T-368 RF Deck Mods

In December 2002, I completed the RF deck modifications for my T-368, model E.  These modifications are based on those published by Tim Smith, WA1HLR, published several places on the web including here.  Several modification possibilities were studied, including those from Electric Radio issue 53 as well as other "descriptions" and "photos" from various sources.  The real purpose of any of the published mods is to remove capacitance seen by the modulator.  In this case, it is reduced from just over 6000pF to about 2000 pF.

The WA1HLR modifications involve changes to the Speech Amplifier and the RF deck.  My speech amplifier already had the clipper set for minimum clipping, and the high and low pass filters in the speech amplifier were bypassed, so I've left it alone for now.  The previous owner told me that the WA1HLR mods had been done to the Speech Amplifier, although I have not taken the time to fully verify these circuits.  The additional modification I made, not part of the WA1HLR mods, was to remove B+ from the plate meter.  This makes it a cathode current meter, as described in Electric Radio issue #53, so you have to subtract about 30mA from the reading to get plate current (20mA screen and 10mA grid).  Modulated B+ is now shunt fed to the tank circuit at the plate of the final.  The existing pie-wound choke (L13 on models D through F, in series with L3) was moved to the rear of the RF deck.  A new hand-wound RF choke was put in series with L13 to feed B+ to the plate of the final.  The new hand-wound choke and L13 were bypassed with 500pF, 20KV capacitors.  The plate is coupled to the tank circuit through two 500pF 20KV capacitors in parallel, each handling half the RF current.  All 500pF capacitors are rated to carry RF without overheating.  The end of L3 that used to be connected to incoming B+ is now grounded.  These modifications put the tank circuit at DC ground potential, and eliminate some potential for arcing.  C10, which bypassed L3, was removed.  The original 4000 pF coupling capacitor (or C7/C41/C42  on earlier models) was taken out of the circuit by moving both silver straps to one side of the capacitor ( or you could short the capacitor with silver strap, or remove the capacitor and add a ceramic standoff).  Some have simply left these original coupling capacitors in place, as they really do no harm if left in the circuit, but will still carry full RF current if not bypassed or removed from the circuit.

A schematic of the changes I made, copied from the link above and modified as applied to my transmitter, is shown below:

A few corrections were made to the schematic to correct the labeling of L4 and L10, and to correctly show the location of C7/C41/C42 in relation to the rest of the tank circuit.  Another item worthy of note is the construction of the RF choke.  I used the audio modifications listed in the Yahoo T-368 group files section, by W3NP, as my primary source for winding this choke.  But I found that using #26 wire yielded far more inductance than expected (nearly 500 uH).  After some careful counting of turns per inch, I determined that #24 wire would yield the correct number of turns per inch and the correct inductance in 5 3/4 inches of choke windings.  I acquired some #24 motor wire from a local motor rewinding shop, and completed my RF choke using about 75 feet of #24 wire on 3/4" PVC, with about 5 3/4 inches of winding on a 7 1/4 inch form.

The impact of these RF deck changes are significant!  They greatly increase the high frequency response of the T-368, and are a very worthwhile improvement relative to the stock audio response.  The Speech Amplifier modifications had already been completed by a previous owner.  Even so, with a Symetrix 528E voice processor feeding the audio chain prior to the RF deck modifications, I could not find an appropriate setting to overcome the limitations in high frequency response.  Following the RF deck modifications documented here, the audio is clean and crisp, much more pleasant for the listener, and significantly closer to that desired broadcast quality, which was certainly missing prior to these changes.

Checkout The Quest Online to see where I now spend much of my time.  God Bless!

© 2005 by Walter Wilson, KK4DF.  All information is offered for non-commercial use without warranty, expressed or implied.