There are just a few of issues. The speaker diaphragm is missing, and the spindle it mounts on is bent round, not too much of a problem as I am sure I can make a new paper diaphragm. One of the front feet is missing part of it. One valve has lost its top cap, but there is a good length of wire so hopefully I can fix a new one on. (I just need to get hold of a screw on type of top cap.)
The good points so far are; The cabinet is in reasonable condition after a good clean and buff up. The speaker windings are intact measuring 750 ohms approx. The audio coupling transformer is also ok, one winding has about 5.8k ohms and the other is about 350 ohms. I have had a run of bad luck where all of my 30's radios seem to have had an open circuit AF coupling transformer winding.
There is a TCC capacitor which is marked 50 MFD it is only 1.5 inches square and seemed very small for this value and period. I wondered if any could confirm this is in fact a reasonable value for a 1930's capacitor of this type. please see photo.
The other thing I wanted opinions on was the Kobra name label. It has me puzzled, to me it seems obvious the name is made up from the makers name KOlster BRAnd and part of the Brand forms the word Radio. But what has had me puzzled is the way it is drawn, and the shaded background, what is that trying to represent.
Herald1360 wrote:That cap has red and black terminals which suggest electrolytic. Where in the circuit is it? Would 50uF be a logical value?
Does that shading look like an American state?
I dont have a circuit and have not traced it out as yet but I guess maybe it could be a cathode resistor bypass capacitor, fairly low voltage but high capacitance, it just surprised me they made 50uf electrolytics in 1931.
I have traced round the shape the best I can with my shaky hand on the mouse to make the shape clearer. My thoughts were the outline of a country, area or something. But I dont think it is an American state. I associate cobras with India. Google found the best match to my new image was a Red Giant Star !!!
kobra map.png (2.44 KiB) Viewed 4632 times
And what is the significance of the way the DIO is drawn in the original image?
The designer of the trade mark must have has some idea they were trying to represent, but what. Mike
its like lookng at clouds we all see something. Where you see map I see an animal.
The DIO of radio struck me first, the marks in the centre of the "D" & "O" look like representations of pupils . The "I" representing the nose. Once I saw that I then saw the head, ears, front legs body etc etc.
Could it be anything to do with Kob Radio? And perhaps the image is an outline of the Organ Mountains (from above) overlooking Les Cruces?
1931: Because of persistent equipment problems and mounting expenses, the Albuquerque Journal proposes taking over operation of KOB and moving it to Albuquerque. The final broadcast from the Las Cruces campus comes on April 24, 1932.
It would need to be 25V or less working Electrolytic in that size package, so then across a cathode resistor, otherwise probably 0.050uF. I've only ever seen paper types in that sort of package as the normal black wax seal on the bottom wouldn't be good enough. If it's rubber / tar then maybe it is electrolytic. If current flows at 12V, it's not paper, even bad ones need more volts to break down.
The shape behind KoBra DiO is probably something Canadian. Not quite right for Canada Or a Scottie Dog!
Brandes Products Corp.; Newark (NJ) and New York, N.Y.; G.Brandes Inc., New York. N.Y.(USA) the parent company was renamed to "Kolster Radio Corporation of America"(see there). 1928 the British branch renamed to Kolster-Brandes (see there).
Brandes was a well known brand for headphones..Brandes Products Corp.; Newark (NJ) and New York, N.Y.; G.Brandes Inc., New York. N.Y. brandes USA English
Brandes was 1908 established in Toronto, Canada; acquired by the American Telephone and Telegraph Company (AT&T) in 1922.
Thanks for the ideas guys, but I fear the explanation for the Kobra trade mark may just remain a mystery. I have re-stuffed the 50uf and the 1uf capacitors, just need to refit them, al the rest look like mica types. I will have to "borrow" some valves for the time being as it seems the 220VSG is the wrong valve, it should be a 210HL triode as the detector, then the amplifier valve is a HL210, and the output valve should be a PM2A. The notes on the Kobra (Wireless magazine May 1932) describe it as having "automatic grid bias" for the 2 amplifier valves. It goes on to explain that if a pentode valve is wanted in place of the small power valve fitted, the bias resistors can be changed, the bias resistors must be the 2 resistors above the paxolin chassis fitted between 2 pairs of thumb screw binding posts.
