Replaced Caps and made things worse...

[Spode]

Re: Replaced Caps and made things worse...

Have you checked RS Components? I don't think they have a minimum order and shipping is usually around £5 I think.

I found a few things I wanted in the end (a heat gun, some more flux) and ended up spending £30 :P

I'll check RS next time. I was pretty tied and relying on Google Shopping for results, I'll admit.

[Spode]

Re: Replaced Caps and made things worse...

Okay - my low ESR caps arrived today. I soldered them in and it made no difference.

There is still a noticeable high pitched squeel coming from the PSU that concerns me.

Is it possible that the main 450V cap is gone too? I can't see any swelling on it.

[seanc]

Re: Replaced Caps and made things worse...

I missed something before. You have the main 400v cap and also a small 15uf 400v cap. What voltage is across the 15uf cap? It's possible that it's dry.

[Spode]

Re: Replaced Caps and made things worse...

I have replaced that cap - I don't think it would be dry. The noise appears to be coming from the side of the board that powers the inverters - potentially the transformer or MOSFET. I will take a drinking straw to it later.

[Spode]

Re: Replaced Caps and made things worse...

I've now resoldered every transformer and toroid coil on the board.

I measured voltages across the 400v cap (300V) and the 35V caps (25V).

I found points on the PSU that should give out 12V and 24V. I get 11V and 26V respectively, even under load, which seem a little off tolerance - no?

I switch on the power, I hear a whine/squeal and after a few seconds I hear the audio on noise, and see the inverters switch on for a second.

I found the on/off pins for the inverter and wired these directly to the 5V supply - this caused the inverters to switch on as soon as I turned on the mains and switch straight off again. This removes the logic board from the equation I believe.

If I'm not mistaken, the logic board makes 5V and passes this to the PSU, rather than the other way around.

What should be my next step?

[Spode]

Re: Replaced Caps and made things worse...

Sorry to bump - but I'm at a loss as to what to try next! I'm trying to learn this methodology here - rather than just a one off fix...

[Behemot]

Re: Replaced Caps and made things worse...

By inverters switching on, you mean the CCFL's light on for a while? It probably means the inverter circuit is fine otherwise it won't turn on at all.

Good start usually is to look whether all diodes, fuses and transistors are fine. Maybe some of them is shorted so it shuts down than. If this is not the case, there is no short and no cold-soldered joint, only two choices are left: shorted driving chip (bad) or shorted transformer (even worse).

[seanc]

Re: Replaced Caps and made things worse...

OK you've not replaced all of the caps. What's up with the attaching the caps to wires?

[Spode]

Re: Replaced Caps and made things worse...

No, I've not replaced 100% all of them. There's a few small ones around. But I can't help but think that the noise I'm hearing is part of the problem - or at least, a clue as to what is going on.

I attached them to wires because there is a solid heatsink over the top of where the caps should be - it's pretty tricky to get in there, and this way I could be certain I had a decent joint. I didn't think it would cause an issue?

Behemot - sounds like I need to a) learn how to test components and b) test some components

I think I know how I would test for blown components - but not sure about shorted ones, that sounds a little trickier?

[Behemot]

Re: Replaced Caps and made things worse...

Diode test on multimeter is good for this. You could test transistors with it, not only diodes; transistors should have ~350 ohm and more. If they have far less, there is either resistor in paralel or the transistor is shorted.

This thing is very practical, you can trace ways on PCB with it and do other great stuff

[seanc]

Re: Replaced Caps and made things worse...

Failed caps can produce high pitched squeals, especially the small ones.

[Spode]

Re: Replaced Caps and made things worse...

Failed caps can produce high pitched squeals, especially the small ones.

Well I can hear the noise is coming from the inverter side of the board. So I guess I could try and replace them all on that side of the board as a start. I need to try the drinking straw method, if I can find one.

Hoping to find some time today to have a fiddle and see what I come up with.

[Spode]

Re: Replaced Caps and made things worse...

Right, I've managed to isolate the noisy component - a transformer labelled "BCK-19-0294".

