HP 8657B attenuator issues?...

[Rax]

Hi all,
I got this HP 8657B for a bit over a song, and although it has recent calibration stickers, there indeed are some issues with it.

Namely, the levels out are missing (no RF signal out) every 5dB of output level setting. Differently put, working my way down from a high level (say, +10dBm), I lose signal out when crossing from 2dBm to 1dBm, getting it back as I cross from -3 to -4, losing it again from -8 to -9, back again at -13 to -14, and so on. Given the attenuator is the one going in steps of 5dB, it's become my primary suspect. This is in module A10.

Getting to it means removing A11 (audio board), which I still have to figure out how to do, but before I get into surgery mode, I was curious if anyone else has sun into this situation with this unit. I'm tempted to suspect A10U1 (an SN74LS273N), though I obviously have to check the (switching) logic levels on its legs.

Thank you for thinking on this with me.

[bdunham7]

I've not yet had the pleasure of dissecting that exact unit, but you're on the right track for sure.  The 8656 types have a mechanical attenuator that physically fails in its old age and needs grommets--those I've fixed.  I don't know how the attenuator assembly on that unit is arranged, but if you get an audible click at the transition and the signal disappears entirely (or mostly) then it may be a mechanical problem and perhaps you can open up the A10 assembly and fix it.  eBay seller part2go in Israel (who I've used and seems OK) has those attenuator assemblies, just in case.