Finding the short.
Help Needed w/ Pioneer SX950
- Thread starter Lexx510
- Start date
merlynski
Curmudgeon Electronicist
Yup!
The next step is checking the unit following the result of the DBT test.
The bulb usually comes on bright at first from the current surge that charges the power supply caps, because they drain down when the power is off. Then it dims down, usually to nothing visible after a second or two. Sometimes it will still glow a bit on a big amp that has high idle current draw, especially with a lower wattage bulb. If it does not dim down (you must turn the power off immediately) there is usually a shorted output transistor or filter cap that can be found with an ohmmeter. If it does dim I usually check the power supplies (leaving power on) to see if they are trying to come up. Unregulated supplies will be present but 10-20% low. Some regulated supplies will show good, some may show low. Each amp is a bit different, sometimes a higher wattage bulb is used after the lower one if it is still glowing a bit. You can set dc offset on DBT to check function, and you can check to see if the bias control works, but must turn it back down all the way. If you leave it turned up on DBT and then go to full power the bias WILL be too high, some times disastrously letting out the magic smoke and destroying otherwise good parts.
MIJ_Pioneer
Super Member
Also a non-lit DBT does not mean you are out of the woods! You still can have faults that can cause your 950 to develop a smoking habit. When powering it up on the DBT be sure to be looking/listening/smelling for any abnormalities. If there is a fault, the DBT should reduce the failure but it's not a 100% component saving operation.
merlynski
Curmudgeon Electronicist
That is exactly why I like to check power supplies on DBT before going on full mains power
. The DBT does not fix anything, but it does protect some things.Thank you everyone. I will keep you all posted. I'm taking this slow, reading your comments, watching tutorials, etc... I love this receiver and certainly do not want to do more damage to it. In the meantime, I at least have my little SX640 for listening. It's a smaller receiver but still sounds great!
KarlK
Well-Known Member
You mean SX-650? I would hardly call it "little" but the 950 I'd definitely call "big"
The 650 does sound great I agree. It was the most popular receiver of that series.
Okay, so I plugged it into the DBT. As I turned it on, the bulb lit up dim then immediately lit up very bright a second later. I shut it down as advised as soon as I could after I saw that it lit up so brightly. So now we can suspect that there's a short somewhere, correct? Where do I start looking for the short and how?
MIJ_Pioneer
Super Member
What wattage bulb are you using? The initial few seconds maybe be the inrush charging the caps.
It's a 57 watt bulb. First it lights up rather dim then immediately turns bright.
merlynski
Curmudgeon Electronicist
First place to look is the output transistors, check Q13 through Q20 with the Diode function on your DMM. They are the transistors on the heat sink. Here is a link on transistor testing: https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ You can test them in-circuit, if they are shorted it will show, however it is better to pull them out. Write down the exact location each one came from as that info will aid further trouble shooting. Then test the driver transistors, Q9, Q11, Q10, and Q`12. Be very careful around D5 and D6, the horse-shoe shaped diodes on the heatsink, the leads break off very easily and there are no replacements available.
I use a 100W bulb on this kind of gear. I feel like a 57W is probably too low. I would try it on a 100W first.
Check the outputs first, like merlynski said. If you have bad outputs, you probably have bad drivers on that channel too. When I fix 950s and 1050s I usually replace all the transistors and capacitors and diodes on the amp board since they’re fairly easy to replace and pretty cheap.
Check the outputs first, like merlynski said. If you have bad outputs, you probably have bad drivers on that channel too. When I fix 950s and 1050s I usually replace all the transistors and capacitors and diodes on the amp board since they’re fairly easy to replace and pretty cheap.
MIJ_Pioneer
Super Member
Please verify but I agree, 57w sounds too low. Try at least a 75w. Make sure they are really rated and not some "looks like a 75w" bulb.
Diodes on these pioneer boards are often overlooked. Good advice.
Diodes on these pioneer boards are often overlooked. Good advice.
I'm having trouble locating Q13-20. I've looked through the service manual and can't seem to find them. I must be missing something obvious. Could you point me to the right direction?
merlynski
Curmudgeon Electronicist
On the mechanical drawing on SM page 43/44 they are in section A5, A6, B5, B6, mounted to the black heatsink, and are labeled differently than on the schematic, as Q1 through Q8, the big silver TO-3 transistors, 2SD427 and 2SB557. I did not realize they are marked that way, but I have seen Pioneer do similar things before.
EDIT: correct typo
Last edited:
I'll try that too, thanks!
Great, I will report back. Thanks.
So the output transistors all tested fine. They were all in the .5-.6 range. I screwed them off to test them just to be sure. I'm moving on to testing the driver transistors as suggested. These are listed on pg.56-57 under the tuner assembly, correct?
merlynski
Curmudgeon Electronicist
No, I don't understand the reference to 'pg. 56-57 under the tuner assembly'.
The driver transistors are Q9 and Q11 for the left channel, and Q10 and Q12 for the right channel on the Power Amp Ass'y AWH-050 on the full main schematic, they are 2SD381 and 2SB536 transistors. The AWH-050 schematic is shown by itself on page 76, the PCB layout is on page 77 & 78, and the parts list (BOM) starts on page 79 of the SM I got from hifiengine.com
I would leave the output transistors uninstalled for now.
Thanks for clearing that up. For some reason they also have transistors in the tuner assembly labeled Q9-12. I figured that is not what you were referring to.
merlynski
Curmudgeon Electronicist
Different PCB sub-assemblies may have similar component ID labels, you have to pay attention to the proper circuit board identifier as well as the component id. Some times the manufacturer will update their unit during manufacturing and change the assembly number identifier of a pcb that has the same function as the old assembly. As an example "Power Amp" describes a function, the AWH-050 is the identifier for the specific printed circuit board that is the "Power Amp" in this SX-950. If the manufacturer made a major change to a PCB they might call the new one 'AWH-052' but it would still be the "Power Amp". Often times a different letter may be put on a PCB to identify a change of component values installed, while the printed circuit stayed the same. AWH-050-a and AWH-050-b would have the same copper traces but different components installed to improve function. Documentation in the field often does not get updated. Sorry if I got carried away with the explanation
.