03-11-2012, 09:18 AM
(This post was last modified: 03-11-2012, 09:25 AM by 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
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
