• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Need help with an Pioneer SX-550

Ok, the DBT is working fine. Upon turning the sx-550 on, the lamp glows brightly and stays bright, but the fuses don't blow anymore. So I have that important tool in my repertoire now, thanks a lot.

I went ahead and measured voltages at different points with a Fluke 87V, with the ground connected to Pin17. All voltages were at least one order of magnitude below what the circuit diagram demands:

35: 0,019
34: 0,36
33: -0,33
32: - 0,44
31: -1,143

30, 29,28,27, 0

26: 0,445V
25: -0,443V

24 23 0

21 20 19 0

16 -0,51
15 1,5
14 1.2
13 0,6

So all values were way too low. The only correct value I measured was at 38, 235V AC. Then I tried to measure around Q13 and Q14, although I didn't understand where C, B and E were. But those values were also all in the one-digit range.

So I tried again to approach the issue from the other side, and worked with the diode tester in turned off state. I measured the following:

r->b | BE | BC | CE | EB | CB | EC
Q3 | 0.70 | 0.69 | open | open | open | >3 (rising)
Q7 | open | open | 0 | 0.66 | 0.60 | 0
Q9 | 0 | 0 | 0 | 0 | 0 | 0
Q11 | 0 | 0 | 0 | 0 | 0 | 0

Q3 and Q7 look odd. Would it make sense to take them out and check them?

Again, thanks for any advice!
 
d_bolle said:
Ok, the DBT is working fine. Upon turning the sx-550 on, the lamp glows brightly and stays bright, but the fuses don't blow anymore. So I have that important tool in my repertoire now, thanks a lot.

I went ahead and measured voltages at different points with a Fluke 87V, with the ground connected to Pin17. All voltages were at least one order of magnitude below what the circuit diagram demands:

35: 0,019
34: 0,36
33: -0,33
32: - 0,44
31: -1,143

30, 29,28,27, 0

26: 0,445V
25: -0,443V

24 23 0

21 20 19 0

16 -0,51
15 1,5
14 1.2
13 0,6

So all values were way too low. The only correct value I measured was at 38, 235V AC. Then I tried to measure around Q13 and Q14, although I didn't understand where C, B and E were. But those values were also all in the one-digit range.

So I tried again to approach the issue from the other side, and worked with the diode tester in turned off state. I measured the following:

r->b | BE | BC | CE | EB | CB | EC
Q3 | 0.70 | 0.69 | open | open | open | >3 (rising)
Q7 | open | open | 0 | 0.66 | 0.60 | 0
Q9 | 0 | 0 | 0 | 0 | 0 | 0
Q11 | 0 | 0 | 0 | 0 | 0 | 0

Q3 and Q7 look odd. Would it make sense to take them out and check them?

Again, thanks for any advice!
Click to expand...
With the bulb glowing bright your voltages will be low. The bulb is protecting the unit from drawing too much current.

Now you need to find what is causing the high current draw.

I would pull all four of the output transistors for testing.

With the output transistors removed check all of their emitter resistors to confirm they are all good.
 
Definitely do that first ^.

Since you were confused about transistor pin-outs I thought I'd give you my take on how I do that. Pioneers usually have a transistor schematic symbol on the board that tells which pin is which and that marking is usually right but it's good to verify it one more way as a double check. If the transistor is still good enough to read in a tester after you take it out you can use the tester to see what the pin-out is and make sure that matches the board. One of my favorite ways is to look at the schematic and see what's connected to each pin. Then follow the traces on the board from each pin and make sure they go to the right component according to the schematic. You could also try to find a datasheet on the transistor and see what it says but I sometimes find mistakes or differences between manufacturers make that method less reliable so I trust old data sheets less than the markings on the board.
 
So I measured the power transistors, and removed the ones that measured open on the left channel. There are in fact destroyed.

I measured the voltages with the dim bulb tester:

Q11 (no transistor installed)
C -17,4; B -16,6; E -0,07
Q9 (no transistor installed)
C 17,4; B 16,6; E -0,07

Q10 (transistor installed)
C 17,4; B 0,60; E 0,02
Q12 (transistor installed)
C -17,4; B -0,56; E 0,02

The base voltages of Q11 and Q9 are way too high.

So I took Q5 and Q7 out, but they measured fine. As I don't have replacements for these here, I reinstalled them.

Base Q7 is 17V, counterpart Base Q8 is 1,2V
Base Q5 is 17V, counterpart Base Q6 is 1,2

So it's somewhere before Q7 and Q5. What could cause these to get the high voltage on the base?
 
Update: I also took Q3 out, that part also tested fine. I wonder if it's not a broken part but a short on the PCB that floods the bases of Q7 and Q5 with 17V
 
Have you checked VR3, D3, and D5? The variable resistors sometimes go bad. If it reads 100 ohms even when you try to adjust it that's a problem. It'll cause your bias voltage to be way too high and would explain why the output transistors blew. Your voltages will be slightly high without the outputs installed because they're not pulling any current and when no current flows there's no voltage drop across the resistors in the circuit. But my money's on VR3. Worth checking D3 and D5 to make sure they're not shorted too.
 
Thank you for the advice. I tested as recommended (test in circuit, unplugged, with Fluke 87V in Diode tester/ohm meter mode)

with Q11 and Q9 removed:
D3: 1,57V
D5: OL
VR3: 24 Ohm

comparison other channel, Q10 and Q12 installed:
D4: 1,58V
D6: 0,97V
VR4: 23 Ohm

So D5 is finished, would this explain the high voltages around the transistors? I'll see what a suitable replacement is and report back here.
 
