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Brought home my new project - SX780

Keymaster

Keymaster

Active Member
I picked up an SX-780 this past week on eBay for $37. Finally was able to meet up with the lady to pick it up yesterday. I had to leave it in the back of my pickup overnight (under a camper-shell) to let it air out. These folks were quite the smokers.

The listing said it would cut on but would not play, and that is exactly right. The receiver lights up and the Source and Tape lights work. But there is no movement of the power needles and the FM needle never moves no matter where you turn the dial. I tried hooking up an iPod to the Aux but I get the same result.

Overall, the condition of the receiver is about a 6. This is mostly due to the wood veneer peeling off in several spots. It's also missing the Bass knob and a couple of switch covers. Fortunately I've seen all of these available on various sites and I won't be worrying about the aesthetics until it's working. I'm a little under the weather this evening so I won't be delving into it until tomorrow. I've already seen lots of information on troubleshooting so I'm looking forward to it.

I look forward to hearing everyone's tales of their repairs in the following days.
 
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Pop the cover and bottom off.
Blow the dirt out of it.
Check the solder joints on the regulator transistors (VERY common issue on these). AND the fuses and all the solder joints.

Luck....
 
Small update

I took the top and bottom off of the unit. Luckily it isn't as dirty inside as it smells. I took all 4 glass fuses out of the bottom and they all ohm out. So that doesn't seem to be the issue.

However, I did connect an antennae to the FM side and I am picking up stations in "Stereo". Still, no movement of the power meters, so it appears that all of my issues are on the "powering" side of things. The FM meter, however, does move, albeit not a whole lot, but does move to dead center when the Stereo light lights up.

blhagstrom, you said to blow out the dirt. Should I just used compressed air? I'm going out shortly to buy some Dexoit since that seems to be the first thing everyone does regardless of the issue. I can pick up some compressed air while I'm at it. I'm hoping Radio Shack carries Dexoxit, unless someone else can recommend another store?

I looked at the Darlingtons and the solder joints "seem to be fine" but I'll be honest, I'm a solder rookie. The only solder joints I'm used to are the plumbing kind and you obviously know when one of those are not correct. :D

Given my criteria so far, if there is a specific piece I should check next, please let me know. And if I need to take any high resolution pics of the top and bottom of the board, I can do that as well.
 
Oh, one more thing.

I wanted to also mention that I don't get the relay click after powering on the receiver. I know it helps to know that as well. Thanks.
 
Yup, compressed air is good. An air compressor used "lightly" is OK. Canned air is fine. A small clean paint brush (tiny household type or large artist brush) to help loosen and brush stuff off. Just be gentle and don't worry too much about getting it spotless.

Look at the main board from the top and find the big transistors mounted on U shaped heat sinks. They stick up from the board. Now find those from the bottom. With it upside down, front towards you, they are on the right side. Check the solder joints on those. Its REALLY common for those joints to get bad and the SX-780 acts just as you describe.

Luckily all you really need to do is re-solder those (in a lot of cases). DO YOU have a solder pencil? A big solder gun is too much. The gun will work but you need to be very careful doing it with one of those.
 
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Did an AK search using the google option above.
"SX-780 solder"
Got this on the top hit, talking about checking the solder joints.
http://www.audiokarma.org/forums/showthread.php?t=422158

This photo shows one place and there is another further forward to look at.

attachment.php


DO a google out in the whole web for "bad solder joint"
Here is one good photo of what to look for.
163_solder-joint-comparison-1342576415.jpg
 
Great pic!

Thanks for the pics! I know the heat sinks you are talking about. I have what I believe is a soldering tool of some kind in the garage. It's shaped like a screwdriver; you plug it in and it has an "ice-pick" type end on it. I believe I have a small spool of very fine solder somewhere out there too. I was having a connection problem with an 80's boombox speaker and I bought both of them about 6-8 years ago. Not sure of the heat of the gun but it does have a fine tip. I can practice on something irrelevant to get some practice.

I'm heading out to the door as we speak to get some spray-in-a-can and some deoxit. When I get back I'll take a look at those solder joints.

Thanks again!
 
Well, I've inspected the said solder joints under the board. They are in a triagle shape and there are three sets of them. The ones circled above look fine, with no visible separation. I checked DC voltage on them, just to do it. I had around 13.5 VDC on two of them and around 26 VDC on the third. When I did this for the other two sets, I didn't get any significant voltages from them. I guess I need to find a post on what voltages to check.

As far as the Deoxit goes, I ended up going to Home Depot and picking up two cans of spray made by CRC. The larger can is QD Contact Cleaner (Quick Drying Formula, Leaves No Residue, Plastic Safe) and the smaller can is "2-26" (Plastic Safe, Multi-purpose, Precision Lubricant). From reading the container directions, I should be able to spray down the entire inside, let dry and see if it made any difference. Are there any precautions I should take before spraying it down?

Thanks
 
Correction!

Well, I need to make a correction on my last post. I did have significant readings on the second set of solder points (for the transistors).

