Pioneer SX727 Preamp issue....

TellMeWhy

Active Member
Hi! I recently acquired a very clean SX727, really beautiful and with a nice soft sound. The previous owener had lovingly "restored" it but not really touched or adjusted it technically, reading between the lines.

The SX727 sounds good, but as is so often, one channel is much stronger and clearer (more presence, more bass, more treble, generally a nicer sound) than the other. I've isolated the problem to the preamp by crossing over the pre-out main-in jumpers, checking a signal direct into the main-in, and checking the various inputs (tuner, aux, phono etc).

So the imbalance must be on the preamp board. Does anyone have a similar experience and any suggestions perhaps of which caps or transistors might need to be replaced? How accessible is the preamp board to work on?

All advice gratefully accepted- it's a very lovable receiver so far but missing the imaging created by a balanced and strong L-R field...
 
No direct experiance with the SX727, so only general comments.

The SX is old, might be due to aged caps.
Also, check bias as part of general house keeping.

Dirt/grime/oxidation or bad solder joints may be the issue here.

With unit powered off/unplugged, select AUX, other tone controls in normal (off?)
positions, volume at normal listening, balance centered. Measure the resistance
from the Left AUX RCA (inside) to C5/C7 node/junction AWK-010
Repeat for the Right AUX RCA (inside) to C6/C8 node/junction AWK-010
This is the passive part of the preamp, it may be difficult to locate the junction
however it will quickly tell you if you need to break out the deoxit etc...
Values should be about the same +/-20%??

Keep in mind that the SX727 may be fitted with 2Sc458LG's, these are known to go noisy
and should be preemptively replaced, KSC1845 or KSC1815 depending on voltage rails.
 
No direct experiance with the SX727, so only general comments.

The SX is old, might be due to aged caps.
Also, check bias as part of general house keeping.

Dirt/grime/oxidation or bad solder joints may be the issue here.

With unit powered off/unplugged, select AUX, other tone controls in normal (off?)
positions, volume at normal listening, balance centered. Measure the resistance
from the Left AUX RCA (inside) to C5/C7 node/junction AWK-010
Repeat for the Right AUX RCA (inside) to C6/C8 node/junction AWK-010
This is the passive part of the preamp, it may be difficult to locate the junction
however it will quickly tell you if you need to break out the deoxit etc...
Values should be about the same +/-20%??

Keep in mind that the SX727 may be fitted with 2Sc458LG's, these are known to go noisy
and should be preemptively replaced, KSC1845 or KSC1815 depending on voltage rails.
Thank you! That's really helpful...Checked the service manual and it seems there are only a handful of electrolytic caps in the Head Amp Unit, and just 6 2Sc458LG's so it shouldn't be a problem to replace them all. I'll try measuring the resistance too.
 
No direct experiance with the SX727, so only general comments.

The SX is old, might be due to aged caps.
Also, check bias as part of general house keeping.

Dirt/grime/oxidation or bad solder joints may be the issue here.

With unit powered off/unplugged, select AUX, other tone controls in normal (off?)
positions, volume at normal listening, balance centered. Measure the resistance
from the Left AUX RCA (inside) to C5/C7 node/junction AWK-010
Repeat for the Right AUX RCA (inside) to C6/C8 node/junction AWK-010
This is the passive part of the preamp, it may be difficult to locate the junction
however it will quickly tell you if you need to break out the deoxit etc...
Values should be about the same +/-20%??

Keep in mind that the SX727 may be fitted with 2Sc458LG's, these are known to go noisy
and should be preemptively replaced, KSC1845 or KSC1815 depending on voltage rails.
Not sure what you mean by the C5/C7 and C6/C8 nodes... those caps seem to be a way apart...
pioneer head unit.jpeg
 
Looking at wrong board, see node in highlight below, might be an error on my part?

sx727.JPG
 
Haven't tinkered with a SX727 so I need to work off the SM.
Chapter 4 Block Diagram suggests the Head Amp Unit is for the
phono stage. The AUX input bypasses this and goes directly to
the AF Unit, this is where you should try and measure, as
previously posted.

