Sunday, February 07, 2010

This and that.

17 Meters was open again this afternoon. I worked Eduardo CO8LY in Cuba. He was calling CQ DX and was way over S9. He sounded more like he was down the block; than all the way just south of Florida. Unfortunately, it was one of those "599 TNX" kind of QSOs. We have worked several times before; and I wanted Eduardo to know his signal was really carrying. I guess New Jersey wasn't the kind of DX he was looking for.

On the other hand, it looks like I am making some kind of progress with the PFR3A. I rewound T1 for the second time and re-installed with no improvement. So I decided that everything in the receiver circuit is probably pretty much just as it should be. That leaves the DDS.

With my trusty VOM and schematic in hand, I began to take some voltage measurements. I checked the 78L05 regulator; and everything seemed fine there. Then I went to check U10, the 812C35 chip. Lo and behold! No 3.5V at the positive leg of C35! This led me to believe of course, that U10 might be defective; or perhaps soldered in badly (it is one of the SMT devices).

So I went to the PFR3 Yahoo group and posted a question; as I was not really sure of the pin out configuration of this device:

Doing some troubleshooting as to why my receiver isn't working; I've come to believe there may be a problem with U10 (or a connection involving U10). I am currently not getting the 3.9V at the positive lead of C35 - only about .9V.

Now, bear with me for a second. If you look at U10, there are 4 solder connections - the "top" and then the three "bottom pins". Using my VOM to check for continuity, I get a short from the left most bottom pin to ground. I am getting a short from the right most pin to the positive lead of C35.

I am not getting a short from the middle lead to the cathode of D5, which I am assuming I should. Is the middle pin the input?

Tnx in advance,

Larry W2LJ

A few minutes later, I received a reply from none other than KD1JV himself:

The center pin of U10 is indeed the input and is connected to the metal tab on the top end of the part. In the PFR-3A, power to this pin comes from the +5V regulator. In the earlier 3 model, it comes from the raw DC via D5.

Your the second one to have a problem here. I wonder if the 50 Mhz clock chip is in upside down? If you look carefully at the silver box between the regulator and the AD9834 chip, you should see a little dot near the upper left corner. If it is in the lower right corner, it is in upside down and could be pulling the regulator output low, since it sounds like its trying to forward bias a diode.

KD1JV

Sure enough, the clock chip is indeed, upside down. I sent another e-mail to the list asking Steve for any recommendations in rectifying this. I have a feeling that I already know what this entails - and it's probably not going to be a simple fix. I do have some QuickChip hanging around somewhere in the shack to help me unsolder this chip.

Maybe there's some light at the end of the tunnel?

73 de Larry W2LJ

3 comments:

John AE5X said...

Larry, who installs the pre-installed parts?

Markus said...

Hi Larry,

Just got a note from HB9... with a photo showing a blown-up U10. Will ask him to check the orientation of U11...

Please let us know about any progress in your repair job.

73, Markus

Larry W2LJ said...

Markus,

The new U11 did indeed take care of the problem. While removing the old one, I did do a little damage to one of the copper foils on the circuit board; but was able to fix that easily enough with a very thin piece of wire.

Other than this, the PFR3-A was completed and works just great! I'd recommend this radio to anyone.

73 de Larry W2LJ