The 4700 is fixed. The 400V issue was caused by missing -15V, That was
caused by a shorted tantalum on the Ohms board. It was less then 0,1 ohm
but not even discolored.
I was curious about the way they die. So a desoldered about 20 tantalums
and measured C, D and leakage current. New ones have a very low D, at 1
kHz well under 0,1. Capacitance did not decrease much like electrolytics
do, but D does increase over age. Leakage current is as good as zero,
(my best leakage tester has only 1 uA resolution so it can not measure
leakage in the nA's)
The worst D i found was 0.67.
They did not leak, so I think they go from good to short in one step.
capacitance does not seem to decrease. In electrolytics this is caused
by a decreasing oxidelayer caused by the decreasing electrolyte.
I replaced a few by electrolytics as a test. The D factor of the 10uF
low ESR panasonic FM caps was better as that of the new 10uF tantalums I
had. Most tantalums measured OK, a few had very high D, non of them
leaked DC.
So I think (based on this to small number test) they get a high D, start
to dissipate and as a result of that develop a short.
Fred
On 07/21/2017 01:59 AM, Todd Micallef wrote:
Fred,
I used the same values and typically went up to the next higher voltage. I
think most ended up as 35v since I bought them in quantity. Some people
recommend the OS-CON family as replacements. Many of the failed tantalums
fit in a tight space so I just stayed with the original type.
Once the in-guard was repaired, I found a few other boards where the
tantalums on the same rail also failed so don't be surprised if more start
to fail.
The 4708 service manual is here
https://xdevs.com/doc/Datron/4708/DATRON-4708-CH-SH_c20030307.pdf
It is now uploaded to KO4BB. The 4808 service manual will be very close
since the 4708 and 4808 share similar designs.
The 4708 uses a LTZ1000 as the reference instead of the array of zeners
used in the 4000 and the lower spec'd 4700. You will have yourself a good
calibrator once it is brought back into service. Be careful around the
displays as there is hardly any room between the glass and front panel.
Respectfully,
Todd
On Thu, Jul 20, 2017 at 12:55 PM, Fred <pa4...@gmail.com> wrote:
Hi Tod,
I just pulled that board (i think) and found a burned 22uF tantalum.
Tantalum caps should have a very long life unless used on the wrong place.
I guess this is an example. I do not even stock them, the last bad one I
had to replace was in a G&M secutest 2 years ago. ( I repair around 10-20
Secutests a year and that was the only bad tantalum)
With what type/value cap did you replace them ?
I found the 4708 manual at KO4BB but that is a user manual without
schematics.
The calibrator should be OK but before I try to start her I always like
check things like caps, connectors, solderjoints etc. It really looks like
new, not even a scratch or dust. It came with a new looking HP8640 and a
new looking HP counter 5248. Both from the same cal lab (part of a company
that stayed there) that cal-lab closed about 30 years ago, but all gear
stayed where it was (I already had received most of their gear a few years
ago). This were the last things because my friend retires.
Fred
.
On 07/20/2017 04:51 PM, Todd Micallef wrote:
Fred,
Shorted tantalums most likely. I had several on mine that went bad as
quickly as I replaced them. I would start with the inguard supply board.
I think the 4708 manual is available for download at KO4BB or xDevs. If
not, I can dig up something. I have a set of the manuals.
Todd
On Thu, Jul 20, 2017 at 10:01 Fred <pa4...@gmail.com> wrote:
Today a friend gave me a Datron 4700. It looks like new. It is stored
(working) for many years in the cal lab he works. He was allowed to
take it home (for me, in return I will repair his SA), He plugged it in,
Fail 1 on the display followed by a sound and a smell.
I opened it but nothing looks burned.
I can not find a service manual, does someone has one ?
On the back the label shows no options but if he remembers it well it
should have all or at least most options. All slots are filled with
pcb's. The "box" in the left upper corner is full and there is a very
small pcb next to it. There is not much room left. Just enough to store
a small multimeter between the huge transformer and that box.
Fred
www.pa4tim.nl
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