HP 8648B repair

[TheDefpom]

Hi All,
I am in the process of repairing a HP 8648B RF Signal Generator, it is giving error 608 in boot up and after much confusion with differences in the service manual and the actual unit I have traced the fault to the A5 "Sig Gen Synth" board, specifically to the VCO which is not oscillating.

I am recording video about the repair for my YT channel, but I cannot see an obvious reason as to why the VCO isn't working (at least not yet, I am still digging into it).

I still have some voltage checks to do within the VCO block itself to be sure the rails are actually getting there, but I have confirmed the VCO control voltage is (although not present) not the cause, as there is a jumper (JP2) which will inject 9V into the circuit, and that still doesn't start it working.

If you have any experience with this kind of oscillator circuit (or repairing the 08648-60115 or 08920-60373 board) please take a look, I could use some pointers, right now I am suspecting the actual oscillator transistor Q13, I found and have ordered some NOS ones.

There are no circuit diagrams for the HP 8648B that I can find, but in a live stream one of my subscribers pointed me at the HP 8920 which uses the same (or at least very similar) boards, so that has been a big help.

Note attached is the block diagram from the HP 8648 service manual, and the circuit diagram of the VCO section from the HP 8920 which is a similar (but earlier) version of the board, component values are different, as is some of the architecture, but it is VERY similar.

[TheSteve]

Have a look at U20(if you have an equivalent in your circuit) for sure, when those amps fail all kinds of interesting things happen.

[TheDefpom]

I have tried probing from C63 before U20, there was no oscillation there, but of course that doesn't mean that U20 isn't dragging it down.

[richardb]

Hi TheDefPom,
I am repairing an 8468. Can you please link to where I can download a copy of the 8920 manual you mentioned - the one I found on the internet is very blurry.
Thank you
Richardb

[TheDefpom]

I will try to find it.

I did put it along with some datasheets etc on my Patreon page for my supporters at the time.

[TheDefpom]

Ok found it: https://www.keysight.com/upload/cmc_upload/All/8920_Component_Level_Information_Packet.pdf