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

Mike. Watterson · 13th Mar 2019, 10:37 pm

Frequency changers are tricky to test on ANY valve tester. I've considered making a dedicated tester with plug in coils. Then measurements for conversion gain and leakage of LO out of RF in.
I've a 2V McMichael and it DOES work on the highest SW band with a DK91, octal adaptor and dropper resistor for 2V to 1.4V. I've two of the "proper" valves and they test fine and work on other bands, however very likely they don't have enough gain for the design at shortest band. No oscillation.

Unusual for the problem to be on 49m. It would seem something in the circuit, not the valve is wrong. I'd one set that switched increasing values of resistors in series with Osc grid at lower bands. Some part is out of wack and it's coincidence a different type of valve works.

13th Mar 2019, 10:37 pm	#2
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Marconiphone 855 Silent on 49m Frequency changers are tricky to test on ANY valve tester. I've considered making a dedicated tester with plug in coils. Then measurements for conversion gain and leakage of LO out of RF in. I've a 2V McMichael and it DOES work on the highest SW band with a DK91, octal adaptor and dropper resistor for 2V to 1.4V. I've two of the "proper" valves and they test fine and work on other bands, however very likely they don't have enough gain for the design at shortest band. No oscillation. Unusual for the problem to be on 49m. It would seem something in the circuit, not the valve is wrong. I'd one set that switched increasing values of resistors in series with Osc grid at lower bands. Some part is out of wack and it's coincidence a different type of valve works.