Beginners Road To Repair

[don@cascadefellowship.org]

There are two connectors from the 'regulator' board to the main board, disconnect those and see if the short disappears.

I have a suspicion it won't.

There are two yellow blobs on the regualtor board near the jack inputs, they're tantalum capacitors, C29 and the other next to the QC sticker.

I would be desoldering those if the short doesn't disappear when you unplug the main board.

I have disconnected the two boards and the short remains. I've taken the 7812 off the board and the short remains.  I'll pop the C29 and C46 capacitors off the board (testing after each one) to see if the short remains.

[CJay]

If nothing else we may have isolated the fault to the smaller board so that's just made the problem a lot simpler to troubleshoot as there are far fewer parts on it

Hopefully it's one of the two tantalum caps.

If that doesn't sort the problem out then it would be useful to be able to see the track side of the board as well as a decent close up of the top side, it helps us as we can trace the circuit out then and make better suggestions

[don@cascadefellowship.org]

There are two connectors from the 'regulator' board to the main board, disconnect those and see if the short disappears.

I have a suspicion it won't.

There are two yellow blobs on the regualtor board near the jack inputs, they're tantalum capacitors, C29 and the other next to the QC sticker.

I would be desoldering those if the short doesn't disappear when you unplug the main board.

I have removed both tantalum capacitors and the short remains between ground and output of 7812.

[CJay]

OK, just spotted another tantalum (they're nasty little things prone to shorting out so they're the first place to look if you spot them), it's next to the leftmost switch, I think it's C32, lift that one.

Be aware they're polarity sensitive and will need to go back in the same way round. .

Can you post a pic of the track side of the board please?

[don@cascadefellowship.org]

OK, just spotted another tantalum (they're nasty little things prone to shorting out so they're the first place to look if you spot them), it's next to the leftmost switch, I think it's C32, lift that one.

Be aware they're polarity sensitive and will need to go back in the same way round. .

Can you post a pic of the track side of the board please?

I've removed the third tantalum and the short remains.  I did not pay attention to their polarity when removing them.  I assume I can test for polarity? Here are the closeups of the board.  I'll have to post them in three posts (two pics to a post).  I just noticed something.  Take a look at the ground pin hole of the 7812. There seems to be another component hole right next to it and they're both connected by solder (there was no component in this other hole).

[don@cascadefellowship.org]

Next two.

[don@cascadefellowship.org]

Last two.

[CJay]

OK, shame and I should have thought to mention about the capacitors being polarised but we can deal with that in a bit.

Handily the two 8 pin chips are connected to the 12V rail with a wire link each so we can lift one end of them to isolate the chips, I've circled them

[BrianHG]

If it is not the caps, I can only see that IC6 is blown pin 5 to 8 as a short, but how?
This is so rare to near impossible with all the other IO connections to that op-amp having huge series resistors.
Being CMOS, the way it is hooked up, you would need an absurd jolt to make that happen.

[don@cascadefellowship.org]

OK, shame and I should have thought to mention about the capacitors being polarised but we can deal with that in a bit.

Handily the two 8 pin chips are connected to the 12V rail with a wire link each so we can lift one end of them to isolate the chips, I've circled them

I've lifted one end of each of the wires and the short remains.

[CJay]

Then I think there's only C38 left...

[BrianHG]

Then I think there's only C38 left...

Seriously, there is like nothing else.  If this isn't the problem, or by some fluke even C40 isn't the problem, we may be looking at a hidden grown 'Tin Whisker' short between 2 adjacent pads somewhere on the PCB.

[james_s]

The best way to learn to repair things is practice, practice practice, with each success (or failure) you will learn something. Look around on trash day for stuff to scavenge, or post an ad in a local forum asking for free broken electronics to practice on. It took me a lot of practice to get to where i was fixing more things than I was breaking worse than they started.

[don@cascadefellowship.org]

Then I think there's only C38 left...

Isn't it always the last thing you check!?  LOL.  After removing C38, the short is gone!  I'm going to re-install the components I removed in reverse order checking after each component to make sure things are still cool.

[don@cascadefellowship.org]

OK, shame and I should have thought to mention about the capacitors being polarised but we can deal with that in a bit.