Here's what I have for each:

*STK-050*

13.5 14.2
o o

26.7
o


39.7 40.4
o o

50.2
o

-39.4 -40
o o

-51.5
o

As far as DC voltage readings on the STK-050's:

With bottom up and front of receiver facing you -

AMP #1

1. -22.4
2. -40.7
3. -3.8

8. -3.8
9. 40.6
0. -19.7

AMP #2

1. -1.29
2. -40.7
3. -90.4

8. -76
9. 40.7
0. 1.17

If anyone can make heads or tales of this it would be a huge help. If there are other points of contact I need to check, please let me know.

Chris
 
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Some more information

By looking at other posts, I decided to record these levels as well.

pa3004 chip:

1. 13.42v
3. 2 mv
4. -.4v
5. -1mv
6. -7.1v
7. -42.8mv
8. .64v

Transistor Q26:

1. 2mv
2. 51.1v
3 -1mv

These reading were started from the farthest point of the protection relay.

I'm exhausted, time for bed. If someone can recommend the next point of action, I'd appreciate it.

Thanks
 
On your transistor readings it would be helpful to delineate which pins are the Emitter base and collector.

On those Regulator transistors (3 with the heatsinks). While looking at the solder points from the bottom wiggle the transistor. A bad solder point will show up by the pin moving. Heat and remove all the old solder then resolder with new stuff. Then take NEW Voltage readings on all 3 pins of each. Once you get the correct readings (this is part of the integrated power supply on the main board), THEN you can get better readings on the STK modules.

Also check the .22oHm or .47ohm emitter resistors on the main board. 4 Ceramic (white) power resistors in a row right in font of the STK packs. De-Solder one side and lif it out of circuit. Then take your readings. No power when testing these.


Larry
 
Thanks larryderouin for your reply.

As far as the transistor readings, I'm winging it a little. According to another post I was using to help myself troubleshoot, it was located next to the pa3004 chip. There was only one item located next to that chip with 3 solder points, so I hope I have the right one. I'm afraid I'm not sure what you mean by Emitter base and collector. Today is the first time in my 40 years that I've attempted to read a schematic.

I will inspect the regulator transistors once more. I haven't tried the "wiggle" test yet, so that may show me something I haven't noticed before.

As far as the 4 ceramic power resistors, I'd like to eliminate everything else before I un-solder anything. Soldering is another thing that I'm very green with. I realize that eventually I will have to do it if the diagnosis requires it.

If it gets to de-soldering those 4 resistors, what type of measurement am I taking? Am I just ohm-ing them out, since the power won't be on?

Thanks again.
 
Just ohm them out. They should read real close to what the spec is on the side. But before you do that touch your leads together and read what the meter says after it settles down. Then subtract that from the resistor readings (unless the meter settles down to zero).

Each transistor consists of an emitter, Collector, and a Base. If you have the manual, look up the board (Page 33 sections 8A and 8B) , and find Q25, Q19, Q20. You'll see a circle with a Line thru the middle, one side will have a perpendicular short line (BASE) and on the other 2 angled lines one plain (collector) and the other with an arrow head in it (Emitter). If the arrow is pointing toward the outside the transistor is a NPN, and toward the center a PNP. Don't Mix them up. This link will help you with determining which lead is which and will teach you how to determine basing on a transistor you know nothing about. Pay particular attention to POST #11. NOT ALL transistors are have the same basing (lead orientation). There are 6 ways to install one, but only one CORRECT WAY.

BJT TRANSISTOR TESTING>>>> http://www.audiokarma.org/forums/showthread.php?t=43186

If I'm reading your outline correctly Q19 and Q20 voltages are OK. No voltages on the board drawing for Q25 tho, but you've got .7v between emitter and base so it's probably Ok. BUT I'm NOT the Final arbiter on this. MARK the FIXER argueably the Best PIONEER Tech here (echowars and he flip flop for AK'er of the Year). If he says that something is such and so, take it as gospel.

Looks like you may have a bad power pack. Also the PA3004 has some weird voltages.

Take and insulate all but the absolute tip of the RED LEAD on your meter. This will keep aWHHHHH SH$#$&#'s away! We've all been there, Done that. The Black lead needs to go to a shiny metal CHASSIS ground. Put a Fresh battery in the meter as they start to give funky readings when used. Take all of your voltages on the PA 3004 and Q26 again. The cap next to Q26 should be replaced as a matter of course as it times the relay's actuation when the unit is turned on. Also Q26 is sometimes suspect.

Look up Dim Bulb Tester and Build one. Use a 75W bulb. This is a current limiter and has saved numerous units while troubleshooting and rebuilding. It's your next best friend next to your meter.

Anyway. Do the Wiggle test on the Regulators, and check the voltages again. You're gonna end up taking one of the power packs out down the road for testing. So find something you can practice soldering on (a scrap board with components.).

Larry
 
Keymaster asked:
"As far as the Deoxit goes, I ended up going to Home Depot and picking up two cans of spray made by CRC. The larger can is QD Contact Cleaner (Quick Drying Formula, Leaves No Residue, Plastic Safe) and the smaller can is "2-26" (Plastic Safe, Multi-purpose, Precision Lubricant). From reading the container directions, I should be able to spray down the entire inside, let dry and see if it made any difference. Are there any precautions I should take before spraying it down?"

