Author Topic: Lafayette Superheterodyne Radio - 1935  (Read 1679 times)

Offline El Sid

Lafayette Superheterodyne Radio - 1935
« on: July 25, 2020, 05:03:04 PM »
A friend of mine, after seeing the Philips radio restoration I did, asked if I could fix his. So I said I could have a look, but no promises.

This is what it looks like:





See anything missing?



Opening it up:





Cabinet pretty stuffed from water:



Give the circuit a bit of a clean:





Lovely valves: (2 x 47 output pentodes, 1 x 80 rectifier, 1 x 55 double diode, 4 x 56/57/58 RF thingies)




Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #1 on: July 25, 2020, 05:12:40 PM »
But there was no indication as to what model it was. It took about 2 hours on the internet, but eventually found it on radiomuseum.org after ploughing through many models. It is a Lafayette M-39 or 41 or 43 from 1935 - can't find out exactly what the differences are. See https://www.radiomuseum.org/r/lafayette_m41.html?language_id=5




Even found a schematic:



So restoring this is quite a daunting prospect. The valves i don't think are easy to get if any are broken. There is no speaker, and it is a field coil speaker. I have no idea if you can just stick something else in - Murphy says no. Almost every single cap will need to be replaced. The cabinet will need a lot of work - it is severely delaminated. I am going to have to think about it.

Offline Family_Dog

Re: Lafayette Superheterodyne Radio - 1935
« Reply #2 on: July 25, 2020, 05:34:00 PM »
The RCA cathedral type radio I have is of about the same vintage, it also had a field speaker. At the time I had a neighbour's son staying with me in order to complete his matric year at school, as they were transferred to Pretoria. The son was a member of a guitar band and unknown to me they purloined my RCA speaker. It did not work of course. But the field was used only for magnetising the speaker, it did not form part of the HT smoothing circuit as was also common those days, so I replaced the speaker with a 10" PM speaker which works 100%.


-F_D



-Eric

That Guy in South Africa...
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Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #3 on: July 25, 2020, 06:20:05 PM »
The RCA cathedral type radio I have is of about the same vintage, it also had a field speaker. At the time I had a neighbour's son staying with me in order to complete his matric year at school, as they were transferred to Pretoria. The son was a member of a guitar band and unknown to me they purloined my RCA speaker. It did not work of course. But the field was used only for magnetising the speaker, it did not form part of the HT smoothing circuit as was also common those days, so I replaced the speaker with a 10" PM speaker which works 100%.


-F_D

You've just helped me out there  F_D - I had a closer look and this one doesn't have a choke, but there are 5 wires going to the (non-existent) speaker, instead of the 4 the circuit diagram suggests. Maybe the coil is meant to act as a choke?

Also looking closely, this one is slightly different in its physical layout - the cans containing the rf transformers and variable caps are different. This must be a variation of the model in the cct diagram.

Offline Rnwza

Re: Lafayette Superheterodyne Radio - 1935
« Reply #4 on: July 25, 2020, 07:29:19 PM »
Settlingn8n to watch with a hot cup of Joe...this looks like fun.
"Music is essentially useless. As life is." - George Santayana

Offline charles

Re: Lafayette Superheterodyne Radio - 1935
« Reply #5 on: July 26, 2020, 09:45:56 AM »
All the best with the restoration. Very good looking vintage set.

I had a quick look, I have some of these valves in stock.

You can use a valve tester to determine the condition of the set's valves.

If possible keep the body, tip, spot resistors. Some of my 12 point one Leaks still have it and the resistance values are still within specification.


« Last Edit: July 26, 2020, 09:49:11 AM by charles »

Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #6 on: July 26, 2020, 03:39:33 PM »
All the best with the restoration. Very good looking vintage set.

I had a quick look, I have some of these valves in stock.

You can use a valve tester to determine the condition of the set's valves.

If possible keep the body, tip, spot resistors. Some of my 12 point one Leaks still have it and the resistance values are still within specification.

Thanks Charles - Iím still not sure if Iíll go ahead with it, but will investigate further. Itís certainly encouraging that valves are available- I though they were unobtanium. Iíll find someone with a tester. Do you have any ď58Ē valves? There is a ď57Ē where there should be a 58

Offline Des Miles

Re: Lafayette Superheterodyne Radio - 1935
« Reply #7 on: July 26, 2020, 06:58:18 PM »

Seems like a real nice project. I agree though, replacement valves may be a problem, great if Charles can assist. The 80 rectifier, if faulty, can easily be replaced by any number of 5v. rectifiers. I've used  GZ34 on a few occasions , simply because I had some, 5Y3GT will also work.

