Pioneer SX-1250 Weak Left Channel After Re-Cap

[DaDima]

After replacing all the capacitors, resistors and transistors on the Left and Right Power Amp boards, I finally got the zero volts needed on the bias and finally got the 2nd relay to kick in - releasing the speakers.

However, the Left channel is very weak coming out of the speaker. Using all the sources - FM, AUX, etc. produce the same weak output to the speakers.

1. The speaker is good - when I switch leads, the other speaker becomes weak.
2. It's not the pre-amp or flat amp (volume, balance, loudness, etc.). I connected the pre-out to another amplifier and the output is equal strength out the speakers, the volume and balance control is perfect too.
3. I connected a DAC into the inputs of the "Power In" (amplifier section) - the same weakness to the Left channel is present.
4. The 4 big filter capacitors (20,000 uF) are new.
5. Full re-cap was done on the following boards:
a) Left Power Amp (new resistors too)
b) Right Power Amp
c) Power Stabilizer (new resistors too)
d) Protection Circuit (new resistors too)
e) Flat Amp
f) Tone Control
g) Power Supply
6. The Speaker Protection Relay is new (on the Protection Ckt board)

Some of the diodes on the Left Power Amp board are the only ones I have not replaced. Maybe this is the cause.

Will other boards (other the Power Amp board) make this effect?

Any suggestions would be helpful.

Thanks.

 

[elnaldo]

Hello. Can you follow a schematic? If so, you could do some passive tests, checking continuity from the Power In connector along the signal path.

Do you have an oscilloscope, any kind of signal tracer, or at least a multimeter? depending on the tools you have, there are different ways to test the signal along the signal path.

 

[ivandezande]

So you messed up on the left amplifier board.

Replacing more things WILL NOT fix your issue.

Find what you did wrong. Use the other amp board as a guide if need be.
Also creating multiple threads about different issues on the same unit isn't great etiquette.

 

[DaDima]

The unit started with the protection relay not kicking in. I tried sorting that out for 2 weeks to no avail - I cannot get the 2 volts down to zero on the left power amp board. So, I replaced all the old parts that's in that board. And it worked - replacing more things did fix the issue. I suspect that even if a component tests OK on the tester, it might fail if it's put under real world voltages and current if it's switched on. After all, its almost 45 years old.
So, having solved that issue (after 2 grueling weeks) and it's now behind us, I started this new thread because it's a new problem and the title of the thread helps zero in on it - and if there's a solution, at least the title goes with it.

 

[DaDima]

This is my first adventure in going inside a unit.
I have about 30 receiver and amps from the 80's and 90's (some "great" ones), but this Pioneer and two Marantz's are my only 70's rig.
I don't have on osci or a sig-gen. But I'm planning to buy both because, after this, I have a Marantz 2285 and 2285b to restore.

 

[DaDima]

I've been studying the "Big Schematic" since Day 1, as well as the individual schematic and all I can do at the moment is power up the unit and take measurements on the connectors. Since the Power Amp board has connectors at both front and back, it's hard to physically take out the board outside of the unit and run it from there, and take measurements on the board itself. Let me post those readings on the connectors.

 

[DaDima]

Oh, by the way, all the power transistors that's on the big external heat sinks are brand new too.
In fact, suspecting that the new transistors are bad, I switched back the old original transistors of the left channel (2SB600 & 2SD555) - with the same results of having a weak left channel.

 

[DaDima]

Let me go through the elimination process again of switching speakers, speaker wires, pre-out leads, power-in leads, etc., and all that jazz - just to make sure that the problem is not outside of the unit or at least not outside of the left power amp board.
Thanks for you, guys.

 

[ivandezande]

The unit started with the protection relay not kicking in. I tried sorting that out for 2 weeks to no avail - I cannot get the 2 volts down to zero on the left power amp board. So, I replaced all the old parts that's in that board. And it worked - replacing more things did fix the issue. I suspect that even if a component tests OK on the tester, it might fail if it's put under real world voltages and current if it's switched on. After all, its almost 45 years old.
So, having solved that issue (after 2 grueling weeks) and it's now behind us, I started this new thread because it's a new problem and the title of the thread helps zero in on it - and if there's a solution, at least the title goes with it.

No, you just happened to fix whatever you messed up the first time as you kept replacing things.
If it worked before, and then you worked on it, and then it didn’t, it was something you did.
The only reason I’m hammering this point is because just replacing stuff is a terrible way to troubleshoot. And you can study the documents and read all the forums, but experience is the only way to learn.

Best of luck. Keep looking and comparing.

 

[DaDima]

I'm not an expert like you. This is my first re-cap and I'm not getting specific 'help' on the replies. I got the parts and the time and it only took me half a day to replace all the old resistors and old transistors. Like I said, testing components using low voltage and low current might be different when real-world high voltage and high current goes through these 45-year old parts. In my case, it worked.
Now, I don't know what of the 20+ old parts were at fault (and I tested all these old parts and they passed before), to me, sometimes a shotgun is better than a sniper rifle - especially if I've been at it for almost 2 weeks and can't solve it. Sometimes, the old parts will fail one by one like dominoes because the new parts will stress them too (like new capacitors). Might as well make all of them new - for now.

 

[Dswankey]

This is my first re-cap and I'm not getting specific 'help' on the replies.

I respect you wanting to re-cap, learn etc but I would suggest trying that out on a unit that is not as valued or esteemed as the SX-1250 you might wind up doing more bad then good.

A good source of information here at AK that you might find useful is the dedicated "Pioneer Audio" sub forum. All Pioneer all the time from mostly die hard Pioneer enthusiasts.

https://www.audiokarma.org/forums/index.php?forums/pioneer-audio.90/

You might find this helpful also

https://audiokarma.org/forums/index.php?threads/pioneer-sx-1250-recap-and-restoration-thread.420950/

 

[elnaldo]

Hello. Can you follow a schematic? If so, you could do some passive tests, checking continuity from the Power In connector along the signal path.

Do you have an oscilloscope, any kind of signal tracer, or at least a multimeter? depending on the tools you have, there are different ways to test the signal along the signal path.

Hello. I was very specific. You mention you don't have an oscilloscope, but you didn't mention if you can read a schematic.

If so, you can feed a low freq sine wave signal, (you can use a phone or a computer to generate a sine wave), let's say less that 500Hz, try 100Hz (most multimeters are accurate enough at those low freqs) without speakers connected (you can test with the speakers for a second to confirm the problem is still there). Then, you can start checking the signal level with your multimeter (set to measure AC voltage, audio sine waves are AC) at different stages, comparing the signal level between channels, from the power amp input connector, all the way to the output. Until you find a difference between L and R.

There are no magic recipes, nobody can tell you where the problem is, you need to diagnose and fix the fail in person. We can tell you what to do, where to measure, but you need to test, measure, give objective data. Replacing components won't fix your problem and will probably introduce new problems.

You mentioned you replaced all the resistors and transistors, well, that make 100 different new chances of a mistake. Very different than diagnosing a unit that was untouched from factory. I make mistakes after 10 years of repairing all kind of devices including very expensive units, so I assume somebody with less experience can make a mistake too.

Take the shortcut, and start measuring the signal until you find where it's lost.