I have a 2445A (w/ DMM, acquired mainly for the rare front cover) that I am now evaluating, and it looks like there is a significant bandwidth problem in Ch 1 (otherwise the scope seems to be in excellent working order). I am testing the bandwidth using a Tek 067-0532-00 calibration fixture (constant amplitude signal generator) for the range from 65 MHz and up, and a TinySA for the range below 65 MHz. Channel 1 rolls off at about 7 MHz (yes, SEVEN, single digits, not a typo) and then has a gradual decline in sine wave amplitude, while channel 2 is essentially flat up to about 75 MHz than has a much steeper decline up to 125 MHz where it then levels out to much more gradual decline.
I have separately verified the output of the '0532' using my 2465, and the output appears to be constant amplitude up to at least 400 MHz (I was surprised to see that my 2465 does not start rolling off till 400 MHz, when the scope is only specced to 300 MHz: that's a lot more headroom than I've seen in either a 475 or a 2236). I did not bother to verify the output flatness of the TinySA, since I was not using it near the top of it's sine wave range.
I have checked the DC response of both Ch 1 and Ch 2 using a voltage reference, and both channels seem to have full amplitude with a DC signal.
I have not made systematic measurements of the attenuator settings, but from my ad hoc testing they all seem to be working as expected, which leads me to believe that the bandwidth limitation in Ch 1 must be after the attenuators.
My plan of attack is two-fold: first, I will remove the pre amp hybrid from channel 1 (U100), clean the contacts with IPA, and reinstall U100 to see if that cures the problem. If that does not cure the problem, then I will swap U100 and U200 to see if the problem follows the hybrid. If the problem remains on Ch 1 even after swapping the hybrids, then I will need to come up with new plans.
My question is about working on the hybrids. I understand that they are delicate and easily damaged. The service manual does not have much to say on this. Specifically I have not found any information on how to safely re-mount the hybrids (how to tighten the screws in a way that won't damage the substrate/destroy the hybrid). If anybody has any advice in how to properly handle the hybrids when reinstalling them, I would really appreciate hearing it.
-- Jeff Dutky