Some of the larger diaphragms I have seen for these magnetic reed speakers have all been made from what looks like a cloth impregnated with latex type stuff. The other type in my KB Masterpiece and Pup radios is more like thin card or paper. I may borrow the diaphragm from this KB Pup radio
Mike, I am wondering if it is a commercially produced Kobradio with the insides removed, and a home made TRF receiver put in it! Look at this, it was sold on Ebay and is almost identical to yours, http://www.ebay.co.uk/itm/VINTAGE-OAK-R ... 7675.l2557 apart from no button in middle of speaker, but look inside the back, and it is totally different. Bob
Hi Bob I contacted the seller of that radio, probably over a year ago now, and said that the insides were not original. He amended the listing, "The electrical chassis inside this radio looks more like 1957 . The speaker is the wrong type probably been replace" The cabinet in that ebay listing is the Kobra 305 which is the AC version with a moving coil energised speaker so no need for the speaker adjustment.
I have now got my Kobra working on 100v DC fed to the HT2 and a 47k resistor connecting HT2 to HT1. I am using an old mains transformer with a 800 ohm primary which I originally made for a DAC9, feeding my bench speaker. The total current draw of the radio is about 4 to 5 mA from the HT and about 400mA on the 2v LT.. There is reasonable volume on the speaker I am using, enough to be heard clearly around a largish room. Interestingly the radio does not seem to mind which valve goes where, it works just as well with the audio amp valve as the detector, and even as well with the OP valve PM2A as the detector.
I have repaired the original read speaker and fitted a diaphragm from another radio, but it is too small so the rim of the diaphragm does not rest on the baffle as it should. The speaker motor really is not much more than the insides of a moving iron headset. The speaker adjustment screw has a rubber bush which pushes on the armature/reed to adjust the distance from the poles. It has gone a little soft so I have slightly re-tensioned the reed to allow for this. I have tested the reed speaker on another radio and it is not too bad, a bit quieter than the test speaker I was using before distortion set in by the diaphragm moving too much.
Next job will be to look at making a new diaphragm. I am not too certain at the moment how I am going to form the ridges around the rim of the diaphragm.
I am a bit short of these 2v valves, I will have to look out for some at Harpenden.
I have tried making new diaphragms in the past, but never successful. As you say, the ridges prove the problem! Maybe get one out of a newer speaker of similar dimensions. There still seem to be plenty 2V valves around on Ebay, but it is always a gamble if they will work. I have dozens and dozens of them, but most do not have numbers on them and it is usually just a matter of seeing if they work, or not. The top cap is all packed up and addressed, and I will post it in the morning! Incidentally, last week, I found a supplier for 2V sealed lead acids (4.5 Ah). I didn't know they were still available. It is 45mm by 23mm and 100mm high. I can charge it by putting a small torch bulb in series with my 6 Volt charger. Saves messing about with regulators or dropping resistors. Bob
Yes, I think I may have been a bit hasty when I said in my first post that making a new diaphragm would be easy. Now I have given it lots of thought I have to admit that forming the ridges around the rim that are necessary to keep the rim ridged is really a daunting prospect.
I have seen an article on the web describing a speaker similar to that. But I cant help thinking if there is no ridge around the rim then the sides will flop around somewhat and not contribute to a good sound.
I have drawn out what I need, that was the easy stage. The dimensions are fairly critical as the cone will foul the motor assembly if it is not tall enough or wide enough.
The image includes an approximation of the speaker motor to show the closeness to the cone. I don't suppose anyone has something like this floating around in their spares?
OK, I have a plan for making the diaphragm, it involves a standard paper cone made out of card paper 14 thou. thick, and adding the ridges to the edge of the circumference afterwards. I have a similar original diaphragm and the thickness of its paper is 15 thou., so I am hoping it will be just about right. I have made a prototype cone out of the paper, it feels quite strong and gives a nice crack sound if the edge is tapped. I am using the prototype to form the basis of the mould I am making to make the ridges around the rim of the cone. How I have made the mould and what I have used will be revealed if it is successful. Or in the meantime you can put your guesses on how to form the ridges here, if my idea fails I may just use your one.
The good news is the new speaker diaphragm works very well. But the construction was not very straight forward. I racked my brain trying to think of a method of forming a ridge around the rim of the new speaker diaphragm, then I had a brain wave and thought “V belt”. The section is not exactly a V but when twisted and glued down with one side flat against the base board it could be used to mould using papier mache to form a rough V shape at the rim of the diaphragm.
Once the belt was cut I started to glue it down to a base board following the outline of a circle I had drawn around the first cone I made. It was very difficult to glue down and it required a lot of force to twist the belt in order to get one side flat against the base. I glued it in about 3 inch stages working my way around. Superglue seemed to hold it but not perfectly flat, but it was good enough. My thumbs were really aching by the time I had finished as each section had to be held twisted and pushed down hard for a few minutes whilst the glue set. I cut the belt to the right length before glueing the final section, it seemed to be getting more and more difficult towards the final section.
