04-11-2012, 08:46 AM
(This post was last modified: 04-11-2012, 08:54 AM by Radio Fixer.)
True Lawrence circuit strays may have an effect but its probably quite small and the 10pF of the scope probe is compensating and probably exceeding that for a compact radio (think I saw a pic of the chassis one time and thats how I remember it). It seems John is adding capcitance so all his frequencies must be initially high. Also we have the inverse sq root factor for freq from capacitance.
John is sparse on info and I'm a little confused: are the cap values he mentions, 110 and 105 pF, the total or additional to whats inside the can? If additional that seems huge to me, I would expect it to be the total.
I have not come across stagger tuning on any small AM radio (on FM sometimes). Some exotic sets with two IF stages may have done it but probably still rare I believe.
What causes IFs to be off tune? Normally its fiddling as the main culprit. Mica caps dont normally move much nor does the coil winding but I did read long ago that it could be down to the permeability of the slug changing. I take it the IFs have been in the warm and dry for sometime?
Think John is on the right track to experiment with some 'junk' IFs and then have a careful go with the ones from the set ... if some solvent and judicious treatment wont move the slugs then he will have to do as you suggest and try in the set and see what happens. If he wants to be cautious he can note the position of the slugs and the number of turns to remove them assuming that he can and wants to take them right out?
Gary
John is sparse on info and I'm a little confused: are the cap values he mentions, 110 and 105 pF, the total or additional to whats inside the can? If additional that seems huge to me, I would expect it to be the total.
I have not come across stagger tuning on any small AM radio (on FM sometimes). Some exotic sets with two IF stages may have done it but probably still rare I believe.
What causes IFs to be off tune? Normally its fiddling as the main culprit. Mica caps dont normally move much nor does the coil winding but I did read long ago that it could be down to the permeability of the slug changing. I take it the IFs have been in the warm and dry for sometime?
Think John is on the right track to experiment with some 'junk' IFs and then have a careful go with the ones from the set ... if some solvent and judicious treatment wont move the slugs then he will have to do as you suggest and try in the set and see what happens. If he wants to be cautious he can note the position of the slugs and the number of turns to remove them assuming that he can and wants to take them right out?
Gary
