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Old 25th Oct 2019, 10:14 pm   #1
Alf Fisher
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Default Marconi CR150 Receiver

I have looked at this receiver before here:- https://www.vintage-radio.net/forum/...ighlight=CR150

I now know a lot more about it. It is a double conversion Rx that could be used in a Triple Diversity Rack Type 211 or as a stand alone receiver. It had the RAF Designation of Receiver Type R1569 but basically it is a modified CR150/3.

It is somewhat deaf and sometimes it drifts into complete deafness and then drifts back again!

I been injecting an accurate 465kHz signal into the grids of V5, V6 and V7 and some stages appear to have very good gain of the order of 25dB and others only about 8dB. As soon as I go near IF2, that fault might change slightly but its still deaf possibly because the other IFTs all have the same problem. MWT Co have thoughtfully filled all the cores with wax so its not going to be an easy job.

The first 465kHz IFT appears to have 2 tuning peaks of around 436kHz and 471kHz which seem very wide spaced to me.

Does anyone have any idea what might be causing this fault. I'm suspecting the old wax covered silver mica tuning capacitors in one or more of the IF transformers and I would be grateful if someone could confirm my suspicions. They have rivets for the connections to the 'plates' and I've read somewhere that the connections go intermittent.

The first IF is supposed to be 1600kHz but the IFT peaks at 1660kHz so that is another problem fault.
I have just checked the first oscillator frequency and it is 1147.435 kHz so it is quite a bit off its intended frequency of 1135kHz. Its no wonder it doesn't work very well with all these faults!

Alf
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Old 25th Oct 2019, 10:54 pm   #2
Sean Williams
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Default Re: Marconi CR150 Receiver

Hi Alf,

I can remeber Chas Miller telling me about tracking across the paxolin bases of the IFTs in CR100s, putting a positive voltage on the AVC line - wonder if this is happening?
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Old 25th Oct 2019, 11:39 pm   #3
Alf Fisher
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Default Re: Marconi CR150 Receiver

Hi Sean,
No it cant be that as the AGC line sits around zero and can get up to -ve 10 volts or more if I stuff a big enough signal in.

I'm suspecting the silver mica caps in the IF transformers as everything seems a bit off tune. As you will see from what I wrote at the top, its a wonder it works at all!

Hey-ho!!

Cheers,
Alf
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Old 26th Oct 2019, 10:00 am   #4
HamishBoxer
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Default Re: Marconi CR150 Receiver

Those silver mica caps will make the set deaf and appear way off alignment.
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Old 27th Oct 2019, 6:45 pm   #5
Alf Fisher
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Default Re: Marconi CR150 Receiver

Thanks Hamish.
You have confirmed my suspicions.

What I shall need to know next is a good way to get the wax out of the cores so they can be re-tuned.

I'm thinking the IFTs will need to be taken off the chassis so some heat can be applied where its wanted.

Cheers,
Alf
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Old 27th Oct 2019, 7:05 pm   #6
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Default Re: Marconi CR150 Receiver

You might find that the alignment after replacement of those caps is ok.

I got caught many years ago with a cap across an IF in a relatively new car radio. (1960,s)The cap was very intermittent.
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Old 28th Oct 2019, 10:40 pm   #7
Keith
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Default Re: Marconi CR150 Receiver

Hi Alf,

Many years ago I acquired a CR150/4 (bought for scrap value direct from a scrapyard weighbridge!). I couldn't get anything out of it. Eventually, thanks to another amateur I discovered that it, like yours, was one unit of a triple diversity system.

This variant, however, didn't have any oscillators - only buffers designed to be fed from a common oscillator unit. Following advice, I converted each buffer into an oscillator using a common cathode double triode circuit. This worked fine up to the top end of the highest frequency range beyond which the 1st oscillator refused to take off.

Otherwise it was a great receiver with its double half lattice crystal filters. I used it for many years.
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