BK Precision 1522 scope: bad input section, no trace

B

Ben Byer

Guest
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...
 
bbyer@spamcop.net (Ben Byer) wrote in
news:57359430.0309220426.16bd10a7@posting.google.com:

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...
Click to expand...
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
jyanik@kua.net
 
It is very common that the input devices, such as the input FET's get easily
damaged in these scopes. When taking measurements, it is very important to
take care to not test high voltage areas that are overspec for the scope
with the probes.

Change the input devices, and check for any other devices in the board that
are defective, or just change all the semiconductors.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
Instruments http://www.zoom-one.com/glgtech.htm
=========================================


"Ben Byer" <bbyer@spamcop.net> wrote in message
news:57359430.0309220426.16bd10a7@posting.google.com...
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...
 
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