newbie solder question

I've been following the electronics newsgroups for some time, and decided to try a small repair job. I have a 20Wx2 small single-board amplifier which has only worked on one channel since it was given to me. Due to its' small size, I'd like to repair it. Anyway, I used a DMM to test each of the 20 or so electrolytic capacitors on the board. I know, I know, you can't really test them in the circuit and expect a 100% accurate result. But as it's a two-channel amp, I figured that at least the equivilant capacitors in Channel A and Channel B should exhibit identical behavior.

I located two 470uF caps which differed in that one of them had zero resistance which did not rise, even on the highest resistance scape of my DMM, 20M ohm, as did all of the other caps. So, I figured I'd swap the two identical caps (channel A and channel B). I tried using desoldering braid (which I've used many times with great results) and found that the solder on this board just wouldn't melt with my 30W iron. Well, says I , I'll just use a bigger stick. I grabbed the 45W iron and still couldn't seem to melt the solder through the braid, although the 45W iron WOULD melt the solder directly. I heated each lead and wiggled and jiggled it loose. When I had both caps removed I put the desoldering braid directly over the hole in the PCB and with a pointed tip leaned on the board until the braid cleared up the solder. This took maybe a minute or more of continuous heat.

When I removed the brad, AAAAGGGHH, it seems to have removed all of the metal around the hole, not just the solder. Now the trace itself to which the cap is connected is on the other side of the board and I could put in a cap with slightly longer leads and solder it on the top. But if there's a way to fix what I've done that'd be even better. When I put in the original cap the solder just won't seem to go into the hole any more, it just sticks to the iron.

Any advice greatly appreciated, I suppose this is how folks learn.

BTW, when I got both caps out they tested identically out of the circuit so I'm pretty sure my problem is elsewhere. The circuit consists of a simple power supply, unregulated, a whack of tiny (mainly 10uF 50V) electrolytic caps, some mylar caps, resistors, a bass/treble IC, two 20W TO-220 amplifier chips. I can very very faintly hear sound in the bad channel. When I turn the volume up and down on the bad channel it makes a sort of "thumping" sound from the bad speaker but doesn't amplify the sound. I've replaced the amplfier chip on the bad channel as it was cheap and simple but this did not help. Anybody have any suggestions what to check next? I do not own an oscilloscope. I have checked and cleaned all of the knobs (bass, treble, balance, volume), they all work as they're supposed to.

Thanks

Dave

Reply to
Dave
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You often have to isolate components or parts of the circuit to diagnose things anyway. Personally, I prefer a very hot temperature controlled soldering iron and vacuum type solder sucker. Work fast and don't let the heat travel beyond where you want it. I know others prefer your method.

If you do lift a pad or trace, cut it with an exacto knife to keep it from lift> I've been following the electronics newsgroups for some time, and decided to

Reply to
Mike Berger

You didn't mention measuring at the same points with caps removed. I usually recompose circuit traces with solid conductor wire. Probably at least a 50% chance the chip is bad. Sometimes I have to double up on irons. I've also lit the Bic next to the iron for more heat. My Radio Shack 250 watt gun for big jobs!

greg

Reply to
GregS

Give up using the braid...that stuff sucks (or, rather, it doesn't). Get yourself a solder sucker if you're going to do stuff like this.

BTW, if I had an audio power amp that didn't work, I think the caps would be near the end of my list of things to check. Start with the power (aka output) transistors and work your way backwards. Unless this is a really esoteric unit, both channels probably share the power supply, so if one channel is working, you can probably assume that's OK.

Reply to
Mr. Land

As I noted before I've used the braid quite a bit in the past and found it to work well... but will pick up a sucker anyways as obviously I need it.

I did start at the output transisters and replaced them all on the bad channel (they're all on an integrated 20W amp chip, NS LM1875). I swapped the caps in an effort to avoid trying to source a 10-year-old base/treble IC which is likely the problem. Other than the output IC's, mixer IC, and rectifier, there ain't a whole lot on the board except caps and resistors. Resistors all checked out OK, move on to electrolytics... No?