Everyone has their own line of approach, I would put only the 80 back and while carefully measuring the HT, slowly power up using variac to see if HT comes up and settles at 200 - 250 volts or so. If OK , I would then use a R2k 10 watt or so resistor in place of the field winding and connect a PM speaker to the output transformer. With all the valves replaced, bring the power up again (still watching HT voltage) to see if there is any indication of audio output and possible RF noise. At least then you know there's a good chance and re-capping inevitably brings about a great improvement in overall operation. If the set is completely dead (audio wise) then obviously will have to fault-find section by section.

Just my thoughts, good luck if you tackle this full on !
I saw Sputnik, well... actually the booster rocket that went into orbit with it!!

Offline charles

Re: Lafayette Superheterodyne Radio - 1935
« Reply #8 on: July 26, 2020, 07:04:12 PM »
I do not have a 58 one but a 80 I have.

I may be able to source a 58 for you.

Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #9 on: July 26, 2020, 11:30:53 PM »
Thanks all for the hints and suggestions.

I tested the transformer with all valves out, and it makes  all the voltages required: 360-0-360, 2,5V and 5V. I then started tracing from there and came up with bad news. What I thought was the output transformer is actually the choke, and tracing the wires on the speaker plug pictured below, I realised that the output transformer was obviously mounted on the field coil speaker (not uncommon in old radios). So not only is there no speaker, there's also no OPT. I suppose one could use a permanent magnet speaker (although not sure how important the choke effect of the field coil is for the power supply), but where to find a push-pull OPT for a 47 valve? Not sure the owner's budget extends to having one wound.


Offline Des Miles

Re: Lafayette Superheterodyne Radio - 1935
« Reply #10 on: July 27, 2020, 12:50:23 AM »

Do you not perhaps have a center tapped primary output transformer, even if it's a modern one meant for EL84 or EL34, 6V6 etc. not as a permanent fix, just as a quick test to see if you can get any life out of the set?  :nfi:
I saw Sputnik, well... actually the booster rocket that went into orbit with it!!

Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #11 on: July 27, 2020, 09:17:54 AM »
As it turns out, I do have an EL84 PP OPT lying around, which is destined for a guitar amp. The Type 47 wants a 7k load according to the data sheet, so not too far off, and should work. A second hand PP OPT may well be a practical solution - even if it's not optimal it's better than f@k@l, and I doubt my mate will want to cough up the couple of R '000 for a custom-made one.

There's another subtlety around the speaker field coil though, which took me a little while to figure out. All current to ground is actually returned through the field coil to the 21H choke and then through the choke to the centre tap of the 360-0-360 power transformer. So the voltage across the coil is actually negative with respect to ground, and this negative voltage is used to bias the grids of the 47s (and some other valves). According to the data sheet the 47s want about -15V on the grid. The 47s use about 37 mA cathode current each, the 55/57/58 about 10 mA, a total of 125 mA. So a 120ohm (5W) resistor should do in place of the field coil.
« Last Edit: July 27, 2020, 09:20:38 AM by El Sid »

Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #12 on: July 27, 2020, 08:35:19 PM »
Did a bit of scratching around on the internet today, and found that the arrangement with the choke and field coil was very common in radios of that era, although more usually on the B+ side. I think that's pretty clever using it for a -ve bias voltage. There are even diagrams with that plug, and the pinouts are standard.

One detail which escaped me that I read about today is that the speaker connection also winds around the field stator to act as a bit of a humbucker (which you can see in the circuit diagram). This was only 15-20 years into the discipline of electronics - clever chaps.


Offline El Sid

Re: Lafayette Superheterodyne Radio - 1935
« Reply #13 on: August 09, 2020, 04:47:35 PM »
Just out of interest, here are some field coil speaker/OPT combinations:


Offline fredeb

Re: Lafayette Superheterodyne Radio - 1935
« Reply #14 on: August 11, 2020, 09:03:27 AM »
Very interesting re field coil drivers . Those 6L6's (?) also look good . CV1917 perhaps ? ( the 6L6's I mean )
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts