Murphy A242 VHF IF Alignment

[JimMac53]

I have finally gotten the Murphy A242 that I'm restoring working on FM. But audio output is weak and tuning is not very sharp. I'm blessed at having at my disposal top of the line test equipment, so I'm using it to align the FM. To start with the front end, rf and oscillator were out and I've managed to re-align it. But in checking the IF alignment, it seems the 3rd VHF IF transformer (just prior to the ratio detector) is seriously misaligned. I would like to realign this transformer, but the ferrite tuning slugs must be waxed into position, or it could be 50 years of dirt and grime. Either way, what is the best way to free these slugs so I can tune it? I know that those slugs can be and are brittle, so I want to be careful I don't crack them or wear out the slot.

[Steve_P]

If it's stuck, don't force it. If it's waxed in position, a gentle bit of heating up should free it.

Before you align, check for faults. If it's not been moved ever, then maybe it's OK? Also, please align IF before RF. The slugs could be off inside the can. A little superglue works wonders...

Needs a good antenna this set though.

Cheers,

Steve P.

[JimMac53]

No, I won't force it. I'm fairly certain I don't have a circuit fault. It does pickup all the local FM stations on its builtin antenna, but the audio difference between AM and FM is dramatic. Volume control needs to be almost on max to fill the room, where AM would be into serious distortion.

I measured the bandwidth of the IF. The 2nd transformer is spot on 10.7 mHz, but the 3rd peaks at 10.9 - 11.0 mHz.

I understand that the IF needs to be properly aligned before the RF and oscillator, but I did them to peak the audio signal plus they were easy. I do have the alignment procedure from the Murphy Service Manual and that's what I want to follow.

How do you go about heating them up to free the slugs? Heat gun?

[Steve_P]

Soldering iron to melt the wax. If that does not work, try a little bit of servisol.

Cheers,

Steve P.

[JimMac53]

Thanks, I'll try that.

[Leon Crampin]

Ear wax remover works on the slugs in Murphy sets of this vintage if you give it time. The IFs are usually stable on these sets - I wonder if one of the parallel capacitors has become low in value.

I have an A272C (a fairly similar but later chassis) which I use every day. I remember when restoring it that the alignment was virtually spot on - a minute tweak of the ratio detector transformer secondary tuning and slight adjustment of the AM RF trimmers was all it needed.

I wish it had the IF gain of the Bush sets (extra valve) and a tuning indicator - but it sounds good.

Leon.

[JimMac53]

Leon,

I'm sure this set has been worked on (or played on) in the past. One of the slugs in the coil in the FM front end was nearly out of the former. This set is one of the later models where the VHF oscillator is below the RF. This was a later mod Murphy did to these sets. Anyhow the coil with the slug nearly out is one of the preset ones and it was no way near where it should be.

But there is no evidence that the IF cans were touched. I went through the Murphy procedure as if I was going to align it, feeding 10.7 mHz into the frontend and watching the AGC voltage. The AGC peaks at about 10.9 to 11.0 mHz. I moved my scope probe to the anode of the IF amp, just before the 3rd IF transformer. The signal peaked nicely about 10.7 mHz. I then fed the 10.7 mHz into the grid of the IF amp, just after the 2nd IF transformer and put my scope probe back on the AGC and measured the same behavior as before -- peaks at 10.9 mHz.

However, I think you may be right. It could well be one of the tuning capacitors. This Murphy has had a rough time of it. Surprisingly all the valves work except the tuning indicator.

In the end, I believe Murphy gave up on this design and went the way of the ECC85, etc. I have to laugh at the alignment procedure which stated that no changes are necessary to the alignment unless valves V1, V2, V3, V4, or V5 have been changed! That's everything execpt the audio, rectifier, and tuning indicator.

[Leon Crampin]

Jim, I have a vague recollection that when Murphy changed from oscillator high to oscillator low (to avoid harmonics in Band III) they changed the oscillator slug material - a purple core, I think.

I wonder if your set has been re tuned to operate with the oscillator low with the original slug fitted?

Leon.

[HamishBoxer]

Try a hair dryer on stubborn slugs

[Top Anode]

It's a very old memory, indeed from 1957. But shouldn't you be looking for a double peak on that transformer...or was that just AM? Over-coupling was the term I think.

[JimMac53]

I'm afraid I haven't had a chance to try any of the suggestions, I've been off doing family stuff for the weekend and just got back. I like the earwax suggestion, but the hairdryer (or heatgun as I have no need for the hairdryer) will be nearby. Murphy did change the VHF oscillator coil, it's a bit longer than the original and the slug's pretuned position is different, but the color looks like the typical gray. Murphy painted a green dot on the VHF assembly to show that it had this mod. There are a few other circuit changes too.

