anyone as dumb as me

Jcricket

Jcricket

not someday, but today
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UGH!!!!!!!!!!
I was given an sx780 today. Lights but no relay and no sound. I was reading about how a couple of transistor could get to hot and create a cold solder joint - an apparent problem with this model.
Anyway, I go looking for these transistors - there are three very close to the relay. One is particulary more colored than the rest - not bad but different. So I get my trusty soldering iron out and go to town with resoldering the joints. Carefully removing the old solder, applying a little flux and resoldering. I am done and going to give it a test. I fire it up and pretty little sparks fly. I left the soldering wick on the board. UGH!!!!!!!!!!!!

By where the sparks were I think it only got one transistor - I hope. Time for a break and maybe a little alan parsons.
Big sigh.

BTW - what are these darlington things on the underneath of the board? Also, I notice both pin 6 and 7 on both darlingtons are not soldered. They are not insulated from the connection on the board, but not soldered either.

MArk
 
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Too bad about the "lightning storm". Has happened to the best of us. I had a DMM probe slip while working on an SX-880 and took out two transistors, including one of the main outputs - which I STILL haven't found a replacement for. All VERY disheartening, to say the least. So, yeah, it happens, don't feel too bad.

Those "darlington things" are your SX-780's main output transistor packs. 6 & 7 don't need to be soldered, that's the way they are, not to worry.
 
meter selection with probe tips and ac/dc cap

Three transistors, three leads each, CAREFULLY read and post all 9 voltages, left to right when looking at the face with the number. Check the fuses, mark a picture with where the sparks were if you wish. Turn off the unit in between readings when moving the hook probes. If using a pin probe insulate all but the very tip of the red probe, the black lead goes to an unpainted metal chassis ground.

Understand what auto ranging is, and how (if NO autorange) to switch ranges (higher voltage to lower voltage) to get to an appropriate range for the reading you are making. Also understand the special symbols the meter displays for various conditions including over-range, which is attempting to read a voltage that is higher than the meter is currently set to read.

When reading, Insulate all but the tip of your voltmeter probe, or use these:
Radio Shack Mini Test Clip Adapters $3.49 Model / Catalog #: 270-334

pRS1C-2160128w345.jpg


In response to your PM's, Here's some serviceable meters:
You can download .pdf's of the manuals from the website to check them out.

Radio Shack 29-Range Digital Multimeter 29.99 Model / Catalog #: 22-813

pRS1C-2265643w345.jpg


42-Range Digital Multimeter with Electric Field Detection 49.99 Model / Catalog #: 22-811
This has a few extra features, like frequency, capacitance and a greater voltage for the diode test.

pRS1C-2265642w345.jpg


get this capacitor for making AC voltage measurements, (the AC ranges on BOTH have a DC response component you will need to block)

1.0µF 250V 10% Metal-film Capacitor $1.59 Model / Catalog #: 272-1055
or this
0.1 uF 250v 10% Metallized Polyester Film Capacitor $1.49 Model / Catalog #: 272-1053

Oh, MAKE SURE that you have good batteries for the meter AND that the meter WORKS (read another battery) BEFORE leaving the store!!! That will eliminate any disappointments....
 
Mark,
Your awesome!!!!
Thanks for the tips.
The transistor that sparked was the one closeest to the darlington or furthest from the relay.The spark occurred(looking at it from underneath) between pins one and two.
I don't see any other spark marks on the board.

I'll get my meter tomorrow and the recomended capacitors.
Thanks again!
MarktheBreaker AKA JCricket
 
I would stay away from the Radio Shack 22-813 DMM. I picked one up and found that it was a bit jumpy with it's readings mainly using the resistance and continuity settings. Thought it might be the leads but I used the leads on a friends Extech 22-816 with much more stability and reliablility looking for open/bad connections. I ended up bringing it back and exchanged up to the Extech 22-816 for more $ but something I know I can trust. They also have a nice 45 watt desoldering iron for dirt cheap that works great! Much easier and faster than using a wick. Just be carefull with the lighter thinner pads with the heat it can transfer.
 
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MarkTheFixer,
Okay here are my findings.
Transistor a - closest to the relay or front of the receiver reading pins from left to right -40,-50,-39
Transistor b 40,50,39
transistor c 14,26,13

The sparks were right at transistor c between pins 2 and 3
Fianlly, I picked up the 22-811 DMM - the 42 range unit

Fuses check fine.

Whats next?
Mark
 
Jcricket said:
MarkTheFixer,
Okay here are my findings.
Transistor a - closest to the relay or front of the receiver reading pins from left to right -40,-50,-39
Transistor b 40,50,39
transistor c 14,26,13

The sparks were right at transistor c between pins 2 and 3
Fianlly, I picked up the 22-811 DMM - the 42 range unit

Fuses check fine.

Whats next?
Mark
Click to expand...

Well, I forgot to say to list the number found on each transistor, but that's not a factor. The voltages look good, so test the voltages on pins 25 and 27 near the protection relay - it looks like it MAY be a popped module.

The lightning was spectacular, but it looks like you dodged a bullet.

