Hello, some time ago tekfan posted the photos of his solartron 7075.
Recently I won this instrument on an ebay auction (edit: not tekfan's one, it's another one
), sold as broken, tekfan was very kind to send
me a copy of the maintenance manual for the instrument, if somebody wants this manual it's now
hosted on ko4bb site:
http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Solartron/Solartron_7075_Service_Manual.zip
I received the instrument in a fair condition, the only evident abuse was a bump on the rear.
After powered on the first thing evident was that the display had problems, the positive thing
was that the instrument responded to test sequence, so the logic was at least partially ok.
The display is built with 4 panaplex modules, 3 model sp-353 (3 digits) and 1 sp-354 (2 and half).
One of these modules was evidently broken, fortunately the displays are mounted on sockets.
Now I would be curious to know how would be possible to break that display when mounted
without crushing the case
These displays light up when excited with a high voltage, around 160V are needed.
I searched in the junk box a transformer with a high voltage secondary and fitted a
rectifier and a 1MOhm series resistor, to obtain about 200V dc limited to 200uA.
With this I tested all the segments and are all ok, only the broken display didn't
light up at all. Lukily I had some old pcbs with the same displays fitted, so I replaced
the broken one with another old display.
The -18V was also absent, the PSU is interesting, it's a switching psu with a working
frequency of around 25kHz. The -18V are obtained from +5V with a voltage inverter
and capacitor multiplier, but the capacitor wasn't there, the terminals seem to be
broken: this instrument must have been subject to some rough handling
I refitted the capacitor and rechecked the display, now all the digits light up but
there was a problem with the cathode drivers: in the first image it's clear.
There are two cathode drivers that multipex trough the display, lighting two digits
at a time.
Apart the missing block, each two digits are lighted by the two cathode drivers, one of
them is faulty and can be seen by the fact that one the digits per couple is only partially lighted,
fortunately the board I used to extract the spare displays also had some cathode drivers,
and the luck is that these Beckman DD-700 are perfect replacement for the broken DS8880.
I noticed a pair of strange things, I don't know if these are from factory or modifications
from someone:
the first thing is that the rear "External Reference High" terminal is disconnected from
his wire and connected to the analog pcb to "Ohm guard" pin.
the second is that some of the single pin connectors are soldered, many others are
not soldered, it's a pain to disassemble the soldered pins without damaging the board
So now the display is fixed and I decided to disassemble all the thing,
after all this is the fun
I washed all the case components, but I didn't wash the boards, I have fear
to have problems, maybe distilled water can be used?
The question is:
will I be able to put all together?
More to come.
Fabio.
edit: trying to show the images inline...