A web search about old radio and amplifier designs as not given me an answer, except that this is not safe.
I was helping a friend troubleshoot a Sherwood S-5500 II audio amp. After we found the problem, a bad tube socket contact, I was disconnecting my scope ground from the chassis and got a good shock. I was between the chassis (left hand) and the scope's ground clip (right). Yeah, I "didn't" keep one hand in my pocket.
This has a two-wire non-polarized-power-plug and a transformer-isolated power supply. After a little investigation of the schematic (attached) and the chassis, there is a 100KΩ 1/2 W resistor from one side of the transformer's primary to chassis. In other versions the resistor is replaced with a capacitor, or two capacitors tied to both side if the power line and their common connected to chassis.
A DMM check between the chassis and line ground showed 5VAC with the power plug in one way and 115VAC with the plug reversed. Disconnection one end of the 100kΩ resistor brought the measured voltages down to 0.9VAC and 12VAC respectively.
I know in the olden days of the "All-American 5" tube radios with a "hot chassis" this was somehow supposed the make them safer. I guess as a current limiter.
OK, why the resistors or capacitors in an amplifier with a transformer isolated power supply?
Ken
I was helping a friend troubleshoot a Sherwood S-5500 II audio amp. After we found the problem, a bad tube socket contact, I was disconnecting my scope ground from the chassis and got a good shock. I was between the chassis (left hand) and the scope's ground clip (right). Yeah, I "didn't" keep one hand in my pocket.
This has a two-wire non-polarized-power-plug and a transformer-isolated power supply. After a little investigation of the schematic (attached) and the chassis, there is a 100KΩ 1/2 W resistor from one side of the transformer's primary to chassis. In other versions the resistor is replaced with a capacitor, or two capacitors tied to both side if the power line and their common connected to chassis.
A DMM check between the chassis and line ground showed 5VAC with the power plug in one way and 115VAC with the plug reversed. Disconnection one end of the 100kΩ resistor brought the measured voltages down to 0.9VAC and 12VAC respectively.
I know in the olden days of the "All-American 5" tube radios with a "hot chassis" this was somehow supposed the make them safer. I guess as a current limiter.
OK, why the resistors or capacitors in an amplifier with a transformer isolated power supply?
Ken
Attachments
-
3.6 MB Views: 48