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Soundcraft power supply recap gone horribly wrong...
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jnashguitar
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4th February 2012
Old 4th February 2012
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Soundcraft power supply recap gone horribly wrong...

Wow... I'm baffled... just recapped my Soundcraft 800B power supply, using Nichicon and Panasonic electrolytics, mostly double the value of the originals. In the process I touched up a few questionable solder joints, then went through meticulously checking continuity and cap polarity.

Just plugged it in (with nothing connected), and I pulled the plug two seconds later when I heard crackling. A burning smell seemed to be coming from the main regulator (LM338K). Ouch!

I'm shuddering to think I might have made a mistake somewhere, and it's certainly possible, but I *really* don't think I did. Is it possible a faulty new cap could have caused this ? Or could increasing the values have put too much strain on something ?

Any suggestions on what I should do next to try to debug this ?

Thanks!

-James
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4th February 2012
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You have induced a short somewhere on the power rails.(cap polarity,poor soldereing)
Recheck your work.......carefully !!.
You may have also destroyed the reg....replace but not until,
You have found your self induced fault.....
You dont need any power on to find that,just a DMM.
If you don't have the tools or knowledge,Take it to a man who does
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5th February 2012
Old 5th February 2012
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Thanks for the advice!

Turns out the smell wasn't coming from the regulator, but from the innocent-looking rectifier next to it (REC4--main power rails). When I probed it, the two middle leads were internally shorted, so I pulled it.

Schematic shows a KBL02 (4A), but this one is a replacement NTE167 (2A). Maybe that was a borderline component that I stressed with the larger inrush from the upsized caps ?

Now that I need to replace the main rectifier, should I stick with the KBL02 specified, or is there a reason to upgrade ?

I can't see anything else wrong, so hopefully that's the whole problem. Unless anyone could suggest other parts that might have been fried in the process (or could have been the real cause) ?

Thanks again for any advice!

-James
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5th February 2012
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Hi
Tsk Tsk, upsizing caps without reviewing the rectifier.
Put in a higher voltage and higher current device AND make sure it has some heatsink (bolt it to the metalwork somewhere, with thermal compound on it). THEN you can forget about it.
Matt S
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5th February 2012
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Matt is always on target.
You sure as heck don't want to put a 2 amp rectifier where a 4 amp is spec'd. Put in a bigger rectifier and heat sink it. Make sure that it's internally shorted and not shorted somewhere down the line.
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5th February 2012
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It's definitely shorted internally--I removed it and tested out of circuit.

All good advice--thanks! I never would have replaced a part with one below spec, but at some point someone replaced that rectifier, and I should have looked up the part number instead of assuming it was OK.

Is there any reason to think nearby caps or the main regulator might have been damaged by that rectifier short, and is there any good way to test ?

-James
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5th February 2012
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Not really. This kind of failure is quite common in power supplies, even ones that haven't had the filter caps doubled in value. +1 to Matt Syson on this one...
best,
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6th February 2012
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Quote:
Originally Posted by Ike Zimbel View Post
This kind of failure is quite common in power supplies, even ones that haven't had the filter caps doubled in value.
Yeah, freaked me out that it fried right as I plugged in the board I had been soldering on, though... was afraid I had made a big mistake somewhere. Glad it appears this problem was isolated and easily fixed.

Thanks, Ike!

-James
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12th February 2012
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Replaced the fried rectifier with a KBL10, and went ahead and replaced the other two while I was at it.

Powered up the supply, and all is groovy. Well... almost...

All the outputs look good except for the +17V, which measures +23V (the -17V is spot-on). I'm assuming that's a faulty regulator ? Is there any easy way to test a regulator, or is the common practice just to replace if there's a question ?

Thanks!

-James
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12th February 2012
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Hi
You can test it if you know how, or replace.
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12th February 2012
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Quote:
Originally Posted by jnashguitar View Post
Replaced the fried rectifier with a KBL10, and went ahead and replaced the other two while I was at it.

Powered up the supply, and all is groovy. Well... almost...

All the outputs look good except for the +17V, which measures +23V (the -17V is spot-on). I'm assuming that's a faulty regulator ? Is there any easy way to test a regulator, or is the common practice just to replace if there's a question ?

Thanks!

