Roberts RM 33 Dead - UK Vintage Radio Repair and Restoration Discussion Forum

Paul_A_R · 9th Dec 2021, 12:07 am

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

paulsherwin · 9th Dec 2021, 12:18 am

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.

Herald1360 · 9th Dec 2021, 12:51 am

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.

GRID LEAK · 9th Dec 2021, 1:21 am

Your fuse is a 100mA anti surge type. Fast acting fuses are suffixed with the letter F.
Regards. John.

Paul_A_R · 9th Dec 2021, 2:51 am

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

Nuvistor · 9th Dec 2021, 11:16 am

Check the BY164 rectifier before powering up with a new fuse, they frequently went short circuit in other equipment.

Sideband · 9th Dec 2021, 1:12 pm

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.....

Herald1360 · 10th Dec 2021, 11:33 am

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.

Paul_A_R · 10th Dec 2021, 2:38 pm

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.

9th Dec 2021, 12:07 am	#1
Paul_A_R Tetrode Join Date: Jan 2018 Location: Penicuik, Midlothian, Scotland, UK. Posts: 54	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
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	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
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	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. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

9th Dec 2021, 1:21 am	#4
GRID LEAK Triode Join Date: Jun 2009 Location: Lancashire, UK. Posts: 46	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
Paul_A_R Tetrode Join Date: Jan 2018 Location: Penicuik, Midlothian, Scotland, UK. Posts: 54	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
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,432	Re: Roberts RM 33 Dead Check the BY164 rectifier before powering up with a new fuse, they frequently went short circuit in other equipment. __________________ Frank

9th Dec 2021, 1:12 pm	#7
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	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..... __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

10th Dec 2021, 11:33 am	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	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. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Dec 2021, 2:38 pm	#9
Paul_A_R Tetrode Join Date: Jan 2018 Location: Penicuik, Midlothian, Scotland, UK. Posts: 54	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.