UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Hallicrafters SX-43 FM drift.

falcon123 · 23rd Oct 2020, 8:58 am

This is my last issue with this set (I promise) although I'm not sure anything can be done about it. the FM drifts badly on this set until it warms up and that can take over an hour when it's in its case in an airconditioned room. Out in the shop on its end with the underside of the chassis in open air it's much worse. I've read "they all do that" but somehow I'm unconvinced that 2 Mc/s was acceptable in 1947,but maybe it was. There's a temperature compensating capacitor, C41, buried next to the first wafer of the band switch. It's a strange affair that looks like a ceramic/mica padder, but it doesn't appear to have an adjustment and I really can't see the very top of it without a dental mirror which I don't have. After chasing the p/n,44A158, in other Hallicrafters sets like the s-76 and s-40 I discovered its value was 3pf. It's connected to pin 3 of the 7F8 tube and has 195 VDC on it. The cap goes to chassis ground. I've limited experience so I'm not really sure it has anything to do with it what-so-ever. but it's the only temperature controlled device that's called out in the drawings and looks suspicious. The distracter is that AM seems quite stable. Anyone have a clue if this could be the issue? Thought suggestions are all welcome.

23rd Oct 2020, 8:58 am	#1
falcon123 Triode Join Date: Jul 2020 Location: Angeles City, Philippines. Posts: 27	Hallicrafters SX-43 FM drift. This is my last issue with this set (I promise) although I'm not sure anything can be done about it. the FM drifts badly on this set until it warms up and that can take over an hour when it's in its case in an airconditioned room. Out in the shop on its end with the underside of the chassis in open air it's much worse. I've read "they all do that" but somehow I'm unconvinced that 2 Mc/s was acceptable in 1947,but maybe it was. There's a temperature compensating capacitor, C41, buried next to the first wafer of the band switch. It's a strange affair that looks like a ceramic/mica padder, but it doesn't appear to have an adjustment and I really can't see the very top of it without a dental mirror which I don't have. After chasing the p/n,44A158, in other Hallicrafters sets like the s-76 and s-40 I discovered its value was 3pf. It's connected to pin 3 of the 7F8 tube and has 195 VDC on it. The cap goes to chassis ground. I've limited experience so I'm not really sure it has anything to do with it what-so-ever. but it's the only temperature controlled device that's called out in the drawings and looks suspicious. The distracter is that AM seems quite stable. Anyone have a clue if this could be the issue? Thought suggestions are all welcome.