SX-950 Relay Bypassed and Burnt Resistors

Pageplayer

Active Member
Hey All,
As I briefly mentioned in this thread Here, I happen to find a SX-950 at a OK price knowing it would need cleaning and a recap.
What I didn't expect is that when I opened it up before powering it up, I found that the relay assembly has been bypassed with a wiring hack, and I also noticed a couple burnt resistors on the power amp board.









So I guess I'll need a relay which I'll order with the recap BOM. I seen a list of Pioneer relay equivalents here on AK a couple weeks ago, but now I can't find it. o_O Can someone point me in that direction?

Seeing as how I'm pretty new to this hobby, I'll take any suggestions on how to approach the repair.

Yikes!
 
Hold off on ordering the relay until a more complete bill of material can be compiled. Get ready with your meter to begin checking components.

Precisely which resistors have overheated? That should provide a reasonable indication of what circuit(s) went over current.
 
I can visually see the insulated sleeve on R40 is burnt and R38 is charred like a burnt hotdog.
Is it ok to test voltages with it plugged into my DBT?
 
I would be wary of powering on the unit with anything but the smallest wattage lamp, maybe 40W as a start. That could still cause heating, so monitoring everything on the questionable channel would be advisable.
 
NO power for a while. Parts will have to be removed to clear the problem.

GOOD job on identifying the components, just be sure that you haven't been fooled by mirror images,
edit - you were - those are R37 & R39 - the tip off is looking at the clusters of the pots. The image you used was of the FOIL side - while the picture shows the component side. It gets us all at first....

D7, D9, Q9, Q11 are probably all damaged,

Check R43, R45, R47, R49 with one leg each disconnected - these MAY have saved your outputs - the output transistors will have to be removed and tested.

R51, R53, R55, R57 all need to be tested, ALSO with one leg each disconnected. Failure here will be open circuit, not shorted.

VR3 (and VR4 on the other channel) need to be new.

Mark each part with it's position when removed for testing or replacement.

There are also other caps and transistors to replace - known troublemakers - since you are in there.
 
Sorry for the delay.. Busy week.
I had parts come in.. I'm going to pull the amp board, replace the charred resistors and start taking measurements of the others. I'll also check transistors with my Dynamic Transistor Checker..
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It depends on your budget, but it might be wise to just rebuild the whole amp pcb and when you order parts buy the parts to re-build the power supply at least. If you can afford to do a complete rebuild it would be a good idea. Sounds like this unit has had a hard life :)
We know all the faulty bits so it is just a matter of time, money like most things.
Who knows the resistor beneath the fibreglass cover maybe okay, some one replaced the R already and used the old cover, you have to isolate and measure.
Comparing one channel to the other is a way to find faults.
Usually charred resistors mean a semi failure(s), starting from the bjts on the big heatsinks. I first blew mine back in the 70's, finally used the newer parts which are so much better. The NTE drivers are still working :)
Still good experience to fault find, so following Mark's advise, it is a way to learn from the experts methods.
 
Here's what I have so far...
Left Channel
R37 = 20ohms.. Should be 150
R39 = Was Open
R41 = 6.84K Should be 5.6K

I replaced all these with new on both channels.
Starting to look at the other now....
Stay Tuned
 
Never seen a resistors drop in value like that, the usual failure mode is they go highZ.
Maybe have another look at that R37.
 
Okay, I guess there's a first time for everything, thats a new one to me....
No harm in double checking though, you're not doing anything wrong, carry on!!

Looks like you have quite the mission to get this one running, should be a good project...
 
How do I check Output transistors?

<<<<EDIT>>>>

I figured this out and found two bad 2SD427's and a bad 2SD557
20180723_183235.jpg
 
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someone may have put wrong value resistor in there .. going off that lovely work around the relay anything might have happened .
 
What am I doing wrong

Sometimes, you do not have to be an expert to be able to prove common consensus is not always right, things just happen. I have seen resistors going down in value several times.
(although, as I am tinkering electronics for many years, it just may be one resistor every five years..... ;) )
 
OK,, moving along. I have pulled most of the resistors off the board and checked the others. I also have another 950 board I'm comparing to when checking values.
As far as diodes, Which Mouser replacements can I use?
 
OK,, moving along. I have pulled most of the resistors off the board and checked the others. I also have another 950 board I'm comparing to when checking values.
As far as diodes, Which Mouser replacements can I use?
Which diodes in particular, theres a few different ones, Zeners ect ect.....
Point them out to us and we can give you some guidance.
 
Be sure that when you arrange replacements that you correct that second one. I suspect you may have meant "2SB557" instead.

I'm replacing all 8 output transistors. ;)
Id like to replace all Diodes on amp board but unable to determine values and equivalents.
 
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