Roberts R700 FM dead - UK Vintage Radio Repair and Restoration Discussion Forum

Superhet Si · 19th Nov 2006, 1:27 pm

I'm tearing out my hair on this one.
I can't get the FM section working. I have checked the AF114's and they seem ok. I think the oscillator is running because it beats with a R606 when tuning. Does this suggest the Mullard module is working ok? Is the fault going to be in the IF strip?

Steve_P · 19th Nov 2006, 5:09 pm

Do you get anything from the speaker (such as a hiss) or absolutely nowt?

Cheers,

Steve P

ronbryan · 19th Nov 2006, 5:37 pm

Si

The Mullard module on the R700 is only used on AM bands. The FM IF strip is separate and uses the 3 x AF114s adjacent to the rectangular IFTs. It's the likely culprit if the tuner seems to be working ok. Shorts on the AF114s can cause the 'HT' to be dragged down, so check at both sides of R23 (330 ohm) to see if the volt drop across it is normal (a few volts).

Ron

Superhet Si · 19th Nov 2006, 9:32 pm

Thanks folks, it was indeed a AF114 that was leaky after I zapped it with 100 volts to clear the screen to junction shorts. another transistor fixed that problem. but the OC71 in the amp section is noisy, as is the one i replaced it with but not as bad. Is there an equivalent?

ronbryan · 19th Nov 2006, 11:32 pm

Si

Although the OC71 first stage transistor may be contributing to the noise, the main problem will be noisy carbon resistors.

I found that changing both of the base bias resistors and the collector resistor for metal oxide types gave a worthwhile improvement. This should also be done for the OC75 stage.

The Towers Transistor Selector lists AC107 or 2N406 as OC71 equivalents. I think an AC125 would be suitable as well.

Ron

19th Nov 2006, 1:27 pm	#1
Superhet Si Pentode Join Date: Jun 2006 Location: Crawley Down, West Sussex, UK. Posts: 151	Roberts R700 FM dead I'm tearing out my hair on this one. I can't get the FM section working. I have checked the AF114's and they seem ok. I think the oscillator is running because it beats with a R606 when tuning. Does this suggest the Mullard module is working ok? Is the fault going to be in the IF strip? __________________ My Youtube channel https://www.youtube.com/user/simonspiers

19th Nov 2006, 5:09 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Roberts R700 FM dead Do you get anything from the speaker (such as a hiss) or absolutely nowt? 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...?

19th Nov 2006, 5:37 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: Roberts R700 FM dead Si The Mullard module on the R700 is only used on AM bands. The FM IF strip is separate and uses the 3 x AF114s adjacent to the rectangular IFTs. It's the likely culprit if the tuner seems to be working ok. Shorts on the AF114s can cause the 'HT' to be dragged down, so check at both sides of R23 (330 ohm) to see if the volt drop across it is normal (a few volts). Ron

19th Nov 2006, 9:32 pm	#4
Superhet Si Pentode Join Date: Jun 2006 Location: Crawley Down, West Sussex, UK. Posts: 151	Re: Roberts R700 FM dead Thanks folks, it was indeed a AF114 that was leaky after I zapped it with 100 volts to clear the screen to junction shorts. another transistor fixed that problem. but the OC71 in the amp section is noisy, as is the one i replaced it with but not as bad. Is there an equivalent? __________________ My Youtube channel https://www.youtube.com/user/simonspiers

19th Nov 2006, 11:32 pm	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,959	Re: Roberts R700 FM dead Si Although the OC71 first stage transistor may be contributing to the noise, the main problem will be noisy carbon resistors. I found that changing both of the base bias resistors and the collector resistor for metal oxide types gave a worthwhile improvement. This should also be done for the OC75 stage. The Towers Transistor Selector lists AC107 or 2N406 as OC71 equivalents. I think an AC125 would be suitable as well. Ron Last edited by ronbryan; 19th Nov 2006 at 11:44 pm. Reason: Equivalents added. 2N406 was 2N404 in error