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steve1811uk · 24th Mar 2006, 10:34 am

I had another look at this and found that I had adjusted C8 incorrectly (LW Local oscillator). According to "Trader" 1449 the tuning range is 1130 to 1970 metres (265kHZ - 152kHz). A local oscillator range of 735 kHz - 622 kHz was required. I adjusted C8 to give this exact range while monitoring the LOsc transistor collector voltage with a scope. After this adjustment I found the sensitivity and selectivity were the best that they had been. Radio 4 was then found to be exactly on the 400m mark of the MW scale.

Regarding the Capital Gold and Radio 5 breakthrough. I did some reading and found that these are in fact images caused by the LOsc not being a pure sine wave. I found some useful info at

http://www.dbugman.com/handbook/tscmh5.html

To confirm that this was the cause I did some measurements

Capital Gold (1458 kHz) could be heard faintly when the LOsc was at 643 kHz. This was found to be due to LOsc 3rd Harmonic distortion, 1929 kHz - 1458 kHz = 471 kHz, the IF that gets amplified.

Radio 5 (909 kHz) could be heard faintly when the LOsc was at 690 kHz. This was found to be due to LOsc 2nd harmonic distortion, 1380 kHz - 909 kHz = 471 kHz, the IF that gets amplified.

Does anyone else hear these images on a Roberts R200 or other radio using a similar circuit ? or could my LOsc be a little too distorted due to a failing component ?

Steve

24th Mar 2006, 10:34 am	#12
steve1811uk Retired Dormant Member Join Date: Mar 2006 Location: Manchester Posts: 33	Re: Roberts R200 LW reception poor I had another look at this and found that I had adjusted C8 incorrectly (LW Local oscillator). According to "Trader" 1449 the tuning range is 1130 to 1970 metres (265kHZ - 152kHz). A local oscillator range of 735 kHz - 622 kHz was required. I adjusted C8 to give this exact range while monitoring the LOsc transistor collector voltage with a scope. After this adjustment I found the sensitivity and selectivity were the best that they had been. Radio 4 was then found to be exactly on the 400m mark of the MW scale. Regarding the Capital Gold and Radio 5 breakthrough. I did some reading and found that these are in fact images caused by the LOsc not being a pure sine wave. I found some useful info at http://www.dbugman.com/handbook/tscmh5.html To confirm that this was the cause I did some measurements Capital Gold (1458 kHz) could be heard faintly when the LOsc was at 643 kHz. This was found to be due to LOsc 3rd Harmonic distortion, 1929 kHz - 1458 kHz = 471 kHz, the IF that gets amplified. Radio 5 (909 kHz) could be heard faintly when the LOsc was at 690 kHz. This was found to be due to LOsc 2nd harmonic distortion, 1380 kHz - 909 kHz = 471 kHz, the IF that gets amplified. Does anyone else hear these images on a Roberts R200 or other radio using a similar circuit ? or could my LOsc be a little too distorted due to a failing component ? Steve Last edited by steve1811uk; 24th Mar 2006 at 10:43 am.