Refurbishment of an HMV Model 1121

 The dial of the model 1121 is novel. The four slats of the dial assembly are tilted so can be read without difficulty in normal lighting, or alternatively, through an edge lighting system using the glass plates as light guides. The paint used was probably luminous when new, containing a radioactive isotope, but over the years this has faded somewhat. The tilted glass also provides another feature in that it provides an exit for the sound output from the much larger than average loudspeaker used in the set. As the speaker is sited behind the dial it re-uses the dial space rather than requiring a second area of front panel as with contemporary sets of standard design.


 This example of the HMV is not in particularly good condition as it has been attacked by woodworm. Thankfully this is restricted to parts that are not immediately obvious, being mainly a lower rear corner and the underside. The first task was to remove the chassis and the loudspeaker/dial assembly and tackle the holes and any remaining infestation. It's said that if one taps a case and quantities of wood-dust fall out of the holes then there is still woodworm activity, but this may not be a true test, as if this is the first time the tapping procedure has been done, wood-dust will inevitably fall from the holes. I almost used the word "sawdust" but I'm not a Zoologist and can't say that woodworm have micro-miniature saws for dealing with their meals. Maybe they have heads faced with emery paper and just nod them up and down or tiny steel-tipped teeth? Tapping did result in little heaps of dust, but I got the impression that there was no-one at home.

What should I use to mask the myriad of holes? I settled on superglue. This was a sort of brainwave as fortuitously I had a couple of tatty tubes of the stuff that had become almost unusable and secondly, the stuff must surely be lethal to woodworm as it contains a goodly proportion of cyanide? What should I use as a filler? I looked around and settled for wood dust having, by then, obtained a plentiful supply. Unfortunately, as the holes were rather deep I had to confine the dust to the finishing material and look for something else with which to block the holes. The solution was cotton buds, of which I have a huge supply to carry out cleaning jobs on virtually everything that settles on the bench. I found that pushing the cotton wool material, suitably wetted with superglue into the holes using the spike of an old pair of compasses (this being just the right shape and size to do the job) did the trick.

After half an hour or so all the holes were filled and after some little more time I'd stained the tops (wood dust mixed with superglue) with black ink so they look like innocent fly droppings. Next I had to deal with the overall finish. As the radio used the typical high gloss finish of the 40s and 50s, which had extensively cracked from internal heat or sunlight, the choice was either to strip the lot and start afresh or to make running repairs. Having got several radios that had been stripped and refinished I'm definitely not in favour of that approach. They look sort of "homemade by an amateur woodworker" so I chose to merely remove the evidence of cracking. I used a fine emery paper wetted with oil to carry out this work. As always some areas of the original finish had completely lifted. Nine times out of ten this will be on the top of the radio where, over many years, a favourite place for standing a teacup has resulted in a circular stained patch or, just as likely, a water stain from an over-watered pot-plant has removed the varnish.

There's no way to make this sort of damage invisible (except perhaps by an antique furniture restorer... which I'm not) so I stained the bare wood to make it match the surrounding finish. To complete the job I applied linseed oil, then polish, to bring back a shiny finish. By no means perfect but it does still look old, which is what I was trying to achieve.

I can't see the point of making a set look like it just left the factory. Some people will strip a chassis and have it chrome plated but I just rub off surface rust and maybe apply some proprietary compound to turn the brown colour a dark grey.

Electrical work is two-fold. Replacement of faulty components and re-alignment of the RF circuitry so that the set will work well.

As a general rule, to restore good performance, all the old paper capacitors should be replaced. Some (those that are leaky), if left in place unchanged, will destroy valves by imparting a high positive bias to the control grid. Some (maybe not especially leaky) have lost their effectiveness and will no longer provide proper decoupling and reduce overall performance, whilst those (leaky ones) in the AGC line will shunt the high value resistors and the set will lose its dynamic performance.

Resistors in these old sets are not as critical as one might imagine. As long as a resistor value is approximately correct, not a lot of difference in performance will be noted. A rule of thumb is to change a component if its more than say 40% away from its marked value. In this HMV all the 47kohm resistors fell into this category (something for which I've never had an explanation as nearly all the old 47k resistors of this style seem to have drifted up to about 70 or 80k). I also changed a 4.7Mohm and a 470kohm that were sky high in value.

The designers of the HMV had dispensed with the customary HT smoothing choke, presumably in the interests of economy, and because mains energised speakers had gone out of fashion (for much the same reasons of cost), had used a simple set of resistors and capacitors. Maybe the word "simple" is not too relevant as four capacitors and three resistors have been used to establish the HT line and smooth it to reduce mains hum.

