switch mode power supply repair Tektronix tds460a

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Hello everyone, I am trying to repair this power supply. I have already replaced the two large smelly bulk capacitors. Also there was corrosion on the heatsinked diodes and the solder side of the board and ran . When I first powered up the scope when I got it worked like normal for about 5-10 minutes then suddenly shut off and never worked after that. I tried this supply back in the scope after I replaced the parts and still nothing.
I did hear a really faint clicking sound from coming from the board almost like a relay. I also hooked this up to a variac out of the chassis for isolation and powered it up and hear the same clicking sound. I measured across about 165v dc at both the bulk caps.
Here are some some pics
 

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Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Oh I forgot to mention i replaced 10 of the Shockley diodes and ran two jumpers to make up for the corroded traces on the solder side. Could I take the voltage pin measurements labeled on the large transformer with the black lead of my meter on earth ground.
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
I went back and hooked up my variac and got the following

pins 16-20(+5.1V)= bouncing around back and fourth to OL

pins 8-9(+15V)= same

pins 6-7(-6.4V)= bouncing around pos and neg

pins 1-2(-15V)= same

Neutral to earth= 76V AC
Neutral to Hot= 125V DC
Bulk Cap 1= 165V DC across
Bulk Cap 2= 165V DC across
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
I wonder if I should have a load placed across some of the pins to see if the values would change. Also, maybe the values that I measured that were jumping around could be some stray current being generated by the coil in the relay. Also I can’t seem to find any info on the web about the rectangular black box thing??
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Ok I think I know what the clicking sound is. It’s the overvoltage protection circuit being engaged. I hear dried up caps could cause this
 

MrChips

Joined Oct 2, 2009
23,524
What you are seeing on the scope is 60Hz line frequency.
This is quite common and typically what you would see if the grounds were not connected properly.

Caution: You need to know where to connect the ground otherwise you run the risk of releasing the magic smoke, blow up your nice oscilloscope and bring your life to a sudden abrupt end.
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
This is always has been so confusing to me!! about electronics is the floating, grounds ,etc. Is there any other way I could manage for now I really want to keep testing for now. I’ll be really careful!!
 

MrChips

Joined Oct 2, 2009
23,524
A Variac® is for controlling the AC supply voltage from 0% to 110%. It is called an auto-transformer and it consists of a single winding.

It is not isolated from AC mains.
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Ok I think I figured it out. A autotransformer uses the same winding between the primary and secondary. A isolation transformer connects the primary to the secondary only thru the transformer core.

Could I verify this with my meter
 

MrChips

Joined Oct 2, 2009
23,524
Ok I think I figured it out. A autotransformer uses the same winding between the primary and secondary. A isolation transformer connects the primary to the secondary only thru the transformer core.
Let's get the terminology correct.

An auto-transformer does not have a primary and a secondary winding. There is a single winding (which you could call the primary winding). The output is tapped off from the single winding.

1613249486700.png



1613249598739.png
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Alright I’ll just poke around with my 60 dollar meter for now. I’m on this thread to try figure out what’s wrong with my power supply. It seems I’m only getting ac voltage on the output from all my testing so far
 

MrChips

Joined Oct 2, 2009
23,524
Alright I’ll just poke around with my 60 dollar meter for now. I’m on this thread to try figure out what’s wrong with my power supply. It seems I’m only getting ac voltage on the output from all my testing so far
First of all, remove the variac because it is giving you a false sense of security.
Next, tell us how the scope and the dead PSU are plugged into AC mains.
 

Thread Starter

Wasoofa82

Joined Jan 24, 2021
41
Maybe I could feed some small signal dc voltage from my dc bench supply into the dc output to see if the feedback circuit is working

I have it plugged in this way directly with a power cord I soldered in. Here is the orange connector that plugs into the supply inside the scope chassis

This way I don’t have to plug the supply at all into the wall
 

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