Need OM-345 for my Marconi 2955A

Back in 2003 a Jim Potts had the same problem

If you care to join the group you might find out whether Jim is still a member, and if so, send him an email to see whether he solved the problem.

Ross

Hello Ross,

Ok!. I am already a member of Amateur-repairs so will check with Jim.

Anyway, if somebody has a clue to locate them or has done any MAR-6 conversion, please, let me know.

Thanks!

HI, I've repaired a Rhode & Schwarz CMT-52 with an available MAR to replace an OM. Should be OK, I expect your OM is used in a freq counter circuit or to drive a prescaler for the pll etc.so the mismatch shouldnt be an issue.

The OM is a 40-860 MHZ 12dB 75 ohm amplifier. pin

Pin 1 is the RHS when viewing the flat side and LHS when viewing the lumpy side.

I cut a piece of double sided board a little larger than the OM, drilled a suitable hole in the middle for the MAR, then used a scalpel to make the right tracks and pads. I made it so i could attach leads (off cuts from resistor or cap leads) to make a plug in replacement.( just be careful when soldering back in that you dont desolder the leads)

Just use a suitable bias resistor for the vcc of the circuit

Depending on the surrounding circuitry you may want to use input/output coupling caps in the board. I used a decoupling cap on vcc side of the bias resistor, I don't recall needing to use a rf choke.

Hope that helps

Greg

Hello Greg,

Thanks for your help!. I will try this way, as I have some MAR-3 coming... problem is that the bad OM-345 are in the levelling amplifier so I am not sure of the results I will get. There are 4 OM345 in the Marconi 2955A. Other two (in the counter circuit) seem to be fine but I am afraid of swapping them as they seem to be fragile and don't want to ruin them...

I will let you know how things finally end.

Regards,

JOSE

Hi Jose, I'm not familiar with the 2955, however I can't see this amp being in an essential circuit as it's 75 ohm and doesn't cover the entire operating range of the instrument. Perhaps it's an autoleveler for a LO before a mixing stage? If so then you may need to make an adjustment as per the service manual for that circuit. (which you would have to do anyway even if you replaced it with another OM) I can't recall the gain flatness of the OM, but I doubt i'ts better than the MAR, so should be fine.

Cheers Greg

Hi Greg,

Well, although it seems incredible, both OM345 are in the main signal chain, going to 1000MHz!. I guess this is one of the reasons for them to fail; they are used out of their specifications and, in order to get acceptable gain from them, I guess they are stressed a bit. After the two OM345 (a 12dB gain stage each, according to manual) there is an OM370 (which is a 23dB gain stage), then it goes to a 0/10dB attenuator and straight to signal output. I can provide you with an schematic of that part, if you want to look at it.

Thanks for your help!

Regards,

JOSE

Hi Jose, schematics of the 2955 would be interesting to see. Are they on your web site? I had a look and didn't see the 2955 there.

I do find it surprising they use use those as the main path amplifiers, the CMT at least only used them in non critical circuits, where good performance is needed they use discrete circuits.

Good luck with your repairs

Greg

Hi Greg,

Please, send me a direct e-mail (to: eb5agv ctv.es ) and I will forward you a link to download it. Once you have the schematics, look at AA4 circuit, where the two OM345 and the OM360L (OM370 in my unit) are located.

Best regards,

JOSE