Re: 2445A calibration


On Mon, 15 Oct 2018 at 15:41 <maxim.vlasov@...> wrote:

Looks like slowly but surely I make some progress. Just found that U2420D
buffer is sinking 11mA. I measure at the U2420D output -8.8V but TP2421 is
-1.25V. 11mA flowing through R2220 (680 Ohm). I've looked at the schematics
and 11mA doesn't seem to be the current to be pulled out of TP2421. There
are quite a bit of loads, but all of them in a range of dozens of KOhms.
Something is pulling up -1.25V rail.
I don't know whether that's unreasonable, but -8.8V out of the op-amp is
well within its control range. A TL074 will go to +12/-12V minimum on a
+15/-15V supply so I wouldn't focus there too much. If you look at the
power distribution diagram <11>(?), you'll see U700/U850/U900 get -1.25V
bias, so 11mA is probably reasonable.

Have you observed this on your scopes when repairing?
Sadly I haven't measured this - if you're stuck, I can pop the case on my
2467 this weekend and check it. Offhand, I don't think 11mA is unreasonable

One more thing - I've got -04 FW version. It seems like the last version
was -09 for 2467 o-scope. I wonder whether I can run the latest FW. If so,
then in the guts of the o-scope there should be a HW identification
register somewhere to tell to the FW whether it's 2445/55/65 or 67, IMHO.
Otherwise the code is almost the same.
I updated my firmware from -06 to -09 without incident. Didn't need
re-calibration that I noticed.

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