Pioneer SX-750 Popping/Crackling ReCap Question

[K-Schick]

When ever I read about these sort of statements I wonder if you were to use instruments to test, would you measure anything different between channels?
Is there anything different in the components that you used on both channels that could explain what you are experiencing? It is possible that you have a bad component, how to isolate it, is a problem.

You are absolutely right. I got in a bit of a rush last night to post that. Instead of posting right away, taking measurements would have been more helpful to both myself and everyone else. Ill gather some test voltages tonight when I get off work, and update you all.

I used the same components for both the left and right channels, so unless I got a bad cap in the lot, that shouldn't be the problem. I totally agree, the tough part will be tracking down the problem child.


On the Crosstalk: I thought thats what it was called, but I didnt know for sure if that was the proper nomenclature. I completely agree that it will not be an issue if it is just inherent in the circuit/receiver. I can live with that, and Ill probably never notice it. I was just asking in case anyone else has happened to experience that, and had found a problem. Im all about addressing issues while Im working on it, as opposed to buttoning it all up, then coming back later and taking it all apart again. But then again, Im kind of OCD sometimes, haha.

On the Caps: I am the furthest thing from an expert on this subject. I am learning as much as I can, and I am just trying to learn from, and to follow in the footsteps of people who have a lot of experience, and have done this several times before. I am not a person that believes that I will notice any magical, gigantic difference in performance or perceivable sound quality from using different Caps, but on the other hand, I am brand new to the vintage receiver game, so I just wanted to avoid buying a crappy brand, or a 'cheapo' or a 'lemon' capacitor. I was asking more for reliability, robustness, material, and, of course, Id like the sound quality to be good. I would prefer that the durability and the sound quality not suffer from a bonehead selection that I made because I didnt know what I was looking for. So, maybe I went overly OCD on the Cap selection, (sometimes I am known to do that), haha. But I am happy with what I got, and I feel confident that they will work and not cause any issues for me. I already have most of my Caps, and I have already placed the order for the remainder, so I have thankfully crossed that bridge, and Im calling it good.

On the PS Recap first: I have read that too, and I agree with you 100%. The only reason I didnt go for that first, was because I was trying to tackle that Popping/Crackling issue before I got too deep into everything else. Do you think the PS not being recapped yet could cause this issue in only one channel? If thats the case, Ill just do that next.

Thank you for your response, and your insight! I appreciate you reading through, and making contributions to help me out.

 

[K-Schick]

Zebulon1,

I was wondering about the Tone circuit too. It sounds like the Left channel is just EQed badly. It still has oomph and volume, but Its just missing some mids/highs. IIRC, when switching the Tone Circuit ON/OFF last night, it did not change the sound I was hearing in the Left Speaker. Would this rule out the Tone Circuit?

Tonight after work I will take some voltages, and do more tests per your post, and Ill report back.

Thanks for your help!! Your time is much appreciated. I could not do this without you guys.

-K

 

[zebulon1]

If switching the tone on and off then it maybe in the left amp.
Does the left channel sound cheesy with the tuner as well, Inputs?

 

[K-Schick]

Yep, we can rule out the Tone Circuit. Sounds the exact same whether the Tone is On or Off. I swapped the speakers Left to Right, and I also tried both A and B Speakers. I tried FM, Tape 1, Aux, and Phono, and its behaves the same, no matter what.

I am taking Voltage measurements now. Ill report back shortly.

 

[K-Schick]

One quick thought: Is there ANY way that the Power Supply could be causing this issue? Should I re-cap the Power Supply board first, then check this? If not, Ill proceed with taking my measurements.

I just dont want this all to be for naught, if theres a chance that the PS could be causing this. If so, Ill re-cap the PS, then take measurements.

Thanks all!

 

[K-Schick]

Test Readings:

DC Offset:
L: 4.5 - 7.5 mV (Not stable, jumped around alot in this range. Stayed mostly in the 6's)
R: 27.9 mV (Very Stable)

Power Amp Board AWH-046:
Pin #.....Stock Voltage......My Voltage
3...................41v......................37.6
8..................-41v....................-37.5
13................-41v....................-37.6
18.................41v......................37.7
21.................21v......................21.6
25.................35v......................32.7
28................-19v ...................-18.7
29................-38v....................-36.9
32.................35v......................32.7

DC Offset is the obvious thing that jumps out at me. When I first got the receiver and took measurements, both channels were around 27 mV. Could it be that the DC Bias was previously set to compensate for some caps that were bad, and when I replaced the caps on the Power Amp, it threw the DC Bias off?

I understand that a lower number is better when we are talking about DC Offset, but the number has changed since I have recapped the Power Amp.

Any input from you fine folks on where to look next would help me tremendously.

Thanks in advance!

-K

 

[zebulon1]

The offset is not adjustable and the readings you took are OK.
What about the Bias settings. There is a procedure in the SM.
So far it's checking out.

 

[K-Schick]

Update:

I have adjusted Idle Current, per the Service Manual.

