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BK Precision 1522 scope: bad input section, no trace
(Ben Byer) wrote in
om:
Hi! I just bought an old-school BK 1522 20MHz dual-trace scope on
eBay, and it works great -- at least, half of it. The trace for
channel 2 works great, but channel one gives no trace at all.
I've narrowed it down to the input circuit board for channel 1 -- the
input board on this scope contains the voltage scale switch and input
jack, a couple dozen passive components, a transistor or two, and
that's about it.
The scope contains two such identical boards. They each connect to
the rest of the scope via a 4-conductor cable. Disconnecting the
input 1 board entirely produces a straight line trace (as if it were
plotting GND); reconnecting it makes it disappear. Connecting the
input 2 board to the input 1 connector produces the expected effect --
the signal on input 2 displays on the input 1 trace.
There are no burned spots, no leaking / puffed out electrolytics, or
any other obvious defects. I've reflowed all the solder joints. How
can I leverage the fact that I have two identical boards -- one
working, one dead -- to find the problem?
Thanks a lot!
Ben
PS -- This was to be my first scope, so all I have is a DMM...
Sounds like the Ch1 board is deflecting the trace off-screen.
That indicates a large DC offset,probably caused by a bad input FET;those
are most likely to be damaged.You could measure and compare voltages on the
xstrs,be careful around the working FET!
--
Jim Yanik,NRA member
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