Nombrex 31 signal generator

[Phil G4SPZ]

My previously-reliable Nombrex 31 let me down badly yesterday, just when I needed it! I discovered, after wasting a good hour chasing a non-existent fault in someone else's radio, that the Nombrex had gone well out of calibration and drifted 20% high in frequency. More than one range is affected.

After searching for a while on the internet, I eventually found a circuit for the Type 31 in a file on my hard drive labelled "signal generators" so, as this does not seem to have appeared on this Forum, I thought I'd share it with you. In the meantime, I'm off to try to find the fault. I suspect C4 or C8, but if anyone has seen this fault before I'd be most grateful for your thoughts. Thanks,

Phil

[cmjones01]

If it's high on all ranges, I'd suspect C4 since it's in series, via S1A, with the tuned circuits on every range.

[Phil G4SPZ]

I was right, Martin, and so were you - C4 was the culprit. For reasons best known to itself, its value had fallen to 0.008uF instead of 0.01uF. I have no idea of the failure mode of this type of ceramic tubular capacitor, but perhaps the signal generator didn't like living in my portable spares box in the boot of the car.

I fitted a polyester replacement capacitor of the correct value, and spent most of this evening realigning the eight ranges, twice as long as necessary because I managed to break one of the ferrite tuning slugs in the process, and had to drill it out and fit another. All seems well now, although I think this signal generator will reside in the safety and warmth of the workshop from now on!

Phil