McMichaels Twin Supervox 20 years in repair

[RadioHist]

McMichael Twin Supervox 20 years in repair…
 
In the Fall of 1995 my Italian friend sent me a McMichael Twin Supervox.  I hit problems in acquiring documentation and encountered some restoration issues I was not equipped to address properly.  So… It has sat in silence till this very rainy week.  It is now on the bench in pieces.  I’m confident the conservation and restoration can be completed.  I do however need some bits of information that I hope some of you can provide.
 

1. I presume that there was some sort of safety cover on the back of the cabinet.  I have not found a photo of this on the Web.  Any photos of this cover would be greatly appreciated so that I can fabricate a credible replica.
   
2. It is obvious that some sloppy repairs were done on the set.  The Trader Service Sheet #557 Rev. 4 shows the Westinghouse Metal Rectifier mounted under the speaker…  In my set the MR is precariously perched as shown in the picture and a 8 ufd. dry electrolytic is laying loose in the bottom of the chassis.  (I presume this is a replacement of C23.) Are there any photos that show the correct arrangement of the rectifier and C23?  How is C23 mounted?
   
3. Our sloppy repairman seems to have misjudged clearances and found he could not replace the chassis side bracket after doing his dirty work.  It seems to me that this bracket is virtually identical to the other bracket except for the mounting of a small phenolic (?) board.  Are they the same?
   
4. The loudspeaker grill cloth in my cabinet is completely wrong.  The Web pictures I’ve found don’t have enough resolution to show the pattern weave…  I need better snapshots.
   

 
This radio is certainly scarce on this side of the Atlantic.  But very interesting to me in that it shows the first attempts to make metal shell valves (Catkins) and has the Metal H.T. rectifier never used in the USA.  I think my set is truly worthy of restoration so that I can exhibit it here at various conferences.  So answers to this query would be greatly appreciated.
 
Yours,
Robert

[westcliff]

Hello Robert,

There are some photos here, http://www.radiomuseum.org/r/mcmichael_t..._rv_8.html showing the missing bracket.  None of the back panel I'm afraid.

Gus.

[AlanBeckett]

There's even a Rally devoted to them.
Someone there should know - http://www.radarc.org/MMRally.htm

Alan

[Yorkie]

The Radio Museum link to the pictures of the set will, I think, answer at least some of your questions, on which I'd comment as follows:

1) Yes, it will have had a back, and there's a pic of the top LH of the back at RMORG, but unfortunately, not the whole back. Sets of this era generated a lot of heat, and I believe it was commonplace in less safety conscious days to remove the backs of sets to allow more ventilation.  To be able to faithfully replicate the back panel, what you really need is a 'brass rubbing' of a genuine one. By that, I mean not a drawing or a picture, but a piece of paper held against the back of the set then a wax crayon rubbed over all of the ventilation slots and other openings (for the mains flex etc) to accurately show the size, location and shape of the holes. Also, a pic of the back to show any labels such as valve types, operating voltages, make and model No. I've made quite a lot of replica backs for a range of radios in my collection using a router jig I designed for that purpose. The hardest part has always been getting good information about the size and shape of any openings, and the label size, text, font, colouring etc.

2) The set was introduced in 1933, and it's my guess that the paper-boxed electrolytic is a wartime replacement in the early 1940s when the set was around ten years old. A lot of appallingly bogged repairs were carried out during that era - sometimes by inexperienced apprentices too young to be drafted into the armed services, or older guys who  were too old or too unfit for active service.  Clearly anyone who was experienced and skilled in radio repairs would have found themselves called up in very short order! Typically, these replacement caps were just bodged  in and left dangling on the wires wherever space could be found for them. I've attached a pic of one I found in an Ekco set I'm restoring (slowly!). It was just dangling under the chassis, the wires having been twisted together (not soldered), and wrapped in insulating tape. (You'll see the tape on the yellow wire in the attached pic).

3) The brackets look the same at each end on the RMORG pictures.

The thing that would worry me most about the set is whether the speaker field coils are intact. There are two, which energise the speakers and also act as smoothing chokes, as you're no doubt aware. One is 1,500 Ohms - the other is 7,500 Ohms. Hopefully they'll be OK, but if not, the 7,500 Ohms one will be a challenge to rewind. I've only rewound one field coil, which was 2,000 Ohms. That took about 2 Km of 38SWG wire (0.006" - about 34AWG).  A 7,500 Ohm Coil would I think be of finer wire than that.

There is two sets of data-sheets available for the set - the Trader Sheet 557, and the 'Broadcaster' which runs to 3 pages. The later has drawings of the underside and top of the chassis rather than simply pictures, as on the Trader Sheet, and might be helpful to have. The Broadcaster Sheet shows C24 (not C23)  mounted vertically to the left of the speaker (when viewed from the rear) close to the end of the chassis. You should be able to find a suitable sized tubular metal can replacement as near to the original as possible. C23 is shown mounted vertically underneath the chassis in the centre. (They're both 8 uF tubular).

I don't know anything about the set - these are just a few observations for what it's worth.

Very expensive set in its day - £18.18.0 GBP - about £1,100 GBP in today's terms.

In the depression years of the early 1930s, it would have been at least two month's wages for a working man, so would have ben out of the reach of all but the 'well off'.

Hope that helps a bit.

Good luck with the restoration Robert!

[Terry]

Wow! That metal rectifier is a mess! Apart from being mounted vertically, which doesn't help the convection cooling, I assume that it is too big to be mounted in the original position and the higgledy-piggledy arrangement of the cooling fins suggest that the bodger tried rebuilding it first in an attempt to fit it horizontally!

Could that bracket at the top be the original one re-located?

At least one of the pictures on the radiomuseum website resolves the question about the missing side bracket!