Manual/schematic wanted for HP oscilloscope 1740a

[colin]

Hi,
I have a HP 1740a oscilloscope wich although old still still very usefull,
only needed a few repairs up until now,
(mains switch, longest range timing cap, vert amp transistor)
however its a bit tempermental of late,
on occasion the traces jitter up and down a lot
other times theyr rock steady
however now the traces also sometimes have a 100hz mains hum both visible on
both vertical and Z
this seems to come and go and wiggling the range switches seems to affect
it,
sometimes a thump helps, but now its anoying.

Im not to sure what could be cuasing this thats close to the front panel,
ive checked the lowe voltage power supplies and the ones that are marked
seem spot on and steady,
my gues its the high voltage electrodes that would affect z and height,
but could be seperate issues, but maybe of the same supply.

I dont have a manual so its a bit hard to know where to poke around,
is there any online on the internet ?
ive found quite a few other manuals that way.

Im tempted to look for a new one but hard to chose,
would be nice to have it working again anyway.

Colin =^.^=

 

[Walter Harley]

Hi,
I have a HP 1740a oscilloscope wich although old still still very usefull,
only needed a few repairs up until now,
(mains switch, longest range timing cap, vert amp transistor)
however its a bit tempermental of late,
on occasion the traces jitter up and down a lot
other times theyr rock steady
however now the traces also sometimes have a 100hz mains hum [...]

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.

 

[David & Cheryl Denslow]

Go to www.agilent.com and search on 1740A. The operating and service manual
is online as a .pdf file.

 

[colin]

Go to www.agilent.com and search on 1740A. The operating and service manual
is online as a .pdf file.

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 =^.^=

 

[colin]

Hi,
I have a HP 1740a oscilloscope wich although old still still very usefull,
only needed a few repairs up until now,
(mains switch, longest range timing cap, vert amp transistor)
however its a bit tempermental of late,
on occasion the traces jitter up and down a lot
other times theyr rock steady
however now the traces also sometimes have a 100hz mains hum [...]

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.

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 =^.^=

 

[Walter Harley]

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.

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.

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.

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.