My heartfelt thanks to you taking time out during the holiday period!! Please do not worry I am a very patient type "slowly and surely wins the race" !!
Take your time and enjoy the day.
I'm off to bed soon!! Cheers Grum.
After giving it more thought it is always possible that there is more than one failed part in the scope. We know for sure that low frequency information is not getting through Ch1 and this points to a failed AD548 but it could also be a part surrounding that chip such as a failed Zener or capacitor. Only high frequency info is getting through and this is capacitively bypassed around the chip.
We know that something is loading the power supply causing a current limit / shutdown mode, but this could also be due to a failed component in the power supply causing it to drive too hard and shutdown.
The two symptoms together could be caused by one part failed in each section mentioned or two separately failed parts in the two separate sections (more unlikely).
If I understand correctly, the capacitor discharge went through the probe ground and somehow completed a circuit back to the capacitor. This surge would have propagated through the BNC ground connector back to earth ground so we can assume that the other end of the capacitor was somehow grounded. It would also have induced a voltage into the center conductor of the probe wire, possibly taking out the buffer chip.
IC's can fail several ways, they can have an input transistor fail from the transient and still draw the correct current or they can destroy the part causing it to present a hard short or a soft short to the power supply. If a hard short, the chip will be colder than normal but some other part will absorb the current and get hot, if a soft short,the IC will be hotter than normal. It can also have a normal temperature and be failed.
Somehow Ch1 must be isolated from the power supply to see if it is causing the load condition.
Alternately you can test the voltage +12AT1 and -12AT1 that supply power to the IC, pins 7 and 4 w/ respect to ground. These voltages are also available across C1085 and C1087 as shown on the attached schematic.
The fact that the other channels use the same +/- supply voltages and work properly points more to the failed chip as open. But something is still loading the supply. Look for any part that seems to be running hot compared to other parts in the other channels and the unit in general.
Testing the power supply can be difficult. Read the attached to see if anything is worth doing as it is a lengthy procedure.
« Last Edit: 2013-11-29, 18:32:14 by ION »
Just because it has a patent application or is patented does not always mean it really works.