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| Vince:
--- Quote from: factory on January 03, 2025, 10:09:09 pm --- --- Quote from: Vince on January 03, 2025, 09:35:51 pm --- --- End quote --- Nice find, correct me if I'm wrong, but doesn't the whole thing slide out of the optional case with the screw on the bottom left on the front panel. David --- End quote --- :o :palm: Oh.... nooooooooo..... you are perfectly right, it just slides out indeed! There was no need to destroy this cabinet for the seller to take pictures ! |O To my defense (and his), I have zero knowledge of these calibration fixtures. There was no reason to believe it was a plug-in... for what purpose ? Why was the cabinet optional ? What are we supposed to do with a bare instrument, with all of its guts accessible for them to be damaged, or the operator to get zapped ? Anyway, you live and learn. At least it made working the thing much easier for sure 8) OK, been working on the thing all day and late at night yesterday. There isn't enough material for me to warrant creating a dedicated topic for this thing, so I will just report here as concisely as possible and not bother you with it any more... CABINET Spent 2 hours to put it back in one piece... not familiar with this thing. Spent most of it trying to fit a square peg into a round hole so to speak, because as I eventually found out, the seller had dismantled the cabinet even further than what I could see, then tried to partially re-assemble it. Turns out he fitted a part the wrong way around, which made it impossible to put the thing together again. Once I had figured it all out, realised I was missing a few nuts and screws. Managed to pinch a few from an old Tek scope, not a perfect match but good enough. However overall, in the end I am still missing one screw, and this one I will never find in my Tek scopes as it's not a common screw. It's one that secures the feet at the bottom. Theses screws are very long and have a very tall and cylindrical head. Luckily the concerned foot is secured well enough with just one of its two screws. So not a show-stopper for now. The exploded view in the manual, and a couple pics in the Tekwiki, did help some. I was a bit intrigued by the fact that the cabinet came with TWO tilting bails ? There is a short one and a tall one. I am confused... a bail at the front to raise / angle it towards you, fine.. but why also prop the butt of the instrument ? So my theory is that the short bail at the rear, you are not supposed to use... I think you just fit the bail of the height you prefer, at the front, and the one you don't use you just "store" it at the rear, keeping it folded / retracted at all times. I don't know... that just intrigued me. CLEANING Those yellow stickers were a pain, took quite a while to fully remove the glue residue but other than that it was not that dirty. It came out pretty nice to me, I very much enjoy looking at it up close and playing with it 8) POWER CORD I went to replace the hard wired power cord with a Euro one, but I changed my mind mid-way when I realised that I could do much better than that : get rid of the cord altogether and fit a socket instead : I realised that this blue part below, that holds the cable strain relief piece, is actually more of a "plug" that fits where a proper socket would go ! Why did they hard-wire the cord instead of just fitting a socket, when the metal work /chassis was already meant for a socket ?! Madness... Then I had a X-mas moment when I realised that a modern IEC socket actually fits the old american cut out ... PERFECTLY ! It's literally a drop-in replacement. The squarish shape of the IEC socket fits e-axc-tly into the american round cutout. it's neither lose nor tight, it's perfect. Same for the mounting holes. exactly the same position. I couldn't believe my eyes. It's too good to be true of course, so I guess whenever the IEC introduced these more modern sockets, they purposefully made them mechanically backward compatible with the american sockets. Genius... So that means I can retrofit all my Tek scopes, marvelous ! :D Plus, that means the conversion requires absolutely zero modification or destruction, it is 100% reversible should that be desired later down the line for whatever reason ! 