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Post by briano on Jun 13, 2022 16:55:03 GMT 12
Hi Peter
Thank you for your further thoughts on those filter capacitors. Tomorrow I will order those 600v 10µF capacitors that you have suggested. Most helpful thank you. Brian.
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Post by briano on Jun 21, 2022 10:11:17 GMT 12
Stand for radio chassis made from scraps of wood. Ready to examine layout, draw circuit, identify components to replace... Brian. Attachments:
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Steve
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Post by Steve on Jun 21, 2022 10:18:30 GMT 12
nice
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Post by briano on Jun 21, 2022 16:56:13 GMT 12
I have sketched some of the chassis circuitry - as per the first photo in this post. The second photo is a schematic of the power supply, detector/audio and the power amp stages. The third photo related to my last question, no. 6. I would really appreciate help in terms of my questions here. Thank you: QUESTIONS1. Would it be best to change the termination of the mains cord on the transformer 220V tag, over to the 240V tag? My thinking is this would place less load on the circuitry since our nominal mains voltage is 230V. (I will be replacing the old mains cord). 2. Shall I change the old wax paper capacitors with the '630v metal polyester film type with axial leads' - as per the 'Components for Sale' on the NZVRS website? (I will definitely change all the electrolytics as per the 'Components for Sale' on the NZVRS website). 3. Is there any need to change the resistors? Most of them read their correct value. 4. Is it normal for one end of the 'Tone' pot be connected to a high DC voltage - power amp Plate terminal? 5. Is it normal for the 'Tone' pot slider to be connected, thankfully via DC blocking capacitor, to the low voltage side of the 5v line? (The other side of this 5V winding is HT). 6. Why is the old 25µF electrolytic wired across the audio input to the 6Q7 valve? My thinking is that this will 'clamp' the audio level going into the top of the volume pot? Limit volume? Some background to my thinking: My thoughts are that, at sometime in the 1940s or 1950s, the old 25µF electrolytic was faulty and that a new 32µF electrolytic was put in its place. The old 25µF electrolytic was not removed, and the repairer at the time accidentally wired the + side of the old cap across the audio input to the 6Q7 valve? My third photo uploaded here should explain what I mean. I remember that ' Richard' back on 12th June commented that the 32µF electrolytic does not look original. The next day ' Peter Walsham' commented on the possibility that the radio originally had a '42' output valve. Looking at an earlier photo, Peter identified the soldering around the 6F6 valve socket wasn't as tidy as the rest of the radio. (Yes, wiring around here, as seen by the third photo, is pretty hairy)! 7. Shall I therefore remove the old 25µF electrolytic, and not replace it? (The newer 32µF electrolytic nearby is wired correctly and would be doing its job). Thank you.... Brian Oliver
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Post by Peter Walsham on Jun 21, 2022 18:10:01 GMT 12
Hi Brian
Answers to your questions: (1) Yes, it would be a great idea to use the 240 Volt tap on the power transformer.
(2) Yes, it's a great idea to replace all of the old capacitors. They will almost certainly be all leaky to some extent.
(3) No, there really isn't any need to change the resistors that measure within about 20% of their stated value. Having said that, old carbon resistors sometimes go noisy, but still read within 20% of their stated value. I would replace the 250K Ohm 1st audio valve anode load resistor anyway, and probably the IF/RF valve screen dropping resistor.
(4) Yes, it is OK to have one end of the tone control pot connected to the anode of the output valve (however, I would prefer to see one end of the 0.05uF tone control capacitor connected there instead (just changing the 'order' that the tone control potentiometer & the 0.05uF capacitor are connected).
