Recapping failure

[Neongreen]

Hiya,
I recently recapped a AOC LM721A; the inverter inside had about 5 bulging caps on a multilayer PCB. I took the easy route and snipped the old caps off and soldered the new ones onto the previous caps' stems, as I wasn't keen to have to deal with desoldering them.

Now, instead of briefly lighting up then turning off, the monitor is 100% dead.

I've checked the resistance across each solder joint and they're all perfectly fine, I resoldered the ones that had a slightly higher resistance.

All caps are on the right way, I double checked.

Any ideas?

[Wizard]

Re: Recapping failure

Are you using low ESR capacitors? Not the jelly beans capacitors that you found locally or direct from idiot electronic supplier?

Cheers, Wizard

[Fizzycapola]

Re: Recapping failure

Soldering legs to legs may have increased the ESR too much. When soldering legs to legs, then taking a measurement, my ESR meter reads ~ +0.01 higher than usual versus directly attaching the gold clips to the caps legs.

[Neongreen]

Re: Recapping failure

I'm based in NZ so purchasing low ESR caps is a devils task; however I've recapped a few monitors and the normal lytics seem to work fine. I've never had a problem with them. I replaced Hermei caps, were they also low ESR?

Is there a way for me to test the ESR? I only have a multimeter handy.

[Wizard]

Re: Recapping failure

give the us the markings on the new caps with other writings on it.

Cheers, Wizard

[i4004]

Re: Recapping failure

>Any ideas?

well, troubleshoot it.
if all you do is swap caps and expect them to work, some of them(monitors) just won't work, as caps were not main problem.

if it's dead, then concentrate on psu and horizontal output(both of which have voltages that can kill you, btw.).

[PCBONEZ]

Re: Recapping failure

Did you put any in backwards?

[Neongreen]

Re: Recapping failure

I'd expect them to atleast function similarly as before I recapped it if the capacitors were not indeed the problem; EG, in this specific monitor it would turn off after an indeterminate period of time.

I have now desoldered the legs and replaced them with capacitors, however this has not solved the problem. Evidently the capacitors themselves are either the wrong way around(unlikely as I checked this several times) or they're the wrong type which is what I'd believe. Are the Hermei 470uF 16V caps that are usually in LCDs low ESR?

I have troubleshooted it; the bulging/leaky capacitors are the only fault I could find and recapping completely killed the monitor. I wanted to make sure there wasn't something simple I was missing or overlooking in relation to the technique.

The new caps are Suntan 470uF 16V radial type caps with a temp tolerance of 105 deg C and have a Y shaped vent.

Thanks all for helping by the way, I appreciate it.

EDIT:
Yep, I'm fairly confident none are the wrong way around but I'll just check once more.

[Neongreen]

Re: Recapping failure

I've double checked each capacitor, and I've continuity tested the entire circuit, and it seems as though the whole thing has continuity(even from the power board to the status LED). I'm going to keep poking and prodding however.

[PCBONEZ]

Re: Recapping failure

Just the 'brand' of cap doesn't help to know ESR.
Need to know the 'series' [a.k.a. "model"] of the caps too.
-
Even then specs for Hermei and/or Suntan may be hard to find.
[Haven't looked yet, no point until series is known.]

The polarity silkscreen on some PCB's is backwards.
Most mark the Negative side but some mark the Positive side instead.
[Asus motherboards are famous for that.]
It's best to make a map of the caps (and show polarity) on the PCB before you disassemble.
[A few snapshots of affected areas works good too.]
It's also possible the silkscreen has an error where one/some of the polarities are marked wrong.

It's possible heat damaged new caps when you installed if you didn't get it done fast enough. (Or one/some were just duds.)

It's not usual but some circuits may need a minimum ESR to function properly.
ESR can be used to create a voltage drop at some frequency and that can be used as part of a voltage divider to BIAS something. - To turn on or off a transistor for instance.
Such use is not common/typical but it could be done.

Most likely the old bad caps damaged something else on the PCB and a shorted or open bad cap allowed momentary function of the screen.

.

[PCBONEZ]

Re: Recapping failure

http://www.hermei.com.tw/factory_e.htm

[PCBONEZ]

Re: Recapping failure

http://www.suntan.com.hk/productindex.htm

[Neongreen]

Re: Recapping failure

Thanks a million PCBONEZ,
I verified the polarity on each cap as I desoldered it, and marked it's location. Should I desolder each capacitor and test them?

The capacitors didn't have any other series markings, but I have found the Hermei capacitors in question are of the LZ series. Through elimination, I have found the Suntan caps are - there's a small chance I'm wrong but without any markings to go on, and just sizes, it's the closest I could get.

[PCBONEZ]

Re: Recapping failure

Now for the particular sizes you have check the ripple and ESR for the LZ against the ratings for the Suntan's to see if the ratings are in the ball park.
Lower ESR is better and higher ripple is better.
If ESR is not listed they are GP (General Purpose) caps and just go by ripple.

>>> Should I desolder each capacitor and test them?
While not a pleasant thought that may be necessary at some point.

If the ratings of the caps you used turns out to be okay and you didn't use too much heat getting them installed then basic tracing and checking the circuit is probably the best next step.

Also look at your soldering jobs and make sure you didn't get a blob or drop of solder on there somewhere that's shorting a/some traces. It doesn't take much.

Same-same any scratches that might have opened traces.

.

[Neongreen]

Re: Recapping failure

The leakage is identical on both of those capacitors, and the ripple current is in the ballpark; it's higher for the Suntans.

Just looking it over, there's one point where a pad/ring thing is missing around a solder point. Looks like I must've lifted it and lost it while I was desoldering. Would this affect it? The solder fulls the hole though so I don't know. Plus it has continuity with the rest of the circut. No solder blobs or scratches which could cause a short that I could see.

[arneson]

Re: Recapping failure

Normally I'd agree that your missing ring would not be an issue, but you say it's a layered circuit.
I also figured you soldered to the leads to eliminate the problems of stressing pads.
You should look very closely at that one cap and examine the hole physically and measure it for continuety.
Then start to look at other component failure as 4004 said.

[Neongreen]

Re: Recapping failure

I'm going to examine the cap and then start looking at other components. I will let you know what I find.

[i4004]

Re: Recapping failure

hehe, i said horizontal output in a lcd thread.
something to note: i usually think about crts when somebody mentions monitor(as i don't really check model etc.) and yeah, with lcds there indeed is less to troubleshoot...

you also probably don't have the electric diagram or service manual, and that doesn't help either....

anyway, you don't have much choice but to check studd you already did and then look elsewhere after that...