Unknown radio. Identified as a Murphy A34.

[cathoderay57]

Just a couple of thoughts. Is the hissing still present with the aerial disconnected, or is it worse or less severe? Also you might want to try turning a few things on and off around the house and see if it makes any difference. Principal sources of local interference can be routers, led lights and various computer/phone power supplies. Cheers, Jerry

[LyntonP]

Hi Jerry The hissing disappears when the aerial is unplugged. I have tried turning off all routers, power supplies, etc but this has no effect.
Last night I was listening to MW on my Trio R600 and there was interference all across the band. Sounded like HT interference from a car ignition running on tick over but the noise blanker wouldn’t shift it.
No I don’t know anyone in the area with a SG other than my old friend the ex tv engineer, but he is in poor health and unable to help.
Lynton

[Nuvistor]

May be worth getting information to make a magnetic loop aerial, reports appear to show they do well if there is high local interference.
Someone on the forum may well have a design for MW.

There are commercial ones but very expensive and you can make your own for a fraction of the cost.

[LyntonP]

I have heard good reports about The Mini Whip active aerial. So small it can be housed in plastic drain pipe. Quite a few offered on eBay which are not expensive. I wonder if anyone has had any experience of them?

[Nuvistor]

Problems with a mini whip is there is no nulling of the interference. The whip picked up a small signal due to its size and relies on a low noise amplifier to boost the signal+the noise.

I wouldn’t try to put you off trying one, sometimes things don’t work the way you expect.

[LyntonP]

Originally Posted by Nuvistor

I wouldn’t try to put you off trying one, sometimes things don’t work the way you expect.

Fair comment
Lynton

[cathoderay57]

OK, understood. There is a BVWS meet at Golborne which is local to you where somebody might be able to help you, but that event isn't until 3 November. I would still recommend leaving the IFs alone (I don't have the service sheet but according to the above posts the IF transformers might be fixed capacitors and non-adjustable anyway). You might want to have a tweak at the RF trimmers on the aerial coils. Do this at the HF (short wavelength) end of each band. You should be able to work out which one is for LW and which for MW from the circuit. Peak up for max noise or on a weak signal; using a strong station will likely invoke automatic gain control and confuse things. Don't overtighten the trimmer screw if the trimmer bodyshell is made of ceramic material as it is easy to crack them. Cheers, Jerry

[LyntonP]

More tests.
Tried connecting an earth from the set to a water pipe. Signal strength improved background hiss remained.
Replaced Mullard substitute with correct AC2/PenDD and rewired as required. Worked fine.
Went back to G5RV aerial. Signals much stronger than internal aerial. Hissing remains but reduced. Looking to get a feed from this into the lounge.
Followed service instructions to peak RF trimmers on aerial coils. Made no difference at all so all trimmers returned to original positions.
Been reading lots of posts on this forum re magnetic loops and ferrite rod aerials. Noted with interest a copy of an article from a book called “Out of thin air”. I have that book somewhere. Need to find it and maybe have a go at the loop described. Not really an aerial for the lounge, but an interesting project.
Now the proud owner of a signal generator from forum member David. A splendid chap who provided me with a great demo and comprehensive information about the unit.
I take on board the suggestions to leave the IF alone which I will do until further experiments are tried.
Lynton

[cathoderay57]

Hi Lynton, seems odd that none of the trimmers in the aerial coil section seemed to do anything. There appear to be 4 trimmers - did you also try the two that I think are atop the tuning capacitor? Be careful though - the third trimmer is the oscillator which looks as though it has HT on it! Worth also checking the 0.1uF fixed paper capacitor between the bottom of the aerial coil assembly and chassis. On the circuit I'm looking at, which I think is the Murphy diagram, it's shown as C8. If it has gone open circuit or become a resistor it won't help things. Cheers, Jerry

[LyntonP]

Hi Jerry C8 was replaced with a new capacitor but on reading the instructions again and another look at the circuit diagram I don't think i touched C1a or C1b. The underside view of the chassis shows the location of the adjustable capacitors but not the two mentioned. I assume these are actually on the tuning cap somewhere. To be fair this is all new territory to me and I do not want to tweak the wrong thing and make the set worse.
Cheers
Lynton

[cathoderay57]

