Tektronix TDS220 oscilloscope startup problem

[Tony Ferrino]

I have a Tektronix TDS220 oscilloscope that has developed this
intermittent startup fault:

Sometimes when the power is switched on the LCD will only show a random
number of horizontal dark/brihter lines of random widths. The LCD
backlight will not be turned on and the mainborad doesn't appear to be
working as there will be no 1KHz/5V square wave on the compensation
probe output. Also the GPIB interface will not be working. After I reset
the mainboard (switch off and on the oscilloscope) several times I
eventually manage to get it started and the oscilloscope will work fine
for then on, until it is switched off. So I suspect this might be
related to a power on reset function (firmware) not being called
properly at the startup or maybe some dodgy power supply connection to
the main board. I wonder if anyone has had a similar problem and/or
could provide some insight about what might be causing this problem.
Tektronix UK will charge £425 + VAT (US$775 + tax) to fix it and will
take nearly 1 month (apparently they have to be sent to Germany to be
repaired). Thanks.

Tony

 

[Lewin A.R.W. Edwards]

Sometimes when the power is switched on the LCD will only show a random

number of horizontal dark/brihter lines of random widths. The LCD

This will be hard to debug if it's really intermittent. But, as I recall
from opening my TDS210, these models use an off-the-shelf 16/32-bit
Motorola micro for which datasheets should be readily available. Trace
back the reset line and see what it runs to. Maybe an RC reset circuit,
and maybe the C is bad.

Or the CPU may not be getting clock.

It's also possible that the micro is starting up just fine, but that its
first act is to wait for something else in the circuit to come up - and
that something else may be the culprit. You need to scope the CPU to see
if it is wiggling its address lines while in this hung state. Some 68K
microprocessors [don't know about microcontrollers] also require an
end-of-cycle signal to complete external memory access cycles. The CPU
will hang if it doesn't get this signal - so again, see what the input
is doing.

 

[Tony Ferrino]

This will be hard to debug if it's really intermittent. But, as I recall
from opening my TDS210, these models use an off-the-shelf 16/32-bit
Motorola micro for which datasheets should be readily available. Trace
back the reset line and see what it runs to. Maybe an RC reset circuit,
and maybe the C is bad.

Or the CPU may not be getting clock.

Thanks for your helpful suggestions. If there is no clock then the
problem should be easier to solve.

Tony