Hi, WOuld some kind soul help me diagnose my switch mode power supplies?

Hi all,

I have three identical Headend UHF channelised amplifiers that all use identical switch mode power supplies. One is working, the other two are faulty but appear to have an identical fault as the output voltages are the same low values on both units.

So I've got my screwdrivers out and a multimeter. Upon taking the backs of all three units there is a 8 wire ribbon lead between the SMPS board and the main board, comprising of 1 black wire and 7 grey ones.

There is a 8 wire ribbon cable that connects the SMPS to the main board, which are the wire numbers in the middle of the image.

There are two columns of output voltages as tabulated below:

Probing for grounds and voltages gives me on both units:

Pin number Good unit 2 Bad Units pin 1 ground ground (this has zero resistance to the metal case) pin 2 ground ground (this has zero resistance to the metal case) pin 3 -3.42v 0.4v pin 4 0v 12.85v pin 5 8.73v 0.76v pin 6 18.32v 7.4v pin 7 0v 8.8v pin 8 0v 8.5v

There is approx 0.5Hz ripple on the secondary voltage outputs of the two bad units and the green power indicating LED can be seen to ripple with the naked eye..

None of the secondary side capacitors are bulging or have brown crud on them and I'm already aware of the bad caps issue, so I replaced all the secondary side electrolytics with Panasonic Low ESR FM series capacitors. This has not made any difference.

I have traced the circuit schematic from the transformer to the output pins of the SMPS board and the primary side from mains input to the transformer. I don't know the exact winding configuration of the transformer, and I have not yet traced the optoisolator board, but this is based on a 3 chip design, TL400, CNY17 and UC3842AD, One is an opto isolator, one appears to be an error amplifer/feedback chip and the third is a precision voltage reference.

I'd really appreciate some comment amongst any SMPS experts amongst you all as to where the problem lies so I can get the two faulty units repaired.

The Primary side is available on

formatting link
and the numbers on the right refer to the pins on the transformer primary

The secondary side is available on

formatting link
and the numbers on the left hand side refer to the order of connections on the transformer secondary

The whole of this side of the circuit is isolated from the primary side, except for the one feedback wire off pins 1&2 which go off to an optoisolator on a small daugherboard on the SMPS PCB.

Any comment?

Also there is a diode between two of the secondary windings that would seem to short ciruit the winding back out onto itself? I have checked my PCB tracing and its definately the case.

I'm of the view that the output of pin 4 should be 5 volts due to the LM7805, so why is the working unit producing 8.73V at this point?

Give the 2.2K series resistor for the LED, and assuming a 10mA current, its suggesting that the voltage coming out at this point should be 24.2 or is there something subtle I'm missing? Also there are noi marked values for any of the inductors.

Regards,

Stephen.

Reply to
Stephen
Loading thread data ...

"Stephen" skrev i en meddelelse news: snipped-for-privacy@brightview.co.uk...

What is these resistances: pin 1 to pin 2 pin 1 to pin 4 pin 1 to pin 7 pin 1 to pin 8

I think you have an issue with the ground/0V.

--
Uffe Bærentsen
Reply to
Uffe Bærentsen

Would this be with the ribbon cable disconnected between the main board and the PSU board?

If so which end should I measure?

Reply to
Stephen

With the ribbom cable disconnected, measuring at Main board end:

With the ribbom cable disconnected, measuring at PSU board end:

With the ribbom cable CONNECTED, measuring on the ribbon cable:

All of the above readings are not steady, so I geuss there are capacitors charging/discharging via the multimeter.

Reply to
Stephen

he

f

the

Units

=A0 =A0 =A0 =A0 =A0ground (this has zero

=A0 =A0 =A0 =A0 =A0ground (this has zero

=A0 =A0 =A0 =A0 =A0 =A0 =A00.4v

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A012.85v

=A0 =A0 =A0 =A0 =A0 =A0 =A00.76v

=A0 =A0 =A0 =A0 =A0 =A0 =A07.4v

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A08.8v

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 8.5v

wo

he

hem

ary

not

ns

r.

have

gn,

ce.

ll

n

seem

ts

any

Reply to
Wicaksono S.

Do you mean the 22uF 50v capacitor that is on the area next to the daugherboard? It is actually near to the connecting pins between the PSU board and the daughterboard.

reason I ask is that there are no electrolytics at all on the daughterboard which contains the UC384x, all there are is the three IC's and a load of surface mount components?

Regards

Stephen.

Reply to
Stephen

--
---
Kripton
Reply to
Kripton

formatting link
and

formatting link
and

you can make a quick esr analog meter to check all your caps sure the small chemical capacitor next to the uc3842 is dry ... my web site contains easy and more complex esr meter to build yourself regards,

formatting link

--
---
Kripton
Reply to
Kripton

UPDATE: I have re-measured thge output voltages on the good working unit.

So here are the correct values:

Also pins 7 & 8 are soldered together at the PSU end, pins 1 & 2 are soldered toegether at both the PSU and the main board.

