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Agilent 54835A scope (4 channel 1GHz / 4Gs/s) repair & uphack
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nctnico:
To me it seems these resistors are some kind of termination network for signals so it doesn't surprise me that nothing changed.

I think it is time to take hires pictures of boards from the 845 & 846 and subtract them in photo editing software to highlight the differences.
Converter:
I investigated the bandwidth of my HP54845 in the factory settings. It seems that it has more than 2 GHz without any upgrade. As the source of the broadband signal, I used an industrial noise generator on avalanche-passing diodes (0.4-4 GHz). This is a very primitive device and there is a noticeable frequency unevenness in the spectrum of the generated signal. Indeed, in its generated spectrum there is a rise in the region of 100-300 MHz and a dip at 1.8 GHz. Nevertheless, this allows us to approximately estimate the bandwidth of the oscilloscope.
nctnico:
The measurements I did showed my uphacked 54845 is just right by looking at the risetime. Unfortunately these scopes won't trigger on signals over 1GHz so maybe your noise measurement shows a more complete picture.
Converter:
I added screenshots.
Unfortunately at the moment I do not have a high-quality microwave generator to make measurements more accurately (yes, I know about the limitations of the trigger). But a rough estimate can be made using this noise generator. There were used 2 scales: 10 dB/div and 5 dB/div. I have roughly noted positions with cursors corresponding to a -3 dB change in the level. This corresponds to a bandwidth of 2.127 GHz.
Converter:
This oscilloscope HP 54845 is not inferior to the bandwidth of my LeCroy WavePro7200.
For comparison, I attach several measurements made in the same way with other of my oscilloscopes (FFT 10dB/div):
LeCroy WavePro7200;
LeCroy WavePro7300A;
LeCroy DDA-120;
LeCroy LC564DL;
Tektronix TDS5054B.

I also want to note that the HP 54845 demonstrated a restart speed of the trigger, which reaches 92 us (in the case of the easiest acquisition settings). This corresponds to processing 11K oscillograms per second, which is 30-50 times faster than all oscilloscopes from my list above (3-5 ms minimum). Faster than this, only the FastACQ-mode in Tek 5054 (7 us) - but this mode is not fully functional, and my analog oscilloscope LA354 (which reaches 1 us).
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