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SX-838 troubleshooting assistance please

goat67 said:
I say you are ready lets hope we got it all covered.
Click to expand...

Ok - I've dumped it into an Excel spreadsheet, and boiled it down to a BOM list.

Mark/forum moderators - do you wish a copy for archival purposes? If so, PM me and I'll email you a copy. Also - is there a "build-sheet" repository on the forum? Would it be worthwhile to bank parts lists by receiver models, in order to help the newbies like me?

Here's the order of battle I envision. First, I replace the transistors and re-cap the power amplifier board to see if we've killed the problem and stabilized the receiver. Now - once that is accomplished, do I check/do voltage adjustments, or do I continue to re-cap board by board? Is there a preferred order to the process?
 
Uncle Bambi said:
Ok - I've dumped it into an Excel spreadsheet, and boiled it down to a BOM list.

Mark/forum moderators - do you wish a copy for archival purposes? If so, PM me and I'll email you a copy. Also - is there a "build-sheet" repository on the forum? Would it be worthwhile to bank parts lists by receiver models, in order to help the newbies like me?

Here's the order of battle I envision. First, I replace the transistors and re-cap the power amplifier board to see if we've killed the problem and stabilized the receiver. Now - once that is accomplished, do I check/do voltage adjustments, or do I continue to re-cap board by board? Is there a preferred order to the process?
Click to expand...

Just a note on how I replace components. I do one component at a time.

For caps I look at the board to see if the cap markings match the board for polarity.
If the cap does not match the board put the new cap in like the old.

For transistors.

I get the data sheets for the new transistors so I know lead configuration.
I then look at the schematic and board to check verify current transistors lead configuration.

I verify new transistors by doing a transistor test. I have a transistor tester so its easy for me.

This test does two things it test the transistor and makes sure you have the lead configuration correct.

Steps to take

Recap and replace transistors on the power amp board.

Turn volume control all the way down

Hook unit up to DBT verify If the unit comes up OK that means the bulb starts bright then goes dim.

Check DC offset note results you can adjust them a bit to see if you can get them to spec but you will be readjusting later.

Adjust Bias pots verify that Bias can be adjusted watch for no big jumps while adjusting pots.

Turn Bias pots back to set Bias to minimum this must be set without DBT

Turn unit off.

If all this looks good you can bring the unit back up without the DBT and do set DC offset and Bias.

I would recommend you these
565-3782-60-2
565-3782-60-0

Now to complete the recap I do one board at a time and check operation.
Always use DBT when powering up after doing any work.

I would do the rest in this order
Power supply
Protection

These can be done in any order and sometimes you need to move a board to get to another and I may to both at one time

Control Amp
Equalizer Amp (phono)
Mic Amp


This is how I do it.
 
goat67 said:
That's the one should be a sticky
Click to expand...

Yeah - it's a nice one, alright.

So, the order is complete, and now I sit and wait. Even with $10 worth of test clips (thanks for that tip, BTW:thmbsp:), it was less than $40.

Now, let's hope I don't blow stuff up when I commence to working on this thing.

On the Offset and Bias thing - a question.......

Is this the "Aligning the Power Amplifier Section" page in the service manual? It talks about adjusting for 0v and 30mv (left and right channels), respectively, on the amplifier board. Other than that, I don't see any explicit mention of offset and bias.
 
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Uncle Bambi said:
Yeah - it's a nice one, alright.

So, the order is complete, and now I sit and wait. Even with $10 worth of test clips (thanks for that tip, BTW:thmbsp:), it was less than $40.

Now, let's hope I don't blow stuff up when I commence to working on this thing.

On the Offset and Bias thing - a question.......

Is this the "Aligning the Power Amplifier Section" page in the service manual? It talks about adjusting for 0v and 30mv (left and right channels), respectively, on the amplifier board. Other than that, I don't see any explicit mention of offset and bias.
Click to expand...

Yup that is the section
Adjusting for 0 Volts can be called DC Offset
Adjusting 30mv is called Bias or Idle Adjustments
 
OK - the transistors Mark specified arrived today and after testing them per instructions, I very carefully installed them, paying strict attention to the data sheets and the silk-screened orientations on the board. Did not do any caps.

Fired up the unit on the DBT, and it looked OK - turned up the volume and it sounded OK, too. I listened to an AM news broadcast for about 30 mins, and then called my wife over to show off. While she was admiring my work, the stupid thing clicked and went into protection. After a few seconds, it came back - and then clicked off again.

Shut it off, let it sit for a while, and tried again. Now it's going in and out of protection more or less constantly - no crackling, popping, or static like before. Damn - it was working nicely for 30 minutes!!!

:(

I'm bummed. What next?
 
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rBuckner said:
Did you do the offset adjustment yet? If it's too far off it can do what you're seeing.
Click to expand...

