I am getting ready to tear into a HP8657a RF generator. The RF output of the generator gradually decreases with frequency below 500Mhz. By the time you arrive at 455Khz it is down 60db. I am wondering if anyone has seen this behavior before and might have some tips?
That is exactly the conclusion I came to. HA!
I have the same model, but luckily it has never had an issue I am aware of. Dead amplifier or attenuator section issue? I have not looked through any information on it as I have not needed. I am sure the time will come some day.
Radios + Tubes + Scopes + Cars= Nothing better!
I’m not sure about your unit since you say is an “A” model but I know for a fact the following: on the HP8657 “B” model, a vast number of units seem to suffer the very same issue and it’s been concluded on several forums that the issue is almost always the Frequency Doubler module and that the module, unfortunately, cannot be fixed because HP did not provide any schematic for it and/or any alignment procedure for it (should there one be required). Some people have reported that if you bypass that (but just to bypass the coax cable) you can still make it work but your maximum range with be lower than before. One note is that you cannot physically take the module away from your unit. It still has to remain inside since the CPU would throw an ERROR (at the start-up) saying the Doubler module is Not present or something of that nature.
@ovi4 So I finally got around to troubleshooting this device. The problem was the reverse power protection module. The last part in the signal chain, The relay inside that module was not engaging. The reason that the high frequency looked good was because it was jumping the gap. I have revised the circuit to exclude it and added a 10db attenuator to the signal chain because the reverse power protector had one in it. The generator is very accurate and working fine. But now it has no protection against reverse voltage exceeding 10V P-P.