Here I found the clue to the demise of the set. One of the resistors in the smoothing chain was open circuit. I replaced this together with the selection of others, noted above, thought to be too far gone, and all the paper-based capacitors.

At some point I decided to power-up the old set using a variac and find out if life was restored.

As the thing warmed up an almighty hum emerged from the loudspeaker. The main smoothing capacitors were useless... I hadn't checked those. Some advocate re-forming of such things, and this may in fact be a satisfactory thing to do, but I'm not sure about this specific problem. High voltage electrolytics are in plentiful supply if you're not a purist. Look in any defunct TV set or computer power supply and there will be a selection of the things. I chose a couple of old chassis and extracted a pair of 100uF, 400-volt components. Never use a capacitor of this value as a reservoir capacitor (that's the one connected to the cathode of the rectifier) as the maximum value of this component (quoted by the valve manufacturer) will be within the range of 4 to 32 uF. Any larger than the specified value will drastically reduce the life of the rectifier.

I fitted my pair of capacitors to the set in the third and fourth positions in the filter and this removed the hum. The new components were small enough to wire in place at the main tagstrip.

Having tamed the set the next job was to check its performance.

This model is unusual in covering the "trawler band", the frequencies between medium waves as well as the standard short wave broadcast band. Wavelengths covered are as follows:-

Long waves.....2000m to 720m
Medium waves....580m to 187m
Short wave 1....187m to 50m
Short wave 2.....50m to 16m

The IF is 465KHz and the first problem I encountered was that the tuning cores were well and truly locked into postion and only with the greatest care was I able to reset them so the the amplifier was centered on the correct frequency.

Before I could proceed to the tuning-up of the front end I had to fathom out the method of relating the dial settings to the tuning capacitor. Because the dial mechanism is part of the overall assembly carring the loudspeaker, and this was now detached, the designers had added a special servicing pointer referencing the main part of the tuning mechanism (a large circular metal plate) with a scale in inches. Now this may not at first seem odd, but odd it is because, rather than use a decimal scale calibrated in tenths of an inch, it's marked in standard imperial measurements. Where else, but in the factory of an English radio manufacture, would you hear the term "Re-tune to 54.5m (27/32in)" I wonder if their engineers ever tried to do this? Trying to count the tiny divisions whilst the set is standing on its end with the dial readings underneath one's field of view is not easy!

Attempting to tune the radio showed that the part of the mechanism on the chassis was seized, which explained why the dial cord (all four yards of it!) had come off its pulleys. Judicious application of switch cleaner and lubricating oil sorted out the problem.

Despite all the tribulations connected with the miniscule imperial dial settings I managed to get the set to track correctly on all four wavebands and, in fact once back in its case, much to my surprise, the dial read correctly.

As with all radio sets of this vintage one needs a reasonable aerial. Positioning and size of an aerial is not critical unless it's used in the vicinity of a television set which unfortunately generate lots of interference. Fluorescent lights may also generate interference. With this model the aerial need only be a few yards long if one's interest is not short waves, however more exotic stations will be heard from a long wire. If interference is a problem the earth socket, at the rear, could be connected to a convenient waterpipe or similar point having a good path to ground. This often improves matters.

This set has a rather special tone control, not just a capacitor across the output. The tone switch in its four positions selects combinations of components which alter not only top cut and bass boost but also IF bandwidth. This can noticeably improve fidelity; in fact is this one of the first high fidelity sets?

I didn't have to change any of the valves as all were quite serviceable.

The part of the dial assembly that was integral to the loudspeaker, like its counterpart on the chassis, was also seized and had to coaxed back to life, by cleaning and the use of a thin oil. The dial slats were pretty dirty because both the fronts and backs, by virtue of their tilting, are exposed to the outside world. Cleaning these has to be tackled with the utmost care so that the lettering, and hence one of the main attributes to the look of the set, isn't removed.

Having finished the main work I connected an aerial and set the tuning to around 19 meters. The first station I heard boomed out of the loudspeaker at enormous strength... it was Radio Japan. Most odd!... as the set's owner lives in Okinawa. I twiddled up the dial.... the next loud station was Radio China. Clearly propagation (or maybe the set?) is better than average.

A few finishing touches were now tackled. I cleaned the control knobs and re-filled their long dulled markings by rubbing a white wax crayon across them. What sort of mains lead should I fit? I decided that a standard modern UK type would be best but I didn't fit a UK-style 13-amp mains as that would be no use in its new home, which may be Japan or the US... I don't know where exactly just yet..

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