TEST GEAR AND
This page is dedicated to my test gear and restoration projects. I've only just started the page but it will soon built into something of interest, I hope. For newcomers to the hobby, I intend to cover some fault finding issues on old valve gear. This will include my Ferranti model 555 mains radio among other things. Valves might be old hat, but restoring valve gear can be rewarding and educational. I also plan to cover some test equipment and its uses. Keep an eye on the page for additions and updates.
There were a couple of faults on this voltmeter. A diode was open circuit and the zero set pot was dirty. There was also an earthling fault on the power supply.
This came from my old mate Simon at GWM Radio in Worthing, Sussex. It's old but works well and will no doubt last forever. Mind you, Simon's old. It's a useful item for the workshop. And so is Simon.
CLICK HERE FOR THE CIRCUIT DIAGRAM (pdf)
This piece of equipment was given to me by Frank, a local radio amateur. The first fault I tackled was the horrendously unstable RF oscillator. Monitoring the output on my HF receiver was like listening to chirping birds. The OB2 stabilizer (stabilizing about 105 Volts to the oscillator) was flashing on and off - not a good sign. I tried a new OB2 but it made no difference. The problem was that the thing wasn't being supplied with enough volts to strike and stabilize the HT rail. Rather than a dropper resistor on the AC side of the valve rectifier, there was a 0.25uF capacitor. Yes, capacitors can be used to drop AC voltage. Don't ask me how, but they work (see circuit below). So, I ripped the capacitor out and replaced it with a wire wound resistor. The stabilizer fired, the HT rail was OK, and the oscillator was a damned sight more stable.
The second fault was no modulation, AM or FM. Using the scope, I discovered that the audio oscillator wasn't working. Using my Avo, I then discovered that there was no HT on the anode of the oscillator. A 10K resistor supplying the centre tap of the anode coil had burnt up and gone open circuit. Why? After some poking about, I found a 0.02uF capacitor had gone short circuit (see circuit below). Lucking, the anode coil hadn't burnt up along with the 10K resistor. The result of replacing the 10K res and the .02 cap was that I had AM - but no FM. The FM fault was simple, a dirty valve base.
Things were looking good at this stage, except that I had very little RF output on any of the frequency ranges. Over the years, the component values had changed slightly. The problem was resolved by adjusting the small trimming capacitors for each multiplier stage. The sig gen now works pretty well, but I really need to change all the .1 caps along with the .01 and .02 caps. This is an easy task, but where the hell you you get caps from? They are like gold dust these days, and they cost as much.