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Old 24th Nov 2013, 11:40 am   #21
esi_snc
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Default Re: Marconi 2022c OM345 Replacement Module

Quote:
Originally Posted by jdavis6809 View Post
the pcb boards are here with all the other components
Please John,
can you publish or to give further information on the construction of the PCB that replaces the OM345?
I have a MI2022E not working because there are three OM345 broken.
Regards.

Alessandro
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Old 30th Nov 2013, 3:32 pm   #22
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Hi John. My 2022 has one OM345 deceased.... unfortunately it was consigned to the bin before I read of posts above...
I would be grateful if I could test one of your modules in my 2022.
I would be happy to pay any out of pocket expences.
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Wendy Mott G8BZY
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Old 6th Dec 2013, 10:44 pm   #23
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

I have a saved search on Ebay for Marconi. Normally I am looking for test equipment. This popped up in tonight's search. The subject of this thread is not my area of expertise so I am not sure it is of any value but I thought I would post it anyway.

http://www.ebay.co.uk/itm/OM345-DIY-...item417a1c4407

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Old 7th Dec 2013, 12:51 pm   #24
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

the device on ebay link has 80mA far too high
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Old 10th Dec 2013, 10:39 pm   #25
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Hi Guys.......... Just to add my two "pennyworth" "old saying from pre decimalisation". Today I successfully fitted a MIMMIC MAR01 in place of IC9, this is a preamp for a Phase locked loop.......I made a mini pcb ......... see attached......
I Checked the full operating frequency range and output levels with my spectrum analyser and it seems ok to me. It has been on test all day, without any grumbles. Now it may not be a "Cure all", but in location IC9 it seems to do the job.
Regards
Wendy Mott G8BZY
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Old 11th Dec 2013, 12:05 am   #26
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

The MAR01 looks like a good device. I think the i/p and o/p is specced at 50 ohms but that is not very important in that position. In the main signal path any small mismatching loss can be made up.
The 2022 has stored values for gain adjustment at various frequencies so can be tweaked a bit if stage gains have changed.
Incidentally, how did you chose the 220R bias resistor from +12V? The spec sheet seems to indicate that about 412R is required. Maybe it's not too critical if the signal levels are small.
Anyway, very neat indeed! I must have a try.
- Jeremy
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Old 11th Dec 2013, 1:40 am   #27
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Quote:
The 2022 has stored values for gain adjustment at various frequencies so can be tweaked a bit if stage gains have changed.

I don't have a Mi2022 here and I've never been inside one but I don't think it has stored values for gain adjustment. Or maybe I've missed this feature somewhere in the schematic/manual?

However, like many sig gens I think it can allow user adjustment of the ALC (detector) target voltage according to frequency and store this info in memory as calibration data but none of this should need touching if the OM345s are swapped because they have nothing to do with the ALC detector accuracy. However, in theory at least, several replacement MMICs 'can' cause the sig gen ALC to go open loop if there is too much or too little system gain for the ALC system to cope with at some part of its operating ranges for frequency and amplitude.

That's why I think each person would have to 'prove' a complete suite of alternative MMICs from the OP by looking at the ALC margins across all the generator's ALC range across all frequencies.

This requires a lot of tests internally and some degree of understanding of how the ALC operates on each range. If I was doing this I'd automate the tests via a PC and GPIB (with a GPIB controlled DVM reading each ALC voltage) to prove the ALC margins were OK across all frequencies and RF levels in the ALC range. Otherwise it would take ages to do it all manually.

I know I'm being a bit of a neghead on this thread but this is a decent bit of test gear. Having looked at a typical OM345 module that has failed I think the easiest and cheapest (and best) way to repair the generator is to simply repair the OM345 and refit it. This way there would be far less risk of falling outside the ALC window somewhere in the generator's ranges and also the spurious performance would be similar to original
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Last edited by G0HZU_JMR; 11th Dec 2013 at 1:49 am.
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Old 12th Dec 2013, 10:34 am   #28
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Quote:
Originally Posted by G0HZU_JMR View Post
Having looked at a typical OM345 module that has failed I think the easiest and cheapest (and best) way to repair the generator is to simply repair the OM345 and refit it.
I'd love to know how you repair, let alone look at, a fully hermetically sealed unit. Please let us know the secret procedure, or am I having a senior moment or three.
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Old 26th Dec 2013, 6:57 pm   #29
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

