ViewSonic N3252W

I bet you're wrong on the capacitor issue. I replace so many caps at work I'm the 'cap guy' that gets most stuff working again for next to nothing. Hints: If a cap looks bad it probably is but if it looks OK it may actually be bad. Look for a brown 'mist' around a capacitor. Those must be removed, the acid mist cleaned off a a new cap installed. It will often have a 'fishy' odor. Without the mist it may still be bad and look fine. The high voltage (300-500Volt) caps rarely fail. The 22 -100uF low voltage caps frequently fail and the worst are surface mount 'lytics. I replace LOTS of those. Many times a 'lytic will have a ceramic in parallel. This might look OK on the tester (Cheapest I know of is the MAT Electronics MUL-3333 at $50) as the meter is seeing the ceramic but the 'lytic is bad. Only way to verify is to remove the cap and test it by itself. It will read better while its warm and as it cools will get worse. Rule of thumb: if there are a lot of the same value and some are bad, replace them all. Look for highest ripple current in the Digikey (and others) catalog. Often it will be a Panasonic FM or FC or Nichicon HL HM HN, HZ for through hole. I like Panasonic FK or FP for surface mount.

G=B2

I don't know anything related to this TV, but bad capacitors which show visible signs of defects are generally a result of only one type of fault (internal heating).

There are numerous other faults/states of decay, but ESR is most often mentioned.

ESR testing is a very important/critical parameter to test for particularly in the high frequency and ripple conditions of SMPS, but passing an ESR test alone, will not prove that an electrolytic capacitor is in good condition (without faults).

There are numerous websites with free schematics available, and many of them are routinely mentioned here in the SER newsgroup. You might get lucky finding a schematic or an entire service manual.

Your listed measurements do show +5V in standby, so your question is a bit confusing. I don't know if the PSU would/should provide other voltages in standby or with the power on.

-- Cheers, WB .............

Does anyone routinely add a small 12V fan/baffles/ducting in such circumstances? as well as changing caps

Any time I'm inside a case, I try to move electrolytics away from power resistors, heatsinks and other sources of heat, just for S&G.. it may help.

Because electrolytics are heated internally in most SMPS (secondary side) applications, I wouldn't think extra venting or supplemental cooling with a tiny fan would add much toward the caps' lifetimes.. they're just destined to fail.

When a fan is near a power transformer, for example, it may lower the surface temperature somewhat, which in turn "draws" more heat from the core, but the core remains substantially hotter than the outer surface.

Electrolytic caps are smaller, and aluminum outer shells, but there is often a (heat) insulating/blocking air layer between the can and the actual layered materials in the center. Air is an excellent heat insulator, so cooling the can wouldn't so much have a chilling effect on the core, IMO.

Perhaps cooling the copper traces on the PCB would be a more effective approach.. the internal aluminum foil ends are secured/in contact with the cap's copper leads, which are in turn soldered to the PCB foil, and copper is a very good conductor of heat, so the heat transfer could be somewhat significant. Definite maybe.

Unrelated drivel: I recently had a small accident with an RCA/China VR5220-A pocket digital voice recorder, where it slipped into the sink while the water was on, so I opened it to get all traces of the water out. I was just a little surprised to see 105 C rated electrolytics inside (all of 'em, and quite a few, too)

-- Cheers, WB .............

One tends to think of electrolytics as /inherently/ failure-prone. But has this always been true? Or are you thinking just of switching and/or "tightly packed" power supplies?

I've been buying electronic equipment, used and (mostly) new for 45 years, and very, very few items have had bad 'lytics, or 'lytics that turned bad. I remember a used KLH compact stereo that needed new caps, and two JVC hall synthesizers that needed replacement PS caps. (The latter had been built with known-bad caps.) And that's about it.

I owned several KLH Model Eight table radios (circa 1960), and none ever needed a new 'lytic.

On a vaguely related note... It's nice to see that more and more products are coming with switching wall warts.

Perhaps the manufacturer used premium caps in all its products, simply to avoid multiple inventory.

Most of that equipment had a better grade of electrolytics. The imported stuff is crammed into tiny containers which can't disiapate much heat. Low ESR caps in switching power supplies have to be very close to both the recitifers, and the load, so they are in a hotspot, which further reduces the reliability. Some CPUs draw over 100 A at less than 2 Volts. If the CPU power supply was on a corner of the motherboard, the voltage drop would waste a lot of power, and create more waste heat.

Sprague, Mallory ore CDE electrolytics? Maybe Sangamo.

Except when they radiate a lot of RF noise.

Or they are cheap, no name Chinese fakes.

