02-08-2009, 02:24 PM
The last two radios I restored (Zenith 216 and Sparton 67) exhibit the same problem. The B+ is WAY to high. In both cases, the original filter caps were dual 8mfd units. I replaced them with 10mfd/450 volt caps (I actually measured them, and they were very close to 10mfd).
In both cases, the B+ reached excessive values with only 100 volts AC input! In the case of the Sparton, the schematic says the B+ should be 275 volts (+/- 15%) with 119 volts AC in. I measure 312 volts DC (B+ to the TUBES, not raw B+) with only 103 volts AC input (from a Variac). There's no way I would every apply normal line volts (123 volts) or even AC from by bucking transformer (110 volts).
In the case of the Zenith, I changed the input filter cap to 4mfd. This helped some, but there was some low level hum and I still had to power it from the bucking transformer, else the B+ would be 360 volts!
I don't think the impedance of the meter (10meg vs. 1000 ohms/volt) would matter in this case.
Has anyone else encountered this problem?
Thanks!
In both cases, the B+ reached excessive values with only 100 volts AC input! In the case of the Sparton, the schematic says the B+ should be 275 volts (+/- 15%) with 119 volts AC in. I measure 312 volts DC (B+ to the TUBES, not raw B+) with only 103 volts AC input (from a Variac). There's no way I would every apply normal line volts (123 volts) or even AC from by bucking transformer (110 volts).
In the case of the Zenith, I changed the input filter cap to 4mfd. This helped some, but there was some low level hum and I still had to power it from the bucking transformer, else the B+ would be 360 volts!
I don't think the impedance of the meter (10meg vs. 1000 ohms/volt) would matter in this case.
Has anyone else encountered this problem?
Thanks!
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