tek 465 alike osciloscope help
 

Hello:

I've been trying to repair my osciloscope, it's not a well known brand
but it's fully discrete and the circuit is similar to a tek 465.
The problem is that i can't see square signals, they are very distorted
on both channels. I tried to follow the signal with another scope but
didn't get far, because my readings didn't show a similar signal to what
was displayed and maybe because i don't know how to properly measure on
a diferential amp.
I tried the trick of shorting the transistors shown on the tek guide on
how to repair scopes and it didn't help me, and i tried replacing the
diode switches with new ones and isolating each chanel but could never
reach a conclusion.

I'm lost.

I made a test circuit to get some reference signals: a 20MHz dip
oscilator module (8 pin metalic can) feeds two 51 ohms resistors, that
go through 20cms of rg174 coax into a bnc male that goes into a bnc tee
were the other ends go to the osciloscope input and to a 51ohm resistor.
This circuit displays the"20Mhz" pics.

I also probed with a X10 osciloscope probe a board that has an 74hc74
flip-flop clocked at 16MHz and got the "16MHz" and "8MHz" pics, these
last one didn't come out the same because i had to capture the event but
it shows the ringing at the rising and falling edges. For the
osciloscope probe i used a tiny (1cm) piece of wire from the tip ground
to the ic ground pin, not the usual ground clip.

The pics are at: http://sweet.ua.pt/~etsteve/osciloscope.htm

Osciloscope history of repairs that i made was a bad bridge on the power
supply (intermitent open diode) and a logic chip that togles the chanels
in ALt mode.

Any help/pointer are apreciated.

Thank you

Best Regards

Steve Sousa