Hi My precious hp8753c has suddenly failed. It comes up with message 'no IF found, check R input level'. It is connected correctly, rf out thru a 3dB split into the R input. Checking power levels on the rf out port shows there is nothing coming out.
Have put together an extension lead for the S3 source unit so it can be run outside the mainframe. The 3.8GHz cavity oscillator power out seems to be ok, ~10dBm. There is however some frequency instability here, approx 5MHz jumps going on, with about 60mV of instability on its supply rail. This in the form of ~1Hz step changes. The supply to this measures about 28.3v across the filtercons fitted to the oscillator barrel.
Measuring the YIG shows no power out. Have unscrewed the YIG from the S3 unit and measured the supply rails on the small pcb attached to the base of the YIG. This shows that the +/-15 and 5v supplies are ok, but the rail marked as -40v is now at +0.9v.
Could it be, perhaps optimistically, that the supplies back inside the S3 unit have failed in some way or rather depressingly, the YIG has failed and is corrupting the supplies. I suppose the YIG could be checked by running it outside the unit on external supplies, though not sure about driving the coil, etc.
Any advice would be greatly appreciated, especially if documentation for the 2x pcb's inside the s3 unit could be had.
The Cavity Oscillator has a third pin, a tune-line pin, that causes it to jump 5 MHz. In normal sweeping operation it jumps back and forth to avoid source spurs showing up in the receiver. Put the source in CW and see if it still jumps, should not.
The -40V line I think is the tune line, so it will move around a lot as the YIG is tuned. If the pll is not working it will rail at 1 diode drop above ground. Find the source tune service test and set the pretune to some fixed frequency (I don't remember the exact test) and then you can measure the Yig output; or just supply it with about -15 V or so on the tune line and it should be in the 3.8-6-8 GHz range.
If the YIG is railed, it's signal will sometimes drop. If you get signal out of the YIG with then the problem is either in the mixer or in the amplifier. To these tests first, post back results.
Apologies Dr Joel for taking so long to get back with this. I have tried the source in CW mode, but the cavity oscillator still skips about by the 5MHz as before. Anyway, as suggested have carried out the source tune sevice test. As previous tests indicated no power out from the YIG, I was suprised when it seemed to be working, with power levels of ~10dBm across the frequency range 3.8 - 6.8GHz, under control from the rotary on the front panel. The line labelled as -40v on the YIG module PCB was actually changing over the range +3.5 to +6v for this change in frequency. That was good, as it seems to indicate that the problem lies elsewhere than the YIG itself. However this situation did not last for long, the Yig after a short while stopped working, the -40v line had gone back to +0.9v fault condition.
Have checked further back in the open loop drive chain, towards the DAC on the A11 'phase lock board'. There are a number of test points on this board, easily accessible from the edge of the card. Checking the '1v/GHz' point (pin 6 of the test point header) showed close agrreement with that measured on the YIG module, +0.9v. After some 'jiggling' about with the A11 card, it all started working again (in source tune mode), pin 6 then shows controlled voltages in the +3.5-6v range. This is the situation I'm in at the moment, an intermittent problem with what looks like the A11 card. I can't corelate anything that I might do to the card with when it works. I suppose I shall have to try and put together an extension lead so I can get it out for a better look. Have tried tracing the circuit associated with what I think is the DAC and amplifier circuit, its quite difficult! Do you have any documentation on this A11 card that I might be able to use.