maybe because the later one has a silicon transistor for Q2 & different resistor too, both are running about 1.3kHz without the rest of the circuit & heater connected, again need to do more testing.
Question is what to set the adjustment at, I would need to remove the crystal to find out the required temperature & fit a temperature sensor to monitor the oven temperature.
David
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Thanks guys, I am amazed some eBay sellers put so little into selling the item, the clean up took me 20mins, he/she could have asked for another $100-150.
Rob
When selling on ebay for profit, the whole idea is to go straight from landfill to buyer with minimal steps in between. Just taking the photos is agonizing enough. ::)
It used to not be this way.
Congrats on the amazing job restoring this! I also have a very large collection of HP gear. I restored and use every piece of it. Sure don't make em like this any more. :-+
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that beast makes me sick :)
in normal counting mode without storage: all ready counting the frequency.
in storage mode: WRONG result.
the manual is here, also a nice programmable test generator from siglent.
PSU: done,
+13V exactly, no ripple.
+20V exactly, no ripple,
-15V exactly, no ripple.
the rectangle at #3 from A23 looks OK also.
the time base ist OK, 10/100/1k/ all ready
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There was a suggestion in another thread, that the neons in the display decoder age, the trigger voltage changes & they stop working correctly in storage mode.
In your counter, have you checked which display boards are giving the incorrect result?
David