I'm struggling a little on how to test this properly. On the back, I can see 5 pins on one side and 4 on the other - it appears to be a pair in and a pair out.

Resistance from left to right with my DMM set to 2000k, trying various combinations is around 1200-1500. Comparatively, the same technique on other transformers shows very little resistance.

Would I be correct in thinking that this transformer is shot?

[rherber1]

Re: Replaced Caps and made things worse...

Right, I've managed to isolate the noisy component - a transformer labelled "BCK-19-0294".

I'm struggling a little on how to test this properly. On the back, I can see 5 pins on one side and 4 on the other - it appears to be a pair in and a pair out.

Resistance from left to right with my DMM set to 2000k, trying various combinations is around 1200-1500. Comparatively, the same technique on other transformers shows very little resistance.

Would I be correct in thinking that this transformer is shot?

It is rare for transformers to fail on smps. Usually, a failed transformer will be indicated by severe charring or dark colouring of the windings and perhaps cooked pcb pads. Your transformer is in pristine condition and it is not the main switching transformer but more likely a feedback isolation transformer.

To me the problem seems to be more that your inexperience with smps repair means that you are plucking at straws or fishing in the dark for the failed component/s. Unless you can get a schematic of the power supply you will have trace out the circuit and draw your own schematic. Even then, once you have replaced the usual suspects (output filter caps) there may be other faults. Because these devices have feedback circuits it is often difficult to get them up and running untill all faulty components have been found. Even I have been forced to admitting defeat on one or two occasions after I have swapped almost everything that can be swapped. Thankfully, that is a rare occurrence.

[rherber1]

Re: Replaced Caps and made things worse...

After my previous post I decided to go back and review some of the earlier posts by yourself and others trying to assist.

Unless you can test all of the electrolytic caps with an esr meter (a must if you are repairing smps) you will be forced to replace them all - as has been suggested by others. Also, if you have replaced some capacitors with new ones then don't extend them on fly leads.

You didn't post the model of your TV but that may not be too important if this psu is used in various brands and/or models. For example it appears that it is used in the Baier LCD27SL. Others have had problems with this psu and here are some solutions on this supply
http://www.tv.quuq.org/forum/index.p...e;topic=2078.0
http://www.tv.quuq.org/forum/index.p...e;topic=3403.0
http://www.tv.quuq.org/forum/index.p...e;topic=3391.0

If you have replaced all the 1000uF caps then it seems that C8 could be a likely culprit.

[Spode]

Re: Replaced Caps and made things worse...

Hi rherber1 - thanks for your response.

Yeah, it's a Baier LCD - 26" I think though.

If I went around changing every single cap - it would cost a fortune. I've already got a perfectly good TV - I just want to learn more and I like repairing things, so blindly replacing everything doesn't really fulfil that!

I think an ESR meter would be a good idea (would this one do the task? http://www.maplin.co.uk/atlas-esr-eq...ce-meter-43095)

Part of the problem with this PSU is that the heatsinks span most of the PCB and I'm not sure how I can remove them. The MOSFETS are attached to them and I don't think I'll get the heatsinks off without removing the mosfets as well!

C8 is a tiny cap, very close to the transformer I'm hearing noise from - so it makes sense that this would be worth changing. It's a bit tricky to get to, but I'll give it a shot!

[seanc]

Re: Replaced Caps and made things worse...

The OST caps on Antec 380 PSUs don't look bad before I change them.
They're all ever so slightly out of spec and surprisingly, the PSU doesn't work, strange things happen or it makes a hissing sound. A full re-cap of the PSU and all works fine.

It's piece of mind more than anything - if you're working with a known bad cap brand/series (Chemicon KZG!) you just replace them to ensure that they're not a cause of the problem.

[Spode]

Re: Replaced Caps and made things worse...

Tried replacing C8 and some other 680uf caps nearby but with no change.

I think removing the caps from their extensions might help - but there is just no room on the PCB!

I'm at the stage of thinking, can I find a generic 24V PSU to whip in there. I know the pinouts for the connections, so it shouldn't be too hard to wire together.