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Post by outbacktech on Jan 9, 2018 1:49:27 GMT -5
Howdy fella's new the the forum and any advice would help; I'm stuck with this D201A RX issue on SSB it has none.. AM works like a treat -115dBm for 12Db SINAD...SSB is crap.. I have injected a signal straight at the input of T500 (6.255mhz) very quiet with no background noise best RX sensitivity is around -70dBm, appears to be quite a bit of loss through the Xtal filter also measured from pins D16-D19 through the filter around 17dB insertion loss have also injected signal at the C305 junction on board A, still the same low RX. V500,501 &502 all check OK, I'm thinking maybe a SSB AGC issue but can't find any details on how it all works, any advice would be appreciated,, Frank...
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Post by 2600 on Jan 9, 2018 23:55:50 GMT -5
Have a look at DC voltages on V500 and V501, especially pin 6 on each tube. R503 and R502 divide the 240-Volt B+ down to about 100 Volts DC on the screen-grid pin number 6 for each 6BA6 tube. That pair of resistors on each tube will deteriorate and change resistance value. Using larger parts, either carbon-film or wirewound will fix the suicide issue when (not if) they go bad. And if either of those tubes has gone bad at some time in the past, R504 2.2k could be damaged, or R509, 1k. This would reduce the voltage seen on pin 5 of those two tubes.
Not the only way to cripple the SSB receive, but one of the most-common ways.
73
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Post by outbacktech on Jan 10, 2018 16:37:29 GMT -5
Ok, many thanks I'll have a look at that this evening, it's really very quiet on SSB RX not even any Psophometric noise which I would expect, just low level hum even with volume flat out, so must be something I'm missing as there aint a lot in the SSB RX circuit after the filter. This is the first D201A that Ive ever worked on so learning...
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