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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

I have identified that Q913 is short circuit between E,B&C and I also
suspect Q923, but until I get them out that's as far as I've got.

Shorted Q913 could cause what you saw. However, did you measure it in place or after disconnecting from the circuit?

Someone suggested I check C914 - I know its 15uF but do you know the
rating ??

All component information is in the service manual. Section 8-9 (Page 113 in the PDF I am looking at) shows 15uF, 20%, 63V. I don't see why it would cause this issue after a knock but now all the components (including caps) did exceed their expected voltage by a large margin so they could be damaged. For example C971 is rated 50V but was stressed to 63V. Some components will survive brief over voltage, some may be damaged. In particular ICs may go into "latch up" and draw excessive current.

Ozan


Ozan

Hi David,

here is the link to the original document made by Hakan (aka Zenith):



Hope this helps,

Leo

I had my first experience with letting the magic smoke out of something
tonight.

The new filter caps finally arrived from Mouser and I replaced C806. I then
put the power supply partly back into the instrument. I didn't bother to
connect the main interface board or the back panel inputs and outputs as I was
only interested in whether the power rails were good. I should have connected
even less and just examined the rails coming off the unregulated supply, but I
did connect up the power cables going to the LV regulator and the horizontal
board.

Most of the power rails were still off as I saw them last time: everyone other
than the +130 and +40 rails were high, +130 is basically spot on, and +40 is
still very low.

While I was probing the power supply however, I heard a clicking sound and
smelled smoke. One of the resistors on the horizontal board had burned up.
Checking the manual it's R578, which is a 1/2 W, 2.5 K ohms film resistor,
which I have replacements for in stock. I checked Q578 in the component
checker and it looks like it survived the experience, as did several other
transistors in the same area, so maybe I didn't kill things too badly.

Regardless, the failure to fix the low +40 V rail combined with burning up a
component on another board has taken the wind out of my sails for the evening.
Maybe I'm learning things, but this is NOT proving to be as enjoyable an
experience as working on the 475 and 475A had been.

One lesson I'm taking away from this is that I should not reconnect the power
supply to the rest of the machine until I am sure that I've fixed the parts
I'm trying to fix.

-- Jeff Dutky

Hi,

I have successfully retrofitted the 5S14N sampling plugin with readout
for use with the 54xx scopes, by basically recreating the 7S14 readout
board and wiring it in, thus creating a 5S14.

See the photos: /g/TekScopes/album?id=261631

If there is any interest, I'm willing to do a little description of
the project, and share the design files.
I have a couple of PCBs which I can give out for the price of the postage.
Mostly probably useful in the EU, elsewhere it will likely be cheaper
just to get them from China.

Let me know if anyone is interested, then I'll take the time in the
coming days to do a writeup on it, as while it is not a complex
project, there were some obstacles, which may be interesting to share.

Szabolcs