Murphy A50 help wanted

[agardiner]

Hello all,

First a quick qualification. I have over 30 years electronic repair experience, but therefore am relatively new to valves.

So I am looking at a Murphy A50 for someone. It has previously been re-capped by someone else, and looks like a perfectly reasonable job was carried out. The wave change switch needed a good clean, and some of the wiring replacing. All of the valves appear to test OK on my emission tester, although this is a rough gauge as I don't have official settings data for them.

The set now works, including the magic eye (although dim with age), but appears to be somewhat RF 'deaf'. The 'installation' switch for the RF amp valve is switched on, and indeed if turned off then the set is pretty useless. I have checked the alignment with my RF signal generator, and it works on all bands and indeed appears to work well with the generator, so the oscillator must be working.

What is interesting and where I am struggling is that on Short Wave it actually works very well, pulling in more stations than my world band receiver. However, as the frequency reduces, so does the performance. On MW, it can just about pull in Smooth at the top of the band (1250KHz strong signal), but by the bottom of the band, Radio Caroline (648KHz strong signal) is virtually inaudible. On Long Wave you can forget it; totally silent even for the strong Radio 4 signal. As already said though, no problem with the signal generator which seems consistent throughout.

So I am struggling to understand the issue, is it likely that the RF amp valve is defective? Incidentally, the selectivity/tone control works as expected.

Many thanks for some pointers!

Adrian.

[PJL]

Most sets of this period work OK without an RF stage. What are you using as an aerial?

[Silicon]

Have you measured the AGC voltage on and off station and on each waveband?

You will need a voltmeter with an input resistance of at least 10 MegOhms.

RF and IF coils can go faulty and the Dust cores that tune the Coils can crumble or fall of their adjustment rods.

Have you checked the resistors to see if they have changed value more than 20%?

Comparing the measured voltages with those on the service manual may help.

[agardiner]

Originally Posted by PJL

Most sets of this period work OK without an RF stage. What are you using as an aerial?

I have repaired a number of old sets and indeed they work very well in my area with just a wire, which is what I have been using.

Originally Posted by Silicon

Have you measured the AGC voltage on and off station and on each waveband?

You will need a voltmeter with an input resistance of at least 10 MegOhms.

RF and IF coils can go faulty and the Dust cores that tune the Coils can crumble or fall of their adjustment rods.

Have you checked the resistors to see if they have changed value more than 20%?

Comparing the measured voltages with those on the service manual may help.

No I haven't; will measure the AGC voltage. I don't have reason to suspect the cores, as they do adjust the signal level when aligning with the generator.

[Leon Crampin]

I had an A50 which was an excellent performer - both in terms of RF and audio. Big brute though...

As I remember, the switch on the top of the chassis for disabling the RF stage on LW and MW is a toggle switch with non wiping contacts. (There's also a separate noise limiter toggle switch, I think on the rear panel). As the RF disable switch may well have been unmoved for a few decades, it's possible that the contacts are oxidised. This could fit your symptoms because the RF stage in in circuit on the two SW bands (the other is for 45 MHz TV sound) at all times, but selectable with the toggle switch only on LW and MW.

As others have suggested, AGC line measurement preferably in conjunction with a generator sweep may give further clues. Some of the disc ceramic capacitors used by Murphy to tune or pad the RF coils may now be suspect, and it's also possible that the coils for the lower frequency bands have lost Q due to moisture ingress. The SW coils have so few turns that this problem is less likely on these bands.

Best of luck, these sets get the very best fidelity from our now inpoverished AM transmissions.

Leon.

You can interchange the RF and IF amplifier valves for test purposes.

[agardiner]

Hi Leon,

Thanks for your information. The RF switch is working fine and indeed does make the problem worse when the valve is out of circuit.

The coils may indeed be the problem due to moisture, as the cabinet has suffered damp and requires a lot of work. I haven't measures the AGC voltage yet, but the magic eye works correctly so this would only be a problem on the lower bands.

Can the coils cause the problem I have, given that the radio is tuning to all frequencies correctly? (as tested with the RF generator)

Will try swapping the valves, thanks for this tip. I don't have reason to suspect the RF valve but possibly the first IF one.

Thanks,

Adrian.