When I had finished fixing the belt, I glued the original prototype cone onto the top edge of the belt using PVA and superglue. The superglue holds the item almost instantly and the PVA reacts with the superglue to form a gell which fills the gaps and eventually also sets hard.
This now formed the basis of a mould, but first I had to do something to stop the papier mache sticking to the mould. I coated the bottom of the diaphragm on the mould with superglue to harden and waterproof the card, I also rubbed superglue into the rubber belt. Then I melted some wax and brushed that all round the belt and base of the cone, that should stop the papier mache sticking, and it will just need a little warming to help free the cone when the papier mache was dry.
Next was preparation of the papier mache, for this I used 2 ply quilted kitchen paper, I have found this type of paper to be very strong, (hence the advert) I peeled the 2 plys apart as I only wanted to use a single ply for each layer. Then I cut it into strips about 1 inch wide. The glue was PVA diluted 50% with water. (in retrospect it may have been better to use wall paper glue as this may set harder, the PVA is still very slightly pliable even 24 hours after making, but this doesn’t seem to effect the sound of the diaphragm)
I wetted the end of a strip of paper and laid it along the edge of the belt, brushing on more glue and stretching it as I went, the top of the paper tended to curl over the top of the belt and onto the diaphragm which was good. Then went over it gently pushing the wet paper into place into the grove and up the diaphragm.
I did 3 layers alternating the height of the edge where it attached to the diaphragm.
As the height of the V belt made a larger lip than I required I went around the edge with a sharp blade attached to a small block and cut about half way up along the whole edge of the belt. The finish of the papier mache where it went up the cone was slightly wrinkled so I used some 240 grit paper to smooth some of the ridges out.
In my shed draw I found a small piece of brass rod left over from another job, I drilled a hole through the centre and another in the edge for the locating screw. Then I turned down one end to form a shoulder to sit on the top of the cone. I didn’t have the means to rivet the brass bush into the top of the cone so used epoxy glue to fix it, carefully ensuring it was perpendicular. (I used superglue to hold it temporally whilst the slow setting epoxy hardened.
I felt that I wanted to tone down the whiteness of paper cone but by experiment on some scrap paper found that any water based paint caused the paper to distort so I have left it as the original colour which is a pale cream and the papier mache is a sort of translucent white, never mind the performance is more important. If I did it again I would add some colour to the papier mache and or even dye the paper first whilst it was still flat. I actuall used 2 sheets of A4 glued together to make an A3 size which was only just large enough, luckily I have an A3 printer so I could print the shape of the cone directly onto the paper scaled to the correct size by the drawing program (Visio) I use. .
To stop any rattling of the diaphragm against the speaker motor supports, I glued some small pieces of soft felt onto the metal supports where the rim of the diaphragm rests.
One other thing I did to the radio apart from re-stuffing the 2 capacitors, was to rewire the connections to the inner coils of the inductive coupler as the original rubber had shed most of its insulation.
The final value of the resistor I used to feed HT1 from a single 100v supply was approximately 140k ohms, this gave a good performance with 60 volts on V1, the RF valve..
When all was put back together we took a video of the radio working, you can see it on Youtube here.
I would rate the audio quality of the radio as easily as good as a small cheep imported 70's transistor radio, and I think it is better than all the other reed speaker sets I have.
The lever on the side of the cabinet which I described as like a volume control is described as; "The big feature of the design is the flexibility of the tuning circuit. By a novel system of inductive coupling the selectivity can be varied over much wider limits than is normally possible" (Wireless Magazine May 1932.)
That's quite an impressive result, and I can't help but wonder how much better it might sound with some period appropriate music.. I have in mind "The Pasadena Roof Orchestra" but there would be others. https://www.youtube.com/watch?v=UIUoy0pM7zw
Given the limitations of recording and broadcasting at that time, I guess that the modern stuff doesn't quite fit within the limitations of the receiver.. I'd bet that 'equalisation' of sound quality would have been very different in those days? https://www.youtube.com/watch?v=VV_k0XZ-zfg
I dont have a AM modulator, but I do remember the Pasadena Roof orchestra from my days when I was a sparks in the "old" Towngate Theatre, in Basildon. I pantry TX is something I have always intended to do but somehow have never got around to it. Possibly it is something to with them being illegal which doesn't help. Mike
Crackle wrote:A pantry TX is something I have always intended to do but somehow have never got around to it. Possibly it is something to with them being illegal which doesn't help.
They don't radiate enough RF to worry about. I've built several over the years with varying degrees of success and none of them have been receivable outside the house. A TRF with reaction would probably cause more interference!
I recently found a circuit for a valve AM modulator which looks quite good.