Signal generator - Now, new to me E4432B - now fixed

[Joel_l]

I used 50 Ohm coax right on my probe points. I would think for relative measurements, good enough.

I also think I found the switches, it looks like they are using pin diodes to steer the signals on the board.

[Joel_l]

Looking at the block diagram and the PCB, I think the red circle on the on the block diagram is the red, blue, yellow circles on the PCB. I removed those components and just shorted pads.I was able to steer the <250MHz and >=250MHz. A part marked G41 feeds into final amp, the other two are marked G2V. I think they are all PIN diodes. At this end of the circuit, shorting the >=250MHz side ( red circle on PCB ) into the final amp did not change anything output wise, still get the unlevel error.

On the block diagram, I expect the blue circle just brings the signal level to what is was through the yellow circle.

What I'd like to check next is if the level coming out of the burst mode is the same as it is at the final amp. I also don't know what else might really be between the two switches.

I may make a better probe as suggested to minimize pulling down the signal. Drive capability might be different along the path.

Time to get more methodical. 

[Joel_l]

After many years you would think I would have learned that when it comes to RF you waste less time being methodical from the beginning rather than trying to save time doing things quick.

Anyway, I made a better probe and now things look more sane and consistent.

I have two shots at J4 showing that the 255MHz path is slightly lower than the 245MHz path. I picked off a few more points into the signal path and the results are consistent. As I go through more stages, the gap gets wider, but they seem reasonable.

I also learned some switching goes on in the synth board. For example, if my first measurment is at 245MHz and I then set 255MHz, I had to turn the RF on to get output from the synth at 255MHz.

So, I think the RF out board is mostly OK ( there is still the multi tone issue to address and not sure where that will be ).

Here are pics at J4 and "A" is I think the input to the burst modulator.



Now I'm off to the synth board.

Thanks for the help so far!

[Joel_l]

OK, I may have found something.

I was measuring around the divide by 2 circuit. Looking at the output of the synth, things looks pretty good. From 500MHz to 1GHz all within 2dBm ( lower as you get higher, seems reasonable ).

The output however is another story. Say I pick 300MHz, the output is about -20dBm with my probe. If I set 600MHz ( same synth output ), I see -40dBm. If I tune the SA back to 300MHz, I see -20dBm. Seems wrong.

So I took ( what I still think are PIN diodes )  out for both paths and shorted the non divided path. I got nothing at 300MHZ as expected, but still only -40dBm at 600MHz.

I noticed that the series resisor right before the PIN diode seems high in resistance ( red circle ). I measure 38KOhm. Everywhere else they are in the 10s of Ohms.

Does 38K seem good? the only other thing I can think of are some amps/buffers? marked A03.

Any thoughts?

I checked the level before the resistor and it is normal. I need to figure out what component an A03 is.

I figured ot the A03 to be a VAM-03. I replaced the resistor that measured 38K and replaced it with a 48Ohm ( closest I had ). the output of the divider is now good. I see about -16dBm at 600MHz. The final output is still not right.

Thanks

[Joel_l]

Well, the SG is now 99% functional. Outputs are all OK, it passes it's I/Q qual through the full range.

I measured all the outputs per the service manual an found some were off. To this point I was only looking at the top of the board, I finally pulled it from it's lower tray and found more caps in the signal path for the filters, touched all those up and the SG started working normally.

Can't say I really did anything purposeful to fix it, but learned a lot about how it works along the way.

@G0HZU - Your thoughts were pretty much right on, thanks for the insights!!

I still need to figure something out with the multi-tone, though now it's much better, still not 100% right, seems the CW freq is still to high, but at least somewhat functioning.

The only other thing I need to do is put that 38K resistor back in the divider circuit and see if things break again.

I put a 38K back it, it broke everything. So I will install a 17.8 Ohm which seems to be what I measure for other resistors in similar circuits on the board. I also noticed that the smaller the resistor, the smaller the unwanted signals when doing multi-tone.

I put a 17.8 Ohm in and things looks much better now. Calling the SG done. Back to being happy with what I got and in the end was not as bad to fix as it could have been. If I would have been more methodical and thought about what I was doing from the start, would have probably gone faster.

It seems there are at least two versions, the older HP that uses the SHF0189 ( H1 ) parts and a newer Agilent version that uses Agilent ICs in many spots. If an Agilent version has a bad Agilent IC, fixing might be an issue, though some here have been very creative in getting theirs working.