Defiant MSH551 Console

Introduction

A friend of my late father, who knew I collected radios, gave me this set. I was planning to sell it but my partner liked it and wanted to keep it in the hall. I was somewhat indifferent about it, but have to agree that it looked good in the corner with a pot plant on top!

As received it was not really in a fit state to display. The mesh speaker grille was dented and the tuning scale was not straight. The cabinet had a number of knocks and bumps, but I was hoping I could do something to tidy it up without refinishing it.

Defiant sets were made for the Co-op. Some used chassis made by companies like Plessey fitted into cabinets made by furniture manufacturers, while others were made my major radio manufacturers. This set uses Mazda valves (6C9, 6F15, 6LD20 and 6P25), which is quite unusual for Defiant sets (most used Mullard valves). Discussions about the possible manufacturer and model number in the Forum did not come to any conclusions. The main users of Mazda valves were Murphy and Ultra, and the general standard and style of the chassis looks more like Ultra than Murphy to me. However the majority vote probably goes to Plessey.

The set uses a dual metal rectifier with two capacitors in a voltage doubler circuit. These capacitors and most others in the set are axial types mounted in clips around the chassis – there is no single large smoothing cap.

The circuit and construction are fairly basic. Low cost Paxolin valve holders are used, and the set does not use delayed AGC (the second diode in the 6LD20 is connected to the cathode and not used). The only gesture towards quality is the negative feedback from the output valve cathode to the bottom of the volume control. The tone control applies negative feedback around the output stage and also acts as the grid bias resistor.

Since I had no service data, I drew this diagram of the audio and power supply stages as I worked on the set.


Repairs

The donor said that the set had been in use until a few years ago when it had stopped working. It therefore seemed likely that there would not be too much wrong with it. After removing the chassis and many years accumulation of dust and cobwebs, I connected my capacitor reformer across the HT. It came straight up with no problems at all. The smoothing caps were OK – that was a relief because it would be quite a lot of work to replace them all.

I replaced both dial lamps, so I had a visible indication that the set was powered. I also connected a test speaker and aerial.

A resistance check across the mains lead with the meter showed open circuit, but a check the other side of the mains switch was OK. I connected the mains and operated the switch a few times, hoping that the voltage would break through the tarnish or whatever on the switch contacts. It didn’t.

I decided to replace the volume pot/mains switch straight away, since it would have to be done sooner or later. The original pot was 2M, but I only had 1M types so that had to do. While replacing the pot/switch I replaced the wax-paper capacitors connected to it. I also replaced the output valve grid coupling capacitor and the tone corrector capacitor. After checking for no leakage between the mains transformer and chassis with the Megger, I fitted a new length of three-core mains flex and earthed the chassis. A capacitor that had been fitted between one side of the mains and chassis was removed and not replaced.

Time for another test. One dial lamp came on and the HT rose to about 280V. After a few seconds the HT dropped a bit as the output valve warmed up, but there was no sound. The voltages around the output valve and triode amplifier seemed reasonable

A screwdriver on the centre and top of the pot bought the expected buzzing. However there was no buzzing on the far side of the radio/gram switch (it was in the correct position). The switch pole was not making contact, and a shot of contact cleaner didn’t help. I bypassed the switch using a lead with a croc-clip on each end, and was greeted with noise. By operating the waveband switch and tuning control I was able to find a few stations, but the set seemed very insensitive. The top adjustments of both IF transformers looked like they had been fiddled with previously. I carefully adjusted them by ear, which improved the signal strength, but also introduced noise and whistles.

There were several more wax-paper capacitors remaining in the set, in the IF and RF stages. I replaced them all. On the next test the whistles were gone and the IF adjustment appeared to be working. I connected an analogue meter across the AGC and adjusted the IFs for highest voltage reading on a medium strength station. The set now sounded a lot better, but still didn’t seem quite right. It was a bit shrill, no matter where the tone control was set.

I remembered that I had used a lower resistance tone control than the original. Since the set doesn’t have delayed AGC, the volume control is part of the detector load and also loads the AGC voltage. I connected a 1M resistor in series with the top of the volume control so that it appeared as a 2M load. This improved the quality and also caused the AGC voltage to rise by about a volt. However the resistor attenuated the audio too much, particularly on weaker stations. Connecting a 0.05uF capacitor in parallel with the resistor solved the audio attenuation while leaving the correct DC load on the AGC. The revised circuit section is shown here.

It’s always worth remembering or noting anything you do that could alter the circuit from the original design, as it gives you some ideas of places to look if the set doesn’t seem to be working correctly.

Just a couple of finishing-off jobs now. The radio/gram switch was removed and dismantled. I scraped the contacts clean and retensioned the wipers, then reassembled and refitted the switch. The dial lamp that didn’t work on the early test had been operating intermittently since. The cause was a common one – the holder is held together with a rivet, and with time and corrosion the contacts become unreliable. The solution is to apply some solder to bridge the relevant parts to make the connections.

