Drake TR-7 Circuit Modifications
High Fidelity AM

by Rick - W3RSW

Following are TR-7 mods for improved audio response in the receive and transmit audio.

I. Easiest receive mod. Simply pick off AF from diode detector. Change C-1116, 0.01uf to 0.001uf, bypass cap on top of AF gain pot. Run minature RG-174 coax from same point (top of AF gain pot) to spare RCA jack on back panel if this is not already factory wired. An additional RCA jack already exists on my ser.# 7130. Specs. listed as nominal 0.1v at 'high' impedance. Can also be used as aux. audio imput to rig's AF amp. Turn internal AF gain to minimum. Turn external amp. controls to suit. Enjoy.


II. More extensive receive mods to internal AF amps. For best results feed external speaker via existing external speaker jack.

1. Bypass one of the empty filter positions with a 125 ohm resistor. This gets you at least 8kc at 3db down plus gentler slopes on receive; the main crystal filter is still in line but none of the other filters are.

2. AVC is already switchable from fast to slow.

MODS TO 2ndIF/AGC board (sect.11 includs all rcv. audio.)

3. Following are three AM detector amp. Q1111 mods that will somewhat increase highs and lows. change C-1166, 1.0uf coupling cap to 4.7uf if space permits. change C-1146, 1.0uf out.coup.cap to 4.7uf, ditto. change C-1147, 22uf filter cap, 10v rail to 100uf/35v. if space permits.

4. Following mods to AF pre-amps and AF out IC. change C-1116, 0.01uf bypass to 0.001, top AF gain pot. change C-1164, 0.01uf bypass to 0.001, wiper AF pot. change C-1150, 0.05uf input coup. Q1113 to 2.2uf/25v. change C-1152, 0.01uf output coup. to 1.0uf/25v change C-1167, 0.01uf bypass cap. to 0.002uf. (note that C-1152 and 1167 are a 1:1 voltage divider in the original circuit; talk about bypassing your highs to ground.) Note also that C-1173, 330pf IC hf bypass determines amp. stability and should not be made much smaller that 100pf. First try without mod. C-1154, 0.1uf combined with R-1180, 3.3 ohms in the output of the SL-61545 IC are for hf phase loading of the speaker line; do not change C-1154 to less than 0.02uf. Same amplifier stability rational for C-1174, 1uf on output side of 250uf speaker isolation cap. Do not change to less than 0.2uf. Try both first without mods. Use the existing 1/4" jack in the speaker line to select an external speaker (the internal speaker is quite tinny.) Optional try changing C-1154 from 0.1uf to .05 and change C-1174, 1.0uf to 0.22uf/35v. These bypasses not too critical in low imp. circuit but don't substitute with very much smaller.

5. Mods to AF amp and IC output 10V supply rails. change C-1151, 22uf/16v to 100uf/16v, Q1113 change C-1155, 22uf/16v to 100uf/16v, IC pin 14 to gnd. change C-1153, 10uf/25v to 100uf/25v, IC pin 1 to gnd. All changes above predicated on new higher value capacitors fitting in old space. Hopefully newer tantalum caps. are smaller than existing ones.

III. MODS to TRANSMIT/EXCITER board 03 for those so inclined to go ahead and clean up A3H 'AM' or SSB transmitted audio. No modifications are shown to change from one sideband with carrier to two sidebands wiht carrier.

1. Make sure you know if your using pin #1 or pin #4 (470k series to pin 1)on the mike input connector. Use pin#1 if you have a relatively low output impedance like my source follower/D-104 or from a 600 ohm line. Use #4 if you have scads of high output, high impedance. :

2. Check R356, 680 ohms, on the audio input to make sure it's not in line to ground. Supposedly not factory jumpered. check C-308 for 68Pf RF bypass: units should be in Pf.(very small) check C-311 for 470 pf; Q301 emitter bypass, first mike amp. Units should be in pf. even though it's transistor emitter bypass (for rf). Interesting in that emitter bypasses usually increase gain. change C-310, 0.22uf to 2.2uf/25v coupling change C-309, 0.22uf to 2.2uf/25v coupling change C-314, 0.22uf to 2.2uf/25v coupiing change C-315, 1.0uf to 10.0uf

3. check C-313, full 1.0uf

4. change C-316, 0.01uf to 0.002uf bypass.

5. change C-312, 10uf to 100uf/35v , filter cap on 10Vrail (or as large as space permits.) Again larger replacement capacitors predicated on fitting in available space or not too far away.



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12 June 2001