Reply to
Dave

My general rule is a solder sucker for standard work and braid for smaller or awkward tasks, especially certain surface mount work. A solder sucker is far more appropriate for removing components like electrolytic caps from an amp IMO.

Do you still have a short? If so that makes things very easy- a short is probably the easiest fault there is to diagnose. Simply trace the two PCB tracks which are shorted and find all other components across these tracks. One of them must be shorted out. Is the capacitor in question on the supply rail? Check that the new IC you fitted doesn't have any solder bridges, and if it has a mica heatsink insulator, make sure it's not damaged. I doubt it's a shorted cap, but it is possible nonetheless.

It's possible the 470uF capacitor is part of the decoupled supply to the preamp section, if so it may have a zener diode in parallel to give a regulated supply. Therefore, check for shorted zeners.

The output ICs no doubt are coupled to the speakers via large electrolytic capacitors, what is the voltage level on the IC side of this cap with respect to ground? It should be close to half supply potential. Compare it with the good side, but take care not to short anything out.

Are both output ICs getting a supply? Are they both getting a ground? Have you measured voltage levels on each pin and compared them with the good side? Have you downloaded a datasheet for the ICs, found the input pin and injected a signal there to listen for output? Even touching a finger to the input pin may give a buzz from the speaker, or a screwdriver may give a click. There's ways and means, even without test equipment!

Dave

Reply to
Dave D

Well back in the "old days" we used "eyelets" - didn't have plated through holes for circuit boards. The eyelets were just miniature one part hollow rivets - or similar to grommets used in sails or tarpaulins. That would be an ideal fix if you burnt the plating off.

Solder Wick for single sided boards - solder sucker for double sided boards.

Regarding the amp. Have you checked the DC voltages between the channels? Is this a single supply or plus and minus supplies? TO220 - five lead power op amp? built in thermal, short circuit, and safe operating area protection? I assume you only have one chip per channel (or "bridged" with two chips per channel?)

If you can identify the chip and its input(S), you might try using a capacitor to bridge signal from the working channel's input into the input of the dead channel. (observing polarity if using an electrolytic). Unplug the input to the dead channel while testing if possible. The cap size will be 10 uf or less for a high impedance audio stage input. That should localize the problem between the input or output of the power amp chip.

If it is like the TO220's I'm using, there's a differential input (inverting and non-inverting) inputs - make sure you drive the correct one).

One assumes you switched speakers at some point.

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I think I'll solder to the trace on the other side of the board, just means that the cap won't sit flush anymore but that's okay, lots of room in the enclosure.

Yes, must buy solder sucker. It's on my list now.

No. Do you mean between the channel A and channel B signal outputs with no signal applied to the inputs? If there WAS a DC voltage differential what might this tell me? Failed coupling cap? There are no output caps, the amp output pin (see below) goes straight to the center pin of the RCA jack on the back.

Both the amp and bass/treble/volume/balance IC's run off +Vcc.

TO220 -

One chip per channel. They're LM1875T's, yes they have built-in thermal, short circuit, etc. As I noted I have already replaced the one on the bad channel (they are a TO-220 package).

Does it matter if I use an electrolytic or not? See pinout below, but I'd connect pin 1 of channel A (working) to pin 1 of channel B (dead). If I do use electrolytics, which end is positive, A or B? Why do I need to use a capacitor to bridge as opposed to a wire?

Here's the pinout:

1 +IN 2 -IN 3 -V(EE) 4 OUTPUT 5 V(CC)

From what I can see, pin 1 is signal in, pin 2 is used as the negative feedback loop, being connected to pin 4 via a 20K resistor. There are no coupling caps on the outputs.

Yes, first thing I tried.

Reply to
Dave

tracks.

supply

and

I checked for solder bridges on the new IC VERY carefully when I put it in, as I did also inspect the mica insulator.

