Power supply dead Humax Freeview F2-Fox-T board

[Martfoxy]

Whilst I was away from home on holiday our Humax Freeview box died - no standby LED or signs of life. I have looked at the power supply board and can't see anything obvious no bulging capacitors and tested diodes which are ok. Can't see a fuse on it even though it says fused. I tested the voltage across the big 68uF capacitor and it was over 300volts.

Can someone point me in the direction of further tests I can do to try and work out whats wrong with it?

 

[73's de Edd]

Sir Maddox . . . .

Your systems fuse was the black cylinder just beside the two pin Molex AC male connector
With 300v being present at the Maxton capacitor, it was good.
If that unit has had no power on it recently and it's charge has bled down, check / ohm out / the two nearby 47k yellow violet orange resistor for value.
Also the nearby 330k orange orange yellow
Also being suspect would be the very nearby black electrolytic cap, but would require test by replacing with another like unit, to test for SURE.

Should you want to skip that, initially, take note of the cluster of 8 E-caps at the end of the board and have the board powered up and do a DC voltage test across each cap to see if any voltages are being present.
Also tell us if they happened to have marked the boards end with any of the voltages of that 7 pin connector.

With your system having a SOFT failure ( no overload condition. . .blowing a fuse ), we can. HOPE for just a value drifted resistor, or an E-caps gradual decline of capacitance or a bad solder connection erosion.




73's de Edd

 

[Martfoxy]

Thanks for your helpful reply.


47K and 330K resistors test ok.


I am getting voltages across capacitors and there are voltages written on the 7 pin connector. Need to find my magnifying glass tonight at home.


I am wondering if the power supply is working ok – and there is a fault on the main logic board. The standby light doesn't even come on.


Thanks again


Martin Fox

 

[Martfoxy]

Voltages written on the 7 way plug are 3,3,G,7,5. (not sure why there are only 5 values - although there are two grounds on the plug). Voltages I measured are 3.3,3.3, G, 11,85, 11.85, G, 26.6.

 

[Martfoxy]

SIr Marty Fox . . . . . . .
(How did I initially come up with Maddox ? )

Looks like that power supply is condition . . .GO.
Next area of concern would be the area that it enters into the main board .


73's de Edd

 

[73's de Edd]

SIr Marty Fox . . . . . . .

Indeed . . . .how could you be so lucky, as to have a working unit AND a test bed to compare against !

TESTING:
Looks like the situation now, just might be the test methodology used.
If you just plopped two power supply's down, side by side, is there a possibility that
you were testing both WITHOUT them being plugged into the main unit

That would be the way that you would be wanting to test them . . . . .under their normal working load.

DETAILS:
Can you pass back the following info on the BAD unit ?
Looking at the E-cap cluster in the output power end of the board, there is a medium sized plastic power 3 terminal regulator.
I am expecting it to be a 7805 or somewhat in that numbering order, with the last two digits signifying its regulated output voltage.
Take an EXTREMELY close up and discriminatory look,at its three terminals in their solder blobs and see if possibly its center connector might be floating free in the solder blob.
Putting a finger wiggle on its case from the top to detect any movement / looseness at its terminal ends lets you see that.
However, a GOOD reflow soldering of all 3 joints, with fresh solder, puts our minds fully at ease.

Voltages:
You gave us the measured voltages and the unit had its printed voltages near the CON2 ?

Now it gave 3.3 as one voltage and that is a suspected norm and being the most demanded voltage of the
unit, with it even utilizing two supply routes branching off from the unit.
It is sourced from the power transformer down at the full wave Schottkey power rectifier D7 and passes up the closest electrolytic above it and then thru a series blue cased inductor and then to the second filter above it and then into the two co-joined connections at the end of CON2.

The next derived voltage will be from the central, cross mounted, 1N54XX sries of diode which is responsible for a raw DC voltage voltage which I am uncertain of that voltage, until you fill me in on the 3 term regulator, currently it leans towards being 12VDC..

At the extreme other end of the power transformer are two diodes mounted side by side, the innermost one is for the supply to the input of the 3 terminal regulator.

The last diode on the outer position seems to be giving the somewhat high voltage of 26 VDC which you were reading.on the outermost connection of CON2

With your forthcoming info of a loaded supply's voltage readings, should help us sort it out.

To date, suspicions seem to lean toward knowing that voltage output from the 3 terminal regulator and it also looks like one or more electrolytic's capacitances may have drastically lowered in value, and they thereby, can no longer provide that particular supply lines s full power demands

Standing by . . . . . .



73's de Edd

 

[Martfoxy]

I will go through this more thoroughly when I have some time but I measured the bad power supply on load and got:


2.9v

2.9v

8.75v

11.86v

35.3v (seems very high).


I did check the connections to the regulator with a magnifying glass and seem ok.


I haven’t checked through the other diodes etc yet will do when I have some more time.


I could compare the good power board loaded voltages as well.


I have got a capacitance meter so could check the capacitors.


Thanks again


Martin Fox

 

[73's de Edd]

Sir Martin . . . .


Appears that your 3.3 supply is pulling down a bit excessively.
You forgot to pass on the device I.D. of the 3 term regulator.

Seems like it just might be a 7805 to me.

Looks like the less demanded upon 12 dc supply might be normal.
That last " high voltage" supply level might be related to a supply voltage for an internal electronic tuner, but they usually get regulated on down to a rock steady, + 33v.

OR . . . it could be related to a supply for the units frontal display panel, but the norm is for that to be a negative supply voltage. The foil paths of both of the ground connectors, show them to be interconnected.

Now all of our questions should be abated a bit with the next test of . . . . .

RIPPLE TESTING . . .

( Pre reminder . . . .was it a 7805 ? )

Repeat your last TESTING procedure, but this time, do your metering in AC voltage mode.
If any of the filters have become LAZY ( decline in capacitance ) this test will cull them out.

They will be showing their degree of loss of effectiveness / decline by the degree of RIPPLE content that they are showing.

( Final chance prompt . . . . . . was it a 7805 ? )

Thasssssittt . . . . .


73's de Edd

 

[Martfoxy]

Got there eventually its a 7812.

The ac voltage is 0.02v on each pin on the connector under load.

Thanks

 

[73's de Edd]

Sir Martin . . . .

Well I certainly wasn't expecting that " zero" RIPPLE figure, unless the supply is so out of whack, that it's not even letting the main unit pull any power.

Since you DO have a capacitance meter the two related filters to the 3.3 v supply are being the ones just above and below the blue inductor. They will test as being paired, unless one is taken out of circuit.

The other filters being highly suspect, is the filter pair associated with the 12v supply, where you are. now only reading 8.5 vdc.

They should be to the right side of the 3.3 supply and there is being a reddish brown 22uh inductor in their supply line.

This fault should be filter capacitors, if the 7812 finally is able to come up to its full potential . . . . .remember, unloaded, it was reading up in that 12 vdc region.



73's de Edd

 

[73's de Edd]

Sir Martin . . . .

I plotted out these board foil / compared to components . . . . . layout as an assist.
To give us somewhat of an assurance of that mystery middle of connector voltage, would you measure
the connector voltages on a good board connected to a system.

I definitely have the 12V regulated DC terminal marked up now.
That other voltage that seems to be a raw 12VDC or 8.5 VDC might be further pulling down under load and actually be that 7VDC as is being marked on the PC board.

73's de Edd