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Old 18th Apr 2018, 7:26 pm   #1
wizzer2k
Triode
 
Join Date: Apr 2018
Location: Wigan, Greater Manchester, UK.
Posts: 10
Default Marconi 2955

Hi.

I have recently picked up a Marconi test set and when doing the self test it fails with the following faults:-

2.0 21
2.1 23

2.2 25
2.3 27
3.2 37

The 2.2 to 3.2 I think are one of the OM345's on the generator board as iIhave seen this fault before and have the same issue with the attenuation not varying properly so not too worried about those codes, but need help with 2.0-2.1 as to which area I need to be looking.

Any help or info is greatly appreciated.
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Old 22nd Apr 2018, 7:05 pm   #2
Biggles
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Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
Default Re: Marconi 2955

The OM345s are definitely troublesome. They are getting hard to find but I believe there are replacement modules available which are little PCBs with a few components mounted on them. I have used them on a couple of 2955s and they seem to work ok. I also had a capacitor on the supply to one of the boards go s/c which caused a lot of self test failures. It burned out the series inductor at the same time. Other common faults are the RF output attenuator relays, which gives a large gap in output levels as you adjust up and down. The line output transformers can fry also. There is a mod out for this. I am not sure whether you can get the components any more. Well worth restoring as a useful piece of test gear. They were the industry standard for many years in radio comms workshops. I used them for years and always liked the simple user interface. I have two of my own which I "rescued" from a previous employer. I believe there is a user group out there somewhere but I have never joined. I also have seen a video on you-tube on 2955 board repairs.
Alan.

Last edited by Biggles; 22nd Apr 2018 at 7:08 pm. Reason: extra info
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Old 22nd Apr 2018, 7:14 pm   #3
wizzer2k
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Join Date: Apr 2018
Location: Wigan, Greater Manchester, UK.
Posts: 10
Default Re: Marconi 2955

Hi yes i have read and seen videos regarding the OM345 modules and have bought in some copy replacements of the 345 i have managed to source a nos OM360 as a spare i read an article that the OM345's can be repaired i have tried this method on my set as one has tested faulty in the alc circuit which has now cured all the self test errors

However the attenuator has been giving me trouble i have stripped it down today and found that one of the relay/coils springs back on its own and thus not latching and keeping in one of the 40db pads. After a little fiddling and twiddling with it it looks to be working now so hopefully be OK for a bit until I can source a replacement unit other than that I am really happy with it and it will certainly come in handy.
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