Roberts RMB no long wave - UK Vintage Radio Repair and Restoration Discussion Forum

Ian B · 29th Nov 2007, 10:37 pm

I bought a very tidy red Roberts RMB at the local auctionrooms yesterday. Mains lead had been chopped, but otherwise it looked untouched. Found Trader sheet 1037 and took it apart.

Changed wax capacitors C2, C6, C11, C16 & C17, fitted new mains lead, and switched on. It burst into life, works really well on mw, and on sw with an extra aerial, but not a sign of anything on lw.

Tried switch cleaner first, no help. I've checked the contacts which should be closed on lw are working, checked resistance of all the associated coils & wiring etc. My prime suspect is either L8 (lw oscillator coil) or C10. I can't check L8 for continuity because it's in series with C10 which is inside the can (along with L11).

Removing the two screws underneath the chassis releases the whole coil assembly, not just the cover as I'd hoped. Or is the large lump of wax in the top holding it all together? I was hoping I could look inside without having to remove it completely as the wires to the base connections are rather tight.

Before I undertake micro surgery, have I overlooked any other likely fault?

Thanks in anticipation,

Ian Blackbourn

Steve_P · 29th Nov 2007, 10:51 pm

Clean the wavechange switch. Then, check for broken wires, especially on the coils. Then try C10, C7, Examine Trimmer C38, C34, L2, L5.

Cheers,

Steve P.

G0PKH - Pete · 29th Nov 2007, 11:21 pm

Can you confirm the local oscillator is working by putting the set next to another and tuning up and down the waveband.

Station X · 1st Dec 2007, 3:45 pm

Faced with this sort of problem I always check whether the Local Oscillator is running and on the correct frequency first. Assuming the IF is 455 KHz and the set is tuned to say 200 KHz or 1500 metres on the dial the LO will be operating on 655 KHz or 458 metres.

Tune a second radio to a station close to 655 KHz and bring it close to the faulty radio. As you sweep the tuning of the faulty radio through 200 KHz you should hear a hetrodyne note.

Ian B · 1st Dec 2007, 5:38 pm

I thought I'd put the result under "success stories" yesterday, but it hasn't appeared. Operator error I expect.

Anyway, yesterday I grabbed a transistor portable to do as Pete (and Graham) suggested and switched on the RMB. It worked perfectly, I hadn't touched anything since giving up on Thursday evening, it had somehow cured itself overnight!

It's all back together and working well on all bands. I just have to try re-stitching the handle, the leather is coming apart at the seams. I don't think that's going to be so easy.

Thanks folks,

Ian

29th Nov 2007, 10:37 pm	#1
Ian B Hexode Join Date: May 2006 Location: Arlesey, Bedfordshire, UK. Posts: 401	Roberts RMB no long wave I bought a very tidy red Roberts RMB at the local auctionrooms yesterday. Mains lead had been chopped, but otherwise it looked untouched. Found Trader sheet 1037 and took it apart. Changed wax capacitors C2, C6, C11, C16 & C17, fitted new mains lead, and switched on. It burst into life, works really well on mw, and on sw with an extra aerial, but not a sign of anything on lw. Tried switch cleaner first, no help. I've checked the contacts which should be closed on lw are working, checked resistance of all the associated coils & wiring etc. My prime suspect is either L8 (lw oscillator coil) or C10. I can't check L8 for continuity because it's in series with C10 which is inside the can (along with L11). Removing the two screws underneath the chassis releases the whole coil assembly, not just the cover as I'd hoped. Or is the large lump of wax in the top holding it all together? I was hoping I could look inside without having to remove it completely as the wires to the base connections are rather tight. Before I undertake micro surgery, have I overlooked any other likely fault? Thanks in anticipation, Ian Blackbourn

29th Nov 2007, 10:51 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Roberts RMB no long wave Clean the wavechange switch. Then, check for broken wires, especially on the coils. Then try C10, C7, Examine Trimmer C38, C34, L2, L5. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

29th Nov 2007, 11:21 pm	#3
G0PKH - Pete Retired Dormant Member Join Date: Aug 2003 Location: Stansted Essex Posts: 263	Re: Roberts RMB no long wave Can you confirm the local oscillator is working by putting the set next to another and tuning up and down the waveband.

1st Dec 2007, 3:45 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Roberts RMB no long wave Faced with this sort of problem I always check whether the Local Oscillator is running and on the correct frequency first. Assuming the IF is 455 KHz and the set is tuned to say 200 KHz or 1500 metres on the dial the LO will be operating on 655 KHz or 458 metres. Tune a second radio to a station close to 655 KHz and bring it close to the faulty radio. As you sweep the tuning of the faulty radio through 200 KHz you should hear a hetrodyne note. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

1st Dec 2007, 5:38 pm	#5
Ian B Hexode Join Date: May 2006 Location: Arlesey, Bedfordshire, UK. Posts: 401	Re: Roberts RMB no long wave I thought I'd put the result under "success stories" yesterday, but it hasn't appeared. Operator error I expect. Anyway, yesterday I grabbed a transistor portable to do as Pete (and Graham) suggested and switched on the RMB. It worked perfectly, I hadn't touched anything since giving up on Thursday evening, it had somehow cured itself overnight! It's all back together and working well on all bands. I just have to try re-stitching the handle, the leather is coming apart at the seams. I don't think that's going to be so easy. Thanks folks, Ian