HP 54540 Dumpster Scope

[veedub565]

Hi, I've rescued an old HP 54540A scope from the trash. One of the attenuators is confirmed dead, which isn't a problem as I have a spare.

I do know some history of this scope and it's been in storage for the past 5 or 6 years. It powers on, and apart from the attenuator it works fine. Question is as a precaution do I bother replacing the mains filter and all the caps in the PSU, is it really necessary?  if it was just a mains filter then I probably would, but this one has the power button built in (it connects to the push rod from the front panel) I doubt I'll find anything like it available now. It isn't one of the Schaffner types that usually go up in smoke, it's Delta Electronics 04EB3S-1.

Caps in the PSU all look good, no leaking, no bulging. They are all glued in place (and to each other) so replacing them all would be a PITA.

Thoughts ? the fan has manufactured in 1997 on it so not that old

[GreyWoolfe]

Remember, looks mean nothing.  The caps can look perfect and have a high ESR.  It makes sense to replace them.  Also if there are RIFA caps in there and the cases show cracks, replace them as well.  My latest HP 3478A as cracked RIFA caps that I have to order and replace.  Hopefully, regular practice of your Google Fu might find a replacement for the mains filter.

[Stray Electron]

  I have a HP 54540c and a HP 54542a and good number of HP scopes that are even older and I haven't any problems with the caps in them so my advice is to leave it alone until you do.  HP seems to have used a much better grade of caps than just about any other manufacturer and bad caps aren't common problem.  My oldest analog HP power was built in 1960 and and still works perfectly. My oldest HP scope is a HP 180 mainframe with a 1601 Logic analyzer plug in and I think it was built in 1968.  I replaced the 5V regulator in it about 15 years ago and it's still working.

[GregDunn]

Assume nothing.     If you can lift a lead and test the caps, by all means do so.  I have serviced my mid-60s HP6920B and HP331A; both had degraded caps inside.  The only external sign that there were problems in each case was some inconsistent voltages on the calibrator and difficulty in nulling the analyzer.  Additionally, this vintage of HP gear used a plastic sleeving over the caps which got sticky and nasty over time, so replacing the caps was desirable just from the standpoint of keeping the inside of the units from attracting grime.

It's your time and money, but leaving degraded caps in a unit can lead to erroneous measurements or possibly circuit damage.

[veedub565]

Thanks for the feedback. If it wasn't for the fact they are all caked in glue I might just swap them out and be done with it. As you say they can look good but still go bad.

There's no rifa caps in this one so that's good. No plastic sleeving on the caps either.

I've searched Google but that mains filter doesn't come up anywhere for sale. A few 2nd hand on Ebay, but then what's the point.

[Stray Electron]

  If by mains filter you mean the X2 or Y2 caps, most people get better grade replacements by salvaging other, newer equipment.  There have been several threads about that and the threads aren't specific to HP or to this model scope.  The value isn't critical as long as you use a part with the same or preferably a higher voltage rating.  High quality servers would probably be a good source if you have access to them.  I find piles of them in the scrap and most are only a few years old and they're made to run 24/7 with no failures.

[veedub565]

No by mains filter I meant what I said "Delta Electronics 04EB3S-1"it's like a module really with an integral button for the push rod on/off .

So pretty specific I would have thought. Googling it doesn't show any for sale, not new anyway. So unless the unit can be disassembled and refurbished.

I did pull a small sample of caps from the PSU and they tested fine, with low esr so I'm leaving them alone.

[tkamiya]

I think some people go overboard believing replacing cap is a cure all and fountain of youth.  There are all sorts of "recap kit" on Ebay, even.

If I see already blown caps, I'd replace them.
If I see leaking or bulging caps, I'd replace them.
If I suspect a particular fault is caused by a cap, I'd test and replace if necessary.

Otherwise, I'd leave them alone.

Few months ago, I bought an ESR meter.  Surprisingly, 20 to 30 year old cap in HP and TEK gears read just as good as new.  What worries me more is heat stress and cracking solder joints.  Out of dozens of test gear and ham radio gear I repaired, there were only handful that needed new cap.  Most were bad switches, bad solder joints, and cremated registers.

It's like doctors....  do no harm.  We could easily damage circuit boards and nearby components.
An ounce of prevention.... applies when there is a clear and avoidable danger.

That's my motto anyway.