Big old Murphy A170 radio to restore.

[Ed_Dinning]

Hi Murphy used a fairly standard range of their own knobs, so if you post a picture in the wanted section you may get lucky

Ed

[E93AFAN]

Guess you will have seen this site "The Murphy diagram dungeon 1950 to 1954"

Looks quite useful and there is a link at the bottom left of the page to the data sheet for your radio that looks like the original Murphy document, good read and interesting too.

I tried to post a URL, but the posting just shows a line of ***** so I assume this is verboten by the administrators!

I wish you the best of luck with your restoration, there is no better feeling than finishing and admiring your work with eyes and ears !

[Heatercathodeshort]

Just a small point. The mains voltage adjuster appears to be set for 220/230V mains. Might be better to readjust it for 240/250V. Less strain on the mains transformer and valves. The mains here is often 245-248. John.

[GreasedMonkey]

Originally Posted by Heatercathodeshort

Just a small point. The mains voltage adjuster appears to be set for 220/230V mains. Might be better to readjust it for 240/250V. Less strain on the mains transformer and valves. The mains here is often 245-248. John.

I did change this before I powered up the set, it is now set for 240/250V.


I have not had time to go back to the set yet. I am currently waiting for some capacitors and resistors to arrive. before I can set to it in anger anyway.



What is the best way of cleaning the chassis? Soft brush and a light hoovering? It seems to be mainly dust and fluff, nothing particularly nasty that I have seen.

I could not find any 50nF capacitors that were not used or extortionately priced. I reasoned that as it is listed as 0.05uF 20% in the book I could get away with 47nF 10% part instead.
Original component spec range = 40 - 60 nF
Replacement component spec range = 42.3 - 51.7 nF
It may not be liked by the absolute purists but I assume this approach will be OK.

[Nickthedentist]

Going for the nearest preferred value is exactly what we all do!

[GreasedMonkey]

Originally Posted by Nickthedentist

Going for the nearest preferred value is exactly what we all do!

I figured as much.

I am just not used to numbers of this magnitude. At work a typical resistor value may be 3mΩ 5%, (not a typo, the lower case 'm' is intentional, milli-ohm rather than Megohms) currents measured in nA and why on earth would anybody need more that 3.3Volts? an 0201 resistor is considered a large component at 0.6mm length and rated 0.05W.

It's nice to look at components I can't accidentally inhale.

[GreasedMonkey]

The Chassis is now out and has had a light dusting with a soft brush.
It looks for the most part to be surprisingly unmolested. I did see an issue with the aerial filter. Inductor (L1 in the murphy book) has 2 flailing tendrils which just does not look correct to me, and I can see no sign of the capacitor (C1 in the book). also, the former has broken at the base where the brass strip passes through to attach it to the chassis.

I have not yet had chance to met a meter on it and see if there is any continuity in the inductor, or had a dig through my bits to see if I have any 470pF capacitors.

[GreasedMonkey]

I have made a start on the chassis, but now am confused by some of the components I find.

I ordered some capacitors to replace the waxies according to what was marked on the circuit diagram as the parts list did not give a voltage rating. But on looking at the existing parts, they are all rated at 350V.

For example, the circuit diagram shows 2.6V across C12, but fitted is a 350V part. Unfortunately the one component not to have arrived yet is the output decoupling capacitor so I am resisting the urge to power it up and make my own measurements.

Should the capacitors all be replaced with 350V rated parts, or is the diagram to be trusted? Are large transients expected or something?

While I want the top of the chassis to look correct and have re-stuffed the can on the top side, I am happy with the presence of a few modern components on the underside as long as it is tidy.

[Techman]

I've got several of these sets, not the exact same models as yours, but they all use near enough the same chassis and valves. They're a strange line-up with all small valves, just having the large octal type for the output. The 6P25 is a direct swap for the KT61 and I've done it, although ideally the cathode resistor should be adjusted slightly. Having said that, all the valves in your set are likely to be good, so no need for swapping and replacing unless you're very unlucky.

Before going too deeply into your set I would just replace C26 (Trader sheet) and stick a length of wire in the aerial socket and see if you can receive any stations above the 'hum' level. You'll need an external aerial on this set, as it won't receive anything much without one. Don't run it for too long just in case that electrolytic smoothing can, the failure of which is causing the hum, starts to get hot and ends up going bang and making a mess - and making you jump at the same time!

