Philmore 12V Power Supply (6): PS123 in the box unit from ebay.com

I purchased a Philmore PS123 from ebay.com for \$10.  The unit was still in the box with original one-page flyer that passes as the operator's manual. The unit's schematic was shown, but doesn't quite match the wiring in the cabinet.   It's almost identical to the circuit I traced out and previously blogged about.  The filter capacitor is 2200 uF, rather than 2000 uF as shown in the schematic. The schematic shows a single 1K resistor from the collector of TR1 to the anode of the zener diode.  The actual circuit has a 5.6K resistor from the collector of TR1 to the negative side of the 500 uF capacitor, then a second 5.6K resistor from the negative side of the 500uF capacitor to anode of the zener diode.

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The reverse engineered circuit is shown below.  

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A photo of the PS123 is shown below. 

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A photo of the interior of the unit is shown below. Note there appears to be a splice in the line going from the diode bridge to the fuse. 

The performance curves of load regulation and ripple vs. load are given in the included flyer. 

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Sheet Metal Inventory and Gauge Table

 I bought two sheet metal squares at Lowe's  today.  I made a mistake and bought a second sheet of 26 Ga, when I intended to buy 22 Ga. 

12" X 18", 26 Ga, Zinc plated steel

12" X 18", 26 Ga, Weldable steel

SWR Meter (9) - Project box design in Fusion 360

 This is my first cut at the SWR project box design.  I used the sheet metal options in Fusion 360 to design a two piece sheet metal box with dividers. The box is 3" wide by 4.5" deep.

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 The division of the box into shielded compartments can be seen with the top removed.  Four pieces are cut from 0.8 mm aluminum: 1) the base, 2) the top, and 3) the dividers.

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The sheet cutout patter for the base is shown below. Dimensions in mm.

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Cutout pattern for the top are shown below.  Dimensions are in mm. 

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Cutout pattern for the basic divider is given below, later versions may need a separate version for each divider.  Dimensions are in mm. 

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CARDBOX CHASSIS AND CABINET BUILD

 

SWR Meter (8) - Project box

 I am going to make the SWR meter enclosure out of sheet metal, and do the drafting in Fusion 360.

The plan for the enclosure is shown below: There needs to be a conducting shield between the tandem coupler and the log amp board.  I'll use bulkhead mounted SMA connectors to feed the RF signals.   I'll also put a shield between the detector board and the MCU/Display section.  A 9V battery will go in the MCU section.  Front panel controls will be a small battery switch, and a cutout for the OLED display.

Philmore 12V Power Supply (5): Counterfeit regulators from China!

I completed the final wiring of the new controller board and tested it out on the bench.  It ran well with no load so I mounted the the board and the LM338 regulator into the cabinet. The photo below shows the control board connected to the LM338 on the left.  It was being driven at 18V by my bench supply, and produced 13.8V output with no load. 

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I wanted to be able to operate at 13.8V @ 2 amps continuously.  For testing I bought a 15 ohm 10W resistor and two 4 ohm 20W resistors which I put in series to make an 8 ohm 20W resistor.  The resulting currents at 13.8V should be 0.92A  and 1.73A respectively.  

After I installed the LM338 and the board in the cabinet and connected the output of the rectifier, it again ran well with no load.  The transformer is a nominal 18VAC secondary, as marked on the transformer itself.  This should give a peak voltage under no load of 25.4VDC on the filter capacitors at no load.  The actual measured voltage was more like 28V however, so the house line voltage is probably a little higher than nominal.  I loaded the supply with the 15 ohm resistor and got 13.5V @ 0.9A.  The input voltage dropped to 24.6V.    When I tried the 8 ohm resistor however, the LM338 died.  I checked my schematic and discovered that I'd wired a protection diode wrong, I assumed this was the reason the LM338 died. 

I corrected the protection diode mistake and replaced the LM338, and began further testing.  Again the unit worked well with the 15 ohm load, and again the 8 ohm load killed the LM338.   I made some measurement between terminals of the LM338, both on a good unit, and on a blown unit.  It appears that the voltage input terminal had opened up in both blown units.  I suspected that the bond wires from the LM338 input pin to the die had fused open. 

I did various test, and destroyed five LM338s in total.   Below is a scope photo of a failure.  In this event the supply is connected to an 8 ohm resistor.  The supply is cycled on, and the voltages at the LM338 are monitored.  The yellow trace is the input voltage.  You can see the voltage step from zero and increase to a steady state as it charges the 9900 uF filter capacitor. You can also see ripple peaks from individual rectified AC peaks. The blue trace is the output voltage of the LM338.  The output voltage rises with the input charging. Once the input voltage gets above the set voltage plus the dropout voltage, then the output voltage settles at 13.8 volts or so.  The cyan trace shows the Adjust pin of the LM338. When functioning properly, this voltage should be 1.25V lower than the 13.8 volts, which appears to be the case. 

