PTC Voltage Stabilization Modification for the SDR-1000


PTC Voltage Stabilization Modification for the SDR-1000

System Dependencies
Minimum PowerSDR Version:
1.8.0
Applicable Hardware: SDR-1000
Content provided by: Brian, K7RE

Application:
This modification applies only to SDR-1000 radios that have the so called PTC device installed on the main oscillator. This modification stabilizes the input voltage to the PTC device by adding an 8 volt regulator assembly to the radio, providing an 8 volt regulated voltage supply to the PTC device.

Symptoms:
The radio drifts in frequency approximately 20-50 HZ for approximately 10 seconds after returning the radio to receive from transmit. This occurs even at very low power levels, but is more pronounced as transmit power is increased.

Cause:
Unless the 12 volt power supply is extremely well regulated, that is beyond the capabilities of typical ham radio supplies, some voltage drop occurs in the power cables that power the radio when larger amounts of current are drawn when the radio is placed in transmit. This voltage variation causes the PTC circuit to change temperature, which changes the frequency of the main oscillator accordingly. After the radio is placed into the receive mode the voltage from the power supply returns immediately to the initial higher value, and after a few seconds, the temperature of the oscillator returns to the initial value as well. The varying voltage produces a varying temperature which then produces a varying frequency of the main oscillator.

Level of Difficulty and Overview:
Only the top cover of the SDR-1000 is removed, no PC boards need be removed or modified in any way. Several parts are assembled on a heat sink and 3 connections are made to the radio. The heat sink serves as a mount for the new assembly, which mounts in an existing mounting hole.

Parts Required:
1 Ea. 7808 or equivalent 8 volt 1 Amp 3 terminal voltage regulator
1 Ea. Heat sink with insulated IC mounting pad
1 Ea. 22 UF @ 16V (or greater) Electrolytic capacitor
2 Ea. .1 UF capacitor
1 Ea. 1 3/8 “ Long 6-32 bolt, 3 nuts or spacers
Wire, heat shrink tubing, solder etc.

First assemble the assembly:
  1. Drill a hole large enough to accept the new mounting bolt in the extreme upper left of the heat sink .
  2. Mount and solder the capacitors to each of the three pins on the 7808 as shown in the diagram.
  3. Solder flexible wires, 22 gauge or thicker, to the 3 pins as shown in the diagram. Place heat shrink tubing over the 3 pins of the IC.


Now install the assembly:
  1. Remove the top cover of the SDR-1000. While you face the front of the radio, locate the black plastic terminal block to left of the radio bottom chassis.
  2. With the front of the radio facing you, remove the wire to the extreme left rear, of the terminal block. This is the wire that runs from the switched 12 volt bus directly to the PTC device.
  3. Remove the upper left mounting bolt of the terminal strip. Replace it with a new bolt about 1 3/8 inches long. This is where the new assembly will be mounted. Either use insulated spacers or 3 nuts to mount the assembly. If using the 3 nuts: One nut tightens the terminal strip, the second nut is placed under the heat sink, the third nut on top of the heat sink.
  4. Run the input of the new regulator from the main DC input of the radio on the (+) and the middle pin of the regulator to the (-) terminals of the 100W pa board. This voltage pick off point allows the PTC circuit to be fed a constant regulated 8 volts, even with the SDR-1000 front panel switch switched off, as long as the 12 volt supply to the radio is left on. This results in 0 warm up time.
  5. Run the output of the regulator to the wire that you removed from the terminal block in step 2. I simply soldered and then used a small length of heat shrink tubing over the terminal spade lug. This allows the wiring to be restored to the original configuration if need be in the future.
  6. Check wiring, replace the top cover. Turn on the power supply. You will note the usual warm up drift for 10-15 minutes, but if the power supply remains turned on, the initial drift should no longer be seen. Make a few test transmissions to see if the initial symptoms have disappeared.
  7. Although an 8 volt regulator is specified, probably any 1 A regulator specified at 5-10 volts output would work, I used the 8 volt variety as that is what I had on hand. Remember that the lowest input voltage to these 3 terminal garden variety types, must be a minimum of 3 volts above the output voltage. There are other types of regulator IC’s available that will regulate with the input voltage much lower than 3 volts. Also probably the insulated mounting pad could be eliminated, and the chassis ground used to establish the (-) connection to the assembly, as the center pin of the IC is also connected to the back metal surface of the IC, and would be grounded through the mounting bolt to the SDR-1000 chassis
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Posted Tuesday, July 29, 2008 by Administrator
http://kc.flex-radio.com/KnowledgebaseArticle50158.aspx