@bob91343
Why you get pleasure out of such activity is really no mystery
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As pretty well everyone here knows full well, it comes from that warm glow of 'a strong sense of achievement'. In your case from being able to improve the accuracy of your HP10811 against a (secondary) Atomic Standard and later discovering months afterwards that both had remained 'in calibration', proving that both were remarkably stable frequency references under the quite reasonable assumption that this wasn't likely to be due to both having coincidentally suffered exactly the same ageing induced error.
I swiftly discovered in my late teens (just over half a century ago now
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) never to trust that even well respected manufacturers 'had done the right thing' in regard of both design and calibration so learned very early on to ignore that old adage "If it ain't broke, don't fix it!".
I became rather interested in high quality audio recording (a logical step from my interest in amateur radio) so had purchased an Akai 4000D way back in 1967/8 which proved to have a mismatch between the record and playback heads which I belatedly discovered when downmixing my stereo recordings into mono to dub onto mono cassettes for portable convenience (I'd already dubbed a hundred or more vinyl albums by then so it was a particularly egregious discovery - no great problem with today's computerised audio processing but a disaster back then).
When I upgraded to an Akai GX630DB a few years later, I was mindful of this issue and the possibility of other 'design errors' so made sure to order a full service manual from Akai before the supply dried up (When I'd placed an order with Rank Xerox for an M8 service manual, they'd been obliged to ship me a photocopy of their own service manual copies - two in fact!).
I'm glad I did because when I'd read an otherwise glowing review of the GX630DB in Studio Sounds magazine, spoilt only by the discovery of a replay clipping issue (at the +8dB VU level) when testing with recordings made at high levels on another brand of machine, it led me to take a closer look at the replay circuitry, notably the Dolby processing boards where I discovered the emitter follower output stage was biased at quarter of the rail voltage rather than the expected mid point voltage. I calculated a resistor capacitor biassing network that would maintain the audio gain but shift the emitter voltage to the mid point to eliminate the asymmetric clipping that had reduced the clipping level by some 6 or 7 dB.
It was only several months later that I saw the original Dolby circuit published in a Wireless World article which revealed that I had only managed to reverse engineer away the penny pinching bodge that Akai had inflicted and restore it back to the full Dolby reference standard. You may well imagine my sense of satisfaction and smugness (don't hold back on the 'smugness'
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)
The story with the Dolby boards didn't end there since the stereo Dolby board used in the record amp chain had been likewise afflicted which neatly explained why a gross intermodulation would arise with high levels of bass, an effect which I'd initially assumed to be a limitation of ferrite over mu-metal used in Akai's Glass Xtal heads.
Since the bass boost part of the recording EQ had been applied before going through the Dolby circuit on the quite reasonable assumption that the Dolby processing ignores the extreme low bass content (the high frequency record EQ boost being applied to the final stage
after the Dolby circuit), this too needed to be brought back to standard.
The only difference this time being that I didn't need to replace the resistor in the emitter follower with a constant current generator circuit as I had done with the replay boards to avoid the onset of asymmetric clipping with output loadings above a minimum limit of 3K impedance. The use of a resistor, in this case 3kR, in an emitter follower compromises the clipping limit in an asymmetric way even for reasonably high impedance loads of 10K (I had decided on this extra modification to the Dolby reference design on the basis of "Why only do a half assed job whilst you have the opportunity to a full assed one to make it more robust against any unusually low loadings that may get connected to the line out sockets?). The output loading on the record boards was well defined so didn't require this level of robustness.
That investment in a workshop manual had more than paid for itself. Not only was I able to add more upgrade modifications (Xtal locked bias oscillator, 12 step switch adjustable bias, HF EQ and record amp sensitivity levels to optimise for a wide range of tape formulations that could be noted against each reel of tape for future reference) as well as allow me to troubleshoot a sudden capstan speed instability due to a shorted out 1N4001 diode in the bridge rectifier part of the servo circuit which controlled the direct drive capstan motor, not once but twice - second time round I used higher voltage 3A rated diodes to avoid any further repeat performances.
The result of that hard won experience with kit that turns out to 'broken by design' (Transam Tuscan S100 bus computer kit, anyone? and of course that classic of "Bean counteritus gone mad", the wonderful Feeltech FY6600 'modify to your heart's content' project) means I rarely apply the adage "If it ain't broke, don't fix it!" unless it's so complex that I wouldn't know where to start looking for things in need of improvement such as that LPRO 101 where there's some risk of making things worse for no effective gain.
I don't need to open it up since I've been able to see more than enough photographs of its innards published on the interweb and in all the various downloaded maintenance and repair guides and manuals I've collected. I'm seeing no hint of any issues in its operation to warrant deeper investigation so I'll wait until the time, if ever, to deal with any problems that may arise.
Right now, the only minor concern is over the lamp photocell output voltage being a little lower than I'd have preferred. It was measuring 4.975v when I first got it some 12 months ago and is now at 4.912v with most of the drop seemingly having taken place as a result of powering it down rather than actual run time. Extrapolating the rate of fall implies it won't reach its 3 volt end point for another decade or two so I'll 'make hay while the sun shines' and ignore the urge to check on certain critical resistors and a decontamination of a quarter of a century's worth of pollution on the glassware inside the physics package for another year or three.
Incidentally, when I checked on the drift rate of the Rb oscillator some 4 hours after re-aligning the traces for another test run, it had drifted by some 55ns and then a further 55 ns or so another four hours later.
Checking the heatsink temperature with my IR thermometer showed it had warmed up by a good half degree C. I guess I must have disturbed my breadboard lash up sufficiently to introduce the half degree error when I'd disconnected the long redundant scope probe I'd been using to monitor the PWM fan motor drive signal up until a couple of weeks ago.
Solderless breadboards have their charms but stability of their connection points ain't one of them, especially when mV levels of voltage stability are a critical requirement of the circuit lash up as it was in this case. Fortuitously, after fiddling with the breadboard at 1pm, that drift has, rather gratifyingly, been slowed right down to 1ns per hour over the past 6 hours.
Soldering the components onto a PCB would fix that issue but since I now want to use a nano 3 for the task, I'll skip that step and commit to getting to grips with the Arduino IDE and write (or modify) a fan controller sketch of my own. I've been putting this step off for long enough. I've seen ample enough 'proof of concept' with my breadboard testbed now to take that next step.
It's not as if programming is an alien art so much as it being over 30 years since I last coded anything in Z80 assembler and even longer since I last used TCL BASIC (that Transam Tuscan kit I'd alluded to earlier) and before that, Sinclair BASIC on the original ZX80.
My main problem is that I've finely honed my otherwise useful skills in procrastination to such an extreme as to be counterproductive in making further progress in a hobby that now routinely makes use of microcontrollers as a cheaper alternative to the more traditional analogue solutions of yester
dayyear. Even my writing style is afflicted by this.
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