Set Top box electronics query

[Blagard]

I have a set top box for freeview which has apparently died. Essentially no power. All fuses are OK (plug and internal fuse). So the next thing I did was to inspect the components for physical sign of failure. All capacitors look fine with absolutely no sign of swelling. The only thing I could see was some discolouration to the circuit board where three pins from a chip share a common connection to the transformer (bottom 3 of the four pin in the picture).

That discolouration suggests to me it is caused by heat.

The chip is marked

Q0370R
AVA08

I have found that the Q0370R is a Fairchild chip. Other than the apparent heat discolouration around the PCB pin holes there is no other visible sign of any issues.

So my question is could the chip have failed? - is it likely ?

Picture to help! View media item 56316

 

[Sam Gangee]

Any component can fail and it's certainly worrying that enough heat has been generated to brown the PCB. However that, in itself, is insufficient evidence. In addition, had the I.C. failed, I would have expected the fuse to melt. So first make sure there are no "dry" solder joints causing and open-circuit connection - especially on the power input socket.

Failing that - most likely, in my opinion, is that the circuit is simply not starting up. To begin with, I would replace C4 and also the very large capacitor, since those are the parts most likely to fail and prevent start-up. If that didn't help, I would measure each of the discrete components 0 diodes, resistors and capacitors. Having eliminated those, the remaining components (transformer and I.C.) are the remaining suspects.

What is the make and model number?

 

[Blagard]

Well thanks for the reply, after closely looking at the joints I did redo a few that had a slight dull look, but to no avail.

The make and model is Technika STBHDIS2010 It was cheap and cheerful!

I quite liked it as it also had a HD output with a HDMI outlet. There is a sign that a faint supply is getting through as the display shows hyphens faintly, but the unit is totally unresponsive. I have looked at it under a magnifying glass to see if I could spot anythng physically amiss. I'll have look at some second hand units as with oldTV's being replaced with built in freeview there should be a good supply of units no longer required!

 

[Sam Gangee]

Based on your new information I'll guess at a faulty electrolytic on the output side of the PSU. Measure the DC voltages. If they seem OK, check them with an oscilloscope to measure the level of ripple.

They will normally be 3.3v, 5.0v, 12.0v and maybe 26v but the latter (tuning voltage) is difficult to measure (microAmps) and won't be the cause anyway.

 

[kbdiy]

.... or alternatively just chuck it away and buy a new one. Hardly seems worth all the effort to repair a cheap Freeview box.

 

[Sam Gangee]

I'm certain that the OP is aware of that option but it's hardly in the spirit of looking after the environment, recycling, saving money or (more importantly, in my opinion) actually learning something and (just possibly) fixing it and getting that amazing feeling of having achieved something.

I encourage people to "have a go", provided that they can do so safely, without spending a lot and when the alternative is to trash the equipment. You'll still learn something even if you fail to get it to work.

 

[Blagard]

Yes Sam, quite so, I do like to have a go before chucking something away.

So far I have fixed a "dead" Panasonic VCR/DVD combo that was a simple capacitor replacement. Then followed similar when I had to replace a batch of six capacitors on my wifes PC which began to shut down on its own (motherboard). Also replaced the CCD on a couple of Canon Ixus digital camaras that went wrong a year to two ago (Thanks to U tube showing how to strip the cameras down!).

Alas I really don't know a thing about electronics so unless the fault is either physically visible (such as swollen capacitors) or has symptoms that indicate what the problem is (the camera's), I may be unable to fix. LOL I did manage to calculate the correct value of a resistor in series when making up a remote power switch with LED and various ports. Photo at bottom.

So with such DIY forums, they can be useful from time to time. I actually work in construction, so I do help others in that area from time to time. Also with safety in mind.

As a lad I used to build my own radio control gear from kits and my first set had a valve in it needing a pair of 67 1/2 volt batteries in series, so 135V DC and a 1.5V battery as well. Weighed a ton and gave one hell of a belt! Later I made up digital servo amplifiers packed with minature components before IC's came out to grossly reduced the component count!

I can get an identical replacement set top box with 12 month warranty for £25 - So that seems to be the best route for me now. Thanks for the help.

View media item 56340

 

[Sam Gangee]

I really don't know a thing about electronics

No problem. Start here: http://www.The-Cool-Book-shop.co.uk/begin.htm

 

[ericmark]

I do remember my dad when he bought a new TV removed the back noted every valve number and ordered a full set. When TV failed he would replace one at a time until it worked again.

However with the introduction of the switched mode power supply much of the DIY electronics became uneconomic. The only way to really test was to replace boards which cost a lot more than valves to keep in a box.

I have come across some poor construction methods a CAT electric forklift where the zenor diode was not one the main board so swapping boards could blow the new board.

I have swapped 741 op-amps on welding sets without knowing what was wrong they were so cheap and the replacement boards were so expensive it was worth trying swapping the op-amps first.

However there is some danger. The old TV could give quite a kick well after being turned off and the same applies to switch mode power supplies.

I remember carrying a radio transceiver from Falklands to UK and back without getting it repaired as no one would give a price and I had a limit. One return I though I would at least find which half of the radio was faulty by pairing each half with another set. It was then I found only problem was dirt on the multi pin connector just unplugged and reconnected and all worked.

 

[Sam Gangee]

Decades ago I was involved with the MOD and they produced Equipment Reliability Statistics, which proved that around 50% of all failures were caused by bad connections. In domestic equipment and most military this is easily solved by applying silicone grease to connections in order to exclude oxygen and moisture.

BBC engineering at one time was recommending it. British Telecom was more wary because silicone vapour can cause problems with open relay contacts in telephone exchanges. However, this problem is restricted to moving contacts where arcing occurs, which "bakes" the silicone to form a hard insulating layer. That's not going to happen in fixed contacts.

More eBooks about repairs here: http://www.The-Cool-Book-shop.co.uk/electronics.htm

 

[Blagard]

Here is an update.

After Sam's initial advice I figures maybe it is C4, as a small capacitor is unlikely to swell visibly compared to a larger one (surface to volume ratio)

So for the almighty sum of £1.18 (includes postage!) I bought ten new capacitors (minimum quantity). Changed C4 and now the box is working again.

Thanks

 

[Sam Gangee]

That's excellent news. We learned something and you got a cheap repair. Just bear in mind that, when one capacitor fails, the rest often follow. I always change them all.