RCA 66X2 SMD - Printable Version
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RCA 66X2 SMD - Eric Adams - 05-12-2020
Hello everyone. I have a strange problem and I need your help: RCA 66X2. AM/SW 6 tube radio. I have what sounds to be a bad case of SMD. All caps have been replaced, resistors and tubes checked and replaced as necessary. The strange thing is that when I test for noise at the IF cans with my signal tracer I get no noise at all. If i test at the 35Z5 and 35L6, I can hear it loud and clear. My question is, can a sound like SMD come from something other than an IF can, such as a tube?
https://www.radiomuseum.org/schemaviewer/usa_rca_66x1_2_3_7_8_9_sch~~1.pdf?
RE: RCA 66X2 SMD - EdHolland - 05-12-2020
Eric,
It could be a tube/valve problem of some kind, or just an intermittent contact - dirty pins or socket.
If I read correctly, you are getting a clean RF signal through the IF, but the noise appears to be generated after (or at) the detector (12SQ7). You could try disconnecting the volume control (centre lead) or just insert a plug into the phono jack. Make sure that jack has clean contacts too, as it could be a source of trouble. These tests tell if the noise is generated only in either the triode section of the 12SQ7 or the output tube. This would determine if the problem was in the audio system. Likewise, temporarily disconnect the IF transformer from the detector tube.
Hope that helps,
Ed
RE: RCA 66X2 SMD - Eric Adams - 05-13-2020
Found it! Thanks, Ed. I was able to find the problem. There was a small 330mmf mica cap connected at the 12SQ7. The problem in locating it was that it was stuck next to the chassis and was covered by the 2 electrolytics that I installed. Replaced it and all is fine now! THANKS!
RE: RCA 66X2 SMD - EdHolland - 05-13-2020
Great! Was it one of those infamous lozenge shaped "deer t#rd" types?
RE: RCA 66X2 SMD - Eric Adams - 05-13-2020
Yup. A little red domino less than an inch long.
RE: RCA 66X2 SMD - EdHolland - 05-14-2020
Ah yes. had trouble with those on my RCA 110K2. I could not get the SW bands to align until a couple were replaced with new precision mica types.