0
\$\begingroup\$

While troubleshooting why an older LCD TV (Viewsonic N3751w) wouldn't turn on, I noticed several capacitors on the power board were swollen. Through my friend Google, I found that on this particular power supply (FSP271-5F01), it was common for the capacitors to go bad. I found a replacement kit, ordered it, and replaced the bad capacitors. However, the TV still did not turn on.

Now here is the part I need help with, and I'm hoping this is the correct forum to ask the question in. While troubleshooting further, with the TV plugged in, I touched the voltage regulator heat-sink to see if there was heat being produced, and received a shock. Not a minor one, but a pretty decent, better than a shot of espresso type shock. I happened to be touching the metal frame at the same time.

I measured the voltage at 86v AC +/-. I measured the voltage in AC because the shock had the back and forth feel of an AC shock, versus the continuous muscle spasm of a DC shock. I am fairly certain there shouldn't be this kind of voltage present here, but as I only have a limited knowledge of circuit boards, was wondering if this is normal, and if not, where should I be looking for the bad component?

Included is a picture I downloaded of the power supply. The red ovals indicate the areas where I get the voltage readings. There is a cover that connects the two heat-sinks. FSP271-5F01

|improve this question
\$\endgroup\$
  • 5
    \$\begingroup\$ It's perfectly normal for parts of power supplies, including heatsinks, to be live. Unless you have a death wish don't touch anything on a live system, ever. \$\endgroup\$ – Finbarr Dec 3 '17 at 20:33
  • 3
    \$\begingroup\$ Once you take the cover off ASSUME evreything is live until you have checked.... \$\endgroup\$ – Solar Mike Dec 3 '17 at 21:01
1
\$\begingroup\$

The whole thing is inside an enclosure, so the end user isn't supposed to touch the heat sink...

Sure, the manufacturer could have added some insulating silpad between the metal back of the power devices (which is connected to some voltage which will shock you) and the heatsink but...

It would have cost 5 cents extra!

Can't have that. Also, omitting the insulator gives better thermal transfer which results in a smaller heatsink. Thus it saves more than a few cents...

If there are 3 devices and they need to be insulated from each other, then you only need 2 silpads... and the third device just gets grease and makes the heat sink live.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ Ok, so while this may not be ideal, it's not outside of the realm of possibilities, and does not necessarily indicate an issue. Would that be a correct statement? \$\endgroup\$ – Scooby Dec 3 '17 at 20:09
  • 2
    \$\begingroup\$ In my experience it is actually fairly common for heat sinks to be live, particularly in TVs. Usually there is some kind of warning on or near it though. \$\endgroup\$ – JCollins Dec 3 '17 at 20:12
  • 2
    \$\begingroup\$ "Sure, the manufacturer could have added some insulating silpad ... but it would have cost 5 cents extra!" - And it would have worse heat transfer properties. So, more expensive and worse. \$\endgroup\$ – marcelm Dec 3 '17 at 20:34
  • 1
    \$\begingroup\$ Yes, you can look at the thermal pads, if they are all insulating silpads and the heatsink is live, there is a problem. However if at least one power device only has grease and nothing else, then the heat sink will be intended to be live. It's pretty common, because omitting the insulator is both cheaper and has lower thermal resistance. \$\endgroup\$ – peufeu Dec 3 '17 at 20:35
  • \$\begingroup\$ Yes, there will be a warning on the case, "Do not open, no user serviceable parts inside" or words to that effect. Saves having to label every single thing that's potentially dangerous. \$\endgroup\$ – Finbarr Dec 3 '17 at 20:42
0
\$\begingroup\$

The heat sink is electrically "hot" because it is connected to the negative out pin of the bridge rectifier BD1. Notice the ground pin from the input cord goes no where! It ends at CN1. Ground is however connected to the chassis. The transistors Q7 and Q4 do not have thermal insulators because the are "Full Packs" The metal substrate is insulated from the back with the same plastic you see on the front. I have tested these thermally and they have BETTER heat transfer to the aluminum heat sink than any other thermal insulator I have tested. Like you, I replaced all the physically damaged capacitors, with out a positive outcome. I tested most components with a DMM both resistance and diode modes. The only thing odd was under Q4, ZD1 a zener diode of unknown value and unreadable marking appears shorted. The melf package is difficult to remove without proper equipment, so that is where I stand now. It would be helpful to have a schematic, but I had no luck for this board. REP

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ I believe these power supplies are often two-layer or even single-layer boards, in which case you can draw your own schematic. \$\endgroup\$ – user253751 Mar 8 '18 at 22:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.