Hi Lynton, thanks for your PM. If you look at the top side of the tuning capacitor you should see 3 sets of vanes: one (C1c) is the oscillator, the next (C1b) is the mixer grid, and the third (C1a) is the aerial tuner. Wired in parallel with each of these tuner sections are 3 trimmer capacitors (C6, C4 and C2 respectively). These are normally mounted on the top of the tuning capacitor frame immediately above the vanes and are usually a fixed part of the frame; adjustment is achieved by screws or nuts on the top side that should be accessible from the top, sometimes through holes in a metal screen. I cannot tell you which is which so with power off you will need to do continuity checks. As said before be careful when making adjustments because C1c and C6 (oscillator) are connected to the AC/TP triode anode and so will be live HT (80v) via L9 and L10. You should not adjust the oscillator trimmer unless the position of stations of known wavelength (BBC5 Live, Talk Sport etc) are miles out on the dial. Adjust C1b (mixer grid) first on a weak signal or noise at the short wavelength end of MW. You should find it makes a difference. If not then it is possible that one of the aerial/mixer coils is open circuit so do continuity checks on all of them. The other possibility is a broken connection to the wavechange switch or even a broken contact. The only other thing I can think of is Murphy sometimes used a corrugated metal screened cable to the top cap (mixer grid) of the AC/TP. Inside there is a centre wire isolated from the outer by ceramic insulator beads threaded onto the centre wire. This isn't very flexible and sometimes the centre wire breaks at one end. Again, a continuity check is simple. Hope that helps. Cheers, Jerry

[LyntonP]

Hi Jerry, great information. I am not sure if the dial is out or not since when I received the radio the dial string was broken and the small pulley on the bottom of the tuning dial (basically a tube with markers on) had fallen off. The tuning dial is quite odd since the wavelengths are only a tiny bit of the dial the majority of the scale are just referenced to stations around Europe the idea being that you would line up which station you wanted to listen to.
I did take the top cover off the tuning capacitor for cleaning but didn’t really notice the capacitors you mention. I replaced the wire to the mixer top cap as it was in poor condition and yes there are a number of small ceramic insulators which I was very careful to replace.
Good advice on the voltage present on the capacitors, I would not have thought of that but checking on the circuit diagram shows that this is the case.
I have noticed that after fitting the AC2/penDD the ‘silent tuning’ feature seems to be working whereas previously all it did was reduce any signal across both bands.
One further question- As you know the speaker has a field coil. Would it cause a problem when working on the chassis to substitute a permanent magnet speaker. I know the set should not be operated without a speaker but it is very awkward using the original when turning the chassis over etc? It would be a bit safer as well!
I have attached a photo of the tuning dial to help my explanation.
Cheers
Lynton

[ms660]

The wavelengths are at the top of the scale, if the cursor and scale etc are aligned properly and the Osc is set correctly you can read off the wavelength relative to the transmission being received, before any Osc/RF alignment the cursor/scale/tuning gang's physical alignment should be checked.

The really nasty trimmer adjustment to watch out for is the trimmer capacitor across the 2nd IFT's primary, you will get a nasty jolt if you happen to touch that and chassis.

You can use a PM loudspeaker but the field coil (or its equivalent) will still need to be in circuit.

Lawrence.

[LyntonP]

Originally Posted by LyntonP

I don't think i touched C1a or C1b.

Because these are the tuning capacitors!!

I had another look today, and again followed the trimming instructions. I have to say I am not very confident doing this, and as such I think I have made things worse!
On long wave R4 is definitely worse, weaker signal and more hiss. However the Irish R1 station is better!
On MW R5 live is quite strong but other stations are not as good.
Bearing in mind that it was all done by ear and things did seem better, but of course I was on the big aerial. I must try and get a feed into the lounge, so that will be the next job.
I was also thinking of a loop aerial so I will start to collect materials to make one.
Lynton

[LyntonP]

BTW I have not touched the IF settings.
Lynton

[cathoderay57]

Hi Lynton, you have a PM with some help on alignment proc. If you previously used the sig gen the key thing is not to have the output level too high otherwise the radio's AGC and cross-coupling will occur and you won't be able to align properly. The sig gen output needs to be turned down such that the received signal at the speaker is only just audible.Cheers, Jerry

[LyntonP]

Originally Posted by cathoderay57

Hi Lynton, you have a PM with some help on alignment proc. If you previously used the sig gen the key thing is not to have the output level too high otherwise the radio's AGC and cross-coupling will occur and you won't be able to align properly. The sig gen output needs to be turned down such that the received signal at the speaker is only just audible.Cheers, Jerry

Hi Jerry
Only just seen this 5pm Monday! Looks like I will be back in the shed on Tuesday.
Cheers
Lynton

[LyntonP]

Originally Posted by LyntonP

I had another look today, and again followed the trimming instructions. I have to say I am not very confident doing this, and as such I think I have made things worse!

With some excellent guidance from cathode ray57 I again attempted to complete the trimming process on the Murphy.
It does seem to have been a great success with a big improvement on both bands.
Stations are much clearer and louder and the dial scales are now spot on.
It also gave me a chance to use my new signal generator which operated faultlessly.
The service instructions although absolutely correct are written in quite a technical way but as a relative novice I initially found a them little confusing. All clear now though thanks to the forum.
Cheers
Lynton