Roberts RT1 Thermistor - UK Vintage Radio Repair and Restoration Discussion Forum

poppydog · 16th Jan 2017, 10:40 pm

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.

Regards
Poppydog

ronbryan · 16th Jan 2017, 11:45 pm

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

poppydog · 17th Jan 2017, 12:22 am

Thank you Ronbryan for that I now know that its 130ohm 1w. I may now have a fighting chance of finding one.

Regards
Poppydog

David G4EBT · 17th Jan 2017, 9:17 am

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.

GeoffK · 17th Jan 2017, 11:25 am

What is actually wrong with the thermistor?

dazzlevision · 17th Jan 2017, 1:50 pm

Quote:

Originally Posted by Hetrodyne

What is actually wrong with the thermistor?

Quite often, coloured paint band markings (where applied during manufacture) flake of these over time, but the component is still OK.

poppydog · 17th Jan 2017, 9:09 pm

Quote:

Originally Posted by David G4EBT

2.50 Euro from this company, but they're in Malta, and I don't know about shipping or minimum order value.

Thank you David G4EBT for that I will send them an email this evening.

Quote:

Originally Posted by Hetrodyne

What is actually wrong with the thermistor?

Quote:

Originally Posted by dazzlevision

Quite often, coloured paint band markings (where applied during manufacture) flake of these over time, but the component is still OK.

Here are some pictures to show what is wrong with the thermistor, surely this component is not ok? (I've gently bent one of the leads apart slightly so you can clearly see) What on earth would have made it do that? The radio works very well still.

Regards
Poppydog

GeoffK · 18th Jan 2017, 1:01 am

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.

Herald1360 · 18th Jan 2017, 1:04 am

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.

Radiocruncher · 21st Jan 2017, 5:44 am

Quote:

Originally Posted by David G4EBT

2.50 Euro from this company, but they're in Malta, and I don't know about shipping or minimum order value.

I bought some of these last year and can confirm that they are correct. From memory I think I purchased 4 and they were here fairly quickly.

Good luck

Graham

poppydog · 21st Jan 2017, 10:43 pm

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

poppydog · 30th Jan 2017, 10:31 pm

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

poppydog · 12th Feb 2017, 11:17 pm

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

crackle · 12th Feb 2017, 11:42 pm

see post 9, this explains the reason for the thermistor, it looks like you were suffering from "thermal runaway"

Mike

poppydog · 12th Feb 2017, 11:51 pm

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.

poppydog · 10th Mar 2017, 8:01 am

All sorted now the thread can be closed regards poppydog

Diabolical Artificer · 11th Mar 2017, 6:10 am

Glad you got it sorted, it's been a bit of slog. Still, fair play to you for your tenacity. Nice one.

Andy.

poppydog · 14th Mar 2017, 11:38 pm

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

16th Jan 2017, 10:40 pm	#1
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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. Regards Poppydog

16th Jan 2017, 11:45 pm	#2
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	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
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,764	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. __________________ David. BVWS Member. G-QRP Club member 1339.

17th Jan 2017, 11:25 am	#5
GeoffK Heptode Join Date: Mar 2016 Location: Essex, UK. Posts: 602	Re: Roberts RT1 Thermistor What is actually wrong with the thermistor?

18th Jan 2017, 1:01 am	#8
GeoffK Heptode Join Date: Mar 2016 Location: Essex, UK. Posts: 602	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.

18th Jan 2017, 1:04 am	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	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. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

21st Jan 2017, 10:43 pm	#11
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	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
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	Re: Roberts RT1 Thermistor All sorted now the thread can be closed regards poppydog

11th Mar 2017, 6:10 am	#17
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	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. __________________ Curiosity hasn't killed this cat...so far.

14th Mar 2017, 11:38 pm	#18
poppydog Banned Join Date: Oct 2015 Location: Cornwall, UK. Posts: 849	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