I actually bought a similar model Bush for a quid at a junk clearance sale late last year. There were no other bids for it and if I hadn't bought it then it would have probably been dumped after the sale. The word at the sale was that it was supposedly working, but when I got it home and powered it up with a length of wire in the aerial socket, the mains hum was so loud that it was only just possible to hear stations above it, so it looks like it's got the same problem as yours. I've not done any more with it and I've not even taken the back off, I just stuck it out in the caravan with all the other junk, to eventually get round to actually looking at it properly at some time in the future. It's fitted with an extremely short length of very modern mains lead, so someone's been recently playing with it before I got it.

Seeing as you've got a similar set with the same fault I'll perhaps have to drag the set in and finally take a look at it. I'm expecting to have to replace the reservoir and smoothing capacitor can, perhaps with two separates - C34 & 35 on your set (Trader sheet).

[Radio Tech]

These are a lovely set to restore and by taking your time and making a neat job of any components you may have to replace will be well worth the effort. If you want to keep the main smoothing cap looking the same then just re-stuff it with suitable axial electrolytic capacitors, but good luck with it though.

Best wishes

Ken

[Andy - G8MNM]

Hi,

Please post a photo of the knobs.

I may have some in my collection.

Andy

[GreasedMonkey]

Originally Posted by Techman

Before going too deeply into your set I would just replace C26 (Trader sheet) and stick a length of wire in the aerial socket and see if you can receive any stations above the 'hum' level.
………….
Seeing as you've got a similar set with the same fault I'll perhaps have to drag the set in and finally take a look at it. I'm expecting to have to replace the reservoir and smoothing capacitor can, perhaps with two separates - C34 & 35 on your set (Trader sheet).

I did briefly power up the set with a length of mains lead poked into the aerial hole. All the valve heaters glowed nicely. The mains hum was so very loud, I unplugged the large internal speaker and tried again with a tiny little one that did not respond to the low frequency so well. Twiddling with the tuning dial, I did manage to receive something from Belfast, but I could not find Radio 4 LW, which I found odd as the transmitter is basically at the end of our garden.

I have restuffed a couple of cans on the top side of the chassis, but work currently on hold waiting for a capacitor to replace C26 (trader sheet) before I apply power again, as advised by the wisdom of this very forum.

[GreasedMonkey]

Originally Posted by Andy - G8MNM

Hi,
Please post a photo of the knobs.
I may have some in my collection.
Andy

Thanks.

I shall try and get into the man-cave and take a picture at some point this evening.

[Mr 1936]

Hi

Regarding 350 V rated capacitors. It was probably cheaper for Murphy to standardise on one voltage rating and buy in bulk, at least for the smaller values. All this means is that some parts are under-stressed, which is no bad thing.

Transients are probably only an issue for capacitors across the mains, and also ones across the primary of the output transformer ("tone correction") where the AC swing can be quite large if the volume is turned up to distortion point or "someone" disconnects the loudspeaker.

[GreasedMonkey]

Originally Posted by Andy - G8MNM

Hi,
Please post a photo of the knobs.
I may have some in my collection.
Andy

Attached are pictures of the knobs.

I am missing 2 if you can help. I am still far away from needing them so no rush.

Thanks.

[Rich Woods]

Have got two original Murphy knobs with the original lettering on them for Volume & Tuner but otherwise identical. PM Me with your details & I’m happy to post them to you if they are any use.

[GreasedMonkey]

Originally Posted by Rich Woods

Have got two original Murphy knobs with the original lettering on them for Volume & Tuner but otherwise identical. PM Me with your details & I’m happy to post them to you if they are any use.

PM Sent.

Many thanks indeed.

[Richardgr]

Inductor (L1 in the murphy book) has 2 flailing tendrils which just does not look correct to me,

I think they are just bits of a rubber band that would have restricted the movement of the core so it stayed in the same adjustment.

[GreasedMonkey]

Originally Posted by Richardgr

I think they are just bits of a rubber band that would have restricted the movement of the core so it stayed in the same adjustment.

I have tried to get a better photo of the tendrils. See attached.
I suspect this is where the capacitor once was.

They are wires coming from the solder tags of the inductor with sort of, ‘half rivers’ left on the ends.

[buggies]

Possibly the remains of a mica capacitor?
Ebay link for picture viewing only - not a purchase suggestion (your cap value unknown):-
https://www.ebay.co.uk/itm/334146156295