But note that about 150 ms after the supply turns on the output drops to 0V.  This must be the instant when the bond wire inside the LM338 burns itself open.  The Adjust pin voltage is tied to the output through a voltage divider, so it falls to 0V too.   Once the output current disappears, the input voltage is no longer loaded, and the input voltage soars from about loaded 23.5V to unloaded 27V or so.  

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Doing some investigations on the web, I discovered that counterfeit LM338s are very common, and that is apparently what I've purchased.    Real LM338s should be able pass 5A continuously, and up to 12A transiently. This is apparently lower current unit (maybe an 3A LM317) that has insufficient bonding between the die and the pins. 

At any rate, I will not be able to use the LM338s that I bought and will have to probably redesign the board.   Very disappointing!

Philmore 12V Power Supply (4): Adding adapter bracket

 I have designed and 3D printed an adapter bracket that will hold a circuit board in place inside the Philmore PS123 power supply.  The circuit board will be a hand assembled version of the LM338 control circuit mentioned earlier. (LINK)

The Fusion 360 bracket model is shown below.   The bracket sits on the bottom of the cabinet. The small holes align with the circuit board mounting holes for the original control circuit board.  The large holes are to clear the screws that mount rubber feet underneath the cabinet. The circuit board is 80 mm wide and 60 mm high. The 80 mm side slides into the slot with components extending into the inside of the cabinet. 

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A photo of the bracket installed in the cabinet, adjacent to partially populated circuit board is shown below. 

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Below photo shows the circuit board as it will be installed.  Terminals blocks on the edge of the board are, from left to right, LM338 connections, power in from rectifier, power out to binding posts. 

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I'll get the rest of the components mounted on the board and wired up tomorrow.  Maybe even start testing. 

SWR Meter (7): Test results of log amp detector

 I ran tests on the response of the SWR detector PCB.  Everything appears to function well. The response graph is shown below

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It looks like I've got the noise problems worked out.  The response is very close to what is given in the data sheet.  There is a jog in the response at about -50 dBm.  This jog is there because I had to switch to an attenuator to get signals down below -50 dBm.  The attenuator changes the load on the signal generator slightly, resulting in the slight offset in response below -50 dBm.

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I am designing the circuit to measure power up to 300W.  The amplitude of the Vr port on the coupler is: 

$V_R=\frac{\sqrt{PR}}{N}$

where: 

• P is the power on the load
• R is the load resistance (50 ohm)
• N is the number of turns on the current transformer of the coupler. (20)

This gives an RMS voltage of 6.1 volts for VR for 300W on the load. 

The AD8307 datasheet define dBm as:

$dBm =10\log_{10}\left( \frac{V_{RMS}^2}{R}\cdot 1000 \right)  $

Using this formula,  6.1 volts is 28.7 dBm.   In order to keep the dBm input to the AD8307 to under 10 dBm,  I will add a 20 dB attenuator between the coupler and the AD8307.

40 meter NE602 DC RX -- 2023 version

 At the request of KA5VZE, AKA "my big brother", I am going to make a replica of my original 1987 "NE602 40 meter RX".   I covered the back story for my 1987 version in the immediately preceding blog post. 

I have transferred the original design into a KiCad project and produced a new PCB layout on the same size 1.5" by 2" board. The details are shown below.  I will order the boards from PCBWAY this week.

The original schematic is shown below.  Except for the trimmer capacitors, all parts can still be obtained for the RX.  (I guess from 1987 to 2023 makes the original project 35 years ago.  Where did the time go?) 

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For the 2023 version, I've added connectors to facilitate working with the board for testing and mounting in the cabinet.   The new schematic, entered into KiCad is shown below: 

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The layout for the PCB, shown below is done on a two sided board, with the bottom side serving as the ground plane. The ground plane is cleared out underneath the oscillator circuit in the lower left and under the tuned antenna circuit in the upper left. 

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The KiCad generated 3D view is shown below, with my added annotation.  

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Compare this to the original layout from the 1987 version and you can see both PCBs are similar. 

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40 meter DC NE602 RX (2): Background

Link to first post about 40m NE602 DC RX 

The original 40m DC RX was built to evaluate the NE602 mixer circuit. This was 1987 and I received a free sample of the NE602 on a blister card.  It looked very useful, because it had the oscillator and double balance mixer all in one chip.  I quickly learned that NE602 overloaded very easily but I still built a receiver up just for fun. draft article submission from 1987

Here's a photo of the original 1987 NE602 RX.  It was housed in a Radio Shack cabinet, using a vernier dial for tuning,  and volume and phone connections on the front panel.  The 9V battery was inside the cabinet, and the antenna connection was through an RCA phono connector through the back panel.

Below is a photo of the interior of the RX.  Click to enlarge.

The original circuit board was single sided, as shown below in a view from the component side.  Click to enlarge.