Lotus AC Bandpass Three or "3-valve all electric"

 I started the refurbishment of this set before I really got underway with my repair business, but rapidly found less and less time to spare, so I wrapped it up in plastic and stored it in the workshop loft. Now I'm working towards retiring from the repair business, I retrieved the set which is now a collection of parts. Initially I'll take pictures of the bits and pieces.

After this I'll describe the start of its mechanical repairs.

 
 The chassis is loose inside the cabinet. Originally the metalwork was painted black. The underside is OK but I had to paint the top and sides as lots of rust was present. The wavechange switch down the centre of the chassis is partly dismantled. The resistors will be OK but the old condensers will probably require some attention.

 

 Below, the set of coils having removed their aluminium cans.

 

 The power supply parts are mounted on the loudspeaker baffle board.

 

 The loudspeaker is now looking better after painting, although when I checked the output transformer the primary winding was open circuit. Maybe this is why the original restoration on this particular set was never completed? Thankfully I have a huge junk box in which there are scores of old transformers, however as it had 20,500 marked on the hidden side once I'd removed it so that may pose a problem. Presumably it relates to the anode voltage and anode current?

 

 The mains transformer is pretty substantial so should be OK. An interesting feature is the "mains aerial". This uses the house mains wiring, and theoretically all the wiring to the power station. I'm not sure if this is a good idea now that there's so much mains-borne noise.

 

 

 

Above, the substantial tuning condenser.

Below, this label was stuck underneath the cabinet, almost as if Lotus were required to display it but decided to hide it away.

 

 

 

Below.. very odd.. two license labels.

 

 

 

 The three valves: a VMS4B RF amplifier, 904V RF amplifier and MKT4 audio output, which has a B5 base and a side contact.

 

 

 Above, what I'd assumed were the original knobs, but as I ventured into the set's refurbishment I realised that this wasn't the case. First the centre mounting holes seemed to be the wrong size for the various spindles and secondly, I happened to glance at a post-war Philips receiver and noticed it had four knobs identical to the two smaller types above. I'd found the gain control, which is a 10Kohm wirewound pot was open circuit, then checked on the shaft length and found the knobs wouldn't have fitted the old pot. Presumably someone had half-heartedly started a refurb years ago then gave up!

 

 Above, the tuning dial and below the new escutcheon kindly made in California by John Gibson from a mould he kindly made from the part in his own set.

When I unpacked the escutcheon I was more than a bit suprised to see when John sent it to me. It was July 2002. I dropped him a line but the email bounced so I searched on the net and found that he'd sold his house in Berkeley and seemed to reappear in Salt Spring Island in British Columbia. Checking this address indicated he'd moved again. Maybe to Burnaby Heights in the same general area?

 

 Here's the circuit diagram for the Lotus Band Pass 3. Click on it to see it full size then enlarge it in a PDF reader.
 

 When I first bought the radio it had no speaker cloth and the escutcheon was missing.

 

 

 The loudspeaker is unusual compared with modern types, having not only a very rigid metal housing, but a substantial locating spider fitted to the front. When I examined the baffle board I found woodworm. It didn't look too bad but the holes in the outside were the tip of the iceberg so I removed the parts and made a new baffle board from plywood (below). The parts yet to be fitted are the metal rectifier, mains transformer and smoothing condensers. The latter will probably need to be replaced.

Below the picture of the new baffle is a view of the speaker with a replacement transformer fitted. I've brought out to a tag strip all the relevant connections so that optimum performance can be worked out.

 

 

 I may have discovered the reason the last owner ditched the refurbishment of this set. It's the wavechange switch. It's a complicated thing using a mixture of parts: quarter inch steel bar, a 5/8th inch ebonite tube embedded with gold-plated shorting bars, a strange worm drive which operates a standard toggle switch, and some special bushes. All bar one part is present and that couples the ebonite tube to the control knob. Given a suitably sized piece of brass and a lathe I could possibly make a new part, but having neither, nor the inclination I need to think of an alternative. Below is the main assembly and the switching strips.
 

 

 After puzzling over how to replace the missing wavechange switch part for several days on and off I found a brass coupler with a quarter inch shaft. I cut off about quarter of an inch of the coupler and sawed it so there were two prongs that exacty fitted the end of the ebonite tube, below. The length of the complete switch left very little room for the coupler and as the thing needed to be rigid, I found a threaded quarter inch brass bush which I fitted to the front of the chassis. The next picture shows the ebonite tube assembly then the new part fitted to the ebonite shaft.

The weird Archimedes screw engages with a toggle switch mounted on the end plate which has the bush for locating the shaft which runs down the centre of the tube. The shaft actually stops near the end of the tube and a complex casting which includes the Archimedes screw locates on the end of the ebonite tube. The design of the whole thing must have given a mechanical engineer lots of fun and the casting people lots of headaches.

Before I can fit the thing I now have to work out the orientation of the shaft for the various switch positions so the toggle switch is activated at the correct time.

 
 
   
 
 
 Below, you can see, temporarily fitted in place without its rear mounting plate carrying the toggle switch, the ebonite tube with its gold-plated shorting bars. The switch has four positions corresponding to Off, long waves, medium waves, and gram.
 

 The table below shows the connections to the wavechange switch with reference to the numbered switches on the circuit diagram.

 PRONG >>

2

3

4

 5

6

7

8

 9

10

11

12

13

14

 15

Off

nc

 nc

nc

nc

 nc

nc 

8

nc 

 nc

 nc

nc 

 nc

nc 

 nc

  Gram

 2

 1

  nc

  nc

 nc

 7, 8

 6,8

 6,7

 nc

 nc

 nc

 nc

 nc

 15

 14

 Long

 nc

 3

 2

 nc

 nc

 7

 6

 nc

 nc

 nc

 nc

 nc

 nc

 15

 14

 Medium

 nc

 3

 2

 5

 4

 nc

 8

 7

 10, 11

 9, 11

 9, 10

 13

12 

 15

 14
 
 

 Below, the completed wavechange switch in position. It hasn't got an ident which would mean more work, and whether the original set had one I don't know. At first it seemed impossible to fit the thing because it was too long. The solution was to slacken the bush on the front of the chassis. This allowed the assembly to slip in at an angle and once in place the bush was re-tightened. The picture shows the switch in the OFF position. Rotating clockwise turns on the toggle switch and joins prongs 2 & 3 plus 4 & 5 which is Medium Waves. Prongs 7 & 8 are also joined which lights up the upper dial lamp. A further 90 degree rotation joins prongs 1 & 2 which is Gram. It also lights up both dial lamps. Another 90 degrees turns on Long Waves and another 90 degrees switches the set off. Alternatively, rotating the switch anti-clockwise from OFF selects Long Waves.

 
 Here's the voltage doubler metal rectifier which fits on brackets on the speaker baffle board. I checked it with the diode setting on my multimeter and found each section had a forward voltage drop of circa 0.7 volts. There are 13 sections either side of the centre connection.
 
 

 The next major task is to test the resistors and condensers.

See the Electrical Refurbishment

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