Hey Rafael,
I second Jared's advice; you'll probably want to have another look at the
PSU. The standby supply in these scopes is notorious for giving up, leading
to dead or hiccuping supply. AFAIK there are two main versions of the
standby supply, the earlier "discrete" one and the later "integrated" one.
You'll find schematics for the "discrete" one here:
(PS).pdf (thanks
H?kan). The later version has an all-in-one switching regulator, which
you'll find in the TDS520B component service manual. This is available from
Tek (as well as from TekWiki, I believe).
The regulation loop in the "discrete" standalone supply is non-obvious (to
me) so I simulated it, and found that a high-ESR C17 wrecks it, though I
don't remember the details beyond that. ISTR that the TVS diodes VR4/VR5
and Q2 get stressed when C17 is high-ESR, so you may want to replace or
check the TVS diodes at least. If they fail open, Q2 will die after a short
while. I should note that without e.g. a curve tracer that goes to >250V,
it's not easy to test the TVS diodes, but maybe you have a suitable curve
tracer at hand - you wouldn't be the only member of this group so equipped
:).
I think you'll also want to look for leaking electrolytic SMD capacitors
elsewhere in the scope. I don't know whether the capacitor plague affects
the TDS744A in particular, but it's wrecked many otherwise good TDS scopes.
Siggi
On Thu, Nov 14, 2024 at 7:57?PM rafael suarez via groups.io <rafik69pe=
yahoo.com@groups.io> wrote:
Good afternoon, I am a humble electronics hobbyist with some knowledge. I
have repaired various household devices, but this time I decided to take on
repairing my TDS744A. I bought this scope online from Japan. When I first
turned it on, there was no image on the screen, though it did display
through the VGA output. A few days later, I turned it on again, and only
the front panel LEDs flickered.
I have recapped the entire CRT board following all recommendations, and I
also replaced the RIFA capacitors in the power supply. Unfortunately, the
issue persists. I don’t want to go any further on my own and am reaching
out to you for assistance. It would be very helpful if you could advise me
on where to look to resolve this issue.
I would be very grateful for any suggestions. I’m attaching a video
showing the issue. Thank you very much in advance.
