AA7BQ
09-11-2004, 11:50 PM
HEATHKIT GR-54 MODS
1. The noise bypass caps, C63,C64 are connected in series across the power transformer primaries w/the chassis connected to the common junction between them. They are wired optimally when the primaries are connected for 240VAC or when the receiver is fed through an isolation transformer. Either way both AC supply lines are ungrounded or in a "balanced" configuration.
If the radio is connected to normal 120VAC wiring where one side is grounded [called the "neutral" wire] & the other side is hot [measures 120VAC w/respect to ground] then C63 & C64 form a ground loop & increase QRM in some cases. The best way to correct this situation is to, first, replace the AC cord plug w/a polarized plug so the AC is always connected to the radio the same way. The conventional way is to connect the hot side to the power switch, fuse, & one side of C63 & the power transformer primary & the neutral wire to the other side of the transformer primary & C64. Leave C63 connected as is & disconnect chassis from C64 & reconnect this lead on C64 to the "hot" side of the transformer primary.
[If a 3-wire grounded power cord is used don't connect cord ground wire to receiver chassis. Connect radio chassis to external ground(s) via a dedicated ground wire. Ground rods, cold water pipes, well casings, etc., make good grounds & are usually tied together externally. Running a ground wire to one of these items in the building ground system & also using the ground wire in the AC cord, which is also connected to these grounds forms a ground loop which invites QRM.]
2. For improved performance on bands 1 & 2 [LF & BCB] remove loading resistors from across L1 & L2 secondaries. Any changes in tuning that result from this can be corrected by the antenna trimmer control, C2. These resistors are located on the antenna PCB, p/n 85-123.
3.A. Replace RF amp., V1, & 1st IF amp., V3, w/6DK6 tubes.
B. Replace resistors, R15 & R19 w/47 ohm, 1/2 watt resistors. R15 is the cathode bias resistor for the mixer, V2a, & R19 is the cathode bias resistor for the 1st IF stage, V3. [These changes are from the May '68 Popular Electronics magazine.]
4.A. Install a 0B2 voltage regular tube on the Osc. B+ line [V2b plate voltage]. Pins 1 & 5 are the plate leads & pins 2, 4, & 7 are cathode leads [only one of each need to be used].
B. Replace the 15K, 2W resistor, R49, in the Osc. B+ circuit w/a 15K, 5W type to handle the additional current from the voltage regulator.
C. Replace the Osc. B+ filter cap, C61 w/a 0.22-.47uF, 350VDC cap. It's necessary to lower the capacitance here to prevent the 0B2 regulator from becoming a relaxation oscillator at certain frequencies [usually at the top end of band E] where osc. current rise causes the voltage on the 0B2 to fall below the sustain level.
5. The receiver should be realigned per instructions in the operator's manual w/one exception: set antenna trimmer control, C2, instead of fully CW as instructed. This helps insure that there's enough adjustment to allow the RF amp to track the other tuning circuits.
6. BFO may not tune low enough in LSB mode. If not, replace C48 w/3-30pF mini trimmer cap.
7. Additional notes. For manuals contact Pete Markevich at www.manualman.com or for schematics showing these modifications or any suggestions or comments contact me at wireaddict1@aol.com. For tubes, sockets pilot lamp bulbs go to www.vacuumtubesinc.com.
1. The noise bypass caps, C63,C64 are connected in series across the power transformer primaries w/the chassis connected to the common junction between them. They are wired optimally when the primaries are connected for 240VAC or when the receiver is fed through an isolation transformer. Either way both AC supply lines are ungrounded or in a "balanced" configuration.
If the radio is connected to normal 120VAC wiring where one side is grounded [called the "neutral" wire] & the other side is hot [measures 120VAC w/respect to ground] then C63 & C64 form a ground loop & increase QRM in some cases. The best way to correct this situation is to, first, replace the AC cord plug w/a polarized plug so the AC is always connected to the radio the same way. The conventional way is to connect the hot side to the power switch, fuse, & one side of C63 & the power transformer primary & the neutral wire to the other side of the transformer primary & C64. Leave C63 connected as is & disconnect chassis from C64 & reconnect this lead on C64 to the "hot" side of the transformer primary.
[If a 3-wire grounded power cord is used don't connect cord ground wire to receiver chassis. Connect radio chassis to external ground(s) via a dedicated ground wire. Ground rods, cold water pipes, well casings, etc., make good grounds & are usually tied together externally. Running a ground wire to one of these items in the building ground system & also using the ground wire in the AC cord, which is also connected to these grounds forms a ground loop which invites QRM.]
2. For improved performance on bands 1 & 2 [LF & BCB] remove loading resistors from across L1 & L2 secondaries. Any changes in tuning that result from this can be corrected by the antenna trimmer control, C2. These resistors are located on the antenna PCB, p/n 85-123.
3.A. Replace RF amp., V1, & 1st IF amp., V3, w/6DK6 tubes.
B. Replace resistors, R15 & R19 w/47 ohm, 1/2 watt resistors. R15 is the cathode bias resistor for the mixer, V2a, & R19 is the cathode bias resistor for the 1st IF stage, V3. [These changes are from the May '68 Popular Electronics magazine.]
4.A. Install a 0B2 voltage regular tube on the Osc. B+ line [V2b plate voltage]. Pins 1 & 5 are the plate leads & pins 2, 4, & 7 are cathode leads [only one of each need to be used].
B. Replace the 15K, 2W resistor, R49, in the Osc. B+ circuit w/a 15K, 5W type to handle the additional current from the voltage regulator.
C. Replace the Osc. B+ filter cap, C61 w/a 0.22-.47uF, 350VDC cap. It's necessary to lower the capacitance here to prevent the 0B2 regulator from becoming a relaxation oscillator at certain frequencies [usually at the top end of band E] where osc. current rise causes the voltage on the 0B2 to fall below the sustain level.
5. The receiver should be realigned per instructions in the operator's manual w/one exception: set antenna trimmer control, C2, instead of fully CW as instructed. This helps insure that there's enough adjustment to allow the RF amp to track the other tuning circuits.
6. BFO may not tune low enough in LSB mode. If not, replace C48 w/3-30pF mini trimmer cap.
7. Additional notes. For manuals contact Pete Markevich at www.manualman.com or for schematics showing these modifications or any suggestions or comments contact me at wireaddict1@aol.com. For tubes, sockets pilot lamp bulbs go to www.vacuumtubesinc.com.