On 3/23/2022 9:48 PM, James55 wrote:
Hi Harvey,
finally a green light to remove parts. I thought you'd never ask... : )
Happens to the best of us....... <grin>
If I am honest, I am not 110% sure that I have isolated all the boards.
You may not have.
Obviously the A10 is directly connected into the A6 interface board and is a headache to disconnect (I know... I've tried), so that leaves all of the various connectors, as well as the physical pins of J's 1 & 2 and P's 5 & 6, which connect the A5 and A7 horizontal sections (which 'are' already removed). Do I need to physically remove any other boards or is it just the connectors, P1690, P400, P1731 and P1711?
Then that says that
1) you need to just disconnect the boards.
2) whatever you couldn't disconnect, and you've checked C1757 and CR1757 for being shorted, and they're not.
3) whatever other boards are connected have a problem on them. Hence the "disconnect the boards" to isolate boards.
As for C1757 and CR1757, both read as almost a short (16¦¸) when connected, yet test fine with a leg lifted.
That points to other places.
Look for the lowest voltage at the +15 volt circuits on the interface board.? Try to measure across each part not part to ground.
Without looking at the schematic for any series or parallel legs, I have taken the following readings on the +15v rail;
Voltages across the components;-
C1751 (bridge rectifier) 10.5v - low
C1757 - 9.53v - low
CR1757 - 9.53v
R1752 - 31.42v - should have 60 volts across it
R1753 - 9.35v - low, but understandable, the +65 is too low.
R1754 - 0.714v
R1756 - 9.39v
R1757 - 0.124v - that says that
Q1752 - B 10.73v C 10.16v E 9.77v - says that VBE is very high and the BE junction may be bad
Q1754 - B 9.38v C 10.7 2v E 9.35v - transistor is not overcurrent
Q1756 - B 10.10v C 10.16v E 9.49v
U1724B Pin #5 - 9.45v
Pin #6 - 9.45v
Pin #7 - 10.82v
Measurements;
R1752 (50k) - 14.35k unless R1752 is bad, may be affected by the op amp input and R1763
R1753 (15K) - 11.82k
R1754 (1.2K - 1.2k
R1756 (16K) - 15.9k
R1757 (0.6¦¸) - 0.6¦¸
@ Mark
Forgive me if I give redundant information. You may have to replace more tantalums on the 15V line. See about the fan board. That could cause the loading. Unplug it to see if there is a wanted change. You can use tantalums as
replacements such as 10mfd or more at 25V or the Nichicon ULD 33 or 47mfd 25V. The latter will be cheaper and work as well. You may still find another bad IC or more. This is a long thread. These are the things that come to
mind here. We look forward to knowing what you find as the problem(s) and it works as it should.
Hi Mark, you may well be correct in your assumptions. Looking just now, it's not clear if the fan board has been disconnected...
There seems to be excessive load on the +15 volt line.
I'm kinda embarrassed as to how long this thread is, and it did cross my mind to close it, considering as to how we had eventually located the crossed transformer output wires, but that said, there still remain a couple of shorts to find and there are gems of information dotted throughout this thread, so I figured to just let it run it's course.
Might as well, this ought to prove educational, I hope.
Harvey
James