-James
Sounds like a shorted pass transistor to me. You can test those with your DMM. They're called out as TR-19 and 21 on your schematic.
Best,
Ike
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12th February 2012
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Could be a resistor that controls the voltage, Should 2 mini-pots one for + one for - right next to each other, or two resistors. If it's the minipots it's an easy fix. I say this without looking at the schematic. That may also have been some of the smoke you smelled, a burned resistor?
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13th February 2012
Old 13th February 2012
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Quote:
Originally Posted by Ike Zimbel View Post
Sounds like a shorted pass transistor to me. You can test those with your DMM. They're called out as TR-19 and 21 on your schematic.
Best,
Ike
OK, Ike . Up to now, I've been impressed with your knowledge and gracious willingness to share. But this is ridiculous...

Really ? You're going to identify the precise failing part sight-unseen ?!?

Sure enough, TR-19 measures 16k between Vin and ADJ... not literally shorted, but I assume low enough resistance to cause my problem ?

You are amazing--I cannot thank you enough for all your help, and this one is downright spooky

BTW, I had to remove TR19 to be sure--in circuit, it was making nearby resistors look suspect, but so far, I haven't found any bad resistors. Will go ahead and replace TR20-22, TR5, TR7, and REG 3-4 for good measure while I'm at it... these are all funky-looking replacement parts (inconsistent mixing of NTE393, NTE392, etc)

Thanks again, Ike!

-James
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13th February 2012
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That is FREAKY and awesome.
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13th February 2012
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Quote:
Originally Posted by jnashguitar View Post
OK, Ike . Up to now, I've been impressed with your knowledge and gracious willingness to share. But this is ridiculous...

Really ? You're going to identify the precise failing part sight-unseen ?!?

Sure enough, TR-19 measures 16k between Vin and ADJ... not literally shorted, but I assume low enough resistance to cause my problem ?

You are amazing--I cannot thank you enough for all your help, and this one is downright spooky

BTW, I had to remove TR19 to be sure--in circuit, it was making nearby resistors look suspect, but so far, I haven't found any bad resistors. Will go ahead and replace TR20-22, TR5, TR7, and REG 3-4 for good measure while I'm at it... these are all funky-looking replacement parts (inconsistent mixing of NTE393, NTE392, etc)

Thanks again, Ike!

-James
Thanks for the kind words.
You are definitely learning much of what there is to learn about power supplies and their failures.
A couple of things:
1) "TR-19 measures 16k between Vin and ADJ" You are getting confused between a voltage regulator, like the LM-338K and, in this case, a transistor.
Both are quite different. The TIP 2955 is a transistor, and as such, has a single transistor junction inside it (note that Darlington transistors like the TIP 110 will have two junctions). You can test the transistor with the Diode test function on your DMM. A good one will read as two diodes conducting from the Emitter and Collector towards the Base, roughly 500mv. A voltage regulator, on the other hand, is actually an IC, made up of many transistor junctions in one case. The only thing you can test for there is an outright short between any or all of the three pins...something I have never encountered.
2) When you change the parts that are mounted to the heat sinks, make sure that you check/service/replace as needed any and all insulating pads, washers etc and thermal compound. I've seen people miss this step before.
Best,
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13th February 2012
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Quote:
Originally Posted by Ike Zimbel View Post
You are getting confused between a voltage regulator, like the LM-338K and, in this case, a transistor.
Both are quite different. The TIP 2955 is a transistor, and as such, has a single transistor junction inside it (note that Darlington transistors like the TIP 110 will have two junctions). You can test the transistor with the Diode test function on your DMM. A good one will read as two diodes conducting from the Emitter and Collector towards the Base, roughly 500mv. A voltage regulator, on the other hand, is actually an IC, made up of many transistor junctions in one case. The only thing you can test for there is an outright short between any or all of the three pins...something I have never encountered.
Ah, right... I do understand what they do (at least partially), just confused the two...

When I test TR19-22, they all measure around 500mv toward base as diodes (as expected current flow is opposite for TR20,22). So by that measurement, they actually seem ok ?

However, on TR20-22 I measure 300k resistance between emitter/collector and base. Yet on TR19 I measure only 12k between collector and base. Is that an indication something's wrong with TR19 ?

Quote:
Originally Posted by Ike Zimbel View Post
When you change the parts that are mounted to the heat sinks, make sure that you check/service/replace as needed any and all insulating pads, washers etc and thermal compound. I've seen people miss this step before.
Best,
Right, thanks--will do. I might have forgotten about the insulating pads, so thanks for the reminder (I see now that the transistor's collector is bonded to its metal back)! My initial testing was without the heat sinks (was careful to power up for only a minute or two, to make sure nothing got too hot).

Thanks, Ike!

-James
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13th February 2012
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Yes on the TR-19 question...it could be bad. The diode test is not foolproof. I have seen transistors pass that test but fail more sophisticated tests.
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19th February 2012
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Success! I replaced all the heatsink-mounted transistors and regulators, and everything's working great!