No - will do. I didn't want to risk doing damage until I heard from you guys.

OK:

DC offset - s/b 0v is .02/.03v
Idle - s/b 30mv, is .014/.015v

Taken without the DBT, pin to chassis ground, 8 ohm resistors across the speaker leads - as directed by manual.
 
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Question 1:
How long until it goes into protect?

Question 2:
How long does it stay in protect?

If it stays in protect it is easier to fix if it cycles in and out it is hard to isolate.

I bet you are glad you got the transistors for the Protection and PS board now.

Now it is time to get a few voltages from the Power supply and Protection Boards

You must be VERY careful not to short out between pins while testing.
What I do is if I think I can not get the mini grabber on I will shut it off
and then get it connected and turn it on.

I like to get Power supply voltages when I start to troubleshoot.

Power Supply AWR-055
Pin
4 44.0v
5 -44.0v
7 -45.0v
8 -13.0v
10 22.0v
11 28.0v
12 32.0v
13 35.5v
14 13.0v
17 51.0v
21 8.5v
22 7.5v AC

Protection Circuit Assy AWM-025
Pin
2 -13.0v
11 35.5v
12 7.5v AC

The following Pins are the inputs to the board and if you can get readings while in protect and out of protect would be great.
This may take you a bit of time.

Out of protect
1
3
5
6
7
8
9

In protect
1
3
5
6
7
8
9


If I was a betting man I would wager that the fault is in the protection board

Go and read the sx-939 protection thread sounds real similar.
His fix was to replace all the transistors on the protection board.
 
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Replace Q7 on the protection board, most "bang for the buck"

you can add the extra diode later.
 
goat67 said:
Question 1:
How long until it goes into protect?

At first power-up, it played for at least 30 mins, and sounded great. Now, it clicks in and out every several seconds.

Question 2:
How long does it stay in protect?

Varies from a second or two to several seconds.

If it stays in protect it is easier to fix if it cycles in and out it is hard to isolate.

It cycles pretty frequently. :(

I bet you are glad you got the transistors for the Protection and PS board now.

YES.

Now it is time to get a few voltages from the Power supply and Protection Boards

I'll do this as soon as I get home from work.

Go and read the sx-939 protection thread sounds real similar.
His fix was to replace all the transistors on the protection board.
Click to expand...

Answers "in-line". I did read the 939 thread - I'll go re-read it.

markthefixer said:
Replace Q7 on the protection board, most "bang for the buck"
Click to expand...

I'll do it, as soon as the second order arrives. Q7 is in with the build kit I ordered on Monday, along with the rest of the transistors you so kindly recommended.

Just out of curiosity (and if you have time), can you thumb-nail sketch the possible scenario here? You nailed the first symptoms, as after I changed those 6 transistors it worked well for a decent stretch of time. Now, the symptoms are back to being very similar to the original ones, except for the fading, crackling, and popping.

Is it your suspicion that the problem transistors on the amplifier caused other components to fail?

Here's a random observation that may or may not mean anything......

When I pulled Q1 - Q4 out of the amplifier, I was immediately struck with how black the leads were. Not like the other ones. Looking closely at Q1 & Q2 on the protect, I see the same black on the legs.

Does that mean anything?
 
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The black on the leads is just oxidation. I think old semiconductors sometimes had silver plating on them vs. the cheap tin/lead. Of course now the lead is gone.

I have had both leaky transistors and electrolytics cause this type of fault. It looks like the caps are still on their way. I recommend recapping the protection board when you get the parts and then see how it works. If still bad the transistor(s) ought to fix it. If it's still bad look for cracked parts and even a cracked board.

I might be coming to San Ramon sometime this spring, if this is still a mess I can possibly swing by and take a look. No plans have been made at all though!
 
it was 50:50 that the protection cycling was caused by the amp fault, the crackling intensity mustn't have progressed to the destructive phase so to speak.

The extra diode? It prevents -60v or so spikes at the relay driver transistor when the relay's magnetic field collapses, which nibble away at the relay driver transistor for years, until the transistor is so weakened that it cannot work reliably. Somewhat heat related, it is the #3 behind the power supply faults and the problematic transistors.

Higher end Pioneer models included the diode as part of the design.

The trade off is a slightly (milliseconds) slower relay release.
 
Lordy, lordy... what an education I'm getting. Thanks, guys.... I really, REALLY appreciate all this.

Since I was smart enough to take Mark's advice and order all the transistors, I think I'm going to go ahead and shotgun the protect board and change all the transistors when I do the caps. It's easy to get to, and these things cost so little - why not?
 
Just checking in......

