Hi everyone,
After a long time on the bench, finally my 2712 is alive and work 100%.
Using the suggestions that I received here, I start the service.
First I have made a visual inspection and look for some cable in wrong connector misplaced or lose but everything was ok.
Then I have checked all voltages and ripple on the power supply and all are OK.
The next step was to change the two batteries, an I would like to tell an interesting thing that happened when changing the battery that is on the GPIB board that powers the RTC.
I have the habit of always measuring the current drain of the batteries when I replace for a fresh one.
For my surprise, the meter shows 4mA.!! I thought it could be some capacitor leaking on the battery line or even some bad ICs.
Based on some previous experiences, I put the card back in the equipment and turned it on. As expected, the current drained from the battery was zero.
So I turned off the equipment and continued to look the current meter. It shows near zero for several hours.
This led me to conclude that when some ICs stay without power for a long time, when they are powered for the first time, they need to be initialized in some form to work properly and not drain this high current.
I removed the current meter and everything is OK with the RTC and battery for a few days.
Well, the 2712 still not work¡ So next I kept checking the 100MHz oscillator, as suggested by Paul / WB6GHK.
BINGO!!
There was no 100MHz. signal on any of the three output connectors and also on the output intended for the calibrator (with it ON of course).
So I removed the 100MHz PCB from the 2712 and removed the shields, which was a very boring job to do.
in the smaller shielded, I observed the existence of a kind of sponge in the position where the crystal is soldered, and that sponge was deteriorated, looking like a bubble gum and with the feeling of being damp.
As a result of this sponge being in contact with the crystal terminals, they oxidized a lot, and broke close to the crystal. Luckily I had a 100MHz crystal to put it there. It does not have the same type of packaging, but it fit perfectly inside the thermal chamber. So with everything still open I gave power on to see if the oscillator was working, and it was !! I checked all the outputs and found the correct levels according to the service manual.
As I can see, this oscillator has no adjustment, and I limited myself to measuring the frequency that was very close to the ideal.
After 15 minutes os warmup the oscillator was on 99,999.988 what I thought was reasonable and I set out to reassemble the shields and install the card in the 2712 to see the result.
After everything was in place, I crossed my fingers and gave power on the 2712 ¡
Fantastic! It is working!!
I followed the menu to proceed with the normalization, and everything was OK.
So to finish, I proceeded with the normalization of the TG which was also ok.
Now I have this fantastic instrument on my bench.
I would like to thank the friends who gave me advice to make the repair, and say that I am very happy to belong to this group. I hope that someday I can help someone too.
Happy New Year for everyone!
Marcus - PP5MS