VR1 (Left Channel) Made little to no adjustment to the Idle Current. It went from 0.1mV - 7mV from its lowest to highest ends of adjustment.
VR2 (Right Channel) Adjusted appropriately, and I adjusted it to 30mV, per the SM.

DC Offest is:
Left: -2 mV (Why negative, does this mean anything?)
Right: 25.6 mV

Both VR's worked when I tested resistance through them. They both went approx 10 ohms at their lowest, to approx 90 ohms at their highest. So, we know that both the VR's are good, but something else is preventing the proper Voltages to get to, or from, VR1.

Does anyone have any ideas on where I can look next?

I am leaning towards Transistors in that channel, but Im not sure where to start on testing them. (Should I remove them to test? Should I just take Voltages?) The SM shows Voltages for their Bases, but not for the Emitters or Collectors.

My suspects are these: (All on Left Channel)
Q1 - 2SA 798-F
Q3 - 2SC 1438-V
Q5 - 2SD 357-C
Q7 - 2SB 527-C

And Output Transistors:
Q1 - 2SD 388-R
Q2 - 2SB 541-R

Should I keep my focus on these, or is there anything else to look into?

Would Voltages on those Transistors help you guys help me, or do you have any other suggestions?

-K

 

[zebulon1]

Did you repaste the outputs?
If not check the left channel ones for condition, if they check out, then swap them for the other channel.
Don't worry about pasting them until you sort the trouble out (You may need new outputs), just make sure you use the old pads and insure the collectors of the outputs are insulated from the heatsink.
Use the DBT when checking the idles for operation. It's called "Tickling the dragons tail".
You should see equal readings and adjustment of the Bias settings on the DBT but you will not achieve the Bias setting as the DBT is limiting the current.
Remember to dial back the bias trimmers to zero mV's before plugging the set into the house power.
Then after you have it repaired you can reset the Idles (Bias) to the spec once on full power.
This should tell you if the outputs or driver board is the culprit.
Some guys might blanket the channels with new transistors but its best to fix it first, or at least figure it out and then decide where you want to go with it.

 

[K-Schick]

Thanks for the help Zebulon.

I have not been using a DBT, so hopefully I have not caused further damage. *Moment of panic*

I will make one immediately. I already have the supplies I need, so Ill do it tonight after work. Then, Ill go through the steps you've laid out.

I have not pulled out, or touched the Output Transistors at all. (Other than unplugging their wire harnesses to remove the Power Amp board for soldering in the new Power Amp Caps).

I completely agree with trying to find the problem first, before swapping out all the parts. I like to know the 'why' of what is causing the issue, so this is helping me learn a lot, on top of hopefully fixing the amp.

Ill report back my findings tonight. Thanks again for guiding me through this!

 

[zebulon1]

Some will see this exercise as long winded but it will accomplish a few things at one time.
First you will have checked the outputs for serviceability.
Swapped them for confirmation.
And the set might end-up working per spec. plugging and unplugging the outputs might be all it needs.
We'll soon find out.
The DBT is a requirement, glad to hear you are not arguing about it.
Note: Whenever you make any electronic, (circuit, component) changes to the set, always check it on the DBT, even if it was a small task (Reinstalling a board ect). You never know what you may of forgot to do.

 

[PTL-Pioneer]

I noticed something when carefully listening. The Left Channel sounds different than the Right. The Right sounds full and articulate, but the Left sounds like it has a wet blanket draped over the speaker. The mids and highs are not as pronounced and articulate as they are in the right channel. They are there, but they just dont sound anywhere near as good, or as "sweet" as the Right channel. This is for all Inputs; Ive tried FM, Phono, and Aux, and Ive put the receiver in Mono mode while checking this. Its hard to hear when both speakers are going, but I noticed it right away when panning Left and Right during testing. I have tried swapping speakers

I am having this exact problem with the SX5560 that i have, one channel is definitely stronger than the other, it comes and goes. Watching this thread with interest, great information here

 

[K-Schick]

I'm completely fine with going through all of the necessary exercises. I like to learn this way, so I really appreciate your time and your help with this.

And yeah, very true. If that's all it needs, then that's a win in my book!

And about the DBT, I 100% agree. It was one of those things that I was meaning to do for a long time, then I kind of forgot about it. I first heard about them while researching vintage guitar amplifier repair.

Bonehead Move:
After reading your last sentence, I remembered something stupid that I did way back when I first started tinkering with this receiver, and I think this may be my fault...

I had the Power Amp board unscrewed and just sitting in place, and a few days later I came back to it, and I forgot that I didnt have the board screwed in, and I turned the receiver on. It sounded weird and distorted (especially in the Left Channel). It did make sound though. I could still hear the radio playing out of the Left Channel, just very distorted. Thankfully, there was not any 'magic smoke'. Since it sounded strange, I remember turning it off, and upon further inspection, I realized that I had not connected the Output Transistors!. Not only that, but pins 2,3, & 4 (the back of the Q1 Output Transistor wiring harness) were grounding out on the heatsink! Possibly even Pins 7,8, & 9 for OT Q2. Stupid. Stupid. Stupid...
The DBT may have come in handy there.

