| Electronics > Repair |
| E4433B RF sig gen repair |
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| microbug:
There aren't any options installed except the high-accuracy timebase (OCXO). I took some more measurements today. I started with the coherent carrier, and noticed the 2.7GHz signal there as well as the correct signal. This made me suspicious, and I tried the same measurement with the vector output board shield properly installed. The 2.7GHz signal disappeared. If it's coupling into this part of the circuit, it could be coupling into other parts too. I measured before the I/Q modulator, immediately after, and after the next IC (U80, a 1GG3-4205 RF amp). Screenshots attached. The following stands out to me: - Across the I/Q modulator, the signal at the set output frequency loses 30-35dB! - The gain block U80 appears to be effective only at the set output frequency. The 2.7GHz signal is relatively constant on either side of the gain block. These measurements were all taken with a piece of coax connected straight to an RF trace, so they won't be very accurate in absolute levels. The very large I/Q modulator losses do seem like a smoking gun suggesting this part needs replacing. Unfortunately I've damaged a DC block cap again, so need to leave it here for today until I can replace it. I will try bypassing the modulator IC with some coax and a 10dB pad when I've replaced the cap. |
| microbug:
I tried bypassing the mixer chip with a 10dB pad. Instrument output looks exactly the same -- 2.7GHz signal. I also tried running it with the shield on the output board (mixer chip not bypassed), and it shifted to 1.1GHz (but still no sign of the signal it should be putting out). I think I might have had a faulty connection in the measurements labelled 2 in the last post. That would suggest that there might be a dead component somewhere later in the RF signal path. If the desired signal isn't getting through, and there's a lot of 2.7GHz around, then I suppose the input to the next stage might just be coupled 2.7GHz, leading to that appearing at the final RF output. I'll do some more probing around this weekend. I'm starting to get a bit fed up with this. The number of parts to check is quite small though -- since the problem is there for the whole instrument output range, it must be a part of the RF chain that is shared. That leaves just a few amplifiers and some PIN diodes to check. If I don't make some progress soon I will consider selling the unit for someone else to have a look at. |
| vaualbus:
Interesting thought, next I could bypass the switched filters bank? Like remove C140 and inject a signal there? But I guess having excluded the IQ is in some regard a good idea. But yeah, this is getting a very weird failure. I would guess that you have already checked all the power supplies (I mean also the secondary one in page 5/5 to the output scheamatics) |
| vaualbus:
Guess by the lack of response that you give up at the end on reparing this? 8) |
| microbug:
Not quite. I am low on motivation but I’ll try a bit more, it’s not satisfying to leave it unfinished. I left my job yesterday (starting a new one in a month’s time) and so don’t have access to their spectrum analyser any more. I have ordered a TinySA Ultra but it’s not here yet, not much I can do until then. |
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