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Steve_P 16th May 2019 2:16 pm

Brenell Mk5 Oscillator
Here is a funny one - The EL84 Oscillator had to be rebuilt as per the manual. The Erase Head reads OK on the meter. The Brenell coil was missing so I subbed a coil from another machine that also uses an EL84 oscillator, Everything else is as per the circuit. it does not oscillate,



Steve P.

Chris55000 16th May 2019 6:55 pm

Re: Brenell Mk5 Oscillator

Have you checked the anode/grid phasing?

The simpler bias oscillators use a simple single winding with a tap at an intermediate point for the h.t. supply connection, which normally ensures the correct phasing if the outer end of the winding is capacitor–coupled back to the grid, as is usually the case, but I think Brenell may use separate windings, in which there's always a possibility of mis–phasing, which results in failure to oscillate.

Is the coil you exchanged identical to the original one? If it's not, you might have to compare the two oscillator circuits carefully in case there's differences in the reaction–feedback coupling.

Another possibility is that the coil you fitted has a lower 'Q' than your old one, so it might be worth trying about 20V increase in the EL84's G2 voltage by adjusting the h.t. feed resistor! The negative G.B might also have been different between the two circuits, in which case you should use the.component values/connections corresponding to the machine your replacement coil came from!

Chris Williams

Chris Williams

Steve_P 16th May 2019 10:36 pm

Re: Brenell Mk5 Oscillator
Working now Chris. Thanks for your help. i turned round the coil. I did not think it mattered but obviously it does.

Thanks again!


Steve P,

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