McMichael 367 TRF portable (1936)

[Phil G4SPZ]

A friend of mine bought this suitcase-style TRF portable set at a ham radio rally, and knowing my interest in vintage sets he offered it to me for renovation, on condition that it would then become part of the collection of vintage radios at Gripton’s Radio Stores, the 1939 wireless shop at the Black Country Living Museum. In short, it has taken me around two years to complete this job, but there are a few interesting aspects that are worth sharing.

The set arrived with a missing battery compartment lid, a missing tuning knob, a physically broken valve in V1’s position plus an open-circuit heater in one of the PM2HLs. This particular set is one from later production, post-November 1936, and has the 7-pin VP210 fitted as V1 instead of the earlier S215A. The S215A tetrode was launched around 1930, so was already a little outdated by 1936 when the VP210 pentode became available. No use is made of the variable-mu characteristic of the VP210 in this TRF circuit. The other valves fitted were PM2HLs (released in 1938, so obviously later fitments, although equivalents to the original HL2s) and a PEN220 output valve.

I sourced replacement valves, lashed up a 108 volt HT battery and a 2 volt Cyclon cell, and started fault-finding. This set is most awkward to work on, owing to the fact that the underside of the chassis is virtually inaccessible even when hinged upwards into the ‘servicing’ position. Added to this, the aerial is wound on a substantial wooden frame inside the deep lid of the cabinet, which also houses the speaker and output transformer. The weight of the lid gives the cabinet a marked tendency to flip upwards, particularly when I removed the chassis from its hinges to gain access to the underside. Even with the chassis removed, the aerial and speaker leads - five of them - had to be extended with temporary wires to enable the chassis to be worked on. And finally, the chassis doesn’t sit in a stable manner any way up, and has to be propped against the cabinet, which with the lid open tends to be unstable as described above. The whole contraption spreads over the entire workbench leaving little room for the Avometer... none of the foregoing make fault-finding any easier!

The immediate problem was significant HT leakage even with no filaments energised, so one by one I replaced leaky waxies - virtually all of them - until the leakage had fallen to zero. Unusually for me, I went to the trouble of re-stuffing the cardboard case containing two 8uF decouplers. The LT and HT current settled down at the expected levels, and the set started to work from V2 onwards, but there was no evidence of gain from V1. All the resistors and the fixed coil windings measured reasonably OK, but I could see that the rubber insulation on the wires leading to the variometer-style reaction coil had perished at the point of entry from the reaction coil’s wooden former, where they flex most in use, and I suspected that the coil was being shorted out by the bare wires touching. To replace the wires dictated removal of the complete variometer assembly. The variometer is ganged with the volume control, a 20 kilohm rheostat which progressively shunts the primary of the first inter-valve transformer. Mechanically this assembly is a bit of a nightmare, but eventually it was removed, new silicone insulated stranded wires fitted, and the assembly reinstalled in the chassis.

On powering up I was delighted that the set worked first time. The sensitivity is remarkably good for a TRF and the volume control works well, with smooth onset of reaction. On strong signals such as Radio 4 on 198kHz, the volume control has to be turned virtually down to zero to prevent overloading. On medium waves the set is selective enough to separate the adjacent stations audible in daylight. Audio quality is mellow yet surprisingly loud at full volume - were these suitcase portable radios designed to compete with picnic gramophones? I made a small adjustment to the single trimmer capacitor to peak up the signals. Luckily the waveband calibration appears reasonably accurate with stations appearing in their proper places; adjustment involves changing the spacing of turns on the frame aerial, not a job I’d relish having to tackle.

The cosmetic repairs involved making replacement transparent screens for the three dials, which had all become partly opaque, yellowed and brittle. I cut these with scissors out of a thin sheet of clear plastic, taken from the front of a cosmetics gift box, which just happened to be around the correct thickness. I pushed small holes through the material and fixed them in place using the original wire staples. A kind Forum member came up with an almost-identical tuning knob in response to my ‘wanted’ posting.

