05-04-2020, 01:02 PM
Still fiddling with this radio... A good job impatient customers are not waiting!
There was still a weird instability, which may now be cured. Symptoms were that the set's gain would jump suddenly high, but also the IF bandwidth was obviously narrower. Occasionally it would break into oscillation, and this could be triggered while trying to adjust the IF transformers. Sometimes it would be somewhere inbetween these conditions, but audio would sound dull, if loud enough. I Found and did the following:
A closer inspection unearthed (pun intended) a blob of solder that had run from a tag strip associated with the #75 valve. This could be provoked into shorting out the detector diode circuit. I also removed the can from the second IF transformer, and cleaned up the trimmers. Lots of dust and debris on these, just like there had been on the first one shown above. It seems better than ever, nice clear audio on broadcast band, and fair enough sensitivity on SW. Selectivity is another matter on SW, - the antenna tuning circuits do not track at all well, and can't seem to be adjusted better with the single trimmer provided for each. Perhaps this is just how things were?
There was still a weird instability, which may now be cured. Symptoms were that the set's gain would jump suddenly high, but also the IF bandwidth was obviously narrower. Occasionally it would break into oscillation, and this could be triggered while trying to adjust the IF transformers. Sometimes it would be somewhere inbetween these conditions, but audio would sound dull, if loud enough. I Found and did the following:
A closer inspection unearthed (pun intended) a blob of solder that had run from a tag strip associated with the #75 valve. This could be provoked into shorting out the detector diode circuit. I also removed the can from the second IF transformer, and cleaned up the trimmers. Lots of dust and debris on these, just like there had been on the first one shown above. It seems better than ever, nice clear audio on broadcast band, and fair enough sensitivity on SW. Selectivity is another matter on SW, - the antenna tuning circuits do not track at all well, and can't seem to be adjusted better with the single trimmer provided for each. Perhaps this is just how things were?
I don't hold with furniture that talks.