If those pin 25 and 27 voltages are not less than 0.1 volt, measure the voltages at the connected pins of each module

BE CAREFUL!!!!!!! IF you short ANY of those module pins to one another, the module is trashed.

Dead modules are not a big deal.
 
Pin 25 is 39 volts, pin 27 is .006v
I meausred these from the bottom of the chassis - that correct? I didn't quite understand "measure the voltages at the connected pins of each module
"

Thanks for the help.
Mark
 
BY the modules I think yo mean the darlingtons???
Here are the voltages from each of the pins
Module a on the left when looking at the receiver from the bottom

pin1 -1.29, pin 2 -39.72, pin 3 -.07, pin 8 -.04, pin 9 39.34, pin 0 -1.22

module b
pin 1 -22.85, pin 2 -39.61, pin 3 39.30, pin 8 39.30, pin 9 39.30, pin 0 -16.36

Hope this is what you need.
Thanks again,
MarkH
 
Replace the "B" darlington / module , IT'S BAD. It's feeding that "pin 25" which is connected to a jumper wire from pin 9 by the module. pin 27 is is connected to a jumper wire from pin 6 then pin 4 and eventually the other module.

usually when I mention a pin, it is referring to a jumper connection usually to another board, but sometimes (like now) to the same board because they couldn't get a trace through.

stk0050 electronix.com $5.39 each, i would do both at once...

MAKE SURE that you get and check the module before installing - there was a recent incident where a stk0050 was ordered, but a stk0050II was shipped.

The two are (MAYBE NOT) INCOMPATIBLE.
edit- see note at bottom done in 2014


While it is out check the two 0.22 ohm 2 watt emitter resistors, they are the two resistors closest to the place where the module's pins are soldered, parallel to the pins. A zero ohms reading from your meter is ok, a problem would be if they were open - blown out - rare , BUT we CHECK anyways...

edit note 2014: Over on DIYaudio there are claims that the stk0050ii will work in place of the stk0050 as long as the extra pins are not soldered into the circuit board.
I have not personally verified this yet - but am noting it for the record.
 
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MArk,
Since these are cheap and I am there now, do you think I should just replace both of them???
MArk

I need to learn to read a little more carefully. I'll do both
Mark
 
Last edited:
markthefixer said:
stk0050 electronix.com $5.39 each, i would do both at once...

MAKE SURE that you get and check the module before installing - there was a recent incident where a stk0050 was ordered, but a stk0050II was shipped.

The two are INCOMPATIBLE.
Click to expand...

That was me that got the wrong modules. I'm embarassed. I haven't installed the replacements yet. The traces where the modules go are such a mess and I dread making matters worse.
 
Parts are ordered and on their way. I'll let you know when they arrive.

Thaks again for all the help.
Mark
 
The module are here. I am going to go remove the old ones and check the resistors. Anything else I need to know before I install the new modules?

Mark
 
I probably should have waited but I didn't. I checked the resistors and they were fine. I installedthe new modules and fired it up. It sounds very good. Do I need to check anything before I close it up?

MarkTheFixer - thanks a million!!
Mark
 
I LOVE IT!!

From screw-up (believe me, I've had more than my share of unexpected lightning coming from something...) through diagnosis, repair... to a successful resurrection!

Great read here! Good job everyone!
 
Jcricket said:
I probably should have waited but I didn't. I checked the resistors and they were fine. I installedthe new modules and fired it up. It sounds very good. Do I need to check anything before I close it up?

MarkTheFixer - thanks a million!!
Mark
Click to expand...
I'd check the bias voltages and maybe the offsets before closing it up. Maybe the stabilizer voltages too, before the others. All just to make sure your back in spec now. Then enjoy and start putting your list together to recap, you might be surprised to see some leaky caps upon removal. You're hooked now!!!!
 
Sounds like a good idea. How?
I am pretty new to this stuff. I think I already checked the offset voltages on the transistors - not sure though.
If anyone can give me a method, I would really appreciate it.
Thanks!
Mark
 
Eagle1 said:
I'd check the bias voltages and maybe the offsets before closing it up. Maybe the stabilizer voltages too, before the others. All just to make sure your back in spec now. Then enjoy and start putting your list together to recap, you might be surprised to see some leaky caps upon removal. You're hooked now!!!!
Click to expand...

You've got your own problems, without making more for me.......

Jcricket said:
Sounds like a good idea. How?
I am pretty new to this stuff. I think I already checked the offset voltages on the transistors - not sure though.
If anyone can give me a method, I would really appreciate it.
Thanks!
Mark
Click to expand...

The BIAS is NOT adjustable on a sx-780. The dc offset adjust procedures isn't in the manual. The pots closest to the protection relay are the meter calibration. The other two further away pots are DC offset adjust and are read at the speaker terminals. If you can get it too high the protection will disengage the speakers, loosing the reading

It's making sound, and the +/- 39.5v were checked already.

You could try slowly adjusting the pots, BUT at this point I would leave well enough alone - it's a selfish reason - I've got just too much going on right now - notice how long it's been since i posted last... and the number of threads looking for assistance has increased at the worst time.
 
Mark,
Good enough!!
Thanks again for all the help.
I'll just sit back and enjoy. I really appreciate the help!!!!!
Mark
 
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