On 3/21/2022 10:43 PM, James55 wrote:
Great reply Harvey.
Made me smile. Thank you.
Focus on the 65 volt supply, while disconnecting the main power supply from the other chassis boards
I take it you mean via the connectors?
and the chassis.
Do you mean disconnect any ground wires?
That's because you keep removing parts. Stop doing that unless you can justify removing them
Fair enough. The two diodes had a leg lifted in order to help find the last 'good point' with the voltage readings. All components are back in now other than the +15v rail, which still has a dead short lurking somewhere.
So, voltages...
The CR1721 +65v lines are reading 173.5v and 86 volts
In the hope that it may illuminate the situation, I just did a series of quick measurements 'across' the components of the +65v section, which are as follows;
R1722 - 128.3v - good since the input of U1724A should be high impedance
R1723 - 18.7v -
VR1722 - 20.2v - GOOD
VR1724 - 23.3v - normaly 56 volts,
C1724 - 23.3v - way low since the input of U1724A should be high impedance
CR1724 - 0.25v - GOOD
U1724 - pin #1 = 2.19v #2 = 3.28v #3 = 3.39v #8 = 2.7v
VR1725 - 2.7v - way too low, possibly because +65 is low
VR1726 - 9.2v - GOOD
R1727 - 26v
R1725 - 5.8v - bad, since the input of U1724A should be high impedance
C1725 - 3.39v - bad, since the input of U1724A should be high impedance
R1726 - 16v - understandable since +65 is low
CR 1723 - 0.573v - GOOD
Q1732 - Base 26.3v, Emitter 25.95v and Collector 86v
Q1734 - Base 25.95v, Emitter 25.39v and Collector 86v
CR1732 - 0.354v
R1732 - 0.354v
R1734 - 25.25v - way bad
Q1736 - Base 25.58v, Emitter 25.3v and Collector 26.3v
CR1733 - 0.326v
R1733 - 25.58v
R1737 - 2.589v
C1735 - 22.03v
R1735 - 22-02v
C1737 - 25.32v
CR1737 - 25.32v
As has been mentioned, 26 volts across 3 ohms would be a current drain of 8 amps.? It's not.? I think that (as has been mentioned), R1734 is open.
That leaves the only way to get any current to the +65 a path through VR1722 (allows 4 ma at worst), and then a possible path through CR1723 through Q1736.
I'd also suspect the op amp U1224A.
Harvey
+65v test point currently 25.32 volts (obviously)
There is lots I wish to add in response to the earlier post however it is very late here.
I'll get back on it tomorrow evening.
James
