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| Marconi 2022E RF signal generator repair and refurb |
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| bd139:
--- Quote from: G0HZU on July 30, 2022, 07:14:44 pm ---Note that the reverse isolation of a typical MMIC won't be quite as good as the original OM345. This might not matter in some locations in the 2022 sig gen but it probably does matter for the IC1 location at least. The OM345 reverse isolation is typically 22dB at about 100MHz and the MAR7 manages just 18dB here. The 22dB reverse isolation of the OM345 may already not be enough to prevent some FM modulation occurring on the AM for frequencies up to about 62.5MHz. IC1 acts as an isolation buffer between the AM modulator and the 160MHz VCO. I've checked several 2022 sig gens and they all suffer from a bit of unwanted FM when AM is selected below 62.5MHz. This includes the ones at work and these all have the original OM345 fitted. There's a good chance the MMIC replacement for IC1 will increase the amount of unwanted FM that occurs here. I designed a high reverse isolation alternative for IC1 but never got around to trying it out. This had about 50dB reverse isolation at 160MHz. --- End quote --- Interesting. I will work out how to measure that when it comes to it. The 2022E is not a particularly wonderful generator specification-wise so I'm not overly bothered about some side effects as long as I am aware of them. I'm not going to be using this as a modulation source however so the modulation generators will be off all the time. I'm intending using it for very basic CW RF measurements such as MDS, return loss and occasionally VHF/UHF filter measurement. My AWG already makes a fairly decent RF generator with some exceptions. I saw your post on the designs on vintage radio forum as well. I actually looked at the W7ZOI termination insensitive amplifier as a design basis of another variant as they have excellent isolation and matching at both ends. This is a similar feedback amplifier topology. I shoved a BFR182W model into LTspice and got reasonable flatness up to 900MHz substituting all the transistors out for those. Obviously no idea on the noise figure. Would have to measure a prototype. I may persist with that design as well. Of course I need an RF generator and detector that is actually usable past 100MHz first. So I need to repair this with whatever I can. |
| bd139:
--- Quote from: factory on July 31, 2022, 12:47:51 pm --- --- Quote from: bd139 on July 30, 2022, 03:35:54 pm ---That’s a a great idea actually. Thanks for the suggestion. Those also won’t fall out when soldering! --- End quote --- They were used in stuff at work, no-one knew what they were and some boards we had made (supposed to be an exact copy) they fitted right angle pin headers, they were a pain to remove, after we found the boards didn't work (eventually I found the problem). Many many years later, some of the leadframe connections turned up in the TEA thread and I asked to find out what they were. Seems there is lots of choice for, pitch, size & board thickness. David --- End quote --- Interesting bit of history. I have seen them before but didn't put two and two together on this one. I've actually ordered two sets of boards here to try them out (as the boards are so cheap!). I have ones with leadframe suitable pads on the edges and some that can just use 0.6mm wire (resistor legs) soldered in as that might be easier to fit for people. Will see what comes out of it! Starting to think I have a PCB problem at the moment :-DD |
| George Edmonds:
Hi If it helps I have attached two photographs of a 2955 AA4 board showing my concerns. The last OM345 replacements on eBay require C37 to be moved to one side, not a good idea. G Edmonds |
| bd139:
Excellent. Thanks for the photos. My design is considerably smaller than those ones from what I can see. They should fit in the same courtyard as the original OM345. The height is also close to the original. I redesigned the board slightly before they shipped from the above. The two sets of boards were ordered with 0.6mm thick substrate and the transmission lines approximated for that board thickness. The thin traces another poster mentioned earlier were a mistake on my part. I rechecked again and again before ordering :) They’re still being manufactured so we’ll see in a few days. |
| bd139:
Mouser parts order for the final crystal oscillator, the OM345 replacements have arrived as have the CTI OCXO modules. All the CTI OCXOs are checked out and have been characterised for lowest current. I'll load up the 2022E power supply with an equivalent load and take some measurements when I get some time. Still waiting for PCBs which is the blocking issue at the moment. |
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