Re: 485 super weak brightness control


Ondrej Pavelka
 

So..... I changed following diodes for BAV21 and there was no change to the
behaviour.
CR1651
CR1646
CR1664
CR1666
CR1660
CR1656
CR1652

No intensity control and no grid bias control. I was desperate but then I
noticed there's another one hiding covered by a cap CR1663 and that was the
one!! Now I have nice sharp trace and working intensity control!

I need to calibrate it, I can get good image but as soon as I change either
of the intensity controls it gets blurry.


Now with the HV supply sorted I can progress further with the list of
problems

Vertical amp Channel 1 is in top condition all LEDs working lightbulb
working
Vertical amp channel 2 has the 1x LED gone and lightbulb gone. Otherwise
cleaning brought it back to 100% working order.

Horizontal A sweep

Works flawlessly from 5second to about 10uS. From 5uS the image starts ever
so slightly moving to the right but I still get enough to fill the full
screen when I use the horizontal position control because there is a
reserve, at 10nS I pretty much run out of the reserve. I can still center
it to have full 10 divisions but there is no reserve at all.
The 5 2 and 1nS settings are not working at all.



Horizontal B sweep.

I don't get any B sweep from 5s to 20ms then I get good B sweep for 10mS to
0.2uS
0.1uS to 10nS is showing only first 4 divisions from the left
5 2 and 1 ns is working correctly.

The delay time position potenciometer is not in best shape, it gets stuck
quite often and needs to be wiggled about to move it further. The latching
is somehow damaged.


Power supply fan.

Is quite picky, it sometimes runs and sometimes it does not.

On Wed, 17 Mar 2021, 19:17 Ozan, <ozan_g@erdogan.us> wrote:


Trust me it's the phones auto correct turning opamp into obama :)
I know I was just kidding. I also had my share of funny auto correct
stories.

I think I had the timebase measurements AC coupled and there was no
change
in level. I haven't checked the DC. I will do that tomorrow when I'm back
at the workshop.
The goal is to find if sweep trigger is not coming (no gate) or sweep
trigger comes but sweep doesn't happen. DC levels are important to
understand the state of the sweep circuit.

At the end most likely it will be a dirty switch. You can make a good
guess which one by looking sheet <10> and see which switches only change in
the last three settings. However, we can also pinpoint which one by
electrical debug.

BAV21 looks OK as a replacement, it has better specs in some areas. The
swing at the restorer input is ~ 100Vpp max, so the diodes in the grid bias
area will not see more than 200V. Close to max spec but could be a good
initial debug step. If others see a problem please chime in.

Ozan






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