UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 4 Metre Converter

runner_bean · 19th Dec 2017, 6:22 pm

Thanks for all the feedback and advice. I have now made quite good progress. My CR100/B28 is working pretty well after selectively replacing various Rs, Cs and perished wiring and it is mostly lined up, except for Band 6 which seems to have a few LO problems.

I have also got the 4 metre converter going after replacing the dead Nuvistor. It turns out that the design LO frequency is 7.550Mhz x3 and then x3 again = 67.95MHz, giving an IF of about 2.25 Mhz on the 4 metre band. This works nicely with my CR100. The design seems to be a personal mash up of various designs including the October 1962 RSGB one (which uses a 1.6 - 1.8 MHz IF) and the 1963 ARRL one (which uses 6CW4 Nuvistors in cascode). I think the design needs refining as there is a tendency to instability and I'm not convinced that some of the coils are right, but there's loads of gain and with a reasonable aerial I'm hoping I will be able to pick something up.

Is it worth me getting one of the used grid dip meters I see advertised? It's all a bit hit and miss at the moment trying to get the coils right as I only have a 10MHz scope!

19th Dec 2017, 6:22 pm	#20
runner_bean Triode Join Date: Oct 2017 Location: Suffolk, UK. Posts: 10	Re: 4 Metre Converter - but which design? Thanks for all the feedback and advice. I have now made quite good progress. My CR100/B28 is working pretty well after selectively replacing various Rs, Cs and perished wiring and it is mostly lined up, except for Band 6 which seems to have a few LO problems. I have also got the 4 metre converter going after replacing the dead Nuvistor. It turns out that the design LO frequency is 7.550Mhz x3 and then x3 again = 67.95MHz, giving an IF of about 2.25 Mhz on the 4 metre band. This works nicely with my CR100. The design seems to be a personal mash up of various designs including the October 1962 RSGB one (which uses a 1.6 - 1.8 MHz IF) and the 1963 ARRL one (which uses 6CW4 Nuvistors in cascode). I think the design needs refining as there is a tendency to instability and I'm not convinced that some of the coils are right, but there's loads of gain and with a reasonable aerial I'm hoping I will be able to pick something up. Is it worth me getting one of the used grid dip meters I see advertised? It's all a bit hit and miss at the moment trying to get the coils right as I only have a 10MHz scope!