The set was then left running for a couple of hours. Since there’s nothing worth listening to on MW or LW, I rebroadcasted Saga 105.7FM to it using my AM-03 modulator. I connected the set to its own speaker rather than my test speaker. Not surprisingly, the set sounds much better playing 1930s and 1940s music (on Saga’s Sunday afternoon programmes) than the over-compressed rubbish on “Classic Gold” and “Virgin”!


Speaker Grille

I removed the tuning scale, speaker board and grille etc, so I was left with just the bare cabinet.

I flattened the grille as best I could. This is painted aluminium and had stretched a bit where the dent was, so I could not flatten it completely. I eased out the dent as best I could, and straightened the individual sections as much as possible. The damaged area was particularly visible because the paint had flaked off and left bare aluminium.

I cleaned the grille in warm soapy water (in the bath) and left it to dry. I then painted it with “Antique Gold” Plasti-kote fast drying enamel spray paint, which was a good match to the original colour. It was left to dry for several days, while I worked on the cabinet.


Cabinet

I had recently ordered a few “Rustin’s Renovators” products for furniture refurbishment from Axminster Power Tool Centre, so this was a good opportunity to try them.

I started by cleaning off all the dirt and old wax polish using “Rustin’s Surface Cleaner”. This needs to be applied generously with a rag, and must be wiped off with a clean cloth before it evaporates otherwise it will simply spread the dirt around. It works well, and on this cabinet (which was particularly mucky) I needed three applications before the cloth came away almost clean. You can tell when you are there because the cloth slides easier over the surface when there is no muck to cause drag – it’s quite a distinct change. Use this product in a well-ventilated area! The cabinet should be left to dry thoroughly for a few hours, especially if the cleaner has soaked into any bare wood through scratches or other damage.

The next product in the range is “Rustin’s Finish Reviver”. This is a mildly abrasive liquid, not dissimilar to Brasso except it is water-based. On cabinets with a reasonably thick but dull finish, this does a good job of improving the shine and removing white rings etc. I had already experimented with the products a bit on scrap cabinets and furniture, so I knew that Finish Reviver was not appropriate for this cabinet with a thin and somewhat cracked layer of varnish. It works well on later cabinets with thicker shiny finishes though.

I filled the deeper gouges and chips with mahogany wood filler, pressing it in with the flat side of a chisel then smoothing and trimming with a sharp chisel when it was partly dry. Always buy wood filler in tubes – with tins the filler quickly dries out once they are opened and part used because of the amount of air inside. Tins may seem better value for money – but they aren’t if you have to throw half the contents away!

I then applied “Rustin’s Scratch Cover and Polish (for medium or dark furniture)” to the damaged areas. It is very effective at darkening the wood where the original finish is missing. With minor scratches it is virtually invisible. Larger damaged areas remain visible because they are not as shiny as the varnished areas, but are much less prominent because they are about the same colour as the rest of the cabinet. Once the damaged areas are darkened, the same product is applied to the whole cabinet, left for a few minutes then buffed with a soft cloth.

“Rustin’s Liquid Wax” contains the same polishes as the Scratch Cover, but in a thicker solution without the colouring. This can be used on top of the Scratch Cover, but it tends to soften and remove the Scratch Cover a bit. It is better to use one or the other, not both.

The areas of damaged brown paint around the tuning scale and speaker apertures etc were touched up with a similar colour car paint. This was sprayed into the can lid and applied with a fine paint brush. The paint I had was a slightly different colour but it blended in well and is not as prominent as light bare wood. Being thin, it tended to soak into the wood a bit.


Other Parts

The rest of the job was fairly routine. The knobs were washed then polished with Bake-o-brite, the tuning pointer was repainted with enamel model paint and the outside of the tuning scale was carefully cleaned with car windscreen cleaner.

Reassembly was, as usual, a straightforward job. By this stage of course the work is just about done.

The set was then left to play for a few more hours, and no problems were found. The round mark on the top of the cabinet that I thought was due to a wet vase or similar being stood on it, actually turned out to be directly above the output valve and became slightly warm in operation. If a previous owner had left a cloth or other item on top it could have become warmer which might explain the damage to the varnish.

Most of my radios on display are not fitted with plugs since they get in the way if the shelf is quite narrow. When I want to play them, I use a long cable fitted with a safeblock.

However this set will be standing predominantly in the hall and is likely to be subject to more “does it work” questions from visitors than most. It’s conveniently close to a socket, so I have fitted a plug and made up a short wire aerial for it. It is tuned to Radio 4 on LW and ready to demonstrate!

Following the discussions in the forum mentioned earlier, one kind member, Paul R K, found a picture of an identical set in a Defiant publicity leaflet dating from 1953-54, which confirmed the model number as MSH551. He scanned and inkjet printed the page for me, and I have scanned his printout to show here (hence the slightly fuzzy quality).


Text and Photographs Copyright © 2004 Paul Stenning




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Last updated 14th April 2006.