1740a oscilloscope positioning problem

[colin]

Hi,
I have a HP 1740a osciloscope wich ive had for a few months and worked
fine till just now, the trace has disapeared off the screen on all chanels
even triger and when i press beam find it only apears on the very bottom of
the screen if i turn the position knob fuly clockwise, althogh beam find
brings the triger chanel to the center. its been intermitent for a day or so
but is now permanent.

unfortunatly i dont have any manuals or anything, is there anywhere there
is a circuit on file or anything for this scope? or any one able to pinpoint
whats wrong from the symptoms ?

Colin =^.^=

 

[Charles Schuler]

Hi,
I have a HP 1740a osciloscope wich ive had for a few months and worked
fine till just now, the trace has disapeared off the screen on all chanels
even triger and when i press beam find it only apears on the very bottom of
the screen if i turn the position knob fuly clockwise, althogh beam find
brings the triger chanel to the center. its been intermitent for a day or so
but is now permanent.

unfortunatly i dont have any manuals or anything, is there anywhere there
is a circuit on file or anything for this scope? or any one able to pinpoint
whats wrong from the symptoms ?

The vertical channel is DC coupled from the input connector (unless AC
coupling is selected) to the vertical deflection plates of the CRT. A
shorted transistor, among other things, along this signal path will cause
what you report. With no signal applied, the voltage difference across the
deflection plates should be zero. You can use a DC voltmeter to walk back
from that point towards the input circuit to determine where the DC error
originates.

If it's a dual trace scope and both channels are pushed to the bottom, then
the problem is after the chopper/alternate sweep circuit (most likely in the
vertical deflection final amplifier).

It would be much easier to fix with a schematic!

 

[colin]

Hi,
thanks, yes its much harder to trace the signals when u dont know which
components are suposed to be taking it .

i managed to find the problem tho, i looked at the delay transmision line as
this is not only very easy to identify but is in the middle of the signal
path to the deflection plates so checking this isolates the problem to one
half of the circuit or the other, or so i thought, the voltages seemed
rather odd to me, 12-13 volts and not exactly similar exept when i presed
the beam find and the both falled to much lower and similar voltage.

so i spent ages looking at the delay line driver (wich is a big custom HP
chip) for the cuase, only to find that there was a pair of comon base
amplifiers at the other end of the delay line and one of these had gone open
circuit so cuasing a considerable dc imbalance, its a pnp device - motorola
824 3-354 - its probably custom HP device so i no idea of equivalent, but i
worked out its taking 12v at 50 ma, so i tried a bfr194, wich was the only
pnp RF transistor i had to hand, and its a SM device so i had to solder it
on the back of the pcb, but its working nicely now.

i gues i shld replace both as a pair but first i wld like to know if this is
a reasonable enough replacement, any one got any ideas ? i had to tweak the
dc balances a bit but have to asume the frequency responses arent going to
be so wildly off to notice much, this transistor probably has way lower cbe
for a start, although in comon base mode i wld hope doesnt make too much of
an overal diference, it might peak a bit now or even have extended bandwidth
but 10mhz squarwaves from very high speed cmos look identical to what
they did before anyway ... i just hope it doesnt go short circuit and take
out the subsequent vert output stage wich is another big custom HP chip.

Colin =^.^=