Murphy B40D RF Input circuitry

[Tyso_Bl]

So this had made its way back on the bench after a break, got as far as trying to do an alignment of the LO and RF stages. Did make it better but still not right very uneven sensitivity from one end of the band to the other.

Have had a closer look and found that the aerial input circuit had been 'got at' , very discreetly done, unnoticeable under normal inspection an extra capacitor had been wired into circuit, and then hidden behind existing wires and coax.

What had been done was to disconnect the RF input tuned circuit primary winding, and fit a new capacitor from the aerial connector to the junction of the first RF stage valve g1, and the input tuned circuit secondary.

I've been driving this from a low impedance source (aerial distribution or signal generator), so my understanding is that apart from the impedance mismatch causing loss of gain that the Q of the input circuit would have been flattened.

I'm going to try and restore the original circuitry, but the question is why would such a mod have been carried out? Was it by any chance a known standard mod for some reason? I can't find any reference to it online, does anyone here have any thoughts?

[trh01uk]

Maybe the primary winding that seems to have been abandoned has gone open circuit?

However, when I look at the B40 manual, and specifically the schematic of the front end, I see two aerial inputs on the B40A/B models:

One is a transformer input which the text says is designed for 80 ohm or 92 ohm coax. That's on pins B and C or PL101.

And there's a second input on pin D of PL101. This is for high-Z aerials and uses capacitors (two) from the aerial input directly to the grid of the first RF valve.

The text goes on to say that the B40C/D is different in that extra capacitors modify the low-Z coax input to become a pi-filter for common aerail working. And there is no high-Z input.

Looks to me like you might either have a B40A/B with the wrong badge on, or someone's been messing with it to try and provide a high-Z input for a long wire, etc.

The use of a (small) capacitor from a 50 ohm source can provide some impedance transformation - do the sums. In the case of the B40A/B its 12pF, which is probably not enough to detune the 1st RF tuned circuit, but as its designed for a high-Z aerial the effect on the loaded Q of that tuned circuit should not be great.


Richard

[Tyso_Bl]

Ah that makes sense, someones tried to retrofit a high-Z input. The capacitor that I removed has turned out to be a 12 Ohm disc thermistor! Hardly surprising it didn't have the intended effect, a little knowledge etc.

[QWENIX]

Interesting matter.

@Tyso can you post a pic of your receiver? And a pic of your model admiralty pattern tag ?

bet regards

Qwenix

[Tyso_Bl]

Mostly for @Quenix

This is the one, it has a very early serial number. I would like to find out what modifications were made that are shown on the modification record.

There is an RF choke at the back of the RF unit, several pies on it, doesn't seem to be shown on the diagram.

I've removed the aerial input socket and wiring, and will modify it to restore original performance, but will not replace MKIV Plessey socket, will use something standard instead.

[turretslug]

I think that the choke is switched into the cathode circuit of the LO valve when the single channel crystal option is selected, to enable feedback to be derived.

[Tyso_Bl]

I see it now, its drawn sideways in the manual, need stronger glasses, or a better printer!

[Alistair D]

Above the admiralty pattern tag there are what seems to be 2 screw holes. Do you know what was fitted here?

Al

[Tyso_Bl]

No idea, the holes are smaller than the ones holding the tag plate on, they do go all the way through, and there is the remains of something recessed in them. A bit over 1mm in diameter I'd say, they look to be original to the radio.

Gone slightly O/T.

[QWENIX]

It looks like the two screws have broken of. I have to rely on my photographic memory. I've seen B40s (here in holland) with an extra wide and small (height) ID tag mounted to the front. This extra tag had text on it which referred to the part of the royal navy raf army or sub division in which it was used.

best regards, qwenix

[Alistair D]

Let me explain. Back in the early 70s I bought a B40 from a surplus supplier.

A few years later I fitted a 2 speed reduction drive from an 1155 to give even more slow motion drive for easier SSB resolution. If memory serves me correctly I had to drill 2 holes in a similar place as those 2. Later on I crated it up and sold it to a radio club but I cannot remember where. I was just wondering if it could have been my old B40.

Thinking about it though I am pretty sure that mine was a B40C and not a D so almost certainly not mine.

Anyway back on topic.

Al

[QWENIX]

I guess something like this offtopic... http://www.rnmuseumradarandcommunica...org.uk/SEA.pdf

and adding a pic of what I meant ...... http://www.rnmuseumradarandcommunications2006.org.uk/B40%20AND%20FAZ[1].pdf


best regards qwenix