HP 8648A - Error 616 - AM Modulation Path Failure.

[BradF]

Hi all. I decided to look at the generator and this post again. I don't believe I got a reply. I thought it would help if I repost this request. ... My HP 8648A during startup shows error 616 - "AM Modulation path failure". Per the service manual due to supposedly bad A3. The motherboard! All voltages from power supply are good per the schematic. I reseated everything. Cleaned inside. Not much in the service manual for A3 repair. Was working fine for years. Went to use it after being in storage and this error came. RF out is ok but will not modulate with internal 400 or 1KHz or external ac or dc. Has anyone solved this? Does anyone have an A3 for sale? Thanks.