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Another Pioneer SX-434 Repair Thread

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New Member
Hey Folks, Long time lurker, first time poster...

I've got a sweet SX-434 that may have been in my family since new in 1975. I stowed it away a few years ago after using it occationally but lightly, but now my son would like a nice sounding unit for his bedroom and I'd be proud to keep in the family even longer. I remembered it had developed an noise issue which sent me back to AK and the fantastic work of @markthefixer, @larryderouin, @xlbee, @bktheking, and countless others who have so patiently enabled folks like myself keep these beautiful machines cranking away. Thank you to all who give their time freely in such a manner.

As for this unit, I'm hoping to get it working well as a daily driver and I'm not planning a full rebuild. I'm chasing a noise that is always present on all inputs and outputs and is complex rather than a hiss/hum/click or pop. It sounds like alien gibberish. Random and almost reminiscent of the ol dial up modem noises but very muted. Here's a list of my notes and tests so far.

---------------------------
DC Offset
All channels jump at powerup to +/-400mv briefly before settling down
A-L 5mv slowest to settle (10 seconds)
A-R -2mv
B-L 4mv
B-R -1mv

Power Supply Tests
Expected, Measured
Pin 1: Gnd, Gnd
Pin 2: 13.4v, 3.1v
Pin 3: 25.5v, 3.8v
Pin 4: -24v, -21.5v
Pin 5: +24v, +21.4v
Pin 6: Gnd, Gnd
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11: 8v, 0v
Pin 12: 6v, +3.1v

Powered on via Dim Bulb Tester @ 110V AC at Load
Audio continues for 10 seconds after turning off.

AUX SOURCE
Inconsistent alien static noise occurs with volume off almost digital interference sounding.
Alien Noise can be increased with volume but stays present w/ vol @ 0.
Headphones/speaker switch to off has no effect.
Left channel slightly softer, with high noise floor, increasingly distorted as volume increases.
Balance to full right, noise still present, but no distortion.
Noise continues when panning away. e.g. bal full left, music left, noise in right
Noise in both channels when music paused.
bal full right = music right, quieter noise in left
tone/bal pots are dirty but make a different, expected noise.
Tone does affect noise tonal balance.
Tape monitor button IN bleads music into volume, makes noise louder
Mono button works, doesn't change noise

PHONO SOURCE
Tried computer input sounds like chaos, didn't expect anything else.
Plugging cable into input jacks makes expected sounds
No access to record player yet.

TAPE MONITOR
Works, Left still quiet and distorted at full range
Right, fairly full range, same noise floor.

AM/FM
Currently Non-Functional.
No additional antenna added.
No sound produced, noise is the same
AM & FM Signal meter does NOT move when scrubbing across all freqs.

----------------------

Having seen how far off the PS Pins 2 and 3 are, I'm planning to start with replacing those Aluminum Electrolytic Capacitors first. I might as well order a slew of the replacement transistors since the odds seem good that are that those are a mess, right? Or at least start with the Power Supply 2SC1384 and 2SD313 first?

Thanks in advance for any thoughts, questions or guidance.
-Scott
 
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Looks like you have some issues in the power supply. Which power supply board to you have? The schematic shows a few possible ones. I'm not an expert on these like some of those other guys but I'd check D6 first. Looks like it should be a 14V zener and if it's bad it could be throwing off both those voltages. Could also be something else nearby though. If that seems ok, I'd check the transistors next. And I'd wait to recap it until the problem is fixed.

On pin 11, set your meter for AC voltage and check it. If you were checking it with the meter on DC, that's probably why you got 0V.
 
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Hey Brian, thanks for the input.

The board is AWR-062.

DOH! Of course I missed Pin 11 being AC. Thank you!

I was trying to avoid multiple orders of parts since shipping is by far the most expensive part of this. I have most of the caps on hand. But I'll test and order the Zener's and transistors next.
 
If you are going to replace transistors, while you are in there replace any that are on this list:

Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements

I've followed AK advice for several vintage solid state receivers now and the difference in sound, from recapping and from replacing "known-to-be-noisy" small signal transistors to be amazing. While following along with others' threads and trying to help out with "consider replacing these...
audiokarma.org
 
Sounds almost like modem or phone interference. Move your phone into the next room,(at least 10 ft away)or turn it off, turn off all florescent lights in the area, turn on the 434 and see what you get. If you have a desktop or laptop computer nearby, turn off the WI-FI and/or modem.
 
