Pioneer SX-3400 distorted sound

ThePlague

Active Member
Hello all, I acquired this SX-3400 not too long ago for cheap. I noticed that the speakers sound distorted when the volume is turned up even slightly and the more the bass, the worse it is. I deoxted everything I could and order a full set of replacement caps. I thought it may be the two main filter caps at 4700uf 35v, but I just switched those out with new ones and the problem still exists. I will go ahead and replace all the caps, but does anyone have any ideas? The idle voltage is pretty close to where it should be. However the right is 2.2v and the left is 1.3v. Also to be noted, the headphone portion works perfectly fine.

Here are the old caps, looks like the originals were replaced with ELNA caps at some point, as are all of the caps on the board (ELNA).
image17.jpeg


Here are the new filter caps installed
image14.jpeg


Here is an overview of the board
image15.jpeg


On the bottom of the board I noticed what looks like burn marks, but I believe it is from someone replacing the parts as its from a soldering iron and the solder is a lot shinier then OEM.
image16.jpeg
 
Just finished the recap and the problem still exists, but holy crap I've never heard a tuner sound that good through headphones! Still stumped. I am using 6 ohm speakers for testing, but pretty sure that wouldn't make a difference.
 
However the right is 2.2v and the left is 1.3v ? clues please .like where you read these voltages .
looks like new outputs are fitted .what are they ?
 
However the right is 2.2v and the left is 1.3v ? clues please .like where you read these voltages .
looks like new outputs are fitted .what are they ?
Idle voltage is to be taken between TP1/TP2 and TP3/TP4. (can be seen in the first image) To adjust Idle voltage you cut the jumper wire. That had no affect.

I did notice that these were replaced and could be the cause?
image18.jpeg


Also do not know if the white around them is supposed to be there
receiever.jpg


And here an overview of the board recapped
image.jpg
 
bias should be between 2mv and 250mv ...you are saying they are reading in volts ..are you sure its volts and not mv you are reading .?
check dc offset
check the ohms of the emitter resistors .
check rail voltages . and check the driver transistor voltages .
on another note are the outputs getting screaming hot ?
 
bias should be between 2mv and 250mv ...you are saying they are reading in volts ..are you sure its volts and not mv you are reading .?
check dc offset
check the ohms of the emitter resistors .
check rail voltages . and check the driver transistor voltages .
on another note are the outputs getting screaming hot ?
Yeah meant mV sorry. The service manual doesn't mention how to check dc offset so I am not sure how to. Are the emitter resistors the ones attached to the piece of metal?
 
And to be quite honest, I haven't much of an idea on how to do these tests. This is my first overhaul/major repair and am just trying to figure out how to do these types of tests for the future. Also, no heat from the outputs.
 
emitter resistors are the large white ones next the heatsink .
first unplug the mains power plug from the wall ..
set meter on lowest ohm setting maybe 200 ..
put one lead on an outer pin the other lead to centre pin .take reading . then leave lead on centre pin and put other lead to other outer pin .take reading .
basically they are 2 resistors in one package joined in the middle .
they should read all the same and about 0.47 ohms
when it comes to reading dc offset do it at the speaker terminals . ideally should read near on 0v .. set meter on high volts dc first then work your way down until you get a reading ..most likely 30-40mv or so .. could vary quite a bit though .
and when reading dc offset set to aux vol to min . no input connected
 
The DC offset on the right channel is 41.7mV and the left is 32.5mV.
Emitter resistors all check out to .5 ohms.
 
could you get voltages at r129
and r130 .
black lead to shiny chassis ground
voltage for each end .don't worry about which end is which i will work that out .
make note whether + or - .
 
*not the one you see in the pictures, there is one right behind it. That one in the picture is inline with R138 I believe.
 
Both channels sound the same? That narrows it down some.

Check the (R135) C119 / D6 +13v for the power amp. It is DIFFERENT than the other +13v regulated (Q29) supply. The rest of the power amp runs on raw DC +/- 28v dc.

The tone controls are PART of the power amp, and nothing else active handles the aux / tuner/ tape signals before that. ... (to screw up the sound)

C85 & C86 are new? and the tone pots cleaned?

I haven't gotten a definitive read on idle current, it is 1.3mV (0.0013V) between TP1 and TP2? and 2.3mV (0.0023V) between tp3 and tp4? I would cut the jumper wires at those levels to get the idle current higher...
 
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Yes, both the same. I will check that out and let you know. All caps are new. tone pots Deoxed twice. That was my read. Ill double check it tomorrow, I cut the left jumper and nothing changed. Haven't touched the right yet though.
 
The only item that I could really think would be the.. not sure of the component name, but the components attached to the heat sink. They had been replaced at some point and the soldering underneath is sketchy at best, but I reflowed it just for good measure. Maybe those components are shot? I'm pretty sure the white stuff behind they isn't normal.
 
i was asking for voltage at 146 to see what the positive rail voltage was at .. i thought it was a convenient spot to take a reading .
i thought you had cut both jumpers . if the voltages below are ok cut the right jumper and see what happens ...
as mark said r135 is a better place to take readings ..one side should be +28v the other side +13v .
make sure + side of c119 connects to r135 .. if its -negative side that connects to r135 then that's the problem ..
lets see those voltages first ..

by the way the white stuff is heat transfer paste is normal
 
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upon cutting the jumper, the idle current / tp1, tp2 voltage DID NOT CHANGE???

pete, start checking from the supply voltages (+/-28v) on up starting at the 4700uf caps, THEN on the other side of R129 and R130 (same polarity, different channels), then the base to base voltages on the drivers (2.4v)@(q21,q23 / q22,q24) and outputs (1.2v)@(q25,q27 / q26,q28)..

Cutting that jumper should have increased the idle current
 
i was asking for voltage at 146 to see what the positive rail voltage was at .. i thought it was a convenient spot to take a reading .
i thought you had cut both jumpers . if the voltages below are ok cut the right jumper and see what happens ...
as mark said r135 is a better place to take readings ..one side should be +28v the other side +13v .
make sure + side of c119 connects to r135 .. if its -negative side that connects to r135 then that's the problem ..
lets see those voltages first ..

by the way the white stuff is heat transfer paste is normal
R135 is in the correct polarity with C119. The voltages are +12.8 and +6.7. With the jumpers cut, its +12.8 and +5.8.
 
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