577 repair status

Craig Sawyers <c.sawyers@...>

In the bowels of emails about a year ago, I described a faux pas with my 577 curve tracer. Basic
summary - shorted tant, unplugged power supply to isolate which board, replaced tant and then
plugged one of the power supply connectors one pin out. Smoke came out and would not go back in

Basically, lots of silicon was toast. Tek designers had their good points (socketed semiconductors)
and bad points (harmonica connectors with no indexing). Luckily all the transistors, bar two dual
FETs, were fine. And the dual FET's are still in production and came from Micross (astonishingly).
Although I had all the repair parts, I didn't have the stomach to tackle it until yesterday.
Replaced the dead silicon, turned it on - and a smell of something far too hot. An inductor - the
sort that is in series with power supply lines on each board - identified because of its dark brown
overheated colour.

So I pulled the supply, and checked for shorts or low resistances on each line. Then pulled each of
the three harmonica connectors in turn to isolate which board(s) were at fault. Eventually found two
more shorted tants, these probably had voltage abuse when I originally plugged the harmonica one pin
out. Also the power supply worked perfectly without the connections to the boards.

The long and the short (!) of it is that it now powers up with no smoke. I have a spot which is off
the bottom of the screen; with beam find pressed the spot can be moved horizontally but not
vertically. Brightness and focus work. Flood guns work in both halves of the screen. Remaining
faults are (i) no response to vertical position and (ii) no horizontal scan. But at least now I have
a situation where the supply is doing its stuff and there is no smoke - so I can now go into much
more serious fault finding mode.

Oh - just to complicate things further, I have had a very nice addition coming - a 576 curve tracer.
The caveat is that it has the dead HV transformer problem. I collect that in about ten days' time,
so I'll be back in diagnosis mode again. That more or less completes curve tracing heaven: two
575's, one with the HV mod (both working and calibrated), a 7CT1N (ditto), a 577-D2 under repair and
a 576 to be repaired on its way. A 570 would be nice, but unless there is a stroke of luck, these go
for silly money and it isn't something that is going to happen any time soon.


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