Detecting alternating current in my PSU heat sink (leak?)

[Gary Tse]

Hi all,

My Samsung TV have flashing problem. I opened it up and found some bulging capacitors which I replaced.
Because the capacitors were bulging, I suspect a problem in the PSU.

Here's a picture of a normal PSU board (can search by BN44-00157A):
http://www.ebay.com/itm/PSU-POWER-S...560278?hash=item3ac106ef16:g:hyUAAOSw8RJXBYOf

After replacing the bad capacitors, the flashing problem still exists; so I looked for other issues.

Then I stumble upon an AC current in the heat sink (left most in that picture) !!! The current was 0.7-1.1 volts! Is this even normal? or does it mean the component attached to those heat sinks are broken/ leaking? I was testing that by attaching the red probe to the heat sink, and the black probe at the ground casing (TV was turned on obviously).

The heat sink with this problem is attached to a bridging rectifier, here's the spec of it :

Glass Passivated Bridge Rectifier, TS15P01G, Peak Reverse Voltage: 600 V, Vf - Forward Voltage: 1.1 V
http://www.mouser.com/ProductDetail/Taiwan-Semiconductor/TS15P05G/?qs=tHU/lV7kTySjz0UsGI/2gg==

My multimeter only has Volts/ Current/ Resistance test (no diode test) so I was not able to test the bridge rectifier properly. I did a resistance test on the +, -, and AC nodes and the resistance looked "OK". Does a current in the heat sink indicate a problem with the bridge rectifier?

Thanks in advance,
Gary

 

[dorke]

Welcome to EP. Gary

From the way you speak of current in volts it is obvious you don't have a lot of experience,do you?
I would like to worn you of the high voltages found in the area you probed ,400VDC is very common.
Take great care!


Which capacitors have you replaced and with what kind have you replaced them?

About your question regarding the bridge :
it could be defective ,but more likely the low voltage you measured on the heat-sink comes from the way it is connected on the board.

 

[Gary Tse]

Hi Dorke,

Thanks for the advise; it's my first time replacing these caps. I've replaced these capacitors:--
2x35v 1500uF (1 of which confirmed blown - bulging)
2x35v 1000uF (1 of which confirmed blown - bulging)
3x25v 1000uF
1x10v 2200uF

As for the others, I replaced them because I didn't know better and bought many spares. When I replaced them I made sure they were the same working voltage and the same capacitance. That's almost all of capacitors in that board; except the 450v 220uF and some really minor ones which I haven't touch yet. The voltage at the heat sink fluctuates between 0.7-1.1volts (AC) so I was wondering if that was the source of the problem.

Anyways, any suggestions for a decent multi-reader brand? I want to get one with capacitance & diode test so I don't have to replace the caps like an idiot. Hopefully one that does the job without making a big dent in the wallet

Cheers,
Gary

PS. The TV works for the first 5-10mins, then starts to "strobe" or "flash". I'm thinking at least my replacement caps did not make the TV get any worse than before. I wonder if heat's got anything to do with it? if anyone has better ideas feel free.

 

[duke37]

You can do a diode test with the resistance range.

If the heat sink is floating (no ground connection) then it will have a voltage due to capacitance.

If the TV works at all, then the bridge rectifier is OK.

 

[dorke]

I agree with @duke37 about the bridge being ok if the TV works.

Please mark the Caps replaced on the board itself.

 

[Gary Tse]

1. 35v 1500uF (replaced)
2. 35v 1500uF (replaced, blown)
3. 35v 1000uF (replaced)
4. 35v 1000uF (replaced)
5. 25v 1000uF (replaced)
6. 10v 2200uF (replaced)
7. 25v 470uF (replaced)
8. 25v 1000uF (replaced, blown)
9. 25v 820uF (replaced)
10. 450v 220uF (not replaced)

I don't know if the heat sink is floating or grounded. Kind of hard to trace because the heat sink is mounted through the board with solder; and I don't know where that line leads to.

 

[dorke]

Check these things.
1.Resistance between the heat sink and the 3 noted points.
2.The voltages should be stable,see if they drop on the flicker.

 

[(*steve*)]

If the caps you used were not low ESR, then their life could be very short. The next time they fail, replace them with the correct type.

if they don't fail then don't replace them

A quick check is to look for the temperature rating of the cap. If it is 85C then the cap is probably not a low ESR type. 105C does not guarantee it is low ESR though!

 

[dorke]

If the caps you used were not low ESR, then their life could be very short. The next time they fail, replace them with the correct type.

if they don't fail then don't replace them

A quick check is to look for the temperature rating of the cap. If it is 85C then the cap is probably not a low ESR type. 105C does not guarantee it is low ESR though!

About the ESR issue of caps.
That applies only to the caps. working in the fast switching parts of the SMPS(flyback).
On this specific board there are many caps. which need not necessarily be low ESR ,as they are filter caps on the linear regulators.

 

[Gary Tse]

Got it; I'll try to get back later today/ tomorrow.

 

[Gary Tse]

ok, finally got round to do the measurements:


I put the row (+ probe) to column (- probe) measurements in below table. e.g. Row 1, the diode at location 1 is 25volts to the ground at 12. Some locations are just too hard to reach; especially when the TV's running. You were right about the heat sinks -- turns out the resistance from mains to heat sink is very, very low.

Not sure there's much to go on here; I don't even know if it's a problem with the PSU or other problem elsewhere in the TV anymore.

 

[Gary Tse]

Thanks for all the feedbacks; I've learned a lot from all of you.

As for low ESR caps -- When I replaced them I used the Panasonic FR/ FM series and aim for high endurance. I don't know how to interpret the ripple current thing anyway.

FR series
https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/ABA0000C618.pdf

FM series
https://industrial.panasonic.com/content/data/CP/PDF/Alumi/rajiaru_fma_e.pdf