Only two minor questions:

1) Now I measure +17.09Vdc and -17.28Vdc coming from the power supply (not connected to the console). Is that close enough, or would you guys try to tweak anything to get the power rails more precisely balanced ?

2) Insulating pads: the original ones are cracking and brittle... probably best to replace. Are these things I can buy in the correct sizes, or is this a DYI job (and if so, what kind of material should I use) ?

Thanks!

-James
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19th February 2012
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Hi
Well done!
The supplies do NOT need to be 'perfectly balanced' as the chips themselves only 'see' the total supply from + to -, the 'zero' being a convenient point of reference for the EXTERNAL circuitry (to the chip).
In any case various losses on the way will change what you have on the supply anyway.
The insulators are either mica, which is thin, clear and 'crispy' anyway, or a grey 'rubbery' feeling synthetic insulator. If this is damaged it should be replaced. Either type is OK and you should buy the 'proper' kit available in decent electronics stores.
If it is mica you MUST use thermal compound with it.
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19th February 2012
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Hi James,
The "grey rubbery things" that Matt was refering to are "Sil-Pads", which are electrical insulators but thermal conductors. They are used without additional thermal compound. I have found isopropyl alcohol to be good stuff for removing the old thermal compound.
Sil pads can be found here:
http://search.digikey.com/scripts/Dk...il-pad&x=0&y=0
Scroll down a bit to see them.
Best,
Ike
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20th February 2012
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Hey, Matt and Ike--thanks so much and I'll definitely get some of the Sil-Pads!

What seems very odd is that while my power supply came with insulating pads, it did NOT have any insulating washers. And aren't the washers actually way more important than the pads ? The heat sink seems to be made of a conductive metal, then it's painted with non-conductive black paint. So, the pads are really just a safeguard in case the paint gets chipped or scratched, right ?

But... the screw holes aren't painted, and the screws are conductive, so an insulating washer is crucial there, right ? Without washers, I'm guessing the only thing that was preventing my power supply from shorting out all those years was the black paint on the screw threads ? That seems destined for failure... so, I'll get some insulating washers.

But... here's the thing: even with insulating pads and washers, isn't there still a very real possibility that the screw mounting the transistor to the sink might touch one of the sides of the hole it's passing through, and that would immediately fry it, right ? Seems like a flawed design...

Of course, a quick DMM test can confirm that hasn't happened before I plug in the supply, but it does seem like vibration over time might jostle things around so that screw in the future gets shorted out against the transistor case. Maybe the best answer would be an insulating washer that extended down into the mounting hole on the transistor, to make sure the screw could never touch the transistor case... is that what I should be using and does anyone have advice on where to find one to fit ? Or am I now being paranoid ?

Thanks!

-James
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20th February 2012
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If you check the data sheets for the transistors, they will probably tell you which parts, if any, of the case or tabs are "live".

Paul
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20th February 2012
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Quote:
Originally Posted by PRH View Post
If you check the data sheets for the transistors, they will probably tell you which parts, if any, of the case or tabs are "live".
Right--with some of these I know the tab is live and must be isolated... what I'm wondering is if there's a good way to insulate the sides of the screw hole (which are also live) ?

Seems like some kind of washer/bushing would do the trick, but getting the right size could be tricky...

-James
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20th February 2012
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20th February 2012
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And now the unfortunate truth has emerged: I probably fried those transistors myself with the seemingly-innocuous step of replacing the old, stripped black screws with new silver ones. Without insulating washers, it was the black paint on the screws preventing the transistors from shorting across the heat sink... d'oh!

Live and learn.

-James
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20th February 2012
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A "shoulder washer" is commonly used where the mounting screw must be isolated from the metal tab of a power transistor or voltage regulator. See here:

Shoulder Washers - heatsink accessories

Electronic component distributors (such as Digi-Key or Mouser) should have these in stock. You might even find a heat sink isolation kit at the local Radio Shack that includes this type of washer.
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20th February 2012
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Hi
Two things.
Transistor mounting kits often come with 'top hat' washers which are insulating and cover the screw threads.
Secondly, if it is a MJ2955 and a LM338 regulator the 'cases' of the two may be joined together and so S/C may have chosen to make the heatsink 'live' if it is inside the box.
The AMEK regulator design would allow this (but usually didn't), not sure if S/C did the same. It is a National Semiconductor inspired design.
Matt S
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20th February 2012
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21st February 2012
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Thanks to everyone for replying!

Paul--that kit looks perfect, but I believe I've already ordered the equivalent parts individually from DigiKey. Good to know I'm on the right track!!

-James
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