Ya, Lar, that's EXACTLY what it sounds like. I do live event audio for a living and the number of times I've had talent walk on stage with a pocket super computer just spewing RF interference to all my microphones is maddening! But, the problem persists even when this 434 is in an electronically isolated location, different rooms, even in different houses with all electronics/lighting turned off.

It's alive!

Since I'm just waiting for a parts order, let's see if I can attach a couple mp3's of the noise for kicks and giggles.
First MP3 music then pause leaving noise only. Volume unchanged.
Second MP3 Noise only. File Volume increased. Volume on 434 off during capture. (be careful, it's nasty.)
 
Rebuild the Power supply 1st. If it's not right, nothing is.

Practically all of the transistors are on the top ten list. If you have a 2nd receiver, an RCA Cable and a .01uf 600v film cap......make a signal probe. Wrap one end of the cap to the center pin on the RCA, solder it on. Heatshrink the end of the free side of the cap. ( Keeps the "AW SH#$" moments to a minimum). Plug the other end of the RCA Canle into the AUX plug on the 2nd receiver, integrated amp, or preamp. Turn on and set volume low but audible.


Turn on the 434 set to Aux and Volume to 9:00. All other settings centered or OFF. Probe from the output's back to the inputs (do all 3 legs on each transistor). The culprit(s) will be quiet on the input side and noisy on the output side.
 
Geez, that's a great idea. I also have an old oscilloscope and signal generator so if I get motivated enough, I can bring out the big guns. ;)
 
Great news y'all. Got to replacing most of the Power Supply components on board AWR-062 and she's back to working very well on preliminary ear tests.

Tested the old transistors and they were all very faulty. Haven't tested the old caps but I'll post those results tomorrow.

Tests:
Speaker outputs DC Offset
A-L Was 5mv, Now 25.5mv
A-R Was -2, Now 14.6mv
B-L Was 4mv, Now 4mv
B-R Was -1mv Now 5mv

Power Supply
Spec, Tested DC
Pin 1: Gnd,
Pin 2: 13.4v, Was 3.1v, Now +13.2v
Pin 3: 25.5v, Was 3.8v, Now +25.18v
Pin 4: -24v, Was -21.5v, Now -20.91
Pin 5: +24v, Was +21.4v, Now +20.7
Pin 6: Gnd,
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11: 8vAC, Was 0v, Now +6.8vAC
Pin 12: 6v, Was +3.1v, Now +13.6 -Double what the spec says. I wonder what that's about.

AM, FM are now working again.
Brief listen shows noises are gone with volume off,
Some noises persist with volume increase but in RF noisy environment. More official tests to come.
AM and FM now work. Clean signal. Signal meter correctly indicates signal strength.
Aux audio is clean as well. Full range without distortion. Balance difference fixed as well.

Pots are still dirty and need cleaning but I'm now wondering how deep I should dig into the AF board once it's out to get to the pots. Hmmmmm.
 
I think your pin 12 issue may not be a problem. From my read of the schematic, you have about 13.4V going through a 220 ohm resistor to pin 12, which goes to the stereo indicator lamp. Since it looks like there's no voltage drop across that resistor that means there's no current flowing through it. When the stereo indicator is turned on, there should be some voltage drop and you will probably see something around 6V but I didn't do the math. If the stereo light turns on try measuring pin 12 when it's on and see what you get.
 
Robin Egg Blue SANYO caps (Value's <4uf) Solid aluminum caps. As they age they lose capacitance and gain resistance. Not good. Fred Longworth (an Early AK Member who's still somewhat active and has a shop in the San Diego Area,) coined the term "Capsistors" for them back around the turn of the century. 2sa725 and 2sa726's are "kill on sight" as are a few other in the TOP TEN Wanted list. Replace with ksa992fta. The above are 1st strike items. The rest is really up to you, but with the 434 being 50+ yrs old, it's prudent just to shotgun all the electrolytics.

Pin 12 is one of 2 ( the other is Pin 6 on Q5 HA1142 or Pin 13 on the Tuner board) output voltages that turn on and off the Stereo beacon lamp. When in Mono, both ends should show following approximate voltages.......... Pin 12 shows 6V and pin 6(Pin 13 tuner board) will show approx 4-6v, and the lamp shouldn't glow even dimly). When there is a STEREO SIGNAL the multiplexer will turn off pin 6 on the HA1142 and the lamp lights. Because you have voltage from both sides when off and a lower voltage on one end while the other end is "hot" the lamp light due to the differential voltage.