I made a new battery compartment cover, cut to size from a piece of veneered radio cabinet which is a reasonable match, donated by my friend and fellow museum volunteer Len. There’s an image of the set on RM.org from which I was able to get a good idea of what it should look like, together with the wording of the brass plaque. I used MS Word to create a similar plaque, printed it onto good quality photo paper and stuck it in place; at some point I may add some tiny screws to improve the authenticity. The battery cover is removed by a flush-fitting pull-up ring handle of the type fitted to boat hatch covers, and I managed to purchase something suitable from an on-line supplier of boat fittings. It’s rather bright in brass, but looks quite good against the veneer. Finally I made a replica Drydex H1044 108 volt HT battery, using a box fabricated from strong cardboard glued together with PVA adhesive and housing a dozen PP3s. The artwork for the battery sleeve came by courtesy of Forum member Bill Morris (thank you, Bill!) which after re-sizing was printed out onto matt photo paper and sealed with two coats of aerosol fixer. The LT battery is less authentic, being a 5Ah Cyclon cell inside a home-made cardboard box with a suitable printed outer wrapper.

These suitcase-sized portable radios must have been sold in relatively small numbers, and would have appealed mainly to the well-to-do since they were priced at 8½ Guineas (around £8.90) which in 1936 represented nearly a month’s wages for many people. Battery life would have been relatively short; I calculate that, at 7mA HT current, the 108 volt battery would have lasted around 80 hours, and the rechargeable gel-type accumulator considerably less than that. The set is relatively cumbersome and heavy. I suspect that today they are probably quite rare, so I am pleased to have restored this one for demonstration and display in a museum where visitors can see it as a rare example of its type.

Phil

[frankmcvey]

Nice job and nice write-up, Phil, thanks.

For anyone else looking them, the brass flush pull you sourced from the chandler is often called a "Military Chest" handle. A Google image search or an Ebay search finds quite a few suppliers.

Cheers,

Frank

[HamishBoxer]

Great job and a very nice set. On the subject of thirties portables I have a "Rolls" but sadly not of Rolls Royce make.

[gary_crutchley]

Hi Phil,

A superb restoration, and a great addition to the museum shop. Thanks for the enjoyable and informative account of the restoration.

Regards,

Gary

[Mr Moose]

Hello,
A nice restoration but not up to McMichael standards I'm afraid as you haven't got your screw heads in line!
I have one of these and all eight screws on the battery cover have the slots in line side to side.
Unfortunately mine has the common problem with these suitcase portables where the battery acid has leaked and made a hole in the case.
Actually the brass ring handle is not recessed into the cover (except for the depression for the finger) but sits proud on top.
Yours, Richard

[Phil G4SPZ]

Thank you everyone for your kind remarks! I'm not claiming this as a full "restoration" because there's a lot more cosmetic work needed to get it into showroom condition, but in its present state it plays the role of a second-hand 'pre-owned' set in a wireless dealer's shop, so a few scuffs and scratches would be expected. I'll apply some black boot polish to the rexine and bring that up a bit shinier, but the rusty catches and handle clips will have to stay I'm afraid.

Thanks Richard for pointing out the differences in the ring pull handle; quite right, and I hadn't spotted that from the photos on RM.org, but it was a case of getting the nearest thing. I confess it took me ages to recess it into the timber and make it sit flush with the cover. Hopefully, no-one else will notice the difference, but I can see you've got sharp eyes...!

[julie_m]

It's a portable radio that has clearly been enjoyed .....

Every one of those marks could be telling a story of happy days out in a motor-car, picnics in meadows by blue, tinkling streams with the juiciest hams and lashings of ginger beer; or perhaps more risqué adventures of a more amorous nature, in a secluded spot late at night!

P.S. Nice to know it's not just me who gets OCD about screw heads .....

[Station X]

Nice write up Phil. Well laid out and a pleasure to read.

The radio looks good too!

[Phil G4SPZ]

Graham,

Your comments are greatly appreciated, many thanks.

Julie,

Interesting thoughts. If only our vintage radios and gramophones could re-tell the stories of what they've witnessed. Who knows, in years to come when people are collecting and restoring vintage iPhones (!) what photos and videos will be found in the depths of their ageing memories!

[gary_crutchley]

Hi Phil,

I was at the museum this afternoon, and popped in to the workshop to look at the McMichael, it looks superb, you did a great job. I will have to pop along one day to see what it sounds like!

Regards,

Gary

[Phil G4SPZ]

Thanks very much, Gary. The radio sounds quite good, although like many vintage sets it doesn't cope too well with the loud modulation applied to many of today's AM stations' transmissions on MW. However it sounds very nice on Radio 4 and the 'proper' 1930s music that we put out from our hidden MiniMod.

See you soon - the museum workshop will only be manned on Thursdays from November to March.