The UC3842AD, TL400 and the CNY17 are on a small daughterboard, which also has a load of surface mount tantalum capacitors and resistors.

Asfar as electroyltics go, there is one on the main board, which is a capxon

22uF 50V job. is this likely to be the capacitor thats gone dry?

formatting link

formatting link

Reply to
Stephen

Yes. The small (> all three units there is a 8 wire ribbon lead between the SMPS board and

formatting link

formatting link

Reply to
PlainBill

OK, I'll get some 22uF 50V 105 degree C ones ordered in then Do they have to be low ESR? if so I'' go for the Panasonic FM series that I used to replace the secondary output electrolytics.

If a capacitor has gone dry, wouldn't there be a clue like bulging tops, leaking brown crud, and a horrible smell when they get hot?

Regards,

Stephen

formatting link

formatting link

Reply to
Stephen

Those symptoms occur some time *after* the cap's performance degrades to the point of compromising the PSU.

Agree with the others, the small cap on the UC3842 is the most likely candidate. Note that it *may* be a surface mount type and not an aluminium electrolytic.

Reply to
who where

you can make a quick esr analog meter to check all your caps the small capacitor next to the uc3842 aways fails to an esr meter even a cheap build from scratch analog one.

my web site contains easy and more complex esr meter to build yourself regards,

formatting link

--
---
Kripton
Reply to
Kripton

^^^^^^

Shoot on sight. Capxon makes crap. Also Lelon, Luxon, and GSC off the top of my head.

Reply to
JW

Wow.... What can I say......

I bought two new new 22µF 50V 105°C capacitors today, total cost 42 pence. (About 0.5? or 50 cents for the rest of the world)

took the first faulty unit, swapped over the 22µF 50V capxon, tested it and it worked.

I then took the 2nd faulty unit.... Swapped over the 22µF 50V capxon, tested it and it worked.

Thank you, Wicaksono, Kripton, Plain Bill and Who Where, you were all on the nail.....

As a token of gratitude, I think I should give some background to the story.....

I am in the reception area for 4 TV transmitters in four different directions and want to get TV channels from all four. Simple aerial group combiners with grouped Yagi aerials would not work as the frequencies used at each transmitter overlap at least one of the other 3. One way of solving this is to used a channelised head end UHF amplifier.

These units have 10 frequency agile filters, with independently adjustable gain, and can accommodate up to 3 UHF aerials as well as an FM radio aerial and a Band III aerial.

These units are still available for £400 each ( around ?400 or $400 each.) via wholesalers.

So I bought 2 off Ebay complete with a programmer. One worked, the other didn't. After haggling with the seller, I got half the money back and was allowed to keep the faulty unit, so it cost me £100 all told.

I kept an eye for more on Ebay. Another one came up, I was the only bidder and bought it for £5 (about ?5 or $5) This was too also faulty with an identical fault. I didn't raise a dispute given it was beer money, and if the worst came to the worst, it could be cannabilised for spare parts.

I contacted the Spanish manufacturers of these units enquiring about a PSU repair kit. "sorry, these units are no longer being made, you need to buy the new version 2 unit. These are £500 each and the only difference is the addition of a SD memory card slot allowing you to copy settings from one unit to another."

So now thanks to the advice given here, I now have 3 working units instead of 1 working and two faulty.....

So now I could combine up to 9 UHF aerials, 3 FM aerials and 3 Band III aerials all pointing in different directions, combine the three units outputs using a passive combiner, and feed that lot into my 17x16 multiswitch.... :-)

I also plan to add CCTV at some point in the future so these units will allow me to combine digital TV with analogue CCTV too....

Thanks again,

Stephen

Reply to
Stephen

We need to get some posting consistency here. With half the respondents top posting, and half bottom posting, it's becoming impossible to follow the thread. Please all stick to bottom posting, as is the normally accepted protocol in usenet newsgroups.

Right, Now that's out of the way. The voltages being quoted make no sense against the drawn schematic that the OP provided. If pin 4 of the ribbon is connected directly to the output of a 7805, then the voltage at that point

*must* be +5v on the unit that is working. It cannot possibly be at +30.2v - period. This indicates that what you have picked as your reference ground to measure against, isn't a valid secondary side ground. Bear in mind that the primary side ground and secondary side ground on a switchmode power supply, are very different. The valid ground for your measurements, should be taken as the centre pin of the 7805. You might think that other places on the board or within the circuit configuration are the same ground, but without a manufacturer's schematic, you just can't be sure, so use a point that you can be sure of or at least 99% sure of. I say only 99%, because occasionally, you will see the output of a three terminal monolithic regulator 'jacked' a little by the insertion of a diode or two in the ground leg, but it's pretty rare on proper commercial equipment.

When you have reached a point where you can read a steady +5v on the righthand pin of the 7805, then remeasure the other voltages, and let's see if they make some more sense,

Arfa

Reply to
Arfa Daily

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.