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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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9th Dec 2021, 12:07 am | #1 |
Tetrode
Join Date: Jan 2018
Location: Penicuik, Midlothian, Scotland, UK.
Posts: 56
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Roberts RM 33 Dead
I picked up a Roberts RM 33 which was sold as dead. The seller indicated they had changed the fuse but that had made no difference.
Incidentally what was fitted was a 13 amp fuse when I assume a 3 amp would be better. With the chassis out of the case I checked the internal Fuse F1 on the power supply board and it read open circuit. The fuse reads T 100mA 250v. I shall order some replacements but is this a time delay or fast acting fuse? Also are there any checks I can carry out on the unpowered radio before replacements arrive to find if there is a fault elsewhere which caused the fuse to go open circuit. Paul |
9th Dec 2021, 12:18 am | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,944
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Re: Roberts RM 33 Dead
If you are lucky, the internal fuse will have failed through old age, and there isn't a fault present.
You can apply power from a bench PSU or a battery, but be careful until you have idenified the problem (if there is one). You may have a bad transistor, IC or electrolytic capacitor. |
9th Dec 2021, 12:51 am | #3 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: Roberts RM 33 Dead
T 100mA 250V will be a slow blow 250V low breaking capacity fuse. A quick blow one would be marked F.
The T type will be to cope with any primary inrush current of the mains transformer.
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9th Dec 2021, 1:21 am | #4 |
Triode
Join Date: Jun 2009
Location: Lancashire, UK.
Posts: 46
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Re: Roberts RM 33 Dead
Your fuse is a 100mA anti surge type. Fast acting fuses are suffixed with the letter F.
Regards. John. |
9th Dec 2021, 2:51 am | #5 |
Tetrode
Join Date: Jan 2018
Location: Penicuik, Midlothian, Scotland, UK.
Posts: 56
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Re: Roberts RM 33 Dead
Thanks. I will order some later today.
I tried some benchtop power and got some static out of a speaker but I will wait for the fuses to do any more. I will hope for some of Paul's luck and just a few tired components need swapping out. I mainly look at 1960s transistors and there is a lot more room and simpler layouts for serious fault finding. This must be late 70s and looks harder to disassemble and assemble. Paul |
9th Dec 2021, 11:16 am | #6 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
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Re: Roberts RM 33 Dead
Check the BY164 rectifier before powering up with a new fuse, they frequently went short circuit in other equipment.
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Frank |
9th Dec 2021, 1:12 pm | #7 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,571
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Re: Roberts RM 33 Dead
Easy to remember 'T' prefix for Time delay or Time lag (anti-surge) and 'F' for Fast blow. There are others but those are the most common.
Fuses are a science in themselves.....
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10th Dec 2021, 11:33 am | #8 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: Roberts RM 33 Dead
https://www.swe-check.com.au/pages/l...e_markings.php
Goes into further detail. Interestingly the letters look for speed to relate to the German words for fast and slow but those for breaking capacity to English high and low.
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10th Dec 2021, 2:38 pm | #9 |
Tetrode
Join Date: Jan 2018
Location: Penicuik, Midlothian, Scotland, UK.
Posts: 56
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Re: Roberts RM 33 Dead
Thanks. I have bookmarked the link explaining fuse markings. Though I suspect I will find it again more easily searching here than through my chaotic bookmarks 'system'.
I have checked the rectifier. They used 1N4001 rather than those marked on my circuit diagram. They test OK but I will probably replace them anyway since they are now disconnected. |