Oscilloscope HP1741A,everything OK,but delayed timebase

[diegotrex]

Hi to all
i just found the time to check and open a HP1741A in good appearance,very clean inside,not very bright.
In main timebase mode everything works well,trigger, vertical gain,timebase and so on,but when i switch on Delayed or Mixed view, i just have a weak foggy light .
I saw on the web heroical efforts to save similar oscilloscopes ,in very worst conditions
https://www.eevblog.com/forum/produ...on/hp-1741a-oscilloscope-restoration-project/

I downloaded the service manual ,very easy to find on the web,i have a decent lab with other oscilloscope and instrumentation.

Please can i have some hint ?Is it possible to have access on the crucial points of the time base boards without dismantle the entire structure?

Are there any components that is worth to substitute with new ones(if generic and easily available)?

Many thanks
Diego

 

[diegotrex]

Hi to all
i just found the time to check and open a HP1741A in good appearance,very clean inside,not very bright.
In main timebase mode everything works well,trigger, vertical gain,timebase and so on,but when i switch on Delayed or Mixed view, i just have a weak foggy light .
I saw on the web heroical efforts to save similar oscilloscopes ,in very worst conditions
https://www.eevblog.com/forum/produ...on/hp-1741a-oscilloscope-restoration-project/

I downloaded the service manual ,very easy to find on the web,i have a decent lab with other oscilloscope and instrumentation.

Please can i have some hint ?Is it possible to have access on the crucial points of the time base boards without dismantle the entire structure?

Are there any components that is worth to substitute with new ones(if generic and easily available)?

Many thanks
Diego

I saw that to pull off the boards is not needed what i was afraid of:to unmount the front mask with knobs and then the side bars.
The boards are connected and shaped in a way that eases the operation.Simply ,the time base shaft can slide off ,once the headless screws are loosen.At this point the time base boards can be easily removed.
The first components i test are two capacitors,aluminum 100uF and solid tantalum 33uF.
It looks that they are good quality components, no leakage at ohmmeter and respectively 97.3uf and 32.4 uF.

 

[BradtheRad]

The circuit that triggers a sweep is working.

I will guess that the delayed sweep is achieved by inserting a one-shot delay into the chain of events. The one-shot is started each time by the normal trigger. The one-shot delay is adjustable.

Problem or problems:

* The one-shot trigger input never gets pulled low (or high, depending).

* The one-shot is triggered but a capacitor never charges or discharges (assuming a capacitor is at the heart of the action).

* The one-shot starts the delay period but never ends it.

* The one-shot reaches the end of a delay, but does not produce a trigger pulse to the sweep circuit.

Does a knob control the sweep delay? Can you examine the potentiometer? Can you find signs of anything broken? Dirty? Disconnected wire? Etc.

Can you locate the sweep delay circuit? The problem may be something simple.

You tested large value capacitors. If the sweep delay has a capacitor involved, I think it is a small value to make a short delay possible (but I could be wrong).