The balance/tone/volume IC (which is basically my entiere 'pre-amp section') has a zener internally, BUT I see that it also uses an external decoupling cap on the internally regulated voltage supply... must check which cap performs this function.

No, no output caps at all. Output pin of amp IC goes directly to RCA plug via large trace.

the

Reply to
Dave

All of the advice given so far regarding the fault, is good stuff. As far as solder wick / solder sucker / desoldering stations go, it's horses for courses. Contrary to what one of the posters said about solder wick " sucking " ( or not ... ), in my experience, this is only the case when you either buy cheap, or use the wrong size for the job. Buying cheap at radio rallies, will just get you old stock, where the flux has gone off, and then, it doesn't suck. Using a solder wick with too big a size, with an iron that's too small tip wise, or power wise, results in insufficient heat transfer to the joint, and then it doesn't suck.

A solder sucker is very good for medium sized joints, but there is a real technique to using one well, and if you use it on a ' poor ' quality board, it will readily suck the print off as well as the solder. There is a real balance between getting enough heat into the joint to melt the solder to a point where it will stay molten enough to be sucked cleanly off the board, and not destroying the bonding between the copper and the substrate.

If you do get yourself a solder sucker, get a good one for which all parts are available - nozzle, neoprene suction washer, circlip ( you'll lose it ! ) etc. Practice a lot on a scrap board to get the heat / time thing right. Clean it out regularly, and don't buy cheap !!

Arfa

Reply to
Arfa Daily

as

then,

Well, my desolder braid is cheap and old, and PERHAPS a bit too thick for the job, but it's worked well recently on similar-sized joints using the same iron.

board,

right.

Thanks for the advice, have not used one before. Unfortunately where I live there are very few stores which sell electronic parts and buying one off of the internet generally costs twice as much when you get done with shipping and taxes and the like.

Reply to
Dave

This is true. I used to use solder wick years ago, and it wasn't very good, so I stuck to a solder sucker. Then, a few years ago, huge amounts started appearing at rallies for like 1 UKP per reel, so I bought loads, as did friends. It was crap. I then read an article in my national ham radio journal about this, and it said that the reason that the stuff didn't work was because even though it was housed in a basically airtight container, over several years of storage, the oxygen in the atmosphere reacted with the powder flux that it's impregnated with, and destroyed its fluxing characteristics. Hence the reason that it had been dumped cheap on the amateur market, as it was of no commercial value at all.

Soon after this disappointing experience, I had occasion to be in a fellow professional's workshop, and saw him using some solder wick made by " Multicore ", and was amazed at its efficacy, so I immediately ordered a reel, and now always have some at the ready. It's not cheap at 30 UKP for a

30m reel ( about $48 ) but it lasts a long time. I also use a Weller vacuum desoldering station, but that's too expensive for amateur use, unless one can be picked up cheap on e-Bay.

Arfa

Reply to
Arfa Daily

My experience with solder sucking bulbs and plungers has been a lot worse than with braid, except when the joint had a huge amount of solder, and even then I sometimes had to remove the final traces of solder by using braid

I once used braid that was over 1/8" wide, and it was terrible because it absorbed too much heat. 0.06" - 0.08" works best for me. I also avoid braid that's tarnished (won't stick to solder) or coarse (not enough surface area, probably low quality as well), and I keep the soldering iron tip really clean and cut off used braid right away because dangling braid just draws heat away from the joint. I cut it even if it isn't full of solder because its flux has been ruined by the heat, keeping it from stick to solder.

When solder won't melt into the braid, I add more solder to the joint because it seems that factory solder has a higher melting point or some kind of film over it.

I don't understand why 45W isn't enough heat, but some irons seem to deliver more heat to their tips than others of the same watt rating do, or they get weak with age. 35W has been enough for me with single-layer boards, except on large copper areas, and 45W should be able to do 2-layer boards or even the smaller components on multilayer boards, even when the lead connects to the internal ground or voltage plane (I normally use 50W).

