Charles wrote: > > After hanging vertically for 36 hrs in a hot upstairs room, more goop seeped > out from under the keyboard. It now works again. Whew. > > While running on the bench for burn-in testing, a cursor problem suddenly > appeared... it would only move every other keystroke. With the technical > description and schematic at hand, it wasn't hard to track down a 74LS193 > up/down counter with a blown (floating) LSB output. Confirmed by manually > toggling that bit and the cursor would move back and forth one position. > Meanwhile I removed the bad chip and put in a DIP socket. Naturally my TTL > collection didn't have an 'LS193 so I'm waiting on that. So I have a 24 > line, 1 column terminal :) > > The monitor was occasionally intermittent (no display at all, no HV, +15 and > drive signals OK). It seemed to change with movement of the wiring harness > from the main board to the monitor, too. I reseated the edge connector on > its PCB and it seemed to be fixed - but then the VERTICAL deflection > collapsed and tweaking the height adjustment caused increasing loss. The 100 > ohm pot to the base of the vertical output transistor had picked that moment > to go open. Changed that out and readjusted everything - so far so good > after another hour of run time. > > This ADM-3A could have been unpowered (and in a storage area without climate > control) for a very long time. I wonder if that contributed to the failures > I'm seeing... hope there aren't any more until I get to use it for a while > on my PDP-8/A (or 11/23+). >
I had a similar cascade of failures on an ADM-5. Each time I fixed a fault, another one arose. At one point, the faults were happening quicker than I could put them right! Initially, there was some logic fault which required a 74LS125 to be replaced. Luckily I ordered several because a short time after that, another 74LS125 failed. Before I could fit the new chips, the raster collapsed to a vertical line and slowly faded out. I initially thought this was due to a bad connection in the wiring harness to the monitor board but I eventually tracked it down to a bad joint on the line driver transformer. This probably arose when I accidentally dropped the lid rather hard. I think a few other faults which I don't recall now also arose during the faultfinding session. Perhaps the ADM-3 and ADM-5 both suffer from these sort of multiple failures? I dug out my ADM-5 just now to see what chips I had replaced and while I was at it, powered it up. It seemed to work ok initially, in that the cursor appeared (after I wiggled the noisy brightness control) but before I could find a loopback connector to make more complete tests, I heard a soft whoosh from the insides and when I lifted the lid, was greeted by a cloud of foul smelling smoke. It seemed like the sort of smoke that mains filter capacitors make but disappointingly, it does not seem to have such a capacitor :-( It is currently sitting outside waiting for the smoke to dissipate before further investigation. I have a second ADM-5 which has a logic fault that I have not been able to trace. While there is an ADM-3 maintenance manual available, I have not been able to find one for an ADM-5. There is some commonality but my problem seems to be in an area where they differ :-( Regards, Peter Coghlan.