Re: 2247A PSU Troubleshooting

Tony Fleming
 

Interesting problems! I assume you measured the caps before you installed
them. I have found couple of times that even good brands have sometimes a
bad one!
Hope you find the solution!

On Thu, Jul 18, 2019 at 7:25 PM Darren Ting via Groups.Io <darren_ting_hc=
yahoo.com@groups.io> wrote:

After google and found this link which having similar symptom for 2245a
[2247 using exact same PSU]

https://www.rc-cam.com/forum/index.php?/topic/4165-tektronix-2245a-oscilloscope-smps-repair-re-cap/

To eliminate any short at final stage output, I disconnect all diodes and
T2204 pin 4. Only CR2208 is connected to allow FAN to run. Even this test
does not solve the problems!!
The only different after disconnect diodes is now FAN run for 3-4 seconds
before stop. HISS sound getting louder.

Base on the links above, my case look like overload shutdown, but not the
same cause [diode shorted].

While measuring pin 12 using DMM, hear even louder HISS this time round
and fuse blow!!!

End up Q2201 Pin G-S shorted with 12ohn, D-S shorted with 400ohm.

I believe those HISS sound before fuse blow could be due to Q2201 having
leakage and overdrive R2201 0.1ohm with enough current to trigger overload
shutdown circuit.

Waiting for IRF830 to come and try see if this solve my problem. Even it
solve I still found this issue seem very tricky to pin point.







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