After buttoning it all up (as I should have done originally), it seemed ok, so I never gave it a second thought until now. Now that Im listening more critically though, I wonder if Im noticing something that was a result of my own past idiocy.

Maybe this will end up being information that helps us pinpoint the problem as we move through this. I would guess that we may be taking a closer look at Q5 and Q7 later on...

In the meantime, Ill build that DBT, turn my VR's to zero, and make my way through testing & swapping those Output Transistors.

Thanks again for all the help.

-K

 

[K-Schick]

I am having this exact problem with the SX5560 that i have, one channel is definitely stronger than the other, it comes and goes. Watching this thread with interest, great information here

Great, thanks! Glad it can be of some use to others.

 

[K-Schick]

Update:
Last weekend I didnt have time to progress much, but I did make some progress:

I made a DBT, and the unit is safely plugged into it at all times during testing now.

Also, I was able to swap the Output Transistors, (Left to Right, and Right to Left).

The problem stayed in the Left channel. It behaved pretty much exactly as it did before. As of right now, I have only taken measurements on the Left Channel during the Bias Adjustment. When adjusting VR1 from bottom to top, the voltages stayed very low, never going above 2mV. I have not listened to the amp, but from this, I believe we can conclude that the OT's are not the culprit, and are probably OK.

I will adjust the Bias on the Right Channel to make sure it still works correctly, (hopefully tonight), and report back my findings.


Let me know what you guys think, and if you have any comments or suggestions on these findings.
As always, your thoughts are much appreciated.

-K

 

[zebulon1]

A wonderful set with a simple amp circuit. Perfect to start the hobby on.
Are you game for more trouble shooting? What are you thinking about the set in general? A rebuild?
The pre-amp wouldn't cause the Idle issue you have so the problem is in the amp.
You could swap components, there isn't many. Or wholesale replace all two drivers and diodes.
The diff pair probably is not the problem as they are in charge of the balance.

One thing if you are careful: On the DBT - connect your meter (DC V's) (Micro clip probe to the collector), (Black probe to chassis ground) to the the collector of the left channel Driver Q3. Adjust the trimmer VR1 and watch the meter. Does it show a good spread from min to max V's?
Nominal is -1.2 volts.
You can use the good channel driver to compare. If fact, do the good channel first and get a idea of what to look for.
Remember to take your time and be careful, no reason to be in a hurry. If you don't have the tooling like the micro clips, get some. They can be found cheap.

 

[K-Schick]

I am absolutely game for more troubleshooting!!!

It really is a super cool receiver. I looked around at several different years of these Pioneers, and I really liked these SX-X50 series. I love the aesthetic of them & how they look, and especially how they sound!

In general what I want to do is a fairly thorough restore on it. I dont need to replace every component on it, but Im game to try a full recap. (I currently have an entire BOM List of caps in my possession that I compiled from MarkTheFixer's lists, and with help from other people's posts). Ive already done a handful of Transistors (super not fun), but Im game to do some more, if it makes this thing sound as incredible as I know it can sound!! I think I may even build and stain an oak cabinet for it.

Im really looking for this thing to be able to pull double duty: I want it to be my daily driver for some nice headphones while Im at work, but Id also like it to be able to beautifully power some speakers too.

As for your next recommendation:
When you say Left Channel Driver Q3, Im assuming you meant the Output Transistor, correct?

Ill check it out and get right back! I have some clips, they arent anything super fancy, but they work, and Im careful with them. I agree, taking your time is very important! (As I learned the hard way). Ill try not to hurry too much. I just get excited when I can tell Im getting close to a resolution, haha.

Ill report back.

 

[zebulon1]

The transistor I'm referring to is a smaller driver transistor on the main board (Q3 and Q4), not the bigger TO-3 outputs on the heatsink.
I think it's a TO-92B style package.

 

[K-Schick]

Ok, so I have taken measurements on Q3 and Q4.

Q3 Collector: When exercising VR1, went from -1.04V to -1.16V. Not quite to the Nominal -1.2V.
Q4 Collector: When exercising VR2, went from -1.07V to -1.2V. easily. I could have easily adjusted it even further than the Nominal -1.2V if needed.

Q3 Voltages with VR1 turned all the way down, on the DBT:
B: -35.95
C: -1.04
E: -36.5

Q4 Voltages with VR2 turned all the way down, on the DBT:
B: -35.6
C: -1.07
E: -36.2

It seems to be that it's the Trimpots, and their adjustability that make this equation different for each channel.

Questions:
Since the Q3 Collector Voltage for the Left Channel was not able to be adjusted to spec like the Q4 Collector for the Right Channel, does this mean that my Q3 is bad? Are there any other tests we can do?

When you say: "replace all two drivers and diodes", which two transistors and diodes are you referring to? And do you have any recommendations for proper replacements?

Thanks again for all the help!

 

[zebulon1]

Clip on to the emitters of Q5 and Q7 one at a time and perform the same procedure.
They are the TO-220 package's