Check the Pin 12 on the P.S. at the pin. You should get approx 6V All the time. If you're getting 13.4 at Pin 12, ohm out the resistor #R-8. Replace as needed.
 
Same voltage on both sides of R8? That would mean there's no current going through it. Is the Stereo bulb burned out?
 
Alrighty, update #3.

Pots are clean.
Replaced most components on Power Supply board.
Replaced all recommended caps on AF board.
Still haven't touched the Phono Board since it's unlikely to be used.
Tested the old transistors and they were all very faulty.

Main Problem: So I found a fatal flaw in some of my testing procedures where I did not always have dummy loads or speakers connected to the outputs. With those reconnected, the left channel exhibits reduced, distorted audio at all volume levels via all inputs in speakers and headphones. When speaker selector switch is at an off position, the headphones audio cleans up beautifully. Aren't Q20-23, the main output drivers? Since those are new, maybe it's something before that. I figure the recommended list of components included most of the transistors in the audio path, but maybe not. I'll list all the items I replaces so far below.

Secondary Problem: Low level Noise still present in all inputs as an elevated noise floor. Can only hear it when source pauses/quiet section.
Noise now responds to volume control. When volume off, dead silent.


Is it inevitable that I need to change out every last component left? E.g. Q1-6, Q14-17 and all the small pF ceramic caps?

Replaced Power Supply Components:
C7 C8 C9 C10 C11C12
Q1 Q2 Q3
D1 D2 D3 D4 D5 D6

Replaced AF Board Components:
C1 C2 C11 C12 C17 C18 C23 C24 C35 C36 C37 C38 C41 C42 C43 C44 C51 C52 C55 C56 C57 C58
R83 R84 R85 R86 Not Replaced, Tested good.
Q10 Q11 Q12 Q13 Q18 Q19 Q20 Q21 Q22 Q23

Tests:
DC Offset
A-L = .04mvDC,
A-R = .004mvDC
B-L = .04mvDC
B-R = .004mvDC

Power Supply
Spec, Tested DC
Pin 1: Gnd, 0
Pin 2: 13.4v, +13.61v
Pin 3: 25.5v, +25.15
Pin 4: -24v, -20.62
Pin 5: +24v, +20.4
Pin 6: Gnd, 0
Pin 7:
Pin 8:
Pin 9: ?, .72v
Pin 10:
Pin 11: 8vAC, 6.8vAC
Pin 12: 6v, +13.6vDC

AF board
Pin Spec, Tested DC
1 ?, 4.7v
2 ?, 4.7v
3 ?, 0v
4 ?, 0v
5
6
7 ?, .005v
8 ?, 0
9
10
11 ?, .008v
21, +25.5, +25.2v
22 ?, 0v
23 -24v, -20.5v
24 0v, .004v
25 +24v, +20.3v
26 +25.5v, +25.2v
27 0v, .04v
28 -24v, -20.5v
 
based on @larryderouin 's comment, I probably don't fully understand that circuit. But from looking at the schematic, I don't see how any current could flow with the bulb open so I think it makes sense that you have 13.4 volts on both sides of R8 and on pin 12. I'm assuming the chip on the tuner board provides a ground to light the stereo bulb but I'm not positive there. Replacing the bulb might fix the voltage.
 
Brian1970 said:
based on @larryderouin 's comment, I probably don't fully understand that circuit. But from looking at the schematic, I don't see how any current could flow with the bulb open so I think it makes sense that you have 13.4 volts on both sides of R8 and on pin 12. I'm assuming the chip on the tuner board provides a ground to light the stereo bulb but I'm not positive there. Replacing the bulb might fix the voltage.
Click to expand...
Yep. Thanks for the feedback. I'll get to the bulb eventually.
 
Welp, I'm scratching my head a bit now. Got the main filter caps and all transistors on PS and AF boards replaced now.

Distorted Left channel is still there when speaker outs are engaged, clean with speakers off via headphones. I seem to recall reading in one of the other forums that this is due to bad output transistors but those have been replaced as well. Any other obvious things I'm missing?
 
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