[cathoderay57]

Hi Adrian, in Post #1 you say you checked the alignment. Not sure whether you made any adjustments? While the oscillator on LW and MW might be on mark, it sounds like the aerial coils, mixer and RF amplifier stages need realigning. The IF stage may be OK if, as you say, freq calibration and SW sensitivity are alright. On my A50 I get full scale deflection of the magic eye on R4 198kHz using a 30ft longwire aerial in the garden. Cheers, Jerry

[agardiner]

Hi Jerry,

I checked the alignment feeding RF on various frequencies on all bands. 130KHz, 400KHz, 1000KHz etc. All spot on. 'Tweaking' the various coils/mixer gently proves they are working and adjustable.

Hence my confusion.

Cheers,

Adrian.

[cathoderay57]

Unless you have followed the alignment procedure set out in the service data here https://www.service-data.com/search....&search=Search your results are probably going to be less than optimal. It's not simply oscillator frequency accuracy against the dial, all of the signal amplification stages need to be aligned on the same frequency to get max gain on all bands. Sorry if I'm labouring a point. I think it is highly improbable that any valves are at fault if the set works well on SW. What normally happens is that either a previous restorer has misaligned the receiver, or else fixed capacitors in the LW and MW tuned circuits might have drifted in value. Cheers, Jerry

[agardiner]

Originally Posted by cathoderay57

Unless you have followed the alignment procedure set out in the service data here https://www.service-data.com/search....&search=Search your results are probably going to be less than optimal. It's not simply oscillator frequency accuracy against the dial, all of the signal amplification stages need to be aligned on the same frequency to get max gain on all bands. Sorry if I'm labouring a point. I think it is highly improbable that any valves are at fault if the set works well on SW. What normally happens is that either a previous restorer has misaligned the receiver, or else fixed capacitors in the LW and MW tuned circuits might have drifted in value. Cheers, Jerry

Points noted - thanks Jerry. Will double check the alignment as per the manual and look at the caps again from the previous restore. After that will consider the coils. I tend to agree about the valves given the shortwave performance.

Adrian.

[MurphyNut]

I have a A50, best sounding table set I have, just replacing all the capacitors got it working really well. It cant be stressed how important the length of an aerial is for these kind of sets, mine is at least 10 metres.

[agardiner]

OK, thanks everyone for their help so far. I have now spent some time checking the alignment carefully and adjusting as needed. I have also found that the cause of virtually no MW reception to be related to the wave change switch. I have now cleaned (again) using IPA, and MW is working pretty well, even without the RF valve in circuit.

However, LW is still dead, although works just fine when injecting an RF signal to the aerial socket, which still leaves me flummoxed. When injecting RF, it tunes at the correct point across the band, but Radio 4 on 198 is nowhere to be found! In case it helps point me further in the right direction, I have noticed that the reception ability drops off as you tune down the MW band. Reception is now excellent above 1000KHz, but it cannot pull in a station on 648KHz which most sets do.

Can someone assist with the AGC measuring? Where would be the best point to take measurement and what sort of voltages would you expect. So far I have measured at the RF bypass switch, getting a voltage that varies from 1 - 1.5V.

Thanks again for everyone's help.

Adrian.

[cathoderay57]