I have resolved the problem of the substitution of the OM345 purchasing from http://www.rte-jhb.com/ the smd board perfectly compatible.
Have received the material from the South Africa to Italy in less than 2 days with DHL!!!!
I have replaced 3 OM345 deceased and now my MI2022E perfectly works.
With the spectrum analyser (ADVANTEST R3261B) I have verified the amplitude of the output and the spurious and everything are in the specifications of the user manual.
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Old 27th Dec 2013, 12:09 am   #30
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Quote:
Originally Posted by Mikebay View Post
I'd love to know how you repair, let alone look at, a fully hermetically sealed unit.
I did it by scraping away the coating with a scalpel. It is a lot easier than you might think. When I started I wondered if I was going to need a Dremel but it turns out that you can remove it with fairly gentle strokes from a scalpel.
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Old 27th Dec 2013, 7:41 pm   #31
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Thanks Jeremy, I'll try that.
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Old 11th Feb 2014, 5:52 pm   #32
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Default OM345 Replacement Module

Hello,

just to let you know

available on ebay now

works with 2022 and 2955 and others

regards

john davis
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Old 7th Aug 2014, 12:22 am   #33
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Three posts moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=108278
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Old 24th Sep 2014, 1:58 pm   #34
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Quote:
Originally Posted by jdavis6809 View Post
5.After 4 and 5 concluded offer boards for sale.
My 2955 was having display problems (fold-over on the right hand side) and all the keypad switches failed. Repaired the problem by changing the capacitors in the CRT H drive section and all the switches replaced (Switches available with element14)

The self test full of errors! Do the above offers still valid? Is boards are available for sale? Or can you give me details on how I can homebrew couple of the OMs?

Peter VU2PJP
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Old 25th Sep 2014, 12:10 am   #35
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Hi Peter, if you have a service manual you can narrow down the errors into smaller areas as it explains what the error codes mean. From this you could possibly work out if there are any common factors which are confined to one board perhaps? Regarding the OM345 a friend of mine says there are some still available on line but I have not checked this. If they are available I would probably buy a few for myself as there are still a few 2955's around. I recently substituted an OM345 with an OM2045? if I remember correctly and this worked fine. Hope this helps.
Alan.
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Old 25th Sep 2014, 5:33 am   #36
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Quote:
I recently substituted an OM345 with an OM2045? if I remember correctly and this worked fine. Hope this helps.
Alan.
Hi! Tnx for the post. Yes! Om345 can be substituted by 2045. OM 345 replacements are available from couple of sources in the Net, but feel little bit expensive and the substitution of these replacement may necessitate re- calibration which is beyond my scope or capabilities. I am probing various other possibilities and daily searching the Web for any new ideas. As of now the RX test part is Ok in the 145Mhz range and it gives out step attenuated signal for my VHF repairs....!! Do I have to disturb it since it is serving me-the big question. Rgds. Peter

Last edited by Brian R Pateman; 25th Sep 2014 at 6:12 am. Reason: Quote repaired.
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Old 15th Apr 2015, 9:53 am   #37
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

Hello all, I'm Kevin from New Zealand. Recently retired (45 years) from Broadcasting here in Auckland. I had the luck ?? to repair 2 Mi 2022c generators. Both had faulty OM 345 amps, out pin voltage at 12v. A sure sign the amp has failed. I got both working by removing the OM345's and fitting MSA-0286 MMIC's and a 360 ohm resistor for bias.

I more recently picked up some work work with the national radio network in Auckland, (right where I started way back in 1963!!!). I went to repair a 450MHz link Rx and discovered that the Mi 2022c gen that they had wasn't working, usual problem, 3 faulty OM345's. I was then informed that there was another in Wellington also not working.

I told my senior that I had fixed a couple of these before and started to try to find MSA-0286's. I found 20 in Poland and we purchased these. (NZ$20.00!!!) My repair worked again and both generators are working again, saving the company NZ$10,000.

Attached are Pix of the way I mounted the MSA-0286, the resistor and the input/output wiring, the OM 345 and the circuit board.

I suspect that these may be the last of the MSA-0286's. There is another newer, functionally and physically identically MMIC to the MSA-0286, Mini Circuits MAR-2SM available from RF Electronics in France. 5+ 2.90 Euro, stock as at 1/April/15 is 899 units.
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Old 2nd Aug 2015, 9:39 am   #38
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Default Re: Marconi 2022 2955 OM345 OM360 Replacement

As for MSA-286 with a 360R bias resistor this gives 20mA bias current

Normally bias resistor should be greater than 10 times the rf impedance ( 10 x 50 ohms with no choke)

Ok to replace one or two but not all

Regards

john davis uk (om345 replacement module)
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