The RF, Balance, and Oscillator tuning trimmers are a bit of a problem. They are really just bolts that screw through a little core. The bolts are at ground in all three cases. In any case the threads are worn and don't hold the bolts securely, hence the capacitance changes with just a slight jiggle. Once they are set properly, I plan to use a bit of finger nail polish to hold them securely.

I am using this radio as kind of an educational tool. I'm an electrical engineer, but have spent most of my career in digital design and specialized software. I played with radios as a kid and have just come back to it 40 years later. So now I'm learning something about the analog world. 30 years ago at university we had one chapter on valve (tube) technology, small signal analysis, and models. What I remember from that was "They (tubes) are kind of like FETs". But I still have my dad's US Navy books on tubes (circa 1940).

I don't think the IF transformers were double peaked. Not sure how they would do that unless the primary is peaked at one edge and the secondary is peaked at the other. Wouldn't that play havoc with the ratio detector?

What do you suggest for cleaning valve pins? The 6D2 is sensitive to how its installed.

[Steve_P]

Bend the pins in slightly, and clean the valve pins and socket with a little Servisol.

It's worth swapping C71 (Murphy manual) by the way.

Cheers,

Steve P.

[JimMac53]

Thanks Steve, I'll try that. C71 has been changed with a 4.7 uF non-polarized cap. Actually all Hunts decoupling caps and electrolytics have been changed. C8 was the one that caused the most problem. It was cracked and it wasn't until I read the service manual that I understood how critical it was (in value). What I pulled out was a 500 pF, which at first didn't make sense according to the schematic (which shows 620 pF), until I read the mod notes and figured out the one I had was one where the VHF oscillator is below the RF. I had always thought that manufacturers would install whatever they had on hand, as long as it was close, so I didn't think the 500 pF was critical. For awhile I tried to get it to work without it at all, and couldn't understand why it was oscillating like mad (and not a the freqs I thought it should). Finally I put in a 510 pF cap and everything settled down nicely and started to work.

This Murphy is nice to work on and play with because I can put it in almost any position and it's stable (won't fall over and crush something). Plus they brought test points out for critical signals. This is a great set to begin on.

I'm wondering if I can use my network analyzer to produce the IF bandwidth. The trouble I see is it's expecting 50 ohm input and output impedances.

[johnn]

I have recently used meths to free the slugs on a Pye VHF2D. They were well stuck with what seemed like varnish!
These slugs have a coarse thread which allowed the meths to go in. Some were freed form the other side. I took out the slugs & cleaned them, as they had a tendancy to restick.
Bush threads are finer for instance.
John

[JimMac53]

John, I have to ask what meths is? Sorry, but I've never heard of it.

[Steve_P]

Methylated Spirits. Also known as Methyl Alcohol.

Cheers,

Steve P.

[AlanBeckett]

Steve,
Not quite. It's mostly Ethanol (Ethyl Alcohol) is mixed with a small quantity of Methanol to make it unfit for consumption, and dyed purple.
Alan

[JimMac53]

Now I get it. Thanks.

[Top Anode]

I'm certainly not trying to pontificate here because my memory is literally over half a century old, but it seems like yesterday that my lecturer at Colchester was talking about the first IF tx being brought to a singe peak and the second one a 'over coupled' double peak. Combined, they made a broad response curve - with a treble peak.

Just for the sake of testing my synapses, I would love to know just how much of that is correct.

To my shame I recall snapping the centre tube while freeing a core. It took a long while to join all the wires and glue the tube. (Got away with it though)

NB, have just seen a program that said that the Marie Celeste had many barrels found empty form her cargo of ethanol , and that a 'cold flash' may have prompted evacuation to a tender - then losing the tow. I had no idea they were refining that stuff as early as that.

[JimMac53]

You may be and probably are correct. The question is: What does this treble peak gain? What is the purpose of aligning the IF's in this manner? It seems to me, naively, that for the ratio detector to produce as little distortion as possible, you'd want a symmetrical peak centered around 10.7 mHz. The exception would be if the receiver used a slope dectector or a phase-shift discriminator which does use a double-tuned transformer.

I've gone through the Murphy Service Manual for the alignment of the VHF IF. The 2nd and 3rd IF are peaked at 10.7 mHz. The exception could be the 1st IF, where after peaking L9 (which is on the anode of the VHF oscillator/mixer), it calls for you to unscrew its core by 1/8 turn.

When I get around to actually trying this, I let you know what I get.