November 10, 2018 at 10:30 am #10578
Hello, just joined. My question is about how authentic a repair has to be. Is it acceptable practice, for example, to 3D print a knob in closely matching brown plastic, rather than leaving it without the knob or with a non-matching original replacement? Or does the thought of a contemporary knob on a wood cabinet cause shudders? 🙂November 13, 2018 at 4:55 pm #10602Gerry O’HaraCVRS Member
Hi Alistair. My own thoughts on this is that it is perfectly acceptable to do what you propose as the knobs are both a functional and cosmetic part of the radio – a reproduction one that matches the others is better than the alternatives (no knob or mismatched knob). The only arguable exception in my book may be the case of an exceptionally rare radio in a museum, where such a cosmetic enhancement may be perceived to cause confusion as to what the radio my have originally looked like. The SPARC museum won an international restoration award with a radio having a 3D-printed escutcheon. – see here.November 14, 2018 at 7:17 am #10603
Thanks Gerry. That’s a spectacular piece of work, and a fascinating article! Something for me to aspire to. I agree with your view on authenticity – there seems to be a delicate balance between functionality, appearance and integrity, and each of us will probably have to make the required compromises in each project, based on our philosophy, abilities, and the challenges the project offers. I’m picking up my first challenge on Saturday – a 1940/41 Marconi 207 cabinet radio with as yet unknown gremlins lurking within. According to the schematic there’s a 1.25V bias battery in there, which may very well be weak 🙂 and that one little item in itself poses questions. Should I put in a modern battery holder with modern battery? Should I hide a modern battery inside the existing one (if it still exists and if it’s possible)? Should I feed a bridge rectifier from the heater supply with a voltage dropper and smoothing caps to feed two series silicon diodes and forget all about batteries? I don’t suppose I’ll know which way to go until I start work on the thing! But I’ll certainly print the missing knobs. For that I use Cinema 4D on a Mac using a digital caliper for dimensions, to design exact models of the originals, then I’ll print them on an el-cheapo RepRap Prusa printer which works surprisingly well. Some post-print work will of course be required, but now that I’ve heard of high-build primer from your article, I’m fairly sure I can maintain the semblance of exterior authenticity!November 14, 2018 at 8:31 am #10604
I use a 1.5 volt watch battery for the bias cell. see photo
Here is a list of suppliers of radio parts, that you might find useful [possible for knobs]
Good luck with the Marconi, they are nice radios.
Dan in Calgary.
Attachments:November 14, 2018 at 9:29 am #10606
Thank Dan. That’s definitely an option. Your retro-fit is very unobtrusive! What is the radio? And thanks for the list of sources. That will be very useful.November 14, 2018 at 9:47 am #10607
I don’t remember what the radio is , as I have worked on several radios that have a bias cell.
Marconi and rogers , and possible others used the bias cell.
Dan in CalgaryNovember 24, 2018 at 3:46 pm #10635Gerry O’HaraKeymaster
I also use 1.5v watch batteries for bias cell applications. However, I usually use the original bias cell holder and the larger size of watch battery (AG13). In order to do this while maintaining the correct polarity, I place some heat shrink around the cell as insulation and insert a small screw into the centre of the holder prior to inserting the cell. The photos show this in a Marconi Model 142 I worked on about 10 years ago (hmmmm… its probably about time I changed that cell!).
Attachments:November 25, 2018 at 6:39 am #10641
Looks like the original radio has a proper battery holder! Here’s a pic of the cell in the 207. The two ends of the cell are held compressed together by a wire. I’ll try to open that up and put in a tiny no. 377 (SR626SW) measuring about 6.5mm by 2.5mm. It’s 1.55V, and I hope that isn’t too far away from the 1.25V spec!
Attachments:November 25, 2018 at 2:16 pm #10643Gerry O’HaraKeymaster
It will work ok. With that type of holder I use a small gob of ‘goo’-type glue to hold the cell in place.November 25, 2018 at 3:43 pm #10644
Thanks Gerry. I’ll open it up and see what I need to do. There is always a way to make it work, and still look OK! Pliable glue would seem to be the best approach with this holder.
I was intent on emptying the cardboard tube containing the four electrolytics – all of which are cacked out – and stuffing them with modern components, but the brown glop filling the tube is rock hard. I chipped a little bit off and applied a heat gun to it, and it melted, but I’m not sure if the heat might also destroy the cardboard case. Any advice?
This is a Marconi 207, and there are two resistors dropping the 390V from the PSU to the screens of the first two stages. The schematic I have comes with a components list, which shows the values of these resistors at 2K6, 2W each. One is burned out and open-circuit, and the other reads 40K, so they have to go. Looking at the current/voltage requirements of the two valves, the 1K3 equivalent of the parallel pair doesn’t make any sense, and the one readable resistor shows a value of 26K, which gives a better V/I environment for these screens. So the correct value is 26K each. Mistakes do get made!
So that’s a warning to not always believe what the manufacturer says in their own docs, even back in 1941 in the days before manufacturers started deliberately telling falsehoods about their products!
Attachments:November 26, 2018 at 7:56 pm #10646
Gerry I really like your idea of using the original cell holder, and it looks like it would be a lot easier to install the battery.
Next time I do one of these I will certainly use your method.
Thanks Dan walker in Calgary
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