UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - AR88

G6Tanuki · 19th Aug 2019, 8:08 pm

Yes, be careful of using the "STBY" position of the power switch - in "STBY" the rectifier is basically unloaded so the filter-capacitors charge to the peak-voltage delivered by the mains transformer. Then when you switch to "REC" you dump this high voltage into the rest of the circuit.

The RAF 'R1556" modification bypassed the 'STBY" position on the switch. I guess the problem was known about back in WWII-days?

Some people wire a 100-Ohm resistor from the cathode of the output valve to limit the current.

As a safeguard, wire a couple of NE-2 neons across the primary of the output transformer = - these will strike if subjected to more than 80-or-so volts, so 'clamping' any switch-on/off voltage surges in the transformer and hopefully saving it.

19th Aug 2019, 8:08 pm	#19
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: AR88 - New keeper Yes, be careful of using the "STBY" position of the power switch - in "STBY" the rectifier is basically unloaded so the filter-capacitors charge to the peak-voltage delivered by the mains transformer. Then when you switch to "REC" you dump this high voltage into the rest of the circuit. The RAF 'R1556" modification bypassed the 'STBY" position on the switch. I guess the problem was known about back in WWII-days? Some people wire a 100-Ohm resistor from the cathode of the output valve to limit the current. As a safeguard, wire a couple of NE-2 neons across the primary of the output transformer = - these will strike if subjected to more than 80-or-so volts, so 'clamping' any switch-on/off voltage surges in the transformer and hopefully saving it.