Tuesday, December 29, 2009

Change of resistance

I hate getting old! Last night my lower back was in agony from the couple of hours I spent hunched over the desk working on the SoftRock 6.2 Lite kit. So I didn't plan on doing any constructional work today. However, Maurice G4DVM read of my problem with the kit and asked me to send him some voltage measurements. It also occurred to me that I had tested the circuit with the power supply set to 9V instead of 12V. So I connected the kit to a 12V power supply, switched on the FT-817 (which was still set to 8.191MHz from yesterday) and this time a carrier signal was heard!

I started taking voltage measurements anyway and observed that the voltage on pin 2 of U2 was low: 0.79V compared to the 2.47V obtained by the author of the instructions. Pin 3 was 1.23V compared to 3.21V expected. More importantly I observed that the oscillator signal stopped as soon as the test probe touched the pad. Around this time I observed that oscillation also stopped if I touched the case of the crystal and it didn't always start up again afterwards. So I probably wasn't mistaken that I didn't detect the local oscillator signal yesterday.

According to the instructions, R6 (which has something to do with the bias of Q2) is not used in most versions of the SoftRock except for 40m. It is supposed to be used in the version for the K3 IF, and the value supplied is 22K. I thought it would be an easy test to remove it - just snip the exposed lead (since it is mounted on end.) If that didn't help I could always solder the ends back together again.

With R6 open circuit there was no local oscillator at all. I then decided to try a lower value. The instructions say a value from 12K to 22K may be used "as appropriate". I found a 15K resistor and tacked it across the pads on the underside of the board. I had a local oscillator again and this time it didn't stop when I touched the case of the crystal!

I absolutely hate replacing components in circuit boards with plated through holes. Removing the snipped in half 22K resistor from the circuit board was easy. But clearing the holes to allow the 15K resistor to be inserted in its place was a job I dreaded. Ironically it would have been easier if the SoftRock kit was all SMT - apart from the fact that I wouldn't have had a 15K SMT resistor to try.

I have a spring-loaded desoldering gun, but I didn't think I could use it as it was designed for larger boards with more space between the components and the holes. I have desoldering braid too, but most times I have used it I ended up lifting pads and traces on the board. I made a real mess of modifying the KSB2 board in my early model Elecraft K2. The board works, but it is a good job no-one can see it.

In the end I did the job using a couple of stainless steel needles from Olga's sewing kit. I heated up a hole and quickly pushed a needle through, then cleaned up the solder that was pushed through. It took a couple of attempts to clean the holes sufficiently for the replacement resistor's leads to pass through.

By the time I had finished the modification and checked that I still had an 8.191MHz signal my back was painful again. So much as I would have liked to finish the SoftRock and see if it works I shall have to pack it and my tools away and leave it for a few days. I hate getting old!
Post a Comment