Re: TEK 465 S/N>25000 Power Supply Issue


I don't have a 465 but I briefly looked at the power supply schematic on sheet <12>.

Started trying to determine the possible issue and determined that I had no
+22vdc at U1524A&B input.
If this is the +22V at pin 8, it is needed for both 55V and 15V regulators.

So continued on going thru the +55 regulator and
determined that if I left pin 1 out of the U1524 base socket the +22 vdc would
show up.
This would disconnect +55V regulator feedback loop, without the feedback loop I expect +55V to rise to an out of spec voltage (not safe). I don't recommend disconnecting pin 1. I can't easily explain why +22V would show up, may be R1524 now supplies more current (see below).

So then re-socketed the pin 1 along with all others and it would drop
+22 again to 2.26Vdc. So I moved to the B section of chip U1524 and removed
pin 7 with all the others in the socket the +55 V would go to 54.9vdc.
What was the voltage at +22V when you disconnected pin 7? If it recovered to +22V you have either a short in +15V supply or a bad transistor Q1548. This is the theory of what is happening: If output of U1524B is pulled to ground it takes current from its supply (pin 8) which comes from +22V. This voltage is generated by R1524/VR1526 shunt regulator. It can't supply current above (55V-22V)/4.7k=7mA without going out of regulation.

Traced it down to the Q1544 transistor when it was removed the U1524 amplifier voltages
would come up to as the markings in the manual for voltages except for the
lead 6 of the U1524B.
Removing Q1544 cuts the current to +15V and also turns off Q1548. The issue still points to either a shorted +15V or bad Q1548. Lead 6 of U1524B is +15V supply, without Q1544 +15V can't regulate so it is normal to have lead 6 ~ 0V in this condition.

Swapped it out the Q1544 with no change. So now if I
just disconnect the base of transistor Q1544 +55 and voltage readings around
the U1524 a&b are as needed along with the +55.
I assume in this state +15V TP is ~ 0V

Thinking that the Q1546 is
causing the issue. As I have basically gone thru and checked all the
resistors, capacitors and diodes involved in the circuit down to the TP1548
+15VDC. Including the Q1546 and don't see any issue.
I also think you are looking at the correct area.

When everything is connected, what voltages do you see:
At 15V TP
At the emitter of Q1546
At the base of Q1546
pin 7 of U1524B
base of Q1548B
Voltage across R1549 (this tells us how much current is pulled from Q1546).

We are looking for a possible high current out of U1524B pin 7 (~ 10mA).


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