A very unusual fault indeed.

[ukcol]

I have for a number of years owned a Murphy consul radio model A40C. This is a complex 15 valve set that I got going when I first bought it. I have since been studying the set and getting it up to the standard of which it is capable.

I have the chassis on my bench at the moment and am working on it while the cabinet is away been refinished by a cabinet restorer.

Now to the fault.

I turned the set on one day and the station being received was nearly drowned out by a high pitch note which I later measured to find it was about 2KHz. The note disappeared when the set was not tuned to a station but did not vary as the station was tuned in. BTW this set has noise suppression but the note was not present off station even with the noise suppression switched off.

The symptom suggested to me I.F. instability and that is where I started to look. I had replaced all the decoupling capacitors during my early work on this set but that does not eliminate a faulty new component or a mistake on my part of course.

Very shortly after I started to investigate the cause of the problem the symptom disappeared and the set started to work normally. Thinking that perhaps the fault was temperature related I switched off and tried it again from cold but the set was still OK. The set worked fine for a couple of days and then the fault returned only to clear again. This merry dance continued for a few days until today when it stayed and allowed me to investigate.

As I said above, I suspect I.F. instability and started my investigation around the MW/LW mixer stage and the I.F. amplifier. All the decoupling turned out to be innocent. I tryed the effect of damping each I.F. transformer primary and secondary in turn and although this reduced the volume of the station and the 2KHz note no other effect was noted.

I decided to look at the problem with an oscilloscope. All the decoupling points were "quiet" but every signal point from the mixer to the detector was modulated with the dreaded 2KHz note.

Getting desperate I decided to look at the local oscillator waveform, reasoning that if for some obscure reason the local oscillator was being modulated then that modulation would appear in the I.F. signal.

Referring to the circuit snip below.

I clipped my CRO probe to test point 22 (the junction of C9 and R2) and the fault cleared immediately and the CRO displayed a clean local oscillator sine wave. I removed the probe and the dreaded note returned. R2 was dry jointed. This left the grid of the local oscillator with no DC path and so the L.O. was squegging. The joint was original and me working around the set must have caused it to mechanically fail. R9 is a 50k component and was well within tolerance at 55.2K but I changed it for a modern 51k 0.6 watt component anyway. Some may be horrified at this but my personal veiw is that it is acceptable to use modern components under the chassis where they can't normally be seen.

I don't think I'll be forgetting that fault in a hurry.

EDIT:

A note on "squegging".

It occurs to me that some may be interested to know the mechanism of squegging in this case. With R2 open circuit the grid of the triode section of V1 is left floating. The grid and cathode form a diode which rectifies the oscillator's sine wave charging C9 such that a negative voltage builds up on the grid of the valve. A point is reached when the negative on the grid has got to the point that the oscillator stops running. The charge on C9 then leaks away and the oscillator starts up again. This process repeats ad infinitum, at 2KHz in this case.

[kalee20]

Wow! Interesting - though I have come across oscillators squegging when they shouldn't.

I guess the 2kHz tone was rather non-sinusoidal!

Thanks for sharing - it's one of those 'gotcha!' faults.

[ukcol]

Originally Posted by kalee20

I guess the 2kHz tone was rather non-sinusoidal!

You guess correctly.