Hameg 312-8 spot problem

[GGMM]

Hello,

Instability of the X-Sweep  (moved 1 cm) adjusted by changing the entire THT regularization chain. (Transistor BF459, CI LM741, and resistance test).
However, there is still a problem to solve, we see the trace return. (No extinction of the spot).
Strange if I put a new opto CNY17-1 instead of the CNY17-2, no trace. The components are practically identical on the datasheet.
I’ll dig this circuitry, some resistors, two transistors and diodes.
An idea for measuring THT without a probe?

see documentation and schematic on PJ
cdt

[jonpaul]

most old 1980s..1990s Hameg CRT scopes are notorious for the marginal Z axis blanking circuit.

The weak points are the HF comp disc capacitors, stressé close to the rated 2kV.

Any leaking or surface contamination misbiases the optocoupler output transistor.

Replacing the 3 caps ( and sometimes the transistors) solves it.

The optocoupler is seldom at fault

Jon

[Kim Christensen]

And what are their designators? IC501, T514, T515 are my guesses for the "THT regularization chain" components you replaced.

Perhaps you could confirm which these components are?  Is the CNY17-1 IC502?

What is THT? The 2000V line? Yea, you'll need a high voltage probe to measure that.

Strange if I put a new opto CNY17-1 instead of the CNY17-2, no trace. The components are practically identical on the datasheet.

Assuming that the CNY17-1 we are talking about is IC502:
The difference between the two is that the CNY17-1 has less gain (Current Transfer Ratio) than the CNY17-2. Gain is fairly critical for the circuit and they even have a trimmer pot to compensate for variations in gain. Try adjusting R592, but it may not be enough to compensate. So you may have made things worse by swapping it out. You could bench test the old one and put it back if it checks out.

[Greybeard]

Replace the 2kV 68nF caps (better use 3kV types):
https://www.eevblog.com/forum/repair/(solved)-blanking-issue-on-hameg-203-7-oscilloscope/msg3891608/#msg3891608

[Greybeard]

BTW:
What's the correct term, "blanking" or "unblanking"?

[GGMM]

Hello,

Thank you barbe Grise for your link.
The first picture of this post is identical with the problem I have.
If I inject a sinusoidal signal I actually have a small bright spot and a vertical bar of the signal height on the left of X.
I will look at these 3 condensators(C545, 546, 547)  this afternoon. For the opto, no worries, it is on support and I had put the old one back.
I think have "blue" capacitor 2KV. (Or look on old TV board i have?)

My multimeter is a 10Mohm of impedance.
so:
To measure the high voltage, I think to make a dividing bridge with a set of resistances of 10Mhom in serie (9*10mohm)
Hence measure v*10/100 volts ohm law v= V*r1/(r1+r2)
With care. (As a temporary probe)
The time of measurement, resistances should hold.

cdt

[GGMM]

helo,

I did not find a 68 ppf 2kv capacitor in my reserves.... But when I was a student, I found a hameg carcass with no Y amplifiers and no power card and part X.
we had patience to realize all this.
...40 years later, I find this and at least I have a 68pf 3kv capacitor to test.
This  oscilloscope  of my construction worked perfectly before I immediately released a new acquisition.
So destruction of it...    LOL
Notice that there was no opto, but made with a transmitter diode and a phototransistor.
cdt

[GGMM]

hello,

I started with a test and changed C545  >> Nok
I then changed the optocoupler and it’s fine  >> OK
So I will order at least 3 * 68pf 2KV capacitors
Thank you all for your help.
Now I have a YOKOGAWA dl1640 to see, but not yet received the memory's.
Gone on a new adventure...
cdt