Quote:
Originally Posted by G0HZU_JMR
A good result! From what you say, it sounds like the radio was deaf on all ranges and not just 0.5-1.5MHz?
Did you spot any obvious duds amongst the old 1SS83 diodes?
On your youtube demo video I did spot a few signals near S9 +60dB. I'm not sure how accurate the s meter is but this implies signals up near -13dBm at the input. For a bit of fun I dug out my diode test rig this evening and tested a 1N4148 diode at 15mA bias with a -10dBm test signal at 950kHz. The idea was to look for distortion in the diode in a 50 ohm system at 1900kHz.
The spectrum analyser showed a second harmonic at -76dBm at 1900kHz. The analyser is protected by a diplexer at the front end so this distortion is caused by the 1N4148 diode.
This means that the 1N4148 diode would generate a second harmonic at about an s8 signal on an equivalent s-meter in this harsh test. I quickly swapped the 1N4148 diode for a decent PIN diode (also biased at 15mA) and the second harmonic distortion term fell about 40dB. It is probably better than this but this is the limit of the test setup at the moment. This shows how relatively poor the 1N4148 diode can be in terms of the even order distortion it can generate in a 50 ohm system. When fitted into a radio it might be different because the diode will be terminated in a BPF. This might makes things worse or better.
I think it is this kind of difference that prompted Ulrich Rohde to experiment with PIN diodes about 25+ years ago. I think the slower 1SS diodes will perform somewhere inbetween. They will definitely be better than the 1N4148 but they won't match a decent PIN diode here.
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I think something was deading it on all frequencies, and also I think it was ahead of where the marker signal was inserted. It could have been the lamp connections, rf input socket connections, or a dry joint on one of the diodes I lifted ahead of the bandpass filters. All the bandpass filter switching diodes were reading consistently with no leakage, so Ive left them in place . May recommission the attenuator now tonsee what happens
Looking forward to see how it behaves on 60 and 20m tomorrow.