Thursday, December 13, 2012

OHR100A Build Part 2 (The Chassis and Alignment)

After re-checking all the soldering and making sure there were no obvious mistakes, I started on the chassis, firstly installing the SO239 socket and then attaching the optional ten turn VFO tuning pot followed by the AF and RF gain switches and the Bandwidth and RIT switches. The wiring instructions were fairly easy and I soon had everything connected up and ready for a smoke test. 

 All ready for the smoke test

The only problem was I didn't have a 13.8v power supply! The RockMites run off batteries and the FT1000 MP V has its own separate power supply. So on to Ebay and I quickly found an old but unused power supply for the princely sum of £15.

Two days later (a miracle in postal terms) the PSU arrived and after wiring in a 1A fuse I was able to carry out a smoke test. Do you ever have that feeling of dread, fingers on the switch wondering if you really will get smoke or just nothing happens? I must have been waiting for half a minute before I pulled up enough courage to switch the transceiver on. I turned the switch and bingo, the radio came to life ....what a relief!

Once I'd got hold of a power supply it came to life!

A quick play on receive and I found some cw, turning on the FT1000 MP and matching the incoming signal I could see that I was tuned in to 14.045 with the VFO fully turned anti clockwise, so I wasn't far off, the VFO toroid just needed some tweaking which was confirmed when I attached the MFJ 259 and using the frequency counter read that I was at 23.400MHz, just off the mark as the instructions quote 23.000MHz.

 So all I had to do was alter the gaps between the windings on the VFO toroid L114 to bring the frequency down to the required 23.000MHz as per the instructions. At the same time I had to tweek the caps for maximum power output between 100mv and 400mv, but this was fairly easy using my DMM. Once happy with the toroid readings it was a case of pinching some of my XYL's clear nail polish and giving the toroid a good coating and leaving for 24hrs!

 The toroid at L114  that needed a tweak

I must admit the hardest part I have found with this kit is the alignment, I've had little experience of this process but with the help of the instructions I managed to get through but it took time. You have to have a frequency counter and a DMM for this project and to be honest I'd never used the MFJ as a FC so it was all new to me and there are no instructions in the MFJ manual about how to set it up for frequency counting. In the end I attached a short piece of coax fitted a PL259 to one end and trimmed the earth off the other end leaving just the central wire. By chance I had a very narrow metal tube that fitted over the wire giving it strength and hey presto it was an excellent probe.

 The homebrew probe attached to the MFJ

The radio is now virtually complete except for the optional LED that I have to fit. I've had a good play but I haven't attempted a QSO yet, hopefully this will be done over the weekend. It's putting out a healthy 7 watts on the dummy load, but I've reduced this to 5 for QRP. First impressions are that the receiver is excellent especially when using the bandwidth, it certainly matches up to other radios I have owned. The 10 turn pot although smooth gives you little idea where you are within the CW sector of 20 meters so I suspect I will purchase the OHR Digital Frequency counter to make life a little easier.

Overall I'm really pleased; I've come along way since the early days of the RM builds and my confidence as a QRP builder has grown dramatically. Reading the reviews on Eham, the kit is probably classed as an intermediate, so hopefully I've passed with flying colours and now can look towards building my ultimate homebrew kit, the K2 !! 

In the meantime I shall enjoy working some QSO's with my trusty new OHR100A. Once I've added the LED I will post some pictures of the completed Radio.

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