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HP 8566B Debug/Rebuild
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MarshallPlexi:
Hello All!

Been a fan of this forum for a while but am just now getting in spectrum analyzer's so I have a mountain of information to process. I picked up what I thought was a fairly decent 8566B from a local shop and found after getting into it that the input attenuator was fried. Luckily I bought a spares 8566B which I thought would help only to find that it's input attenuator was fried too.

I'm picking up a new attenuator tomorrow, and until then, I have nothing I can do to help, so I thought I'd do one of the common mods that have been recommended for the A20 board, and that is to replace C7, a 10uF/25V cap. I didn't have the exact cap so I replaced it with a 220uF/50V cap thinking that more voltage is better and more capacitance should help to stabilize the supply more.

I also installed a 10dB attenuator between the CAL signal and the input of the LIMITER. To my shock and amazement, I can now run RECALL 8 and I see a 100MHz signal there. Granted the front end attenuator is out of circuit, but this is more than I've seen in the past. The shape of the signal is odd. See attached pic.

Here is what I'm getting.

When I run RECALL 8, I get the image of the kinked 100MHz signal, and when I open up my scan to around 3MHz, I get the correct-looking signal.

Do you have any thoughts on what could be causing this?

The final image is what I'm seeing when I run the RECALL 9 routine. I'm not able to adjust the line into view.

I was also able to get MUCH further into the SHIFT+W calibration routine this time so I feel like I'm making progress!

I'm looking forward to learning more about this incredibly built piece of equipment!

Thanks for any council you can provide!




squadchannel:
Definitely, the C7 that was replaced will have an impact. Why did you even change the capacity?
C7 is the capacitor that hangs in the tuning circuit of the RF section YTO coil; installing a capacitor 20 times larger would naturally have an effect.



I am not familiar with RF, so I don't know. :-X
MarshallPlexi:
Holy crap. You’re right. I’ll change it back tomorrow morning and report back!
MarshallPlexi:
I couldn't sleep thinking about it so I had to get up and swap out C7 for a proper 10uF cap. It's sorta better.

IMG_5869 shows the scope when RECALL 8 is pressed. The kink is clearly gone but no longer visible because it's out of frame.

IMG_5870 shows the scope when the RES BW is set to 300kHz

IMG_5871 and IMG_5872 shows the scope when the center freq is selected between 99.94 and 99.96MHz. It's a jump across and not a smooth transition.

It seems like progress, though.
MarshallPlexi:
New attenuators in use and unless I totally misunderstand how the 8566B is running things, that's not a constant step in attenuation.

Thoughts?

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