HP 8648B ERR 616 AM Modulation path failure

[petercut62]

Hello,
I've got my RF Signal generator HP8646B in trouble.
It display at power on : 616 AM Modulation path failure.

According to the troubleshooting section of the service manual : Either the main board A3 or the detector has failed.

As it's still doing AM fine, i think something around AM detector has failed or partialy failed. May be the AM diagnostic side is faulty...

I've teardown the unit. Fortunately for me the power supply is in bad shape, full of dirt and some resistors are almost fried. So i will repair the power supply first.

For AM side, as there is no schematic of the A3 mother board on the web, i don't know where i must have a look first :
- Cabling
- Follow the AM signal path trying to find a kind of detector

Any advices are welcome.

Thanks

[FloridaJo]

In my 8647 manual it says:
"The AM path detector indicates a failure at the output of the A3 board.
Either the signal output has failed, or the detector has failed.
Replace the A3 board."

[mmagin]

Sadly they don't provide board-level information on that series.  I'd first check supply voltages (including for excess ripple) and re-seat (cleaning if necessary) all the relevant connectors.

[SoundTech-LG]

full of dirt...  yes, they are! Dirt is a main culprit in these, particularly if they are ON 24/7.

No Filter fan draws air in from the front panel holes around the soft keys  , and dirt / contamination destroys the key membrane printed circuit switches too. I have repaired 2 of these now. Happened to find the membrane switch assy on ebay. It is possible to repair, but far easier to replace!

[Zinger Liu]

I got the same issue. I measured the A3/J11-1 and A3/J11-2  for the frequency. it was 1 MHz instead of 200 kHz.
I'm not sure is that correct?