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Leon Crampin · 7th Jun 2017, 9:56 am

Well done, your findings and fixes match closely the work I have done on various VHF61 receivers which use the same chassis.

A couple of observations: The usefulness of the eye on VHF can be enhanced by attenuating the DC signal to it from the FM detector with a couple of high value resistors. The set can then be tuned accurately on strong VHF signals. Watch the EL84 for grid current. Connect a DVM from g1 to chassis and monitor the DC potential with the volume control at minimum. Around 200mV is fairly normal for this valve, but if it increases steadily over a half-hour period, the set may suffer from excessive HT current draw. My two "fixes" for this are either to use a Toshiba 6BQ5 which is a far higher quality equivalent to the EL84, or substitute the more easily obtainable 6CH6. Using this valve means a minor re-wire to the base, and shunting the cathode bias resistor to give a total of 100 Ohms. The 6CH6 is reliable and gives equally good, if not better audio.

The question of AM performance is interesting. Why is it so lousy on these sets - with 2 IF stages? Obviously a fixed ferrite rod is stupid, as is the coupling of the external AM aerial with a shunt capacitor and resistor to attenuate the signal. The Germans knew far better and used a panel-rotatable rod and in some sets, a switchable set of aerial coils for external connection.

One day, I'll go through it all. I suspect proper AM aerial coils and some attention to the AM IF stages to reduce the damping on the tuned circuits may be the answer. Any suggestions or observations are welcome.

Leon.

7th Jun 2017, 9:56 am	#3
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF 62 Well done, your findings and fixes match closely the work I have done on various VHF61 receivers which use the same chassis. A couple of observations: The usefulness of the eye on VHF can be enhanced by attenuating the DC signal to it from the FM detector with a couple of high value resistors. The set can then be tuned accurately on strong VHF signals. Watch the EL84 for grid current. Connect a DVM from g1 to chassis and monitor the DC potential with the volume control at minimum. Around 200mV is fairly normal for this valve, but if it increases steadily over a half-hour period, the set may suffer from excessive HT current draw. My two "fixes" for this are either to use a Toshiba 6BQ5 which is a far higher quality equivalent to the EL84, or substitute the more easily obtainable 6CH6. Using this valve means a minor re-wire to the base, and shunting the cathode bias resistor to give a total of 100 Ohms. The 6CH6 is reliable and gives equally good, if not better audio. The question of AM performance is interesting. Why is it so lousy on these sets - with 2 IF stages? Obviously a fixed ferrite rod is stupid, as is the coupling of the external AM aerial with a shunt capacitor and resistor to attenuate the signal. The Germans knew far better and used a panel-rotatable rod and in some sets, a switchable set of aerial coils for external connection. One day, I'll go through it all. I suspect proper AM aerial coils and some attention to the AM IF stages to reduce the damping on the tuned circuits may be the answer. Any suggestions or observations are welcome. Leon.