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Pioneer SX-450, Phono has low volume in right channel

gleegum

gleegum

New Member
I has this SX-450 that works fine except that the right channel in phono mode has low volume.
I've tried a different turntable and swapping cables but the problem persist.
I did a repair on this unit some months ago, it was blowing fuses, two output transistors were shorted.
Also fixed the power switch, it had the typical flickering lights issue.

I never tested the phono input until now, So I just discovered this issue.
All other modes AUX/FM/AM works fine except for the phono input.

Left channel works fine.

Any guidance what to check?
Thanks!
 
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Ok, thanks for the replies.
Could anybody please point me the position of the transistors to check?
I'm looking at the service manual but I'm not sure which components are the phono section. Are they in the power pcb?
 

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They are on the "RF Assembly" AWE-074 board. That is essentially the right-hand board as viewed from the top/front.

However, before swapping components it might be worthwhile to verify that the signal path is reliable. Clean function switches can make all the difference.

Of course, this is all contingent upon the turntable having the same signal level in both of its channels. Swapping the the input RCAs from left to right should verify that.
 
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If you determine that the 2SA725s are a problem, the commonly accepted replacement for those through the years has been the KSA992F. Since those are becoming difficult to find, another acceptable replacement is the BC560C. PN4250A is another possibility.
 
Thanks so much guys for sharing your knowledge. I'm gonna check those components, but first, as you said, got to be sure that the pots and phono input connector is clean and working.
I'm going to post results after checking everything. thanks again!
 
Let's see, I've replaced some transistors and capacitors in the phono section but problem persist, slighty better volume now, but still low and "dirty sound".
I've replaced:
Q203 ---> it was C1222, replaced with C1845s
Q204 ---> it was C1222, replaced with C1845s
Q201 ---> it was A725, replaced with A992s
Q202 ---> it was A725, replaced with A992s
C209
C210
C207
C208

Also selector and phono rca inputs were cleaned.
Cables were swapped and also another turntable was tested.

I have some leftovers from a rebuild kit that I got for another unit, so all those transistors and caps are new, and the right replacements according to the seller.
I've noticed that in the rebuild kit instructions it says that the phono section has 6 transistors, I replaced four as indicated in the picture above, where are the other 2?

I've uploaded a video where you can hear the problem, video starts with left channel, then I swap to the right channel, you can clearly hear the difference.
VIDEO HERE:
 

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The kit instructions are for a different unit, right? So they wouldn't correspond to this one, which probably has a different number of transistors?
 
EastPoint said:
The kit instructions are for a different unit, right? So they wouldn't correspond to this one, which probably has a different number of transistors?
Click to expand...
The kit instructions are for a SX-450, I have 2 units, although they are not exactly identical as one is 220V and the other is 110V. But I'm assuming the pcb are the same.
 
Ah, I gotcha. Must be an error then. If your phono still doesn't work, maybe it's a low voltage from the power supply issue?
 
Did those parts include C203/204 and C217/218? Those are 4.7µF and 0.47µF respectively.

It wouldn't hurt to post the E-C-B voltages at the four transistors in the EQ section as well (Q201-Q204).
 
Watthour said:
Did those parts include C203/204 and C217/218? Those are 4.7µF and 0.47µF respectively.

It wouldn't hurt to post the E-C-B voltages at the four transistors in the EQ section as well (Q201-Q204).
Click to expand...
I'm going to measure them and post results. thanks
 
Watthour said:
Did those parts include C203/204 and C217/218? Those are 4.7µF and 0.47µF respectively.

It wouldn't hurt to post the E-C-B voltages at the four transistors in the EQ section as well (Q201-Q204).
Click to expand...

Ok, this is the measurements I got:

Q201 ----> E=14.8V C=1V B=14.2V
Q202 ----> E=14.8V C=0.4V B=14.2V (When touching with the probe Base the volume went up a little, still too low and noisy)
Q203 ----> E=0.4V C=25V B=1V
Q204 ----> E=0V C=34.2V B=0.4V

I see that these voltages are wrong as in the schematics Q204-203 Collector should be 19.4V, Base should be 1.5V.
Q201-202 Emitter should be 15.7V and Base 1.5V or 15V I'm not sure if there's a mistake in the scematics.

about the caps that you ask, did you mean if I replaced them?

EastPoint said:
Ah, I gotcha. Must be an error then. If your phono still doesn't work, maybe it's a low voltage from the power supply issue?
Click to expand...
I'm gonna try to measure it. thanks
 
It would be advisable to verify the pin orientation on the replacement transistors. The E-B-C arrangements are probably different from the originals.
 
sruchris said:
Did you replace caps C204, 208, 210, 218?
Click to expand...
Yes, replaced, problem persist.
Watthour said:
It would be advisable to verify the pin orientation on the replacement transistors. The E-B-C arrangements are probably different from the originals.
Click to expand...
I was really careful with pin orientation, I followed the silkscreen as I think in the SX-450 there's only one orientation error in the power amp pcb. Anyways I'll double check.

I've replaced C218, C204, C237, C208, C207, C210, C209, Q204, Q202, Q201, Q203

This is the phono area right? (see pic) And I've highlighted with green the area where the problem should be right?
Why Q204 (Right channel right?)Collector has 34V where it should be 19.4V? Why Q203 (left channel right?) Collector has 25V and this channel sounds fine?

I've cleaned pots, but if they were the problem wouldn't all inputs (AUX/FM) have problems?

Thanks
 

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gleegum said:
I have some leftovers from a rebuild kit that I got for another unit, so all those transistors and caps are new, and the right replacements according to the seller.
I've noticed that in the rebuild kit instructions it says that the phono section has 6 transistors, I replaced four as indicated in the picture above, where are the other 2?
Click to expand...

Are the leftover two transistors KSC1845s? If so, I'd bet they are for Q205 and Q206.
 
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