HP 8648B AM suddenly very noisy

[route20]

Picked up a an 8648B about a month ago and it's been excellent, until today when out of nowhere the AM output became a mess:




It occurs with internal or external modulation (mine does not have the programmable internal modulation source and is limited to 400Hz and 1kHz). The above captures are using internal 1kHz, below is 400Hz:



All captures so far have been at 100% AM depth, it decreases as depth is lowered. Here is 1kHz at 80%:



No errors on power on self test or returned via GPIB  SYST:ERR?. Happens across entire frequency range.

Hoping someone has seen something like this before on one of these units as the service manual is pretty useless (all adjustments done through ATE software)

[gnif]

@route20, aka @dos... care to explain why you created a puppet account to post this?

See rule #8
https://www.eevblog.com/forum/chat/forum-rules-please-read/

[route20]

@route20, aka @dos... care to explain why you created a puppet account to post this?

See rule #8
https://www.eevblog.com/forum/chat/forum-rules-please-read/

Just go by something else now (did not see a function to change username) and didn't realize it was a problem as on other forums it's only an issue if it's for some malicious reason like ban evasion or spam. You're right, I should have checked the rules first. Can my old account be deleted so I only have this one? There's nothing worthwhile there.

[gnif]

You can request your username to be changed, however creating a new account to operate under is not allowed.

Edit: Account banned, but user is welcome to continue here under their old account.

[dos]

So I found one of these at work (probably hasn't been used in decades) and hooked it up to an E444 series SA and observed exactly the same behavior, starting to think this may be just how they are and I didn't really notice it initially, or I had a lower modulation depth set or something. Either that or they both have the same fault. All I know is the newer RF generators at work don't do this at 100% and neither does my 33120A function gen, but I have heard that noise was a problem with these 8648s. I think this is case closed unless another owner of an 8648 chimes in with a clean capture on their SA or scope's FFT.

Apologies again for the duplicate account, forum staff