In circuit tests of these components will be quite meaningless because of
the low impedance components connected to them and the other semiconductors
connected across them, such as CR556 across Q556 b-e junction. The only
accurate way to test is to remove them and test out of circuit.
Before you start removing parts, have you replaced the fuse and checked for
55v across C558? Q552 and Q556 comprise a blocking oscillator which provides
primary excitation for T550. This oscillator cannot function without the 50 volt bias
at T550 pin 4 (or across C558). Q554 and Q548 are control circuits which regulate the
blocking oscillator output to provide a stable +95volts.
The voltages across C721, C741 and C761 all look good, so they are not the problem.
Dave
On Sat, Aug 29, 2020 at 07:38 PM, Harrison wrote:
I will check out voltages across C721. Since my last posting I did some
reading after which I decided to look closer at the HV Oscillator and HV
Regulator circuits. I did an in circuit check of Q556, Q552, Q548, Q544,
C548, and L554. I used my beckman DMM in the diode mode or the SZBJ BM4070
digital LCR tester. All seemed to check out okay except the Q552 (part of the
HV Oscillator circuit). I plan on removing it, doing an out of circuit test
and replace it regardless. But if I understand the manual correctly that
circuit has a direct impact on the oscillation and output of T550.
On Sat, Aug 29, 2020 at 10:28 PM, Harrison wrote:
Read out the voltages across C721. Also read out C741 and C761. Used the
Beckman HD110 MM with the common to ground. C721; three of four pins read
zero, both AC and DC. The fourth pin read 55.5V DC and 0.240V AC. C741; four
of five pins read zero, both AC and DC. The fifth pin read 11.4V DC and
0.068V AC. C761; four of five pins read 11.3V DC and 0.078V AC. The fifth
pin read zero, both AC and DC.