8) That really made my day. The original screws were therefore too long for the new IEC socket, so I put a piece of heat shrink tubing around the now protruding screws, to keep them 100% safe from the live terminals of the socket. Yes, I only had green tubing of the required diameter, sorry for the eye sore... :-\ QUESTION : as you can see (and in accordance with the schematic), the earth wire from the socket / cord, is NOT connected straight to the chassis. Instead, it's connected to it VIA a pair of diodes in //, front to back. Never seen that before, what's the reason for this ? To me it's just a safety hazard... imagine if just one of the two diodes goes open-circuit for whatever reason. Though unlikely of course, it's still less reliable than not having diodes there. So if just one diode is open-circuit... for half the sine-wave, the earth connection is GONE and you could get zapped. :-\ Of course if they did it, I am sure they had to, for some reason... just curious what this reason actually is ! I guess it was a requirement to achieve the performance / accuracy of the instrument ? Maybe in the very low voltage settings, the earth was introducing noise that was unmanageable and it was impossible to achieve the desired accuracy ? Just a shot in the dark... I have no clue whatsoever ! So just in case, I checked the diodes, they are fine. FUSE HOLDER Was missing. Sadly I had no perfect replacement. Have a dozen salvaged ones but only one model with the suitable outside diameter. The flat on it is too small compared to the flat in the chassis, so it wiggled quite a bit. Tightening the large retaining nut fixed that, but for how long... luckily one is not supposed to mess with the fuse holder all day long eh... TRANSFORMER I checked its wiring. Luckily it was wired for 240V not 120V, despite the US power cord. There was no sign of rework on the transformer pins either. So it clearly was wired for 240V at the factory. Which is strange... so maybe that US plug was not so US after all ? It looks like one, but... scaled down. Maybe it was not american then, maybe it was from some European country that had this style of plug back in the day, and the instrument later. Whatever.... the point is, my biggest worry was now cleared : the prospect of having a fried transformer and fried internals... was now going away fast ! :D I measured the primary winding and it read 36 ohms or something, which was further encouraging for sure. TESTING OK so it was looking good now eh ? Cleaned, re-assembled, getting power to it is now sorted, and transformer is wired properly and in good shape. So time to give it a test ride ! :) At first I used the dim bulb tester just in case, with just one bulb since it's a low power instrument. Bulb didn't light up at all and no smoke nor fire-works to be seen. So I got rid of the bulb tester. The pilot light on the front panel does light up but can only be seen if you look straiiiight at it. If you look at any angle, it looks like it's not lit. Normal ? Or maybe a weak bulb ? Then I heard a little buzzing sound... yeahhhh, the electro-mechanical chopper thingy is alive, what a relief ! :phew: Then I scoped the output and yeah, a nice clean square wave, it's alive !!! :D Frequency is 1kHz as it should. Scope measures it at spot on 1.00kHz which is a miracle given that the manual states the tolerance is 20% ! I guess Tek likes to undersell and over deliver... I went through all the ranges, and it looked accurate to me on the scope. It basically works just fine... But is it as accurate as it's supposed to be ? The whole point of this fixture is to be accurate, so it's important to check if it actually is... The front panel allows you to output DC voltage rather than a square wave. This means I can measure it with a DMM, much more accurately than the scope. So I did that, and here are the results (I let it warm up for 15+ minutes). I don't have a fancy super accurate bench meter like all of you have.. for now I make do with my best handheld DMM, a Metrix MX56. 0.025% +/- 1LSD , 50,000 counts. As we can see, the very worst we get, is 0.24%. What are the specs for this instrument ? It's 0.25% ! Yeah, we have a winner ! :-DMM 0.24% is spot on an order of magnitude worse than my MX 56, which means it's likely that all my measurements are actually truthful (enough). But what does 0.24% mean in practice ? Well that fixture is meant solely to calibrate a scope calibrator, so let's put it into a valid context : a user expanding his signal vertically to fully fit the graticule. Other than the 545A scope which had only 4 vertical divisions, most 500 scopes had 6 divisions, that's 30 minor divisions. So 0.24% of that is...0.07 ! Much less than a tenth of a small division... I don't know about you, but the best my eyes can discern on a graticule, in the best of conditions (sharp thin trace, and my face glued to the screen), would be about a quarter of a small division. Smaller than that is getting difficult to me. I am sure there are super humans on this forum that can see a micron on the screen, but I am not one of these people.... Hell... the WORST... really worst would be a low B/W scope like a 502. These have a 10x10 graticule. So that's 50 minor divisions. Accuracy then falls from 0.072 to 0.12. Barely more than a tenth of a small division. I am sorry but I sure as hell can't discern that ! And that's really the worst, wooorst scenario ! In practice most of the time it will be more accurate than that. So... I think that we have a winner here what do you think ?! :-+ I am glad I took that gamble, for 35 Euros + shipping it was well worth it !! |
| Vince:
