Tuning Wand

[pwdrive]

Most of the old timers will remember the tuning wand as an aid to finding out if a tuned circuit (IF's and others) are not tuned correctly. For anyone not familiar here is a short description:

"Tuning Wand. This tool is used to check for resonance in tuned circuits. By inserting it into the magnetic field within a coil, the inductance can be slightly changed. One end of the wand contains iron that increases the inductance; the other end contains brass that decreases the inductance. If a tuned circuit is at resonance, either a decrease or an increase in inductance will reduce the signal level. A tuning wand can easily be made by installing iron and brass ends on a piece of 1/4" or 3/8" diameter insulating rod. The rod needs to be long enough so that the hand does not disturb the circuit being checked.wand. An old tuning core can be used for the iron end. Cut the core's threaded rod to a length of about 1/4" and screw it into a tapped hole in the insulating rod. A piece of shrink tubing will help protect, and insulate the powdered iron.
The brass end, about 1/2" long, can be either a cylinder, or a piece of solid rod. The cylinder will give greater inductance shift than the solid rod. It can be made from a piece of tubing and installed over the turned, or filed, end of the insulating rod. If a solid piece of brass is used, it can be installed by using a short piece of threaded brass rod tapped into both pieces. A piece of shrink tubing should be installed for insulation"

Lawrence.

[Radio Fixer]

Thanks Lawrence.

This experience fits in with your input:

I had to adjust the RF trimmers on a Stewart Warner 950 TRF with 4 gang tuning capacitor, in a way recommended in the Service Manual.

It was pretty easy, well with a large scale mV meter across the speaker terminals. I don’t think I could have done it by ear as Stewart Warner suggest.

I used an insulated hacksaw blade, the “earth stick”. This was moved close to the detector gang, with an un-insulated part of the blade in contact with the frame, and the meter dipped by a few dB. This having no trimmer is the reference and when repeated for the second and third gangs (the ones with trimmers) then these had to dip by about the same amount. This is bringing the stick up towards the end fixed plate at the manufacturers recommended frequency of 1 mHz (gang a little less than half open say). The first gang didnt matter because there is the front panel antenna capacitor and I kept this peaked.

I had to tweak the two middle trimmers by a turn on one and a turn and a half on the other, so not a lot but it makes quite a few dBs difference to the overall gain. The earthed blade is just adding an extra tiny amount of capacitance and this was repeated with the gang opened a fraction more. Now the effect was to give a slight rise in the meter reading for the three sections in turn. Having to tweak the trimmers was to be expected as it had had a complete rebuild and I had changed the shielding on the connections from the RF stage anodes to the tuning caps. These were by running the wires in bent over pieces of brass that I changed to solid brass tubing.

Gary

[pwdrive]

Hi Gary, yes the ancient black art of alignment, well done, the HRO has brass discs in some of the high range coils, I seem to remember back in my tv days some of the vision IF's had aluminium (I think) slugs, possibly the last vision IF?
Tracking multiganged can be difficult especially on a four gang, as you say the meter beats the ear every time.

Lawrence.

[Alistair D]

Thanks Lawrence.

This experience fits in with your input:

I had to adjust the RF trimmers on a Stewart Warner 950 TRF with 4 gang tuning capacitor, in a way recommended in the Service Manual.

It was pretty easy, well with a large scale mV meter across the speaker terminals. I don’t think I could have done it by ear as Stewart Warner suggest.

I used an insulated hacksaw blade, the “earth stick”.  This was moved close to the detector gang, with an un-insulated part of the blade in contact with the frame, and the meter dipped by a few dB. This having no trimmer is the reference and when repeated for the second and third gangs (the ones with trimmers) then these had to dip by about the same amount. This is bringing the stick up towards the end fixed plate at the manufacturers recommended frequency of 1 mHz (gang a little less than half open say). The first gang didnt matter because there is the front panel antenna capacitor and I kept this peaked.

I had to tweak the two middle trimmers by a turn on one and a turn and a half on the other, so not a lot but it makes quite a few dBs difference to the overall gain. The earthed blade is just adding an extra tiny amount of capacitance and this was repeated with the gang opened a fraction more. Now the effect was to give a slight rise in the meter reading for the three sections in turn. Having to tweak the trimmers was to be expected as it had had a complete rebuild and I had changed the shielding on the connections from the RF stage anodes to the tuning caps. These were by running the wires in bent over pieces of brass that I changed to solid brass tubing.

Gary

Just found this old thread while searching the forum.

Gary, does the 'earth stick' have to be earthed to the frame of the capacitor or is hand and body capacity enough to produce the effect?

Al