Electronics > Repair

Repairing bad power supply for Tektronix 212 vintage analog oscilloscope, need h

(1/3) > >>

SittingBear:
Hello EEVBlog Repair Forums,
The issue for my oscilloscope is the 0.4A fuse blows the instant I connect to AC mains. The physical power switch is set to "OFF". There is no obvious visible damage. All diodes and bridge rectifiers are OK. Forward voltage of 0.55V measured with multimeter across diodes and bridge rectifier. Outer 2 bridge pins are 1.1V. No capacitors read 0-Ohms nor do I get a low ohm-reading from Positive output to Negative outputs; typically greater than 1Meg-ohms in either polarity with multimeter across positive and negative supply rails and AC mains input.

Here is the
--- End quote ---
of the power supply circuit. I also took the liberty of photographing the circuit board in question. It is completely disconnected from all others as to avoid partial damages of the digital components in the repair process - the problem persists even when disconnected.


--- End quote ---


--- End quote ---


I've been skimming through the Tektronix Oscilloscope Troubleshooting Guide to find a few possible issues, though I have yet to find the root of the problem.

Any help is greatly appreciated!

Vito_R:
Recheck all rectifier diodes makes sure they are also not leaky when you reverse the leads.  If you get a reading desolder one leg and test again.  If you are sure then the next thing I would check are all those disc capacitors and tantalum caps known to sometimes short when then go bad, and anything across the mains that could short out your circuit before the switch.

Vito_R:
Looking at your pics again I would also clean off those terminal pins on the board labeled P4  P6. Lot's of corrosion and looks like some kind of arcing or short occured between 2 terminals? Doesn't hurt to clean it all off and recheck.

WarSim:
If the power switch is off all it can be is the filter, snubber or bridge.  The arc over point could defiantly be a symptom.  Given the arc is between P4-2 and P6-1.  I would have to suspect the filter and snubber.  Both a meant to control voltage spikes at P6-1. 


Sent from my iPad using Tapatalk

tautech:
Parts of the PCB are a mess as far as mains is concerned.
Firstly it needs a good clean on the top side.
The green/blue around some connections and pads is corrosion ie. salts that are very conductive at mains potential.
The arcing could be a result of back EMF from the common mode choke when the fuse blows.
Make up a "dim bulb tester" to limit the energy on power on.
Remove the bridge and double check it for correct operation, maybe with a small AC wallwart.
I can't see a reason why it could not be powered with bridge removed for testing.
Other suspects are shorted caps and a shorted common mode choke.

When the repair is completed it would be wise to use a PCB lacquer.

I also note a few Tantalum bead caps that give me the heebee geebees although they are after the switch.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version