Last edited:
Those 3 parts make up the bias spreader which puts the correct bias voltage on the bases of Q5 and Q7 so D5 being open is definitely a problem. Maybe not your only problem though. So once you replace D5, test voltages everywhere again while it's on a dim bulb tester. Don't try to set the bias (VR3) while it's on the dim bulb unless it's to set it to zero. You should only set the bias to whatever value is in the service manual once it's off the dim bulb.
 
Replaced D5 - it actually fell apart when I pulled it out - with a 1N4007. Voltages then were fine on the pads, left similar to right. So I put in new power transistors (TIP41C and BD244C)

On DBT the bulb stayed dim, so I dared to plug it in regularly - and it finally works. Well at least the fuses don't blow. FM reception is very weak, and I get a pop-brrr on the right channel (like when you have water in your ear) when I turn it on and off. And it takes 2-3 seconds until I get sound when I turn it on.

But the issue with the fuses blowing is finally sorted.

So my next steps will be to set the bias voltage, continue to replace all electrolytics on the power section, and then maybe finally try to learn how to align a tuner.

But the first step is done - the unit is alive! Thanks everyone for the advice!
 
Update: I set the bias voltage, I had way to low voltages of about 3mV, now it's within spec. The issue that it needs to warm up a few seconds before playing is still there. Both on headphones and speakers. I also have bass rattle on the right speaker. But I'll check the schematics and see what might cause it.
 
d_bolle said:
Update: I set the bias voltage, I had way to low voltages of about 3mV, now it's within spec. The issue that it needs to warm up a few seconds before playing is still there. Both on headphones and speakers. I also have bass rattle on the right speaker. But I'll check the schematics and see what might cause it.
Click to expand...
I think if you monitor either pin 32 or the emitter of Q16 You will find that the voltage ramps up from zero at turn on. The -32 at the emitter of Q16 goes to the amp section where it allows the bias to slowly ramp up. It does this to help eliminate the turn on thump since there is no speaker relay.

Of course pin 32 will only rise to -19 or so. The -19 goes to the muting assembly (AWM-101) which also slowly turns on the audio. It also quickly mutes the audio on turn off.



550bias.jpg
 
d_bolle said:
Update: I set the bias voltage, I had way to low voltages of about 3mV, now it's within spec. The issue that it needs to warm up a few seconds before playing is still there. Both on headphones and speakers. I also have bass rattle on the right speaker. But I'll check the schematics and see what might cause it.
Click to expand...
Try reversing the speaker connections to make sure the rattle problem isn't your speakers.
 
mrk229 said:
the muting assembly (AWM-101) which also slowly turns on the audio. It also quickly mutes the audio on turn off.
Click to expand...
I suspect this assembly to be responsible for the weird noises on turning the unit on and off. I guess it's the one hovering next to the large caps in the middle. I need to replace the electrolytics in that assembly anyway, I'll see if that sorts it out.
 
Brian1970 said:
Try reversing the speaker connections to make sure the rattle problem isn't your speakers.
Click to expand...
yes I tested with speakers and with headphones, also with headphones the wrong way to rule out it's my ear that's rattling. Pretty sure it's in the signal. Still need to verify it's both on A and B output.
 
Back on working with the SX-550, one question regarding a 2SA733 in the AF Mut Assy AWM 101: I'd like to replace this transistor with a modern equivalent to find out if it causes the rattle I'm still hearing. I have 512-KSA1013YBU and 512-KSA992FTA available.

These guys recommend to use the 992 as replacement: https://hudsonvalleyhifi.com/technical-data/transistor-substitutions
But my data sheet (not sure if I got the right one gives me a maximum collector current of 500mA for a 2SA733 (Fairchild Datasheet), whereas the 992 only has 50mA. The 1013 is rated to 1A instead, but I'm not sure if the gain and frequency characteristics would match. What would you say?
 
d_bolle said:
Back on working with the SX-550, one question regarding a 2SA733 in the AF Mut Assy AWM 101: I'd like to replace this transistor with a modern equivalent to find out if it causes the rattle I'm still hearing. I have 512-KSA1013YBU and 512-KSA992FTA available.

These guys recommend to use the 992 as replacement: https://hudsonvalleyhifi.com/technical-data/transistor-substitutions
But my data sheet (not sure if I got the right one gives me a maximum collector current of 500mA for a 2SA733 (Fairchild Datasheet), whereas the 992 only has 50mA. The 1013 is rated to 1A instead, but I'm not sure if the gain and frequency characteristics would match. What would you say?
Click to expand...
If you look at the circuit there is a 430K resistor in the collector of the 2SA733 making it a low current/low power circuit. The 992 will be fine. Double check the pin-out.
 
Have you replaced any caps on the AWM-101? C5 could be a trouble maker.

If you still have trouble you can temporarily bypass the muting circuit with a couple of clip leads for testing.

POWER OFF, SPEAKER SWITCH OFF, VOLUME CONTROL ALL THE WAY DOWN.

Connect pins 1 to 2 and 4 to 5. Turn the unit on, give it 30 seconds or so to stabilize and then turn the speaker switch on and test.


550mut.jpg
 
You must log in or register to reply here.
Back
Top Bottom