Marconi 2955 Service Monitor Repair - OM345

[Jay_Diddy_B]

Hi group,

I am going to share some details on how I replaced a damaged OM345 amplifier module in a Marconi 2955 Service Monitor.

Acknowledgement

I would like to acknowledge the valuable information that I found on this subject, particularly in this thread:

https://www.vintage-radio.net/forum/showthread.php?t=100840

The failure of the OM345 modules is a well known problem, in several Marconi products including the 2955 Service Monitors and the 2022 series of RF signal generators.

The supply of replacement OM345 modules is pretty much depleted. There is a replacement module available on eBay. I decided to make my own.

Marconi 2955 failure

The Marconi 2955 was failing a large number of self test routines. This was traced to IC1 on the AA2 assembly:



The amplifier IC1 and IC2 is used in the frequency counter circuitry, so the gain and flatness is not critical.

Test Fixture

The OM345 test fixture is described on the OM345 datasheet:



I built a simplified version of this, using single-sided circuit board:





I removed a good OM345 from the Marconi and measured its gain with a HP8714B Network analyzer:




The gain is nice and flat.

To be continued...

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

My first attempt to build replacement module was based on the work done by G0HZU on the UK Vintage Radio Forum.

Here is the schematic:



The board design:



A photograph of the assembled board:



The gain measurement:



I tried this in the Marconi 2955. It fixed the issues with counter and unit passed all the self-test routines.

To be continued ...

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

Although the replacement module worked in this location, it may not work well in other locations. I wanted to explore using a MMIC amplifier instead of the transistor.

I have some Mini-Circuits MAR7 amplifiers. This part has similar gain to OM345. It runs at a higher current 22mA.



Link: https://www.minicircuits.com/pdfs/MAR-7SM+.pdf

Here is the schematic:



The board layout:



Photograph of the board:



Measured gain:



I placed this in the Marconi 2955, IC2 position on the AA2 board assembly. The 2955 passed all the self-tests.


Conclusions

The transistor version is probably o.k. for applications operating at lower frequencies or where gain flatness is not required. A replacement based on a MMIC is probably a better choice for more demanding applications.


50 Ohms versus 75 Ohms

The OM345 is a 75 Ohm amplifier. No attempts were made to get a better match. All the measurements presented here were take in a 50 Ohm environment.


Regards,

Jay_Diddy_B

[Samogon]

My Marconi waits the same cure also was planning to use mmic from the same manufacturer.
Nice topic and most valuable that you did measure gain across frequency range. Guys on vintage-radio didnot do that.

[Ringmodulator]

Great work. Thank you!

[roumen]

Thank you! Good work!

[coromonadalix]

Very good work   

Oh  some moduule began to show up on Ebay:
https://www.ebay.fr/itm/OM345-OM2045-REPLACEMENT-MODULE-MARCONI-2018-2019-2022-A-B-C-D-E-2955-R-S-ETC-/283314072654

[KD0CAC John]

I am looking at one on line , that has self-test fail #21 , 24 .
Anyone have some ideas ?
As I remember from looking quickly a pdf , 21 low power about 300 mhz , 24 high power 900 mhz .