Need OM-345 for my Marconi 2955A

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Hello!

I have a very nice Marconi 2955A but has a couple of damaged Philips
OM345 amplifiers... I wonder if you could help me locating a source for
them.

Another possibility is use a Mini-Circuits MMIC. I have read that a
MAR-6 could be used. Has somebody done this mod?. If so, how did you
hook it up in the unit?. Did you use any adapting PCB or network?

Thanks for your help!. It is a pity such a nice 2955A remains unused
:-(

Regards,

73 JOSE - EB5AGV/EC5AAU
http://jvgavila.com


Re: Need OM-345 for my Marconi 2955A



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Back in 2003 a Jim Potts had the same problem
http://groups.yahoo.com/group/Amateur-repairs/message/13456

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

Re: Need OM-345 for my Marconi 2955A


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!

73 JOSE - EB5AGV/EC5AAU
http://jvgavila.com


Re: Need OM-345 for my Marconi 2955A


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
1=input
2,3=common/ground
4=vcc
5=output

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

Re: Need OM-345 for my Marconi 2955A


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
http://jvgavila.com


Re: Need OM-345 for my Marconi 2955A


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


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Re: Need OM-345 for my Marconi 2955A


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
http://jvgavila.com


Re: Need OM-345 for my Marconi 2955A


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





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Re: Need OM-345 for my Marconi 2955A


Hi Greg,

Please, send me a direct e-mail (to: eb5agv <at> 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
http://jvgavila.com


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