Reply to
larry moe 'n curly

That's far too long to hold an iron to a joint. Aside from using functional (i.e. quality) wicks, something you need to keep in mind is tip cleaning and tinning.

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As for wicks, I've tried lots of different brands and from my experience Tech-Spray's Pro Wicks and Easy Braid's Quick Braids give the best bang for the buck. I've never had a spool of Pro Wick fail me, even after many years in storage. Quick Braid is a little less expensive, though both are priced very reasonably. Pro Wick performs better, with a little quicker heating and a faster solder draw. Avoid Pro's Tool wicks like the plague.

A good compromise between an expensive professional desoldering station and a cheap solder sucker is one of the powered solder suckers. These have a heated barrel like the desoldering station guns and a simple vacuum chamber like the inexpensive solder suckers. The wattage of these irons is typically 30-45W. They can be had for around $20-$30US.

Reply to
Ray L. Volts

I would not be without my flux pen.

I never tried to make some. Just alcohol and regular flux.

greg

Reply to
GregS

BTW, one use for the flux--I use the paste stuff, must try the liquid--is to 'activate' old desoldering braid. When doing a lot of work on old boards, I keep the tin of flux open, heat the braid and dip the end into the flux between uses. It activates the braid and makes for much easier desoldering.

Is that also the way to use the liquid stuff? It would probably be a less messy than my procedure.....

jak

Reply to
jakdedert

right!

Reply to
GregS

So, Dave...

This topic seems to have drifted off of the subject of helping you troubleshoot your amp and onto desoldering techniques and tools...(not that that isn't great content).

So have you fixed it?

If not, what's this 10-year-old mixer IC you mention? Do you have a pinout of it? How do the DC voltages on all the pins of the output IC compare? If you have a meter-based meter (was that redundant?), can you see any DC fluctuation on the input or output pins of the output IC's when you spin the volume control? Check both channels and compare them.

I've seen speaker "thumping" occur when DC was getting to an input it shouldn't be.

Reply to
Mr. Land

Um, not yet. As you noted below, I also thought about DC getting to the speakers being the cause of the thumping. That's what started me on my capacitor-testing adventure in the first place. Which led to my frying the PCB traces trying to remove the caps and associated solder with solder braid. So I've got to procure a new pair of 470uF caps (with nice long leads) that I can solder onto the other side of the board. After (hmm, maybe at the same time) I procure a solder sucker and some new decent-quality solder braid and some liquid flux (from other posters) to clean up the mess I've made. I see that there is quite a bit of technique involved with this de-soldering stuff... I'll get there.

The IC in question is an LM1036N, combo volume/tone/balance control. It's available from Digikey for pinout of it? How do the DC voltages on all the pins of the output IC

I didn't think of using an analog meter, that's a good way to roughly compare outputs... I'll make up a CD with continuous tones to feed a signal via the unit's RCA inputs and test the signal at various points moving backwards from the output stage. I have a feeling it's the combo volume/tone/balance IC but then I had a feeling it was the amp too and it ain't.... I think I suffer from "jump to unsubstantiated conclusion" syndrome and I just have to slow down and think things through before running off to the store to replace another part.

I'll keep folks updated (a retain-my-pride way of saying "I'll likely have more questions") via this thread.

Thanks for your help so far...

Dave

Reply to
Dave

Have you had to have a go with this hateful lead-free stuff yet ? If you're a soldering professional, used to lovely shiny joints that'll see you out to the grave, this stuff is gonna make you weep ...

If you only assemble for the US market, I guess that you're not going to have to get to grips with it ( at least not yet ) as the US has sensibly declined to get involved for the moment, but if any of your products are destined for Europe, or one or two other places that have signed up to this particular piece of eco-nonsense, then you're soon gonna be finding out alllllllll about it .... :~{

Arfa

Reply to
Arfa Daily

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