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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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19th Nov 2006, 1:27 pm | #1 |
Pentode
Join Date: Jun 2006
Location: Crawley Down, West Sussex, UK.
Posts: 151
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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?
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19th Nov 2006, 5:09 pm | #2 |
Dekatron
Join Date: Sep 2003
Location: Bolton, Greater Manchester, UK.
Posts: 6,644
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Re: Roberts R700 FM dead
Do you get anything from the speaker (such as a hiss) or absolutely nowt?
Cheers, Steve P
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19th Nov 2006, 5:37 pm | #3 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,959
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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 |
Pentode
Join Date: Jun 2006
Location: Crawley Down, West Sussex, UK.
Posts: 151
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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?
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19th Nov 2006, 11:32 pm | #5 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,959
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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 |