I re-capped the protect board, power supply, and the amplifier board today. While I was on the protect board, I shotgunned the transistors and replaced them all. I realize there is danger in doing all of them at once, as it increases the odds that I might make a mistake. However, I figured that because they were all easy to reach, were cheap, and were all ECB configured, it outweighed the negatives.

Ok - so far, so good. I powered it up after the protect board was done, and played it for 30 minutes without issue. I then shut it down and did the amplifier. I've been listening for 35 minutes so far.......

Keeping my fingers crossed!!


EDIT: 42 minutes, and it went back into protect. :(

Could it be one of the original transistors I replaced on the amplifier board going bad?
 
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Answers in-line.

There is a big difference on the protect pin 9 depending on if it is protect or not.

Here's what I have on the amplifier board:

Pin 01 +40.2
Pin 02 -40.2
Pin 09 -41
Pin 12 -7.8
Pin 14 +36.5
Pin 17 -41
Pin 24 -40
Pin 25 +40.4
Pin 26 -40.3
Pin 29 +40.2
Pin 34 +40.2
Pin 37 -40.1

While the symptoms are similar - it goes in and out of protect - it doesn't do it as rapidly as before. It goes into protect, and then comes out and stays out for a lot longer than before - a few seconds before, now half a minute, a minute, or more.

goat67 said:
Power Supply AWR-055
Pin
4 44.0v 40.3
5 -44.0v -40.5
7 -45.0v -41.3
8 -13.0v -8.1
10 22.0v 14
11 20.0v 29.2
12 32.0v 33
13 35.5v 36.2
14 13.0v 13.7
17 51.0v 48.2
21 8.5v 7
22 7.5Vac 7.5Vac



Protection Circuit Assy AWM-025
Pin
2 -13.0v -7.5
11 35.5v 36.4
12 7.5v AC 7.4Vac

The following Pins are the inputs to the board and if you can get readings while in protect and out of protect would be great.
This may take you a bit of time.

Out of protect
1 .3
3 .3
5 .3
6 .26
7 .26
8 .26
9 13.1, but it seems to range up and down, and when it gets into the upper 20's, the unit goes into protect

In protect
1 .3
3 .3
5 .3
6 .26
7 .26
8 .26
9 36.5
Click to expand...
 
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Well rats! :sigh: Probably something breaking down on the amp board, like the differential transistor pair if it has one. I'm not familiar with this exact model to be able to guide you specifically. Time for an oscilloscope maybe!
 
Uncle Bambi said:
Answers in-line.

There is a big difference on the protect pin 9 depending on if it is protect or not.

Here's what I have on the amplifier board:

Pin 01 +40.2
Pin 02 -40.2
Pin 09 -41
Pin 12 -7.8
Pin 14 +36.5
Pin 17 -41
Pin 24 -40
Pin 25 +40.4
Pin 26 -40.3
Pin 29 +40.2
Pin 34 +40.2
Pin 37 -40.1

While the symptoms are similar - it goes in and out of protect - it doesn't do it as rapidly as before. It goes into protect, and then comes out and stays out for a lot longer than before - a few seconds before, now half a minute, a minute, or more.
Click to expand...

Power Supply AWR-055
Pin
4 44.0v 40.3
5 -44.0v -40.5
7 -45.0v -41.3
8 -13.0v -8.1
10 22.0v 14
11 28.0v 29.2
12 32.0v 33
13 35.5v 36.2
14 13.0v 13.7
17 51.0v 48.2
21 8.5v 7
22 7.5Vac 7.5Vac



Protection Circuit Assy AWM-025
Pin
2 -13.0v -7.5
11 35.5v 36.4
12 7.5v AC 7.4Vac

The following Pins are the inputs to the board and if you can get readings while in protect and out of protect would be great.
This may take you a bit of time.

Out of protect
1 .3
3 .3
5 .3
6 .26
7 .26
8 .26
9 13.1, but it seems to range up and down, and when it gets into the upper 20's, the unit goes into protect

In protect
1 .3
3 .3
5 .3
6 .26
7 .26
8 .26
9 36.5
Click to expand...

Pin 9 on the protect board is OK what you are seeing is the relay voltage drop

I want you to verify that the transistors you installed are correct please list what part numbers you used

Q1 =
Q2 =
Q3 =
Q4 =
Q5 =
Q6 =
Q7 =

Can you take pictures of the board so I can see how they are installed?

From the voltages I get this but I want to verify your work first.

What is Not OK is Pin 8 on the power supply -8.1v should be -13 volts

So

D8= 13V 1/2W Zener Diode. Part# 512-1N5243BTR $0.04ea

Also I do not like that Pin 10 is at 14v instead of 22v please re-check
I have concerns that something is pulling voltage this down.

BTW is this a multi voltage unit? what I mean is on the back can you set the voltage?

I hope Mark comes by and adds his thoughts unless you have a 13v Zener you are on hold for parts.
 
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