The point to the measurement is to rule out any channel inbalance
due to dirty switches/pots etc. The measurement may indicate "no fault".
I could have suggest "clean pots etc..." but wanted a measurement to
back this up.
 
Clean, (Deoxit) the pots (Buttons, Pot and switches) before getting crazy. The signals go thru every one of those controls.
Click here For the Deoxit guide.
Don't forget to follow up with Fader 5 lube.
 
Haven't tinkered with a SX727 so I need to work off the SM.
Chapter 4 Block Diagram suggests the Head Amp Unit is for the
phono stage. The AUX input bypasses this and goes directly to
the AF Unit, this is where you should try and measure, as
previously posted.

The point to the measurement is to rule out any channel inbalance
due to dirty switches/pots etc. The measurement may indicate "no fault".
I could have suggest "clean pots etc..." but wanted a measurement to
back this up.
Thank you mbz, really helpful, will sure give that a go...
 
Clean, (Deoxit) the pots (Buttons, Pot and switches) before getting crazy.
Disagree that a simple resistance measurement, that takes like 30seconds is "getting crazy". Yes there are lots of switches and
combinations and while dirty switches/pots is very high on the list there are other high probability causes, caps, transistors, resistors out of
spec,,, Maybe I'm being a bit defensive (apology) however I prefer a test approach rather than saying "yeah, I had one of those, TRxxx was
faulty, suggest replacing it". Better to say test TRxxx.
 
mbz; I agree with Zeb1 on cleaning the unit 1st. The x2x and x3x series also are problematic for needing repeat cleanings after a short time (less than 6 months) due to switch location, type, and skill of the previous cleaner. These tend to collect dust/dirt, etc., fairly easily. On the SX-727 all of the tone, loudness, tape monitors, etc. switches are located on the AF BOARD! These are highly suspect until proven otherwise. I have a SX-737 that i have taken the switches and pots apart and burnished the contacts, and still have to clean with DeOxit yearly. Same with my SX-939. The Tape monitor switches are the worst as they don't get used much if at all on most units. The simplest and easiest thing for a non technical person who isn't familiar with electronics is to clean the controls.

Both Zebulon1 and I are very familiar with the various units in the x2x and x3x series of receivers including the SX-727. Cleaning of the controls is the 1st thing we both , along with quite a few of the senior TECH's such as Mark the Fixer and ECHOWARS advocate to rule out control problems. Once that is ruled out, then if the unit still has the problem, will we delve into more technical solutions, depending on the acquired skills and willingness of the owner.

Once the controls are ruled out, then he can take voltage readings if he's comfortable with it.

See notes at bottom for Switches on AF Unit.
SX-727 line drawing.jpg
 
Agree first hand experiance counts for plenty, just I'm against the blanket "clean the pots/switches" approach, having worked on a few
yamaha CA-1000's types with problematic switches a plenty, which to clean? all? Why not do a simple health check with 2 resistance (not
voltage) measurements? Maybe takes 1-2 minutes to find test points and measure.

switches are located on the AF BOARD!
Not sure why this is mentioned and ! and graphic included.

Chapter 4 Block Diagram suggests the Head Amp Unit is for the
phono stage. The AUX input bypasses this and goes directly to
the AF Unit, this is where you should try and measure, as
previously posted.
Think I was pointing in the correct location. Highlights in post 5 was the wanted test points.

Must be wrong time of the mont.. apology to all.
 
Hey, I shouldn't of used the "Crazy" adjective in my post.
Just a general misplaced phase I use.
My troubleshooting tends to include a percentage of probability (Illogical). If the known probable cause is easy to reach, I'll go for it, hoping for quick success.
If you want, I could list all my misplaced troubleshooting judgments but it would use way too much bandwidth. Although my previous suggestion isn't one of them.
Cleaning the controls, in most cases (Two out of three :)) will correct an imbalance. But as Larry stated, it is without a doubt, the first step in troubleshooting this particular issue.
 
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