(5) The 0.05uF (that you say is a DC blocking capacitor) is the tone control capacitor, It shouldn't have been connected to the 5 Volt rectifier heater as you have shown, as it will introduce hum in to the output. It should have been connected to the HT buss instead. IE: there should be nothing else connected to that 5 Volt 'line' (pin 4 of the 80 valve). To clarify further the connection of that capacitor. The tone control capacitor & tone control potentiometer should be connected together in series (as shown), but the capacitor should connect to pin 3 of the 6F6, and the top end of the tone control should connect to the HT buss. Note: The value of the tone control capacitor is high. This will attenuate the high frequencies too much (in my opinion) and could well be reduced in value to 0.02 or even 0,01uF to be less severe on the 'top cut'.
(6) You're dead right about that 25uF capacitor across the input to the volume control. If that capacitor was 'OK', there would be no audio for the 1st audio valve (the 6Q7) to amplify - the audio from the detector will be shunted to ground instead. Remove this capacitor. The newer 32uF capacitor is connected correctly.
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Post by briano on Jun 21, 2022 19:05:38 GMT 12
Hi Peter Thank you for your very prompt, and most detailed reply. Thank you. All carefully noted, including the advice on the tone control circuitry. I really value your help. Kind regards, Brian
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Steve
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Post by Steve on Jun 22, 2022 0:17:45 GMT 12
Nice work on the schematic - its interesting to see one drawn like later transistor radios were done... its not normally how valve schematics get laid out and its actually quite nice seeing it done that way.
I'd take that 0.05 tone cap and drop it to 0.02-0.04 as Peter suggested, but I'd take the other end of the cap to ground, not HT. Thats pretty standard for a tone control of that era. Often the body of the pot will have an earth wire soldered to it as well - I don't know if they did this for noise shielding or safety - but swapping the cap and the pot as Peter said will keep the high potential DC out of it.
Also, the suppressor grid of the 6F6 should be connected to the cathode not the grid.
Looking good so far... it'll be singing in no time.
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Post by Peter Walsham on Jun 22, 2022 9:37:47 GMT 12
Hi Steve
I would like to make a comment about your preference to connect the tone control capacitor to earth. I, personally, prefer to connect the 'non signal' end to HT because of two reasons. (1) It removes the 250 Volts potential across the capacitor (2) If this capacitor happens to short out (very unlikely with a new capacitor, but I have seen it happen) (and if the tone control pot happens to be at the maximum treble cut end of its range) then the full HT voltage is placed across the output transformer primary winding, causing a high current to flow through the winding & the likelihood of the output transformer failing - and/or worse, having the rectifier valve fail & burning out the power transformer as well. By having the tone control network connected across the output transformer totally eliminates the possibility of this happening.
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Post by briano on Jun 22, 2022 11:10:35 GMT 12
Hi Steve Thank you for your kind comments on my schematic. In terms of layout, I seem to prefer the way the British draw diagrams compared to the Americans. I am a 'visual' sort of person and if I place the 'load' above the valve (or transistor) I can look at it like a voltage divider. In the case of the valve, the signal on the grid varies the current (electron flow) flowing up through the valve, and with the anode being the centre of the 'voltage divider', providing the variation of voltage at this point. Oops, oh thank you for the correction to my pentode symbol. The pencil from the suppressor ended up at the wrong place, ha ha!
Thank you too for confirmation on Peter's suggestion on the tone control capacitor value. I have had a close look at the body of the pot to see if they have an earth wire connected. No it doesn't. However, a continuity test shows the metal backs of both the tone and the volume pots are at earth potential - most probably through the fastening at the front.
Thank you for your encouragement in my project. I really appreciate your help. Brian.
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Post by briano on Jun 22, 2022 11:11:41 GMT 12
Hi Peter Thank you for your thoughts on the pros and cons on where the tone control capacitor should connect to. I appreciate your thinking on this. Brian.
