Murphy A242 VHF IF Alignment

[Herald1360]

A single peak centred on 10.7 MHz would tend to produce some amplitude variation depending on the deviation level (mod level) of the signal. A flatter peak made up from three individual staggered peaks would do this to a lesser extent. Maybe it was thought that this would introduce some distortion into the detected signal.....

I suppose it would only be on weaker signals where the limiting was marginal or non existent.

This doesn't help with recollection confirmation, though.


Chris

[JimMac53]

I wouldn't think a ratio detector would need a flat response. It reacts to the instantaneous shift in frequency and is somewhat immune to amplitude variation, which is its big advantage, I'm led to believe.

[JimMac53]

I tried the ear wax remover, but to no avail. Essentially soaked it all day, periodically removing what debris there was, and adding more. The good news is I haven't done any damage and it's clean. I'm a bit timid at applying a lot of torque for fear of breaking the slug or former or something. I don't think it would require lots of torque to turn the slug once it's free. So I'm backing off and giving it a rethink.

[AlanBeckett]

Jim,
Was it sort that fizzes (Hydrogen Peroxide) or the oily sort (smells of cloves)?
I would guess that the former is better than the latter.
Alan

[JimMac53]

It's the Hydrogen Peroxide type (Earex) and did foam a bit when I left it. The FM side still works, but the AM side is now not working, no stations, lots of hetrodynes (all bands). I need to leave it and start fresh, as I'm tired and frustrated. Not a good combination for working with electricity. I'm going to go work on the cabinet.

[AlanBeckett]

Jim,
Good idea. I usually combine it with lots of photos (for future reference) and some beer
I would think some gentle warmth when you get back should do the trick. I'll keep my fingers crossed for you.
One thing I have done in desperation. Take the can off and hold the former when you try to turn the core. That at least avoids ripping the wires off.
Alan

[dave_n_t]

If you can access both ends of the recalcitrant slug, try using trimming tools in both ends at the same time (remember to turn both in appropriate directions ). This should double the amount of torque you can safely apply.

dave

[JimMac53]

I just gave the cabinet its second coat of Danish oil. Looks great. In the end I should have at least a good looking poorly working radio. One thing I like about this hobby is there is always something else to do that uses the other half of the brain.

I think I'll try the gentle heat approach. There is another slug on the bottom side of the former (secondary) and I think it needs to be freed too. I noticed that the FM audio sounds distorted when I'm tuned, what I think, is directly on station. That either side of that the audio sounds better. When I'm tuned into Radio 2, I'm still getting a bit of Radio 3 (sometimes that's a nice mixture). I think this supports my theory that the IF is misaligned. When I'm directly on station (IF at 10.7 mHz) the ratio detector is close to one end of its range (distorted audio). When I'm slightly mistuned (IF at 11.0 mHz), then the ratio detector is in the middle of range (audio ok). Just a theory. Think I'll go work on the HMV.

[JimMac53]

Ok, I got that b*****d slug to move. It was a combination of WD40, alcohol, dental picks, and gentle heat. I first attacked the bottom slug. This one proved easier to free. No heat needed, just patience and lots of cleaning and lubricating. And cotton swabs. Once that one was free, I went back to the top slug. I tried cleaning and using dental picks to chip away the hard gunk, but it still wouldn't move. Got the heat gun out with its wrap around shield, low setting, while applying torque. After about a minute it started to move. I let it cool and it re-siezed -- couldn't move it. Heat gun again and filled the top of former above the slug with WD40. Freed it up again. Tuning is going to be way out of wack, but it's an opportunity to learn something new.

[Top Anode]

Okay, so with a perfectly clean slug and tube, what's the approved method used to lock them again?

I know it won't please the purists, but I imagine a tiny blob of silicone (bathroom) sealant might do the trick. It would be vibration resistant while being able to be pulled off with tweezers when needed.

Car restorers use masses of it I'm told, saying that it keeps the moisture out as well as bonding.

[Brian R Pateman]

Some silicone sealants evolve gases and other substances which might be detrimental to the insulation of the coil wire etc.

