@thinkfat
I finally decided, some 23 months after its purchase from testpoint1, to pop the lid off my precious LPRO-101 just yesterday to check out a couple of one off suspicious wobbles I'd seen on the DSO the night before. I've been tracking its drift against my home brewed GPSDO for several months whilst trying to fine tune the fan cooling algorithm that's meant to keep the baseplate temperature stabilised to within 50mK of the 36.01 deg C setpoint I'd chosen to allow stable temperature control up to a maximum ambient limit of 32 to 33 deg C. These disturbing events were the final motivation to risk my doing more harm than good by following DeVries's Rev 7 October 2011 repair guide.
Whilst I had it open, I noticed the largely missing foam insulation between the lamp and the vapour cell assemblies. There had been no sign of foam debris so I assumed testpoint1 had simply hoovered it out to get the lamp voltage back up to a more workable level. Although the absence of this foam insulation compromises the temperature control of each section a little, it does at least have the merit of providing a quick fix to restoring an LPRO to a working state.
That was a minor issue, easy to fix with polyamide foam, readily available as "Magic Eraser" cleaning sponges. It's exactly the same stuff that's used to insulate boilers and hot pipework but sold at a high cost per gram to the cleaning obsessed public (luckily, less than a gram's worth is needed in this case - it's very lightweight stuff
I removed the lamp to check for rubidium splash marks to see whether I needed to give it the hot air treatment but all I could see was the frozen blob of rubidium right where it was supposed to be, at the pinch (considering its normal horizontal orientation, Gawd knows how it defies gravity but I guess capillary forces must normally keep it in place other than for when being dropped on its end whilst still above 40 deg C). The lamp envelope had a brownish tint similar to that of those 15W pygmy oven lamps, only a touch darker.
Undismayed, I treated the glass with IPA to clean off some 25 years' worth of atmospheric contaminants, and the same for the protective window and the front glass of the vapour cell to test my hypothesis that the cleaning associated with the hot air remelting exercise plays some greater or lesser part in this heat treatment improvement. In my case, no need for the heat treatment, it does at least seem that cleaning the glass alone offers a small but useful improvement.
I did try tuning the capacitor on the lamp housing but the very narrow slot on the end of the brass tuning slug forced me to resort to the smallest of flat bladed jeweller's screw drivers to stand any chance of turning it (it was very stiff to start with and even if I'd had a plastic trim-tool with metal blade tip that could be filed to fit the slot, I doubt it would have been strong enough - it might be now that I've worked the extreme stiffness out).
At first I'd assumed the the tuning slug would be grounded but this seemed not to be the case. In hindsight, I realise it was isolated to eliminate sliding contact losses, working on the same principle as butterfly vaned ceramic trimmers where only the stator plates are connected to the circuit. This means I need to buy or fabricate a purpose made trimming tool to tune this particular circuit
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[EDIT 2022-07-05] When I had another go at tuning the lamp oscillator cap whilst I had the lid off yesterday to fit the missing jumper link and adjust the servo amp gain to drop the amplitude down to 4.5v pk-pk and tune the cavity and the SRD bias pot, I took a closer look at this capacitor tuning slug (both visually and with a DMM continuity test - something I should have done to begin with
). It turns out that my supposition that the slug was a "piggy in the middle" component part had been total and utter bollix.
That tuning slug is in galvanic contact with its ground terminal. It would seem that at 60MHz, the lamp oven's ground isn't quite at RF ground potential, hence the disturbance to the servo signal whilst tuning it.
I'll save my further observations and conclusion on this lamp oscillator adjustment exercise for a later post and just add that a cheap flat bladed jewellers screw driver is safe to use. I found one that was just the right width with a blade thickness that was barely shy of being an interference fit in the slot - it was just a case getting it precisely aligned before it would slip into the slot.
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I gave up when I realised I was on the losing side of this particular adjustment battle. I
think I left it parked where it had been before starting this doomed enterprise but I'm not certain. I'm guessing this is why DeVries neglected to mention the adjustment procedure in his otherwise excellent repair guide.