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Steve
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Post by Steve on Jun 23, 2022 15:55:59 GMT 12
Hi Peter, thanks for your thoughts on the connection to HT - I had not considered the potential failure mode you point out there - and you're right I think. I was simply referring back to the standard 'way it always was' - I can't immediately recall seeing any sets where the tone control went to HT - although I note this circuit already has a simple tone compensation cap strung across the primary of the OP TX. I will take a bit of notice of this as I look at circuits over the next while... interesting! And Brian, I agree about looking at the circuit like a voltage divider - it does look cleaner, although I guess by now I am just used to seeing it done a certain way. My brain is stuck in the 1930s Cheers, Steve
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Post by briano on Jun 24, 2022 14:01:11 GMT 12
THE CABINET For my grandmother's radio, I have the capacitors and a couple of spare dial lamps on order through NZVRS. Meanwhile, I thought I would take a look at the cabinet. I thought it was in pretty good shape. However, upon inspection, I found it was not so good. Any advice I can receive would be most appreciated. The cabinet is very dry. The front and sides, are a nice golden colour wood grain, and have some scratches. The surface is rough to touch when rubbing ones hands across it. The top is a disaster! It also has some scratches, appears darker, and looks like to have either varnish or shellac flaking off, leaving patches of bare dark wood. I'm in a quandary as to the best way to approach this. I think for the sides and front, I could give a gentle clean with meths water sparingly with 0000 steel wool. Maybe a light sand next if needed. And then apply either a wax or the white vinegar/olive oil trick? I don't want a high gloss finish, rather a low sheen look. For the top, do I scrape any remaining varnish off, see what the colour looks like, and apply either a wax or the white vinegar/olive oil solution? In a post from last May, 'Steve' had used 'Briwax' on a Pacific model 18 cabinet and looked good from his photo. THE ESCUTCHEONS Now, looking at the dial escutcheon and the magic eye escutcheon they look like a dull dark metal, maybe coinage bronze? Any advice on cleaning these, without making it look too new or too bright would be appreciated? CONCLUSION For sentimental reasons, I don't want to botch this up, so I feel I'm not ready to tackle the cabinet yet. Any advise would be of most value, thank you.
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Steve
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Post by Steve on Jun 25, 2022 11:12:23 GMT 12
My thoughts - and please note that I am NOT a cabinet expert, but I've done a fair few of them in my time - and the panic still hits me every time I do one that I might screw this one up. I haven't yet (not everyone might agree). Cabinets are actually a lot more forgiving than you might first think.
First, decide on a finish you want to apply. The original yours has was nitrocellulose lacquer, and it's unfortunately buggered. There is no coming back from the damage you've got there, so in my untrained opinion it will need to come off and be refinished. A scraper for most of it, and acetone in some steel wool or a rag to finish, with maybe a light sand at the end are the best way in my opinion. You'll find the old finish will scrape off very easily without too much pressure, at least on top anyway. You can always resort to a chemical stripper if you need to - I try and avoid those if I can.
The wood underneath also looks a little stained, so you might want to use oxalic acid to clean it before re-applying anything. I've done this on a few cabinets with mixed success - but mostly positive. Pick a warm day, mix the acid as directed on the package, slather it on evenly all over and keep it wet for a while (if you have dribbles, you'll create stripes). Then wash it off thoroughly and 'rinse and repeat' as necessary.
Once cleaned and de-stained, I stain or paint anything that needs it - in your case, you will need to use a fine brush and some black lacquer or similar to infill the stripes on the grille bars. Then I will either:
A) If going with NC lacquer I'll apply at least two coats of shellac as an undercoat / sealer then a very light sand to de-nib the finish and flatten off any raised grain 'hairs'. Maybe another coat of shellac after that, but generally I'll leave it at that. If you are keen, you could keep going with the shellac and french polish the cabinet - I never have because its a lengthy time-consuming process.
Then I'll spray multiple light coats of satin clear lacquer with about 20 minute intervals to allow it to dry but not fully cure (so the next coat can chemically bond to the one underneath). After a few layers I stop, let it cure for a day or two and decide if it needs more - if it does then it needs a light sand to flatten and key the surface. That's generally as far as I go with that.