I use either Loctite or nail varnish.

Regards,

[AlanBeckett]

TA,
Do you really want to lock them? Both the methods suggested are pretty permanent.
I've seen/used two methods over the years which are not so permanent. The first, used by many manufacturers, was to put a piece ofd thin elastic (band) down the thread. The second, used in much rougher (industrial) environments was to use extreme pressure grease. In its normal state it's very thick and sticky, but does flow, as its name suggests, when pressure is applied.
Alan

[Station X]

There are three reasons for "sealing" cores. To prevent rotation, to discourage tampering and to indicate where tampering has taken place.

Where the slugs are attached to brass threaded rod with a lock nut, sealing is not really necessary. To seal slugs with a screwdriver slot I have used nail varnish, typewriter correction fluid (Tipex), emulsion paint, Waxoyl (designed to stop cars rusting), silicone sealant and candle wax. All work just as well, but I have not subsequently tried to move the core.

If using candle wax drop a small lump into the bore and melt it with a hot piece of wire or the tip of a small soldering iron.

[MichaelR]

Originally Posted by Station X

There are three reasons for "sealing" cores. To prevent rotation, to discourage tampering and to indicate where tampering has taken place.

.

I agree with the sentiments but would add that the method of sealing must be such that subsequent realignment ( readjustment of the cores) is possible. Replacement of a valve can require an adjustment of an IF so the whole point of the adjustable cores is future realignment need.

Mike

[JimMac53]

All good points, but I'm not worried about tamper indication and I'm not really worried about movement much either. These radios, until someone puts them in the loft or barn, live in the most benign environment possible. The Murphy notes already state that re-alignment might be needed after replacement of V1 through V5. I am being taught how finicky these old radios are, but just how sensitive are these cores anyhow? I'm not planning on locking anything down in the immediate future and would prefer something akin to grease that doesn't harden over time, but would provide some token resistance to movement, but allow future re-alignment. The trimmers that are in the frontend of this radio will need some type of "torque stripping" to keep them from moving and they are indeed quite sensitive.

[Boom]

I use Vaseline. It is just thick enough to grib the cores but doesn't set so the cores can always be moved at a later date. I'm not sure that cores move about anyway when they are left sitting in a set.

[JimMac53]

Thanks Dave. I think I'll try that.

[Top Anode]

Some silicone sealants evolve gases

Yes, good point. That vinegar smell can really cut sometimes.

I would think that the only real reason a slug might move again in a cherished and static radio, would be if the tube was clean and dry and had a speaker nearby...as is so often the case. There might be a hint of mechanical resonance that would vibrate the slug round.

I'd imagined the silicone blob to only be on top of course, putting it in the gap would be rather permanent - if the car people are correct.

However, reading on this forum about coil insulation breaking down, I can see how one should shy away from volatile/caustic substances.

[Tim]

I don't think these radios were fantastic on VHF anyway. Also make sure the two VHF triodes are in good condition(or better still new)

[JimMac53]

Tim, you hit the nail on the head and I agree with you. There are still some things that have me puzzled, but I've gone through the alignment procedure and I'm disappointed with the results. In the end I did manage to crack one of the ferrit cores, but it was a good crack and I was able to epoxy it back together. I also found the valve base for the VHF detector (6D2) was worn out -- poor contacts -- so I replaced it. Once I did that, aligning the 3rd IF went just like the service notes indicate. Since the 1st and 2nd IF didn't indicate a problem, I left them alone. The final stage of VHF alignment is the oscillator, balance trimmer (minimizes RF radiation), and RF stage. Oscillator and balance trimmer went according to plan. But the RF stage is touchy. If you just look at the trimmer crosseyed, it changes. It doesn't help that the screw is loose. I used a bit of loctite to give it some friction, got it set, and used some torque stripe to set it. After all of this gain is still low. It is dependent of the antenna connected to it. It works better with the HMV 1132 antenna than its own internal antenna. Also the magic eye tuning indicator is barely moving, hence my theory that something is still wrong. It could be the two VHF triodes in the front end.