1Mc/s oscillator board fault on Racal RA17W.

[Racalrob]

All.

Recently I was asked to repair a Racal RA17W by the Internal Fire Museum of Power in Tan-Y-Groes.

I ascertained that the set was not very sensitive and the calibrator was none working.

Upon connecting a frequency counter to PL3A, the frequency displayed was 2Mc/s instead of the normal 1Mc/s. I got a trimmer tool out and proceeded to trim C2. However, as I was touching the slotted brass part of C2 I noticed that the frequency was jumping all over the place between 1 and 2Mc/s. I connected up a scope to PL3A and saw the output was only at 300mV instead of the normal 2V.

With the scope still connected to PL3A, I adjusted C3 again and suddenly the RF output, adjustable by L2, stopped working.

I measured the voltages on the valve bases and all was okay, checked the valves in a VCM163 and they were okay, the resistors and caps were in spec even changed out the crystal but to no avail. I decided the only course of action was to remove the board.

Upon removing the board from the set, I removed all the components, wiring, trimmer cap and tuneable inductor.

Re-tinned all the feed through tags.

Cleaned the board with IPA.

Cleaned the trimmer cap vanes with IPA.

On examining the base of the trimmer cap, I found that the brass strip from the slotted shaft was displaying signs of either corrosion or oxidisation.

Cleaned that up with IPA till it was gleaming.

Re-built the board with new components, wiring and installed back in the set.

Tested for any HT shorts and then powered up the set

The frequency was spot on 1Mc/s and then adjusted the inductor till I got 2V output.

I think the problem was the corrosion or oxidisation on the base plate of the trimmer cap with the result that the corrosion was altering the resonant frequency of the tuned circuit and causing it to go high, but resting a trimmer tool on the tuneable cap was altering the frequency of the circuit and causing it to go lower.

Once the 1Mc/s oscillator was working correctly, I found out that the calibrator was still non working and the sets sensitivity was in the region of 35uV for 100uA meter deflection. That is fault finding for another day.

Rob.

[Herald1360]

I like the idea that valve sets have internal fire!

[Phil G4SPZ]

I haven't visited Internal Fire for a few years and their radio display was quite extensive then. I presume it's improved since. Must pay another visit on a crank-up day soon.

Well done on fixing the Racal. I have experienced intermittent faults with trimmers on other sets and they can be a devil to find.