B) If I'm using briwax I will stain any areas that need it, if they need it - or use different toned waxes on different areas. I'll generally apply that (relatively liberally) with either a cotton rag or a fine steel wool (I've gone off the steel wool approach though - I don't like steel in my finish) - and then I'll wait till the solvent in the wax has started to flash off and then rub it out a bit. Then give it a good brush over with a shoe brush or similar to finish it later on.
Thirsty wood (like my 18 was) might need another application.
My Oxford 56M (https://www.vintageradio.co.nz/model/oxford/56m) was done in lacquer. I masked it off, did the black trim, then removed all the masking and sprayed a very lightly brown-tinted clear satin over the whole cabinet. I think the black is probably a little too stark, and maybe a lighter shade, or more clear mixed in to make it a little more translucent, might have been better - although it presents well in person.
The wax finish on the 18 is pretty nice I think - but I don't know if it will ever come off, which could be a problem for its next restoration/rejuvenation in 80 more years time. I've never experimented with removing the wax. It's something to keep in mind for your great great grandchildren when they re-restore the old family heirloom.
Also, don't polish the escutcheon - It is my understanding (my standard disclaimer applies: I wasn't there, so I can't say for sure) that they were typically artificially aged or coated as part of the manufacturing process, and not brightly polished.
Hopefully that helps, and hopefully far more skilled cabinet restorers than I will chime in and correct any misconceptions I've created 🙂
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Post by briano on Jun 25, 2022 14:02:37 GMT 12
Hi Steve Thank you for your most detailed, knowledgeable, and helpful reply.
Interesting about it having a nitrocellulose lacquer finish. I have never heard of that. To take off that finish, are you referring to the whole cabinet, or just the top? I quite like the look of the sides and front.
You certainly have an eye for detail, and like your attention to the the black stripes on the speaker grille bars.
QUESTIONS 1. In regard to your option 'A', When you refer to spraying light coats of satin clear lacquer - does this come in a spray-can or do you need proper spray gear? (I have not had a lot of luck with polyurethane finishes. I sometimes get blemishes, like bubbles or specs. Although I have always used a brush).
2. Alternatively, If I used your option 'B', would the Briwax go straight onto the cleaned and de-stained wood?
3. With your Oxford 56M (1937), I notice the top is a dark colour. Did those cabinets come standard with a darker finish? (The lacquer finish of your Oxford 56M looks really amazing).
Thank you to on the advice not to polish the escutcheon. Sounds good. Cheers, Brian
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Steve
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Post by Steve on Jun 25, 2022 19:27:55 GMT 12
Nitrocellulose Lacquer (or Duco) was developed for the automotive industry back in the 20's I think. Today its acrylic lacquer, also automotive - although I believe you can still get nitrocellulose lacquer if you look hard enough.
1. I buy my lacquer from Spraystore, and I either spray with a proper gun and a compressor or, if its just a small cabinet I use Preval sprayers (Spraystore sells them - small self-contained spraying systems). They will make up whatever amount of lacquer you need, in whatever mix of sheen you need. I tend to mix it 50/50 with their lacquer thinner when spraying it. And never spray in high humidity or you can get blushing (a white appearance in the finish). I almost never use poly - it tends to look plasticy and wrong on these old sets.
2. Yes, I apply the Briwax direct to the timber with no undercoat - unless I am applying a stain first.
3. Only the trim is black, not the whole top. I don't have any before photos of this set, but the cabinet was in a VERY bad way - I think, from memory, that the trim was a dark colour - but I can't recall for certain. Its likely that the cabinets came with some kind of different trim finish colour on them - but I don't think yours did by the look of it - so it will come down to your preference in terms of colouring away from the original.
And re: your suggestion of just doing the top - it can be done, but I don't know how successfully you would be able to protect the rest if you were going to destain it... if I am doing one part, I tend to just do the whole lot - but its worth a try.
Cheers, Steve
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