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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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16th Jan 2017, 10:40 pm | #1 |
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Roberts RT1 Thermistor
I have recently acquired a rather delightful Roberts RT1. However, there is a thermistor and its in a rather sorry state, it is a VA1040 according to my schematic. Would anybody be able to tell me what the resistance of this should be and what wattage? Also maybe even point me in the right direction as to where I can source one.
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16th Jan 2017, 11:45 pm | #2 |
Octode
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Re: Roberts RT1 Thermistor
There is a Mullard VA1040 data sheet on the Electrojumble web site - it's the last one in the document.
http://www.electrojumble.org/DATA/Mu...o%20VA1040.pdf Ron |
17th Jan 2017, 12:22 am | #3 |
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Re: Roberts RT1 Thermistor
Thank you Ronbryan for that I now know that its 130ohm 1w. I may now have a fighting chance of finding one.
Regards Poppydog |
17th Jan 2017, 9:17 am | #4 |
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Re: Roberts RT1 Thermistor
2.50 Euro from this company, but they're in Malta, and I don't know about shipping or minimum order value.
Might be worth an email? http://www.fabian.com.mt/en/products...30-ohms-1w.htm Hope that might help. Good luck in your quest.
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17th Jan 2017, 11:25 am | #5 |
Heptode
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Re: Roberts RT1 Thermistor
What is actually wrong with the thermistor?
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17th Jan 2017, 1:50 pm | #6 |
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Re: Roberts RT1 Thermistor
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17th Jan 2017, 9:09 pm | #7 | ||
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Re: Roberts RT1 Thermistor
Quote:
Quote:
Regards Poppydog |
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18th Jan 2017, 1:01 am | #8 |
Heptode
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Re: Roberts RT1 Thermistor
The radio has got some corrosion on its metal parts so this may also have weakened the thermistor, or tension on the leads during manufacturing may have lead to its eventual failure. It could be squeezed back together with the tension of its leads holding it together and then a dab of epoxy to hold it. The thermistor was not fitted to the nearly identical output stage of the early version of the R200, the difference being the RT1 had OC72 output transistors and the R200 had OC81 output transistors.
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18th Jan 2017, 1:04 am | #9 |
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Re: Roberts RT1 Thermistor
The thermistor is in parallel with the bottom bias resistor for the output pair so that as it warms up and goes lower in value, the bias is reduced to help stop thermal runaway. With it open circuit the radio will still work and sound OK, but output quiescent current will be a bit high and there is a risk that high volume use could lead to output device failure.
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21st Jan 2017, 5:44 am | #10 | |
Hexode
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Re: Roberts RT1 Thermistor
Quote:
Good luck Graham |
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21st Jan 2017, 10:43 pm | #11 |
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Re: Roberts RT1 Thermistor
I would like to order some as well unfortunately they don't seem to be replying to my emails regarding shipping. I will send one more email tonight and see what happens. In the meantime I have been doing a little more research and was wondering if I could use this:
http://www.mouser.co.uk/ProductDetai...1CsrvHxGnljg== The only thing I couldn't find on the one from Mouser was the power rating (watts). Also the other concern was whether it warrants the £12 postage charge Regards Poppydog |
30th Jan 2017, 10:31 pm | #12 |
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Re: Roberts RT1 Thermistor
I sent 3 emails to the company suggested in Malta and have had no reply, so I am assuming they are making too much money and don't want to bother with a small order such as mine. I was hoping someone would have been able to advise on the one from Mouser that I previously posted about but nevermind, I have ordered a supposedly identical one from the far east and will see what that is like when it arrives.
Regards Poppydog |
12th Feb 2017, 11:17 pm | #13 |
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Re: Roberts RT1 Thermistor
My thermistor arrived yesterday from the far East (very quick). I haven't fitted it yet, I want to ask one more question before I ask the Moderators to close the thread.... Yesterday I had it on via my power supply, even though I said the radio worked well I haven't had it on for very long until now, after about five minutes it went silent - the power supply had turned itself off. I made sure I hadn't shorted battery lead connections and tried it again. To cut a long story short I watched the current being drawn rise very quickly after a couple of minutes until it reached 70mA and then the power supply turned itself off again, for some reason (luckily) I had the power supply's OCP set to 70mA. Is there anything other than the absence of the thermistor that would make it do this? I am a little concerned that it happened so quick and just wondered if I should check anything else as we don't know how long the radio has been run for in its current state with the thermistor like it is, and I assumed it was safe to run the radio with the thermistor like it is. I did however clip some test hooks to the remaining leads on the old thermistor, to the new thermistor and it stayed around 35mA for around 3/4 of an hour at roughly half volume and it seemed fine. Am I worrying over nothing?
Regards Poppydog |
12th Feb 2017, 11:42 pm | #14 |
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Re: Roberts RT1 Thermistor
see post 9, this explains the reason for the thermistor, it looks like you were suffering from "thermal runaway"
Mike |
12th Feb 2017, 11:51 pm | #15 |
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Re: Roberts RT1 Thermistor
I understand Herald's post Mike, I'm just concerned it happened so quickly, as some say they can happily do without it.
Regards, poppydog. |
10th Mar 2017, 8:01 am | #16 |
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Re: Roberts RT1 Thermistor
All sorted now the thread can be closed regards poppydog
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11th Mar 2017, 6:10 am | #17 |
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Re: Roberts RT1 Thermistor
Glad you got it sorted, it's been a bit of slog. Still, fair play to you for your tenacity. Nice one.
Andy.
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14th Mar 2017, 11:38 pm | #18 |
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Re: Roberts RT1 Thermistor
Thank you very much for that comment Dr Wobble. I'll let you into a secret, as I think you won't tell anyone else . In post 13 I said that after about 5 minutes it started drawing large amounts of current and turned itself off, that turned out to be operator error. Father Christmas brought me a digital bench power supply that is variable and also has the capability of storing 5 pre-set voltages, to cut a long story short I was putting 9v, that was previously saved, into a 6v radio In future I will not be pre-setting any voltages, I will be typing them in according to the voltage I need. It wasn't until after I put the new thermistor in and was ready to try it again that I realised what I had done, after almost doing the same thing again. I will never make that mistake again
Regards Poppydog |