Centronics ZAT502HD ATSC Set Top Box

[mechguy]

I watch some basic TV using an ATSC tuner set top box which outputs HDMI to my LCD TV. The brand is Centronics, which seems like a fairly low cost (ie cheaply made) product. The UI is a bit crude looking compared to what I've seen on TVs that have the ATSC tuner built in (but it works!). Anyway, the set top box has always had an odd behaviour, which is that turning the device on using the remote would sometimes not "stick", that is, it would turn on momentarily and then immediately turn off. It didn't do this all the time, but it did this randomly since I've had it, which is 2009. Recently though, it started acting up and I had to repeatedly press the on button until it stayed on, taking 20-25 attempts. This all happened around last weekend, around May 11-13 2024. I did see in the news that there was high activity for solar flares and that this could cause issues with telecom/electronics etc. On the Saturday I figured maybe that's what it was, but then when it happened again the next day I decided to open up the case and have a look at the power board. See the attached photo. Nothing really seems out of place or damaged. I put everything back together, powered it up and had to press several times until it would stay on. I then tried again several hours later (turned on and stayed on right away), and then the following day (again no problems). So it's a bit of a mystery as to what the culprit might be.

Anyone experience this kind of behaviour with other devices? ie device turns itself off immediately after powering on?

[mechguy]

Well the "it fixed itself phase" was short lived. The set top box started exhibiting the same behaviour again. I just keep pressing the power button (either on the remote or on the front of the box) repeatedly until it stays on. Takes about 20 button presses before it holds and stays on. So it seems like some poor joint somewhere that is getting intermittent contact, but I took it apart again and looked at all 3 boards on both sides, and nothing stands out, I don't see any cracked solder joint etc. But looking at all the different sizes of capacitors on all the boards, getting replacements for all of them would be pretty much a waste of time since I could get a replacement ATSC set top box for very little money, so that's the route I'm going to take.

[lotas]

Try replacing this 33u with 47u...

[mechguy]

Sorry for the delay, I didn't get around to this until today... I didn't have any 47uF 50V on hand but I did have some 33uF 50V so since they are the same as the original, I thought I'd try that.
Result: no difference. I see a pattern in the behaviour when trying to power this set top box on. After about 9 or 10 times pressing the power button, the standby LED turns off for a split second, but then comes back on. (normally when this box is ON, the LED is off). So I keep pressing the button and after another 9-10 times, the LED blinks. Repeat this about 4 or 5 times in total, and then the standby LED stays off and the unit comes on and the TV tuner starts working. Then, if you turn it off, and turn it on again within a couple of minutes, it works right away. But a few hours later, you again have to press 40-50 times to get it to turn on.

After removing the old capacitor, I measured it as 35 uF and 2.2 ohm ESR. The new one was 34uF and 0.8 ohm ESR.

I found a sticker that was attached to one of the bigger capacitors, but pretty sure this is for the whole power board: sticker reads Sino-American SF410A-12, and some specs on input/output V and A. Some googling revealed that Sino-American makes all kinds of power supplies. Come to think of it, you can see this model number in the power that you edited

Is it worth replacing the 33uF again with a 47uF? Or is there another direction to take from here? What about the 2 big ones in the top right of your edited pic? Those are 200V 10uF

[lotas]

Yes, check these two large capacitors, they may also have a problem, although they look good, but the legs on the bottom sometimes rot... (this happened with the Samsung air conditioner, two capacitors are connected in series...) and it was also difficult to start it.

[mechguy]

Ok so I pulled the 2 200V 10uF caps, they measure OK. No sign of bulging, the legs looked fine.
One was 10.2uF ESR=2.8ohms, the other is 10.2uF ESR=3.9ohms.
These caps are marked JP, where the J and P are "combined" (the straight part of the J is the same as the straight part of the P)
other markings are CE-TKM, 105 C, 08-02

So what's the course of action, keep removing caps and testing them?

[mechguy]

My first question would be whether those ESR values seem high for the application. These caps are 200V rated so I take it they are seeing the line voltage on the primary side. So would ESR matter that much for those caps.

[mechguy]

Ok to close out the thread...
I pulled all the caps out and tested them, and since it wasn't actually as expensive as I thought it was going to be, I ordered replacements for all of them.

C3, C4: rated 10uF 200V; measured 10.2uF, ESR=2.8ohms; new caps: 10uF, ESR=1.0ohm
C8, C20, C21: rated 33uF 50V; measured 34uF, ESR=0.8ohms; new caps: 30.4uF ESR=0.6ohms
C9: rated 220uF 16V; measured 196uF ESR=0.9ohms; new cap: 193uF ESR=0.5ohms
C11: rated 56uF 16V; measured 51.4uF ESR=1.2ohms; new cap: 51.4uF ESR=0.47ohms
C14, C19: rated 680uF 10V; measured 787uF/675uF ESR=0.7ohms; new caps: 640uF ESR=0.2ohms
C16: rated 220uF 10V; measured 234.2uF ESR=0.44; new cap: 233uF ESR=0.3ohms

Capacitance didn't seem to be an issue, they all measured well within range of nominal values. The ESR values, the new ones all measured lower than the original ones but since this is a basic tester, I'm not counting on a great deal of accuracy there, but at least it allowed for a comparison.
The set top box works normally again, ie it powers on on the first try, no need to press the button 40 to 50 times.
In the end I don't know if it really was the capacitors or if there was a joint with a hairline crack causing intermittent issues, but I learned a lot about SMPS along the way. I didn't mess with the ICs, the optocoupler or the Y1 capacitor.