Sick HP 8657A

[Zenwizard]

I have an HP 8657A on the bench that is mostly working. All the RF functions are working as well as the internal modulation functions. It is the external mod functions that I need and seem to be out to lunch. Voltages are healthy when I hook up a function generator the Ext Low light will not extinguish and modulation start. No matter what kind of signal I put in the front end FM or AM. I have confirmed that I have modulation making it to TP8 on the A10 but this is where troubleshooting has ground to a hault. I have not been able to locate a schematic to trace things further. If anyone has a schematic or an idea of a common part that should be looked at I am looking to save this RF source. If not I might be on the look out for the B version.

Zen

[Swake]

Have downloaded the service manual for this device from Keysight. It contains the schematic for the A10 board. But title for that board is audio/power supply board. This does not match your description about RF modulation. Something is wrong, could you double check please.

[Zenwizard]

Turns out this is an indication issue. The RF is being modulated just fine but the circuit that says there is enough signal for the modulation is contently saying signal low. It looks like i might need a front panel for this one. There is really only a block diagram of how the logic is laied out and it is serial date going up to the front display board.
So the good news is the RF circuits are all good but I have a logic line that is stuck.

Zen

[Tony_G]

I took a quick look at the manual on the Keysight website and I think Service Sheets 17 & 19 might be the schematics you're looking for.

I highly recommend going over to ArtekManuals and purchasing their high-quality scan of the manual - It will make your life way better.

TonyG

[CounterEMF]

Zenwizard
I have two of these units and was confronted with the same issue when I first obtained mine The odds are high that there's nothing wrong with your generator. On page 3-56 of the Operating Manual you'll find the proper input level to be 707 mVrms +/- 20 mV. Deviating beyond those will set the either the "LO EXT" or "HI EXT" annunciator lights. Careful level setting should solve your problem.