You might want to read through the Deoxit and Tuner cleaning sticky posts, if you haven't already seen them:

Deoxit guide:
http://www.audiokarma.org/forums/showthread.php?t=207005

Tuner cleaning guide:
http://www.audiokarma.org/forums/showthread.php?t=144163

Real Deoxit is readily available at Guitar Center or Fry's, if you have one near you.

The CRC non-residue cleaner will clean off grease and dust but won't clean off the corrosion in the contacts. The lube type CRC should be kept off the board and tuning capacitor, it will attact dust and throw off the radio alignment if it gets on the capacitor. It might be OK to use inside the pots, most people recommend Faderlube

I would go ahead and reflow the solder joints on the three heat sinked transistors, even if you can't see a cracked joint. Once the soldering iron is warmed up, it will only take a couple minutes. A better job will be done on them if you remove the old solder with solder wick, or a desoldering gun, then use rosin flux core 60-40 solder. Radio Shack has a neat desoldering gun for $13 that works well once you get the hang of it:
http://www.radioshack.com/product/index.jsp?productId=2062731
http://www.youtube.com/watch?v=uSfTTgn-ewY
Keep the tip clean with fine sandpaper or steel wool and retinned once in a while and it works great! Later, if you decide to recap the unit, it will speed up the removal process.

Hope this helps!
 
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Thanks, Marty. I gave re-flowing a shot, but still no luck. I don't have the equipment to remove old soldering.

I'm curious, with the Darlington's being notorious for going bad on these series, could these two items be my issue?

With my limited ability with soldering, I'm contemplating whether or not to try this myself or to take it somewhere to have a professional look at it. Luckily, I'm only about $48 into it, so as long as repairs don't get ridiculous, I wouldn't mind so much. It would be nice, though, to pick up a couple of STK-0050's and try soldering them in myself. I would be so stoked to be able to repair this on my own. I would be equally bummed to turn this into a huge paper-weight.

I need to see if they sell a "Schematic's for Dummies" book at the bookstore. I am familiar with the terms resistors, relays, transistors, etc but in all honesty, I don't know how they work together. And I certainly have trouble looking at a schematic and making heads or tails. I'm much better with actual photos of the equipment and having good instructions to go with it. Thanks to this forum I have found a few of these.

Once again, thanks to everyone for their help. I will be mulling over my next move. :)
 
Wooooooohoooooooo!!!

I AM SUPER PSYCHED!!!! After mulling over what to do, I was leaning toward the STK-0050's. I saw several on auction sites for very reasonable prices. I did a search on this site to see if anyone had bought them recently and found a guy with a similar problem. Someone suggested that he remove the STK-0050's to see if you can get the protection circuit thingy to click in. So I decided to desolder the one on the left first, just to see. I cut it on and what do you know, it clicked!!!

Of course, the receiver is upside down, so I look and I see one of the power needles moving! I put my little test speaker (small surround speaker) to that channel, and I had music!!! The channels appear to be a little off from where they should be on the FM dial, but that is small potatoes right now.

So I'm hoping that the STK-0050 is indeed the issue and removing it just didn't allow something else to cause it. I would like to order two of these and install them to see if that gets me where I want to be. Can anyone recommend a reputable site that I could order some from?

I'm on cloud 9 right now, so please bear with me. Thanks in advance. :banana:
 
Just ordered a couple online, complete with heat sink compound. Arrival date is 2/11 to 2/14. Can't wait! Will let everyone know how it goes. I may just replace the one I pulled out first and see if that does it. Then I can save the other one if the 2nd one goes out on me down the road.
 
I'd be more inclined to install both new modules and save the "good" old one as a spare. The characteristics of the new modules will likely differ from the originals.

As Marty advised, it would also be prudent to check the emitter resistors feeding those modules, and test the power supply voltages through the unit. The power supply regulator transistors tend to run hot and can stress the solder joints and foil traces around them,so close inspection of those areas is critical.
 
Watthour said:
I'd be more inclined to install both new modules and save the "good" old one as a spare. The characteristics of the new modules will likely differ from the originals.

As Marty advised, it would also be prudent to check the emitter resistors feeding those modules, and test the power supply voltages through the unit. The power supply regulator transistors tend to run hot and can stress the solder joints and foil traces around them,so close inspection of those areas is critical.
Click to expand...

Thanks Watthour. I believe I read that you have to check the emitter resistors while the power packs are removed, so I can check the two on the left side, since I removed that power pack last night. Does the receiver need to be on to check these, and am I looking for a DC voltage....seems like I saw where you have to ohm them out, but I'm not sure of exactly what reading I need to see on those? Luckily they look good physically, as I have read that some folks have had them burn up.

I was really surprised how good my little surround sound speaker sounded on the side that was working. It was so good to see that power needle moving to the beat. I played with the bass and treble and they worked well, as well as the loudness. If I can successfully get the unit up and running I will begin the process of replacing the missing knobs and give it a good cleaning.
 
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