Anyway, whilst I was checking the board over for obviously failed components - the complete absence of any electrolytic capacitors makes finding any obvious failures by visual inspection alone a forlorn task (anything else that shows any signs of failure is likely to have already stopped it dead in its tracks and mine was still functioning ok (ish)), I noticed the jumper headers which reminded me of your findings. Unfortunately, I couldn't recall where these particular nuggets of useful information had been stored (or whether I'd even kept a copy of these notes) so I decided not to experiment since they matched what was in DeVries's repair guide in spite of the pk-pk servo voltage being only 900mV to begin with, maxing out to 1.5v pk-pk after adjustment.
It was only this afternoon, as a result of yet another "Unicorn Hunt" with DDG, that I came across an image you'd posted that linked back to this topic thread which I'd obviously read back when it was still active otherwise I'd have still been trying to track down the jumpering (and other) info you'd so kindly provided. However, putting that information to use will have to wait for another day or two since I'm seeing a very encouraging improvement that at least suggests I managed to do more good than harm this time round
I'd like to let the test run another day or two before I subject it to yet another cool down cycle (or, as I view it, another nail in its coffin).
Besides this, taking the lid off involves the ticklish job of releasing it from a fan cooled heat spreader before undoing the add-on mcu board piggybacked onto testpoint1's own add on breakout board attached via the J1 10 pin connector's plug retaining screws so as not to damage the connections to the three regulators bolted onto the heat spreader plate (a 7812 feeding a couple of 7805s, one of which is currently dedicated only to supplying 1mA to a 5K 10 turn tuning pot feeding the EFC pin via a 3.3M resistor - that internal 10 turn trimpot is an extremely useful feature in simplifying the padding circuit to just a single cheap resistor
).
Hopefully, when I next disassemble it from my fan cooled heat spreader and mcu controller concoction, I'll not have to resolder a broken connection (it had been a poor solder connection direct to the end of the 33k input resistor rather than a vacant adjacent hole I should have used and solder blobbed to the input resistor instead
) This was the BITE wire connection which resulted in an immediate green "Atomic Lock" status indication on power up as opposed to the red "Powered up - awaitng lock" indication on the two wire bi-colour status LED. Initially, I'd hoped, despite being sure I'd connected it the right way round, that I'd simply reversed the non-polarised two pin connection but since it had remained green after the DSO showed that it had locked, I was resigned to having to hammer yet another power cycle nail into its coffin.
However, that was not until I'd given it another half hour or so to confirm that I'd at least managed to raise the lamp voltage from its previous 4.848 volts reading prior to my attempt to improve it to a more healthy looking 5.2xxx volts after its initial 5.9ish volts excursion from cold followed by the usual slow climb to where it now shows 5.2458 volts some twenty hours later, and possibly still on the rise before ultimately starting a slow decline of about 1mV or less per week as it had always done during the previous 18 months or so of running it 24/7.
Assuming for simplicity's sake that this trend remained constant, that implied a remaining lamp life of 36 years and that was before I'd "rejuvenated" the lamp! Of course, I doubt such an optimistic estimate and figure there might be another 12 to 15 years left on a lamp that's almost certainly clocked up more than 20 years to date.
IOW, unless the lamp voltage is already below the 4 volt mark the strategy of intermittent operation to maximise the remaining lamp life seems to my mind more likely to do harm than good by the thermal cycling stresses imposed on the electronics.
I've attached some 'scope screenshots taken over a sixteen and a half hour period. There were 12 bmp images in all and I was going to drop two of them after converting them into jpgs to fit within EEVBlog's 10 files max and paltry 5MB posting limit until I realised I really needed to show all 12 making the file compression exercise redundant ('d still have to split them into two postings and worse still, the compression loses the original date/time information which is important in this case - I'm assuming the attached files will retain their original date/time stamps despite the renaming to get past EEVBlog's stupid filter... which the server refuses to accept for failed security checks
).
I've given up trying to preserve the original date/time stamps. Instead I'll list the time stamps here, starting at 3am Saturday morning
1st lot
03:00
06:45
11:13
12:00
12:00
12:10
13:27
14:00
15:30
17:00
2nd lot
19:30
00:13
The final image shows the result of what seems to be yet another 'event' where it suddenly shifted about 5ns to the right where it has been keeping station within a ns or two for the last 3 hours. I guess I'll be checking out those jumpers tomorrow after all and having another go with the lamp exciter oscillator setting.
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