Hi Adrian. The low signal strength at the LF end of MW and the absence of R4 LW are probably unrelated. The symptoms that you describe for MW are typical of a set that has lost "Q" in one or more sets of MW coils. This sometimes happens if the set has been stored for long periods in damp conditions because the damp seeps into the coil former and affects its "Q". The "Q" of a coil is defined as its inductive reactance divided by its resistive losses at a specified frequency. The losses seem to affect the LF end more noticeably. On other receivers it is usually possible to trim the sensitivity semi-independently at the LF end of the band by adjusting a core screw within the coil. Unfortunately there is no such adjustment on any of the coils of the A50. The dampness effect on the coil formers is irreversible in my experience but others have suggested baking them in an oven can improve things to some extent. However, that's pretty drastic and I don't recommend it. For the LW problem, the first thing I would check is that you have tuned the A50 local oscillator to the correct frequency. The Intermediate Frequency (IF) of the A50 is 465kHz. Usually (but not always) the receiver local oscillator (LO) runs at a frequency above the desired reception frequency by the same amount as the IF. I think the A50 is a typical set where the LO runs above the received frequency. Therefore, to receive a signal on 198kHz LW the LO should be running at 198+465=663kHz. To check this, if you have a transistor portable, or even another valve set, place it close by the A50 and tune the transistor (or other) to 663kHz (452m) MW. Sweep the tuning dial of the A50 around 198kHz and you should hear a warbling note from the other set. If you don't get the warble then either the LW oscillator isn't running, which can be the result of a weak AC/TH1 (even though it runs happily at higher frequencies) or else the LW oscillator is tuned to the wrong frequency. Although you say you are getting a response from the set on LW when you inject a signal from the sig gen it is possible that you are tuning to a harmonic from the sig gen or the image frequency. The rule of thumb for alignment is "if the oscillator frequency is higher than the received signal (as in the A50) the lower frequency position on the sig gen dial or the higher frequency (shorter wavelength) position on the radio dial is the correct signal at which to align". Hope that helps. Cheers, Jerry

[hilitevr]

In the years Soon after the end of WW2 I was working as a radio engineer for a Murphy dealer, we used to get lots of pre war madio's in for repair. It was common to find very poor IF gain And very flat tuning. One of our older engineers use to take the IF coils home and bake in his oven, on re-fitting them the gain was up to normal. This always worked so why not give it a try?

[Silicon]

I don't want to distract you from the investigation of the tuning coils.

However a possible failure mode has occurred to me.

Later version of the A50 were wired so that the RF valve could be cut off by applying a positive voltage to the cathode. This voltage was derived from the HT by using a switch and a resistive potential divider circuit.

If the wafer switch insulator becomes slightly conductive, when the switch is open an increased voltage will be applied to the RF valve cathode and that will affect its gain.

[agardiner]

Many thanks for the suggestions.

Cathoderay57 - I will try the radio trick to check the oscillator, and also have a high impedance frequency counter I could connect. I don't think it is a harmonic but worth double checking. And I might see if I can locate a spare valve to try. As for the poorer gain possibly due to damp in the coils; this would not surprise me as the cabinet had to be treated for some damp damage. What would you do in this respect? Leave alone given the reasonable performance on the higher frequencies?

Hilitevr - I have decided to get and replace the first IF valve. Although it works, I noticed this morning on my valve tester that its emission drops off after around 5 mins of use. The RF one is solid so I suspect the IF valve is on its way out.

Silicon - Thanks for that, will have a good look and perhaps try disconnecting the switch to test.

Thanks again; will report back on findings!

[cathoderay57]

Hi Adrian, where you live you should be able to receive Radio Caroline on 648kHz. I don't think there's much else worth having at the LF end of MW these days - presuming R5 Live is OK on 909kHz - you obviously only need decent reception on one freq so you might not need it on 693kHz.? The snag with the oven trick is how much dismantling you want to do. Putting the entire coil can in the oven even on a low heat isn't going to do any wax capacitors or resistors much good, if indeed there are any inside. Removing the coil from the can runs the risk of severing the fine coil wires. Some of the coils are coated in wax - dunno if these ones are, but it wouldn't be a great idea to melt it. If it were me, I'd leave it as-is. Re: LW, you might like to try attaching the aerial wire to the top cap of the AC/TH1 and see if you can pull in R4 on 198kHz. If so the fault might lie in the RF stage. All it takes is a broken wire on a coil somewhere so also worth checking continuity of all the LW coils. Cheers, Jerry

[hilitevr]

If you don't fancy the oven, why not try using a hairdryer on the supect coil f.or an extended period, nothing to lose

[agardiner]

Thanks to everyone who has assisted thus far. Got a very busy week this week, so won't be able to look further at the Murphy until the weekend.

Will post back next week on progress.

[agardiner]

So a quick update for everyone who has helped.

After trying out all suggestions, I have come to the conclusion that the coils have lost Q. The oscillator does work and is on frequency, but unless I inject a signal into the set, then there is nothing on LW. Bottom of the MW is poor but by 750KHz the set is good and performs well. Will live with that! Great reception on the rest of the band and an input jack to boot.

Thanks again for everyone's help.