I fired up my SX-42 tonight, as I saw there was a 6 Meter opening. Only to discover that It is completely deaf on that band, as well as the highest FM band. All the other bands are fully functional. I flipped the bandswitch around several times, nothing.

Any thoughts or theories?

Arthur,

Bands 5 and 6 use the 10.7 Mhz. IF so issues in this IF will cause it to be deaf on those bands. A 7F8 approaching end of life may also not be oscillating on the higher frequencies so a bad tube is another possibility.

Rodger WQ9E

Rodger- Wouldn't you know, that tube is ice cold. I think I am going to try to swap out, and see what happens. Thanks!

Bought a "new" 7F8 off of the auction site. For $6 shipped, hard to go wrong. I will let you know what I find when it arrives. Thanks again! A

scrape the pins with an xacto knife and try it.
loktals and miniatures tend to get oxidation on the pins.
open heater is a rather rare failure mode.

Going to try this over the holiday weekend.

Scraped the pins. Used a QTip and some Deoxit. Still nothing on those bands. The filaments light, but is there a possibility that the tube is bad?

As tubes age, they sometimes won't function well as the frequency of operation increases. A common example of this is the 12BY7A tube which is used as a RF driver in many vintage transceivers. The tube will test fine in a tube tester (which tests at DC or line frequency) depending upon the tester but will fail miserably in the upper part of the shortwave range. This is one of the reasons I rarely use a tube tester because they are a great producer of type 1 and type 2 errors when testing.

You can use another radio to see whether the 7F8 is oscillating on the two VHF ranges of your SX-42. I can't remember whether the SX-42 uses low or high side tracking on the upper two ranges but as a quick check put a test portable FM radio with its antenna fairly near the 7F8 tube (you can use a clip lead from the test radios whip antenna to extend near the 7F8 glass envelope). Tune the test receiver to a blank frequency in the 89-91 Mhz. range. With your SX-42 set for the highest range, tune it from 75 through 103 Mhz and you should hear your test receiver background noise quiet as the SX-42 HFO passes through where it is tuned if the HFO is operating.

If the 10.7 Mhz. IF and discriminator detector is working in the SX-42, you should hear a distinct hiss from it when the SX-42 is set for FM reception, if not the problem apparently lies somewhere in the IF strip or detector area.

Rodger WQ9E

Thanks- I will try this. However, when I say there is no sound on those two bands, there is NO sound! I may just wait for the tube I ordered, which shipped today. Hoping it is something this simple. A

OK,

With no sound on those bands I would suspect:

1. 7H7 limiter tube
2. 7H7 second limiter and AM detector
3. Switch contacts that choose between the 455 Khz. AM detector and the 10.7 Mhz. AM detector or FM discriminator.

With a sudden failure, I would suspect one of those two 7H7 tubes.

Rodger WQ9E

Thank you- off to shop for a 7H7! A

Cleaned and DeOxit'd the pins on both 7H7 tubes. Reseated them, The top two bands came roaring back to life! There was a lot of black stuff on both tubes, which came off both with an emery board, and QTips soaked with DeOxit on the pins.

Oh well, a few cheap tubes are en route, extra backups!

Thank you, guys!

A