Another R&R of the RF Board, this time to correct what may have killed the Bandpass Filter decoder IC:

Seems that JRC used two back-to-back pairs of diodes as a front-end protection circuit. Devices used were silicon; when you get more than 1.5V RF at the Antenna connector all sorts of interesting and undesirable intermod products appear in the receiver passband. Not uncommon at all if you live close to a MW broadcaster or run the radio in a multi-multi setup.

Counsel at the time was to remove these diodes. In the unit I have, someone did - and in doing so, possibly caused the 7445 bandpass filter decoder to be taken out when a nearby lightning stroke put a sufficient voltage pulse into the set.

I did a bit of research into receiver protectors and the various ways to implement one. Many still utilize a diode (or diodes) somewhere in the current path. One had an NE-2 bulb paralleled with a 1mH inductor; the inductor keeps the DC (static) potential of the antenna low while the neon bulb shorts a voltage spike to ground by way of firing. This didn't seem to be a viable solution for a couple of reasons - one of which being that the addition of the inductor pretty much killed sensitivity of the radio below 2MHz.

What I eventually settled on was a gas discharge tube in parallel with a bidirectional TransZorb. Think of a Zener diode with a very high amperage rating and the ability to conduct in either direction. The decoder IC specifies 30V max switched voltage at its outputs; the other devices (potentially) in the path of a pulse are the 1st mixers - a balanced pair of U310 JFETs. Their gate-to-source maximums are spec'd at -25V. A 13V TransZorb (right arrow) and a 75V Gas Discharge Tube (left arrow) were installed in the original vias. Should the TransZorb ever open after repeated conduction cycles, the GDT will still offer protection from large spikes - and is what Drake, Kenwood et al did in their various equipment lines. Additional measures including switching the feedlines to ground or disconnecting them altogether when storms are forecast are used, and the combination of all should keep the front end free of incident.

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