Hi,
After having repaired several scopes (not only Tek's, but other brands also) with this symptom they invariably had leaky DC restorer diodes. Testing these diodes for leakage must be done at high voltages, because at normal DVM or component tester applied voltages they usually seems to be ok, but this is not true. Since the main work is to reach and remove the diodes, I would replace all of them, period.
This also applies to the focus circuit DC restorer if one is used. The symptom there is lack of control over the focus.? The way the DC restorers circuit operates may cause stresses in these diodes that make them to fail. I don't know the exact explanation.
Regards,
Ignacio EB4APL
toggle quoted message
Show quoted text
El 16/03/2021 a las 23:33, Ondrej Pavelka escribi¨®:
Thanks so much for your reply, it makes it much easier to approach.
I cleaned up the HV test point and I get now some 2800V. What makes me
little bit nervous is I don't know if the cause was just the contact
cleaning or if something else have changed. Nevertheless it changed in the
right direction. I think the whole section under the plastic cover needs
cleaning it's covered in black dust.
I will de-solder the diodes in the DC restorer and measure leakage current.
The intensity is too bright
I'll get back to you when I follow up on the other measurements tomorrow.
The HV probe is labeled as HV-40 but can't find any manufacturer markings.
It's definitely from 80s or 70s
On Tue, 16 Mar 2021, 22:42 Ozan, <ozan_g@...> wrote:
I checked the DC recovery diodes in the HV supply as listed in other
thread
(all under the plastic cover and they are ok.
Any other suggestions?
Note that checking the leakage of those diodes is important, just a DVM
diode check doesn't discover problems.
It looks like the behavior of your 485 is different in different settings.
As the first issue are you debugging the intensity? Do you have too bright,
or too dim brightness, does intensity control make any difference?
HV supply voltage of 2000V is low. What is the impedance of your 1000x
probe? Looking at the schematic ¡°CATH REG¡± can only adjust ~ +/-100V so
this is outside adjustment range. Should be the first thing to fix since
everything else with intensity depends on it.
Feedback loop around U1624 uses +50V at the top of R1642B as the
reference. Is +50V correct?
If you look at pins 2 and 3 of U1624 what voltage do you measure (should
be essentially the same and close to 0V).
Once you fix the HV supply if intensity is still problematic:
There are multiple posts about DC restorer issues I recommend you search
"DC restorer" "Grid Bias" for useful information.
The ones I know well are: Topic #180156: Although different equipment,
this topic has some DC restorer debug. Topic #180235 is a 485 with
intensity control issues. In particular looking at the circuitry around
intensity control pots. If you review both chains from start there is some
relevant information.
For time base issues, in the problematic 3 fastest settings how does the
"A Sweep" and "A gate" signals look? They are easily available at the back
of the scope with a BNC connection.
Ozan
--
El software de antivirus Avast ha analizado este correo electr¨®nico en busca de virus.
