short wave recption

[James Vlasblom]

Hello, I am relatively new to the art of restoring old radios and I have a couple of quick and easy questions, (I hope) 😉 . I am currently working on a GE model JK-70 which was dead but in beautiful shape when I got it. After replacing some caps and a open resistor I heard the wonderful sound of the CBC at 690 AM. This set has the broadcast band which is what I guess is now called AM, and 5 other bands; short wave, 31m, 25m, 19m, 16-13m. I don’t hear anything on these bands but don’t know enough to know if I even should. Also, is it normal on these radios for the volume control not to turn completely down? At the lowest setting I can still hear the station, perhaps the potentiometer is failing? Any help would be greatly appreciated.Oh ya, I am also only measuring half the voltage expected on the plate of the 6sq7 (2nd detector, A.V.C. & A.F.) tub.
Cheers

[Gerry O’Hara]

Hello James,

Whether you hear anything on the short wave bands on your set (assuming it is working ok on these bands) depends on the type of antenna you are using and time of day – in general, the shorter wavelength bands (higher frequency) are more ‘lively’ in the daylight hours and the longer wavelength ones (lower frequency) during the evening/night-time. You should try to obtain a textbook that provides some info on shortwave propogation to gain a full understanding (look for old copies of the ARRL handbook etc or try ‘googling’ on teh web for things like ‘shortwave propogation’).

Of course you could have a problem in the set (very likley if you can hear nothing at all on all the shortwave bands). This could be due to many causes from the frequency converter tube being ‘tired’ and failing to oscillate at frequencies higher than the Broadcast Band (worth trying changing the tube for starters), followed by faulty components in the switched coils (eg, dirty switch contacts, broken coil winding) or poor alignment (is there evidence of someone ‘fiddling’ with the trimmers?). Have you checked the DC voltages around the tubes to see if they are ok/reasonable? (especially the frequency changer), as well as the AVC line (need a VTVM for this).

I have a JK-66 which has a set of pre-sets – these occasionally need cleaning or they stop working (not sure offhand if the JK-70 has this arrangement or not).

Gerry

[James Vlasblom]

Thanks Gerry, any info is helpful when you are at the beginning of the learning curve.
James

[James Vlasblom]

Just a quick follow up. I am now hearing faint reception on the B short wave band. I only listen at night time, I am looking forward to a daytime trial. The problem with the volume not turning down all the way was resolved with the replacement of the 6SQ7 (2nd detector/AVC tube) This makes sense although I do not as of yet fully understand how the automatic volume control works with tubes. Advanced tube theory is still new to me, haven’t got to that part in my old radio books yet. The only thing I am now waiting on is a new 6U5 indicator tube. Gotta love Ebay.
Cheers
James Vlasblom

[cvrs]

you might attempt to use a 6E5 instead of 6U5 , they are a bit easier to find so may come cheaper. I think the green display may move a little less but someone brighter than me 🙂

Also check inside the tube socket if you find there is a resistor on the schematic that you can’t find. Usually there is a wafer type cover with holes that the tube pins protrude through. come careful work with a pick or something should pop the cover out to expose the resistor. Chances are good that this one has drifted out of tolerance. ( from about 1 meg? check schematic) I assume one could actually check the resistor before they even open the socket if they know where it is. You wouldn’t expect it to be hidden there if no one showed you.

In my experience If I can see a shadow permanently burned into them that is visible with the tube out of the radio, it often seems that those ones have sat in a radio for many years and now have a weak display. You might still see them working properly in a darkened room if this is the case. I have heard some talk that sunlight is damaging to them I haven’t made a point of covering them when not in use, but might consider just that if they are standing in a window where the sun is on the face all the time.

Phil

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