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Old 4th Dec 2017, 1:39 am   #1
Stefan_O
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Unhappy Marconi 2022C, no signal below 250 MHz anymore

Hi,
after replacing an OM345 for the <62.5MHz range a few years ago I now noticed it stopped working below 250 MHz at all:
When I enter 126 MHz I'll get 252 MHz at the output and modulation is about twice as big. When I enter for example 63 MHz I'll also end up with 252 MHz and modulation is about four times as big. Signal power is also weaker. Below 62.5 MHz no signal at all, but I can pick up 160 MHz inside the device.
I assume the problem is a switch somewhere. I cannot hear any relais when switching below/above 250 MHz (I do hear one at the 62.5 MHz border), but I don't know if there is any mechanical relais to switch that.
Is this a common problem? Any idea how to fix that?
Best regards
Stefan
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Old 4th Dec 2017, 8:46 am   #2
Dickie
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Default Re: Marconi 2022C, no signal below 250 MHz anymore

If it's not working below 250MHz then the 125-250MHz divider path or its switching is probably not working. This is IC11 on the AB1 board and its associated control and diode switches. IC11 (Plessey SP9131) is not cheap, so its worth some detailed fault finding before condemning it.
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Old 4th Dec 2017, 12:30 pm   #3
Stefan_O
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Default Re: Marconi 2022C, no signal below 250 MHz anymore

Thanks for the fast answer. Unfortunately I cannot find any datasheet of the SP9131, so I'm not sure how exactly this part is working.
Could find a more readable service manual now, switching is done without relais.
So for now I will check if the switching levels are correct, then check the OM345 (IC9) in front of the divider and hope it's one of them.
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Old 4th Dec 2017, 4:56 pm   #4
Stefan_O
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Default Re: Marconi 2022C, no signal below 250 MHz anymore

Update:
Switching seems to be fine (4,5V high level, 200mV low level at Point F).
I measured input/output levels of IC8, 9 and 10. While IC10 is amplifying, both IC8 and IC9 have much less output level than input.
Is it possible that output >250MHz works even if IC8 is broken? Range selection does not have any influence on the measurement.
Is IC9 a good candidates to replace for fixing?
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Old 4th Dec 2017, 5:23 pm   #5
Dickie
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Default Re: Marconi 2022C, no signal below 250 MHz anymore

Its quite possible it will still work with IC8 broken, there is plenty of gain in the signal path to make up for it. The ALC will crank itself up to compensate as far as possible.
Given the poor reliability of the OM345s, they must be suspect. There is a thread here that describes a way of "fixing" them which works some of the time.
https://www.vintage-radio.net/forum/...=100840&page=2
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Old 4th Dec 2017, 10:19 pm   #6
Stefan_O
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Smile Re: Marconi 2022C, no signal below 250 MHz anymore

Thanks, that's the thread I started when the first OM345 gave up (IC2), I replaced that one with a self-etched PCB using a BFR540. Now I soldered the 3k9 resistor across the capacitor at IC9 and it's working again over the full range!

During the test I found out that above 250MHz I cannot get output level above 400mV, between 400mV and 2V Signal level remains constants, I've compared 250MHz and 249,9MHz.
Could this be because IC8 is also broken and the AGC cannot compensate?
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Old 5th Dec 2017, 7:55 am   #7
Dickie
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Default Re: Marconi 2022C, no signal below 250 MHz anymore

It's more than likely!
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