The switch mode SMPS power supplies are running at high frequencies for efficiency, and by design. The high frequency is vaguely similar to an ultrasonic cleaner, or an ultrasonic muscle therapy unit.. if the muscle therapy is set too high, or is allowed to sit on one spot, damage to the muscle tissue can occur.

The electrolyte in the caps is going thru much the same treatment.. heated by high freq and relatively high loads, internal heating results, and electrolyte will likely be vented gradually until the cap eventually avalanches into a rapid failure mode.

In addition to the severe duty they undergo, manufacturers want cheap PSUs, and low quality caps are cheaper.

It's really a good feeling to have an old piece of equipment that still operates fairly well, and hasn't been repaired.. or barely needed a minor repair (dial lights). I especially like to see a video display on an old TV or monitor.. it just amuses me, and then I shake my head, thinking of today's equipment. It's been a while since parents routinely gave their used still-working well TV to the kids or grandkids when they buy a big new one.. but TVs were about as durable as furniture. Many of them required 2 adults to move them, and they used a lot of power.. I remember the last round-screen console color TV I saw, was rated at 300 or 350W consumption. The power transformer in that TV probably weighed as much as a 17" LCD monitor does now.

Capacitors that are subjected to 60Hz line frequency are less likely to be operating on the edge of destruction. It seems that as long as there is a small amount of moisture in the can, they function as intended. Not only that, but capacitors of yesteryear were huge, and far less susceptable to damage from moderate loads. Old aluminum can electrolytics were often marked with nominal peak current ratings.

I would expect that you are correct regarding the common stocking of the 105 C rated caps found in a 3V device.

-- Cheers, WB .............

Think they always were a weak link if used in arduous conditions. Things like WW2 and after military portable comms radios. Overhaul consisted of replacing all the electrolytics regardless.

Actually the measurements I am concerned about were: CN102 On (Standby)

I would have expected the values of these lines to be closer to 5V. The more I read up on this TV, it might be a problem with Q7 and ZD4. Does anybody have any actual experience with this TV?

Spare electrolytics were reformed at a set interval in the US military. I've seen 30 year old Sprague electrolytic capacitors that were still perfect.

The voltage that you see at 3.74 is almost certainly the 3.3v standby supply for the system control micro. I would NOT under any circumstances, hit this rail with 5v. Although it sounds a little high, such rails often are a few decimals above the 3.3v nominal value. The 23.75v is the 24v nominal supply for the backlight inverter. Other 12's and 5's are the general supplies for the analogue and digital circuitry.

Whilst it is true that electrolytics do cause a lot of problems in switchmode power supplies, I have never been an advocate of just blanket changing them for the hell of it. It is easy - particularly if you are not highly skilled with a soldering iron of the right power rating, and with the right size and shape of tip, to create more problems than you think you are fixing. By all means, suspect electrolytics - they can be responsible for some bizarre fault symptoms, although I have to say that this one doesn't 'feel' like an electrolytic problem - but do the proper checks on them first - that is initially, an ESR check, which in my experience, will find over 90% of bad caps, followed by looking for hash on the rails with a 'scope. This will find any that have gone low value without their ESR changing significantly (rare, but it does happen. Much more usually, the other way round) without having to remove them from the board to check with a capacitance meter.

Before going any further, I would also check that you haven't got any problems with any of the user switches sticking or leaking, particularly if this TV is used anywhere moist or greasy like a kitchen. I have a 'cheapo' LCD in my kitchen, and from time to time, the volume 'UP' button gets leaky, and the volume then goes up and up, until you rush over and switch off. When it's doing it, a couple of buttons still work, but most are locked out. How the micro responds to this type of fault condition, depends a lot on whether it is a 'straight' xxx by yyy switch matrix connected to a pair of digital ports, or a series switch - R - switch setup, connected to an ADC input on the micro. With this type of arrangement, I have also had leaky decoupling caps along the chain, causing the micro to sense erroneous 'button push' conditions. Good luck with it. Try to use a structured and disciplined approach to troubleshooting the problem, and try to avoid shotgunning component groups, unless you are confident that you have one or more problems among them, and are totally confident in your soldering abilities.

Arfa

Very true, which has me wondering. I've been watching these "Nano" scopes on Ebay - see

While pretty much useless for most tasks given their limited bandwidth and sampling rate, I wonder if they'd be good for checking ripple across electrolytics? Since they're battery powered, you could use them in hot circuits. Has anyone used one of these scopes for this?

It says the input is DC coupled. You need AC coupling, or a very wide adjustment of the vertical position to see just the ripple.

True, that would be best, but even DC coupled, looking across a filter cap that would normally have 50VDC across it, it would be fairly easy to see 5 volts of ripple. Would probably be trivial to modify the scope or probe and put a small ceramic cap inline.

A small ceramic capacitor? For a scope with an attached probe and no publihed input impedance?