--- Quote from: Vince on January 05, 2025, 03:17:48 pm ---POWER CORD [...] Why did they hard-wire the cord instead of just fitting a socket, when the metal work /chassis was already meant for a socket ?! Madness... --- End quote --- Ah, I might have the answer to my question here ! Was on the phone a minute ago with an old retired EE friend of mine... I told him about it and he said that here in Frog land in the '70s or thereabouts, legislation (local or Europe-wide I don't know) explicitly FORBID the use of detachable power cords ! So manufacturers of TE or any appliance, were forced to hard wire the power cord instead ! :palm: That explains why every '70s TE I own drives me nuts with this cord.. I hate it. So it's all sussed out now. Tek made the cutout in the metal work to fit a socket for its domestic market, but when they shipped it to Frogland they removed the socket and soldered a cord instead, and disguised the resulting gaping hole with this blue "plug". Which further confirms that this unknown to me power cord / plug style was indeed not american at all... wrong assumption... |
| Vince:
--- Quote from: Vince on January 05, 2025, 03:17:48 pm --- QUESTION : as you can see (and in accordance with the schematic), the earth wire from the socket / cord, is NOT connected straight to the chassis. Instead, it's connected to it VIA a pair of diodes in //, front to back. Never seen that before, what's the reason for this ? To me it's just a safety hazard... imagine if just one of the two diodes goes open-circuit for whatever reason. Though unlikely of course, it's still less reliable than not having diodes there. So if just one diode is open-circuit... for half the sine-wave, the earth connection is GONE and you could get zapped. :-\ Of course if they did it, I am sure they had to, for some reason... just curious what this reason actually is ! I guess it was a requirement to achieve the performance / accuracy of the instrument ? Maybe in the very low voltage settings, the earth was introducing noise that was unmanageable and it was impossible to achieve the desired accuracy ? Just a shot in the dark... I have no clue whatsoever ! --- End quote --- RTFM !! :-[ Replying to my question and sharing in case someone else might be wondering as well... The manual has a paragraph devoted to explaining this to us. Looks like my guess was right... incredible luck : I also found this paragraph which explains how critical it is that the load impedance be precisely 1Meg as the instrument is designed solely to feed a scope input. My DMM of course is 10meg not 1Meg, so might redo all my measurements to see if that makes any difference. Logically I guess the voltage should drop a hair on all ranges. Most ranges are either spot on or on the low side of things, so it will only makes things worse I guess. So I might need to tweak the reference voltage just hair to brings things back up. We shall see... |
| factory:
--- Quote from: Vince on January 05, 2025, 03:17:48 pm --- --- Quote from: factory on January 03, 2025, 10:09:09 pm --- --- Quote from: Vince on January 03, 2025, 09:35:51 pm --- --- End quote --- Nice find, correct me if I'm wrong, but doesn't the whole thing slide out of the optional case with the screw on the bottom left on the front panel. David --- End quote --- :o :palm: Oh.... nooooooooo..... you are perfectly right, it just slides out indeed! There was no need to destroy this cabinet for the seller to take pictures ! |O To my defense (and his), I have zero knowledge of these calibration fixtures. There was no reason to believe it was a plug-in... for what purpose ? Why was the cabinet optional ? What are we supposed to do with a bare instrument, with all of its guts accessible for them to be damaged, or the operator to get zapped ? Anyway, you live and learn. At least it made working the thing much easier for sure 8) So... I think that we have a winner here what do you think ?! :-+ I am glad I took that gamble, for 35 Euros + shipping it was well worth it !! --- End quote --- I have to admit I don't know as much about these modular calibration fixtures, but have seen a choice of cases, including the one you have for one unit and a double width case for two units. The constant amplitude signal generator is taller than the others, it has a simpler box case, I haven't got a picture showing the case. Edit: I've taken a picture, the covers are completely different, as I said already it's taller than most of the cal fixtures. You did very well with this one, seems to be a winner. :-+ David |
| Vince:
--- Quote from: factory on January 05, 2025, 08:12:50 pm ---[...] but have seen a choice of cases, including the one you have for one unit and a double width case for two units. David --- End quote --- Oh wow... interesting, so it was indeed a plug-in system, they made modular bays for their fixtures ! :o Must be as rare as hen's teeth, imagine a collector trying to find a double-size bay ! :o Definitely not as common or well known as the usual TM500 / 5000 systems... |
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