Manual/schematic wanted for HP oscilloscope 1740a

Sounds like bad electrolytics to me... I'd replace 'em all (except the tantalums), they're probably all the same age and all going bad at the same time.

I've done that with other vintage HP gear and had great results.

With electros, as a general rule, getting an exact match in value is not important to the cal of the instrument. E.g., if you see a 120uF and can't find that value, replace it with a 150uF or even 220uF if you have to and you'll be fine. That's been my experience anyhow.

Go to

and search on 1740A. The operating and service manual is online as a .pdf file.

on

manual

thanks very much found what I needed, manual with schematics. I now remember looking there last time I had to repair it but only found a operators manual. must be a recent addition. it not on the boat anchor site either.

I love looking at circuits for some reason, shame acrobat is so slow looking at those scanned pages, has it got slower this last release? certainly a lot bigger.

Colin =^.^=

usefull,

same

can't

Thanks, I looked again at the low voltage supplies this time a bit more carefully and there was a barely visible intermitent mains hum drop out type ripple, I looked at the resorvoir cap and the ripple didnt look too bad when I compared it to the others, but then it dawned on me the ripple was at 50hz not 100hz, ie half wave, but had a full wave rectifier ! strange thing is the diodes cheked out ok on a dvm but after finding the schematics and checking for anything else replacement was the cure.

Good advice on the electrolytics, although when I put a quieter DC fan on my HP freq counter, and thought id check the supplies, they looked very noisy, I managed to find some replacement electrolytics in a big box of them I havent used for ages, The supplies were now nice and quiet, however after a few minutes the fuse blew, the 'new' electrolytic had gone short circuit !

I found several more caps in that box that were complete 0ohms short circuit. I dont trust any of them now so il just use as I need, however all the shorted types were of one make/type.

Gues il buy some new electolytes to replace them with, but maybe not today, maybe not tomorow but someday soon ...

Some of the caps I beleive are tantalums in steel tube type cases wich look like electrolytics, but have a hard ceramic type seal at the end as oposed to the rubbery type on electrolytics.

Ive had the same advice several times before but its so much more rewarding to find the actual culprit, I did however do that for a VCR only to find the problem was a tantalum after all, and to make it more confusing it was just a bit too high in value, it was a timing cap in the drum servo/sync detector ciruit.

Colin =^.^=

I've never seen the old steel/glass tantalums fail. Doesn't mean it can't happen, but it's not where I'd look first.

Tantalum caps do tend to be specified for better precision than regular (Al) electrolytics, though personally I still wouldn't trust one in a precision timing circuit. Aluminum elcaps are often specified at -20%/+80%; tantalums often +/-20%, although it's possible to buy +/-5%. So, for instance, if I see a 10uF that measures 15uF, I don't even blink.

The main concern is effective series resistance (ESR). As a cap ages its ESR climbs. It can still measure nominal capacitance with a capacitance meter, but under real circuit conditions its performance suffers; it's no longer an effective AC bypass.

You can buy inexpensive ESR meters, that can even be used in-circuit in many cases, that will help identify this problem.