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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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24th Sep 2022, 10:13 am | #1 |
Triode
Join Date: Jun 2022
Location: Birmingham, West Midlands, UK.
Posts: 19
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Champion 784(M) TRF. Which thermistor to use?
Hi everyone.
I'm fairly new to radio repair and wondered if you could help me with thermistors. I've started with a simple radio, a champion 784(M) trf, and was thinking of fitting a thermistor to it, but I can't seem to find out at what voltage they are rated. Is there a type of thermistor that is commonly used in vintage radios. It needs to run approx 200mA at 240v Thanks in advance for your advice Gary |
24th Sep 2022, 6:06 pm | #2 |
Moderator
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Location: Exeter, Devon and Poole, Dorset UK.
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Re: champion 784(M) TRF. Which thermistor to use?
Why do you want to fit a Thermistor?
If you looking to replace the dropper resistor for the valve filaments you may want to consider a wattless dropper like a Capacitor. If you just want to protect the scale lamps from switch on surge thats a different thing again. We need to know what you trying to acheive. Cheers Mike T
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24th Sep 2022, 7:30 pm | #3 |
Moderator
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Re: champion 784(M) TRF. Which thermistor to use?
If you want further information on Wattless dropers look here:
https://www.vintage-radio.com/repair...per-calcs.html If you want a device to drop the bulk of the filament voltage you could use a Barretter such as this one. http://www.r-type.org/exhib/abo0045.htm don't forget if you are using any kind of resistance to drop the filament voltage (That includes a Barretter or Thermistor) its going to be dissipating 20 to 30W Cheers Mike T
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Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk |
24th Sep 2022, 9:04 pm | #4 |
Triode
Join Date: Jun 2022
Location: Birmingham, West Midlands, UK.
Posts: 19
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Re: champion 784(M) TRF. Which thermistor to use?
Thanks Mike. I'm using the champion to learn about valve radios. I wondered if the inrush current was damaging the valves for some reason, I did get it working originally, but then the UF42 valve gave up, and since then I've ordered two more but cant get the set to work. Unfortunately the valve tester I have doesn't test this type of valve (pip). As for the dropper I did try replacing the dropper with a capacitor, but i couldn't get the voltages across the heater and the B+ right. In the end I tried two separate capacitors. The valve heater dropper worked fine, but the B+ didn't work at all (that was going to be another question on the forum, can you use a capacitor as a dropper in a circuit with a diode in series with it?). In the end I put a dropper resister back in for the B+ and that seems to have sorted it.
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24th Sep 2022, 11:37 pm | #5 | ||
Hexode
Join Date: Jun 2009
Location: Cheshire, UK.
Posts: 363
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Re: champion 784(M) TRF. Which thermistor to use?
Hi Gary and welcome to the forum
The topic of thermistors comes up quite frequently here. You can find previous discussions by using the search box up at the top of the page. But here are a few links to save you the trouble: https://www.vintage-radio.net/forum/...ead.php?t=8018 https://www.vintage-radio.net/forum/...ad.php?t=75574 https://www.vintage-radio.net/forum/showthread.php?t=135863 Quote:
Quote:
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25th Sep 2022, 12:12 am | #6 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: champion 784(M) TRF. Which thermistor to use?
Quote:
A capacitor will not work in series with a diode, at least not usefully, since the the current it has to pass is pulsating dc rather than ac. A single series capacitor will work more or less OK though if the ac heater current swamps the dc (B+ supply) current. As you found, a separate capacitor (of a suitable value) for the heater string (a resistive load) string will pass the required ac current wattlessly. It will give the set a lousy power factor, but for the few VA involved that's not really much of a problelm. A separate resistor for the rectifier anode feed (chosen to result in the correct B+ voltage on load) will work fine and will still dissipate a lot less power than the original all resistive dropper network.
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25th Sep 2022, 2:16 pm | #7 |
Triode
Join Date: Jun 2022
Location: Birmingham, West Midlands, UK.
Posts: 19
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Re: Champion 784(M) TRF. Which thermistor to use?
Thank you for your welcome, comments and links. I look forward to reading the articles.
Gary |
25th Sep 2022, 4:03 pm | #8 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Champion 784(M) TRF. Which thermistor to use?
These early 50s Champion radios do not require a thermistor. It's been working 60 + years without one. I have a similar one that still has all it's original valves and I've had a few similar models likewise.
Make sure your UF42s are definably faulty. In my opinion fitting a capacitor dropper is like putting a fox in with the chickens. If you must you need an excellent quality cap and a parallel 1m ohm discharge resistor wired across it. Take care. John. |
1st Oct 2022, 1:18 pm | #9 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
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Re: Champion 784(M) TRF. Which thermistor to use?
are you sure the valve bases are 100% ok? If it uses those McMurdo ones with the little forked contacts then they can drive you crazy if they go brittle. Put a valve in, it works then dies. Change the valve, it might work again for a while then die. Ad nauseum.
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1st Oct 2022, 2:23 pm | #10 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Champion 784(M) TRF. Which thermistor to use?
Just Curious: Was the original speaker faulty? Sometimes dust gets in between the voice coil & the magnet if there is no dust cap. But that's a neat job with the alternative speaker.
Q. You always have to use a motor-run capacitor as a dropper? I just visited the pages on how to work out the dropper value and accepted that type was the only correct sort. Is it because when they fail they go open-circuit & the current flow stops? Just wondering as I am not sure what type is in here. If one of the valve holders has an intermittent contact. Have you tried dosing it with contact cleaner? If the contacts are not broken you just might be lucky. Before you have to drill out the rivets holding the old base in. |
11th Oct 2022, 9:12 pm | #11 |
Triode
Join Date: Jun 2022
Location: Birmingham, West Midlands, UK.
Posts: 19
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Re: Champion 784(M) TRF. Which thermistor to use?
Hi everyone and again thank you for you comments. They are appreciated. The capacitor is only being used to drive the valve heaters on this radio, but it is a motor run capacitor. The B+ is being powered by a dropper. The swapped over speaker is completely my fault, as I damaged the original, and then ruined it trying to fix it, a lesson learned. I've looked on line but can't find a replacement, so I've replaced it with a different one. The speaker driver transformer is original though, just transferred to the new speaker. 3 ohm. Thanks for the advice on the valve holders, they are as you described. I'll check the connections.
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