What did I get myself into? Murphy A40C.

[B.Taylor]

Be careful when soldering the components back onto the tags or you risk melting the soldering you have done inside the box.

[stevehertz]

Yep, get those tags nice and shiny and pre-tinned, again, quickly.

[Katar83]

Well, I've messed something up or fixed and found another issue in the process.
Soldered everything back together and when I switch it on now, I get static ok for a second or two with what appears to be correct volume and then throbbing/pulsing sound from speaker
Sounds like capacitor charging/discharging but bit lost now what this might be.
I've checked all wires going into condenser block and all are connected correctly.
I've quadruple checked condenser block and all seems fine and connected correctly.
If I turn the tone control between the settings(I assume its off then), radio seem to work fine, just quiet.
Oh and I think I found reason why R52 was open circuit, with neither of AC4/PEN valves in the set, it gets very hot, rather quickly, although I think that's unrelated.

[stevehertz]

Well, if it worked prior to you refitting the cap block, then in theory it has to be related to that? As has been suggested, has a lead come unsoldered inside the block as a result of soldering on to the tags?

[Katar83]

Block is fine but I've solved it, got confused by the 'underside' schematics with the block pictured, its basically missing a 'dot' for the ground for C8 but its there in the main schematics, so added that and radio works, need to tidy it all up again so will get that sorted and report back. I knew it was something silly!

[Katar83]

Started looking into SW. From day one SW reception was either non existent or barely there, sometimes when I've hit the tuning capacitor I was able to find quite few(mainly foreign stations).
Well, found the problem now. Fixed it too.
This radio is an absolute monster of a puzzle at times

I went over the whole set again and found 4 more of these too(basically wires came off at slightest push), along couple more of faulty silver mica caps, one looked like it delaminated, was showing correct capacity as far as I squeezed it.

It's finally at least functional. Cord and more voltages checking next.

[stevehertz]

Short wave stations are mainly foreign stations.

[Katar83]

Yes, it appears so. Got next two questions on this set.

The R52, through which the anode of S.W. H.F. Amplifier, V12, AC/VP2 is fed was o/c when I got this, I've replaced it with a 2W 15k resistor, but its clear that this is running too hot and wont last so I'm thinking I'll put a much bigger ceramic one in there instead. I take it, this is down to missing field coils. The R54 also gets warm but nowhere near as bad as R52 so should be ok I think?

Also, how does the noise suppression works on this set? Is it just removing the inter station noise? Is it suppose to work on all bands? The switch on this does absolutely nothing it seems but there are some voltages present on the switch contacts and its probably next thing I'd like to investigate.

[cathoderay57]

Something is amiss. R52 should be dropping 95 volts so at 15k it ought to be dissipating 0.6 Watts. If a 2 Watt resistor is getting excessively hot then something is wrong. The most likely cause is if C71 is leaky. It could be a duff AC/VP2 but less likely. Check the voltages either side of R52 and work out how much current is passing through it. Are you sure that's a 15k resistor? Neither of them look like it.... Jerry

[cathoderay57]

According to Post #7 of this thread https://www.vintage-radio.net/forum/...d.php?t=147650 muting is achieved by making the suppressor grid (Grid 3) of V5 the AC/SP1 pentode 20 volts negative with respect to the cathode. This action cuts off the anode current (but not the screen current) thus muting the audio. I can't tell from the circuit whether this function is intended to work on all bands. I can imagine it would be fairly useless on SW. Jerry

[Katar83]

Originally Posted by cathoderay57

Something is amiss. R52 should be dropping 95 volts so at 15k it ought to be dissipating 0.6 Watts. If a 2 Watt resistor is getting excessively hot then something is wrong. The most likely cause is if C71 is leaky. It could be a duff AC/VP2 but less likely. Check the voltages either side of R52 and work out how much current is passing through it. Are you sure that's a 15k resistor? Neither of them look like it.... Jerry

Well, looks like I have dead valve, valve anode was dropping eventually to ~22V so at that point if my math is right resistor was dissipating more than 2W. C71 was ok but I replaced it anyway, then swapped the AC/VP2 and that immediately settled to around 110V which is what it suppose to be. Thanks for pointing me in the right direction!

[Katar83]

Fixed another issue, an open L26 coil in the a.t.c. circuit. Still non wiser how the N.S suppose to function here or how I could test it.
Also whoever thought it was a good idea to put resistors and capacitors inside the valve cap hopefully was fired
Think I'll get the cords replaced now.

[PaulR]

The only Murphy I have restored is an A46. I found that the wires were easier to remove from tabs as they weren't wrapped round as in many other sets.

As a matter of interest was that the case with your set or was it just something that happened in the one I have?

[ms660]

Originally Posted by Katar83

how does the noise suppression works on this set? Is it just removing the inter station noise?

A description is given on mag. page 197 in the link below if that helps:

https://worldradiohistory.com/UK/Wir...ld-1938-03.pdf

Lawrence.

[Katar83]

Thanks, will go over it, see if I can understand anything out of it!

Redid dial cords now, the main one was super easy, the S.W. one was a bit fiddly, having a lot of spare cord, definitely helps. Indexing works well now with the new cord.
Cleaned up both dials, can just about see the red lettering on the main dial for LW.
Replaced 3 bulbs for the main dial too, I think I'm at the end of what I can do with it, just need to fit new power cable and need to have a think about aerial for it and see how it works with something better than a few meters of copper cable.
Currently at home I get BBC on LW, maybe couple stations on MW(barely) and a few on SW, this is improved when set it is in the garage but still feels like the whole thing is missing aerial.

[Katar83]

Originally Posted by PaulR

The only Murphy I have restored is an A46. I found that the wires were easier to remove from tabs as they weren't wrapped round as in many other sets.

As a matter of interest was that the case with your set or was it just something that happened in the one I have?

I'd say half were wrapped around, then another quarter with wires through the tabs so easy to pull out and remaining quarter just soldered against the tab. Looks like you were lucky.

[ms660]

Originally Posted by Katar83

Thanks, will go over it, see if I can understand anything out of it!

No problem, basically when the interstation mute is operating, V5's suppressor grid (g3) is made -ve with respect to V5's cathode, the -ve voltage is developed across the two resistors in the cathode circuit (R24 & R25) the voltage developed across them will be approx. 20 Volts, the bottom end of the cathode resistor R25 is connected to ground as is V5's suppressor grid via R26, the switches, R43 and R42, the cathode of V5 is approx. 20 Volts +ve with respect to ground which means that V5's suppressor grid is 20 Volts -ve with respect to the cathode, -ve 20 Volts on the suppressor grid with respect to the cathode is enough to cut off the current flow to the anode, hence the inter-station noise is muted, the valve data for the AC/SP1 (V5) can be seen via the link shown below, it includes information about using g3 (the suppressor grid) for noise suppression and details of g3's cut off voltage etc:

http://www.r-type.org/pdfs/ac-sp1.pdf

Lawrence.

[Katar83]

Thanks Lawrence, that's very helpful! I'll go through all this info during this week but a quick update for now! Managed to source an excellent condition AC/ME magic eye which arrived today and its working like brand new valve!

Original one was barely glowing and could only be seen in dark really. This was at 1pm, beautiful green