PA100D Complete and working!

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PA100D Complete and working!

Postby W4DJW » 10 Aug 2012 03:09

I just finished the PA100D and gave it a test run on the Genesis G11 and the Flex5000A. It works great. 5 watts in gives 75 watts out on 40 meters and 20 meters. Direct keying from the radio works good too. The only issue I have is the LCD is very dim. I ran the brightness and contrast to both ends of the scale without any good results. I can shine a flashlight on it and easily read the settings without any issues. Anything I messed? Anyone had this issue in the past?

Dave (W4DJW)
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Re: PA100D Complete and working!

Postby 5B4AIY » 10 Aug 2012 10:35

Hi, Dave,

The reason your display brightness and contrast do not work suggests a problem with the PWM circuitry. The following explanation may help to understand how this section of the amplifier works. You will need to print out the schematic: Juma-PA100 Control Board CPU & LCD Sheet 2 of 2.

Let's deal with the Contrast first. Pin 63 of the microprocessor is the PWM o/p pin, and this outputs a random pulse train whose average width is modulated. R39 (100K) and C70 (2.2 uF) form an integrator that smooths the pulse train into a DC voltage whose value varies from roughly zero to approximately +2.5V across R17 (100K). This DC voltage is buffered by IC3-A connected as a non-inverting voltage follower and the resulting buffered DC voltage is applied to R19 (1K) and then to pin 3 of the LCD connector.

As you vary the contrast setting from the User Configuration menu with the UP/DOWN buttons, first check the DC voltage across C70. You should see it vary continuously across the range. If not, then check the connection from pin 63 to R39 and C70. Equally, with the amplifier OFF, using the ohms range of your multimeter check the resistance to ground across R17, it should be 100K anything else needs to be investigated.

If the variable DC voltage is present across R17, then check the DC voltage across R19, it should be the same. If not investigate IC3-A and the LCD connections.

Now let's examine the Brightness circuitry. This is again a random pulse train from pin 62 of the microprocessor whose average pulse width is varied. Once again, the pulse train is integrated by a similar C-R network comprising R41 (100K) and C71 (2.2uF). This voltage is buffered by IC11-A and fed to Q3, an IRF110 FET. This combination configures IC11-A and Q3 as a voltage-controlled current sink. The gate-source of Q3 is inside the feedback loop of the amplifier, and the gain of the amplifier cancels out the gate-source voltage offset, so that the voltage across the source resistors, R31 and R32, should be the same as the input voltage. As the voltage increases, the voltage across the parallel combination of R31 and R32 whose combined value is 5 ohms, increases and this causes the current drawn through the backlighting LEDs of the display to increase.

To check this, once again verify that the integrated DC voltage across C71 varies as the backlighting is varied from the Configuration Menu with the UP/DOWN buttons. Check the DC resistance across R18 with the power OFF, it should be 100K. Check the DC voltage at the output pin 1 of IC11-A, and verify the voltages across R31 and R32.

Check carefully that IC3 and IC11 are correctly oriented! If you have them inserted backwards, then obviously they will not work. These ICs usually have either a dot or pip alongside pin 1, or, unfortunately, a 45 degree chamfer on the edge which contains pin 1. In other words, hold the IC so the chamfer is on the left, and pin 1 will be the top left pin. Unfortunately manufacturers also tend to slightly round the other edge, so it can be very difficult to positively establish the edge which is chamfered. Have a really close look with a good magnifying glass to be absolutely certain.

If you need any further help, let me know. You might also like to try my modified firmware for the PA100D, if so let me know, you can either download it from this website, or you can send me an email (adrian04@cytanet.com.cy) and I'll let you have the latest build and it User Manual.

73 Adrian, 5B4AIY
Last edited by 5B4AIY on 11 Aug 2012 06:56, edited 2 times in total.
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Re: PA100D Complete and working!

Postby W4DJW » 11 Aug 2012 02:57

Hi Adrian,

I followed your direction and found the issue in about an hour. It was the solder joint pin 2 on IC11. As I was measuring the voltage on the pins I saw the display backlight come on. A quick touch up on the legs of the IC solved the problem. Thanks a lot for your attention to detail. My PA100-D is 100% now. :D

Image
73 Dave (W4DJW)
W4DJW
 
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Re: PA100D Complete and working!

Postby 5B4AIY » 11 Aug 2012 07:02

Hi, Dave,

Glad to have been of some assistance. Most of the problems associated with the kits have indeed been missed soldered connections or open solder joints. The quality control is excellent, and there have been very very few bad components, especially if suitable anti-static precautions have been observed.

If you do contemplate updating your firmware to my modified version, then carefully check that your serial port is working correctly. Full instructions for this are contained in my firmware upadte package, along with, naturally, the HEX load file and the illustrated User Manual.

73, Adrian, 5B4AIY
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