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HP 1741A oscilloscope - restoration project

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SeanB:
Metric at least has defined threads and pitches, and taps and dies are available for them all in a single series ( ok it is a 20 page pdf) for ease of use.

Kilroy:

--- Quote from: MuchMore on January 27, 2014, 12:43:08 am ---Since the new flyleg is at least coated with copper is it a longtherm issue off rupping on the goldpats?
I could get a Nickel-solution to make it like the other once..??
--- End quote ---

The biggest thing to watch out for is that surfaces of anything coming in contact with the gold is a totally smooth, polished finish. The slightest bit of surface roughness will tear the gold right up. Just because it feels smooth to the hands does not mean it is so. Check everything under at least a 12X loupe to be sure...smooth stuff can look particularly horrible under magnification. I use very fine Japanese waterstones for polishing wiper surfaces for these types of repairs. You can *not* use fine wet/dry sandpaper....3000 grit wet/dry will leave a much worse surface than a 3000 grit Japanese waterstone finish. Always carefully compare the finish on the original contact surface with the finish on any potential replacement material before you consolidate the repair. Any impatience or shortcuts in the procedure will reward you with a ruined switch surface.

The HP 1740A/1741A machines are beautiful instruments, perform well beyond their specifications, nice to work on when/if the time comes, and well worth the trouble to repair them when they do require a bit of attention. I would plough ahead with the repair if everything else looks good with the scope. The experience alone counts for something.

Incidently, since the board requiring the repair is the main sweep pcb, I would check the Sprague 109D 100uf 30v wet tantalum capacitor when your are in there. After much use this cap had a tendancy to fail, which kills the slower sweep speeds. It's usually not particularly catastrophic when it happens, but since the sweep board will be out anyway I would just go ahead and replace it. If it is anything other than a Sprague 109D then it has probably already been replaced....hopefully with an equivilent milspec substitute.

MuchMore:

@Kilroy
    Hey

    mhmm polishing the pad is a very good idea... however I was a Little bit fast. I already put in the new flyleg and glued it back together. (was a real pain in the ass job) 
    I don't know if it is very practical to polish them when they are inside the Frame, or if it would break another one, and there is no way to separate them now (the epoxy is really tough)
    I have to think about that.

    About the cap: you mean C11 ?? I can't read the name on the part. If you mean C11 than yes it look really crusty, but the slow timebases work just fine (for now).
    I don't have the exact replacement part, however I can write it on a list for the next Mouser order.

    So it seams you know which parts are up for an replacement. It would be amazing if you can write a small list containing parts that are prone to fail?


 
for now the scope works, but the knops for the vertical Settings are very "funky", means: you turn them and the line jumps around like crazy, but it is stable when you leave it alone. Is there an easy way to clean the pots, or do I have to replace those? (I have not jet removed the main vertical board)
and the second Problem is, when I Change the Volts/div the ground Level jumps around. Is this an calibration issue or an Hardware failure??

 
I really appreciate your help,    (and I like the positive Response to the metric System)

lowimpedance:
 Neat refit job on the switch there.
Sounds like you need to clean the vertical switches as well.

 Couple of things to check are the power components on the power supply (including the connectors for any fatigued joints and re flow any suspect looking ones. Also make sure you lubricate the mechanical surfaces of the timebase switches don't need much but will help with the "feel" of the switch detents when rotating. One other failure I have had was a power resistor in the vertical amp hybrid 'bias' go high in value killing all vertical signal getting to the vertical deflection stage.

Kilroy:
Looks like you used a wiper off something else for the repair so you should be fine, no worries there, and you saved futzing about fabricating a special part.

C11 is the right one. Often fails open when it goes and you don't get much, if any, warning that it is near death. Suddenly your slower sweep speed is a flickering flat trace originating from somewhere mid screen.  This cap is specified at 20v which I suspect is probably towards the low end of the requirement...possibly to keep leakage and ESR down as low as possible, since this tends to go up for equivilent capacitors having higher voltage ratings.

You can replace the original Sprague 109D wet slug with a good solid tantalum if you prefer. I like the Kemet T140 as a substitute...not inexpensive but very low ESR, low DC leakage, good heat performance and stability. I usually go up a bit in the voltage rating but these HP 1740A/1741A machines are very solid, and would go for years and years with no trouble on the originally specified components so if you want to stick with the original 109D 20v cap there's no reason not to.

Sounds like you need to go over all the controls and give them a good clean and flush with isopropyl.

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