Fred -
The first thing that comes to mind is that ONE of the PA tubes is DEAD. You can check by measuring
the voltage across R32 or R33 in TUNE. Other possibilities are an open Screen or Cathode resistor
(R30, 31, 32 or 33)
Another possibility is that the HV power supply (AC-4), isn't. The HV should be about 650 VDC at
full load (0.340A), and about 700 VDC at no load. Low Screen voltage on the PA tubes can also cause
this. Both are voltage doubler supplies, so a single diode or cap failure can result in ~ half
voltage. These voltages can be measured at the underchassis shield beside the PA area. The
rearmost feed through is the +650 VDC and the middle of the five is the +250 VDC.
The capacitors of these supplies are well past their expected life, and should be replaced either
individually or wholesale with the Heathkit shop AC-4 kit.
73, Garey - K4OAH
Glen Allen, VA Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs www.k4oah.com
fjmel...@aol.com wrote:
//Bias and plate current is low on Drake B Line. Adjustment on back of AC 4 is turned up full.
Plate current is only .210 mills. Bias is .04 on plate meter.//
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/Any ideas where to start to look for a cure./
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/Thanks /
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/73 de Fred WD8ADG /
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