Re: Tektronix 2215, pulsing/chirping noise, no trace.

Samuel Rey
 

Alright, I've tinkered with it some more and I feel like I am getting
close. I measured the caps and checked some for shorts (I straight up
removed the Rifa caps) and they seemed fine. The diodes also seem fine. I
decided to take a look at the Q933 MOSFET, and decided to check for a short
there, as I hadn't checked for a short after the rectifier. Lo and behold,
0 Ohms to ground at the source. Suspicious! I started checking all
components going to ground thru the FET, and none of them were shorted
after desoldering them. About halfway through this, though, I decided to
take a *short*cut. (Pardon me.) I fed in 5V limited to 1A with my PSU from
the source to ground, and checked which component got hot. And it was T933!
Unfortunately I ran out of time so I'll desolder and check the transformer
when I get back to the scope on Friday. I also suspect U920 might be dead,
so if I can buy a replacement at my local components store I'll get one
just in case.

Sam

El jue., 11 jul. 2019 a las 16:00, <tekscopegroup@...> escribió:

Most switching power supplies need a load on the output to be able to
operate normally, so just as a reference by testing the pre-reg section
with its output disconnected is most probably not a valid test. The full
wave rectifier that you mention deals only with the mains AC input to
convert it to DC for the operation of the switcher which then generates the
+43V, so I doubt that the problem is there as it would rather blow the main
fuse or would simply do nothing. But in any case have you checked the
voltage across the (big) 75uF input capacitor? If for any reason that
voltage is low or has a lot of ripple because that cap has aged and/or
dried out I would think this might prevent the pre-reg from starting. On
the output of the pre-reg there is only one rectifier diode that if I
remember correctly is labeled CR907. Check that one as well, it might be
leaky or open/shorted, and if in need of replacement has to be a high-speed
fast recovery rectifier diode.



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