--- In TekScopes@..., "cmjones012003" wrote:
I could just about see the left hand end of the trace by winding the horizontal position all the way over to the left, and the scope was clearly still working otherwise: the bit of trace I could see responded to input signals and adjustments of the controls. I noticed also that pressing the 'beam find' button put the display back in the centre, but shrunken as I'd expect.
I tried the scope again a little later on and the display came back, but tried it again today and it worked for a few minutes then suddenly jumped off to the right again.
I do have the service manual for it, but before I start digging in, are these the typical symptoms of U800 failure? Searching the group, it seems that U800 failure tends to push the display to the left, and I haven't seen anyone document how 'beam find' behaves in that case.
Mine did exactly the same - and the effect of the beam finder was similar too. I bought a replacement from a helpful man in California (Ebay seller "2465b"). He advised me that the original IC was mounted with two lock washers between the package and the board. This is a BAD IDEA as it creates an air gap and prevents any cooling that may be had from contact with the board - bolt the replacement directly to the board surface. A little heatsink compound seems to help as well. I also reamed out some of the case cooling holes directly beneath the position of U800 to improve the incoming airflow.
There's usually several second-hand U800s on Ebay - but avoid the ones made by Maxim. Not as reliable as the original Tektronix ICs, I understand.
John
G4FDD