Handily the two 8 pin chips are connected to the 12V rail with a wire link each so we can lift one end of them to isolate the chips, I've circled them

Now how do I find out the polarity of these tantalum caps? And, equally important, how do I tell which way they go back in the PCB?  There's no indicator on the board for polarity like the other caps and I'm not sure how to read the schematic to determine which way they go in.  For example, I can see that C32 feeds into the positive pin of IC6A while passing R63.  Can I assume that the IC6A side of the tantalum cap is supposed to be the positive side?

[CJay]

If there's no polarity marking on the board they may not be polarised but if you can post a pic of any writing on the caps we will be able to work it out.

Oh, put the links back one at a time and check to kake sure the sort doesn't raeappear (I don't think it will), the reconnect the larger board and check again.

Put the regulator back in and check, then I think you can probably power it up...

[TimNJ]

You should find another point on the circuit which you know is connected to either end of the capacitor and test continuity between that point and either of the capacitor's pads on the PCB. You should be able to determine which pad is for + and which is for - based on that. For instance, the schematic tells you that pin 6 of the BBE 2153 chip is connected to V+ (the positive supply rail). Put  one of your multimeter probes on pin 6, and the other on either pad of C38. If it beeps, then it's connected.

Based on the fact that one tantalum already popped, I'd probably replace them with aluminum electrolytics. 100uF/16V is readily available in that size.

[CJay]

You should find another point on the circuit which you know is connected to either end of the capacitor and test continuity between that point and either of the capacitor's pads on the PCB. You should be able to determine which pad is for + and which is for - based on that. For instance, the schematic tells you that pin 6 of the BBE 2153 chip is connected to V+ (the positive supply rail). Put  one of your multimeter probes on pin 6, and the other on either pad of C38. If it beeps, then it's connected.

Based on the fact that one tantalum already popped, I'd probably replace them with aluminum electrolytics. 100uF/16V is readily available in that size.

No tantalums popped, it was an electrolytic. I'm now not sure the yellow capacitors were tantalums either so let's not confuse matters.

[don@cascadefellowship.org]

You should find another point on the circuit which you know is connected to either end of the capacitor and test continuity between that point and either of the capacitor's pads on the PCB. You should be able to determine which pad is for + and which is for - based on that. For instance, the schematic tells you that pin 6 of the BBE 2153 chip is connected to V+ (the positive supply rail). Put  one of your multimeter probes on pin 6, and the other on either pad of C38. If it beeps, then it's connected.

Based on the fact that one tantalum already popped, I'd probably replace them with aluminum electrolytics. 100uF/16V is readily available in that size.

No tantalums popped, it was an electrolytic. I'm now not sure the yellow capacitors were tantalums either so let's not confuse matters.

Here's a close up of C29, C32 and C46. The text is very hard to make out but it seems there's just a number.  Disregard the sharpie mark on the left side.  I was doing some research online and found a post that indicated that when looking at the number side, the negative might be on the left.  So I marked then with a sharpie.

[TimNJ]

Ah I'm sorry for the confusion. Misread a post above.

Those are definitely just standard multi-layer ceramic capacitors (MLCCs). They're fine!

105 is just the capacitor code. 105 means 1uF.

See here: https://synthrotek.com/wp-content/uploads/2012/08/Capacitor-Codes.pdf

[don@cascadefellowship.org]

Ah I'm sorry for the confusion. Misread a post above.

Those are definitely just standard multi-layer ceramic capacitors (MLCCs). They're fine!

Ah yes!  After googling some images for tantalum and for mlcc I can see that, even though they may be similar, you can see some obvious differences.  I don't believe I have to worry about polarity with C29, C32 and C46.

[CJay]

What he said, they're not polarised

[don@cascadefellowship.org]

If there's no polarity marking on the board they may not be polarised but if you can post a pic of any writing on the caps we will be able to work it out.

Oh, put the links back one at a time and check to kake sure the sort doesn't raeappear (I don't think it will), the reconnect the larger board and check again.

Put the regulator back in and check, then I think you can probably power it up...

I've put the ceramic caps and the regulator back in and reconnected the boards.  So far the short has not reappeared.  Time to buy a new electrolytic cap and see what's what!

[Dubbie]

Nice one.

These repair threads are always fun!

[StillTrying]

I don't think there's much reason why you couldn't test it without the 100u. Connect the DMM to 0V and 12V points before applying power, and switch off quick if it's not 11.5V to 12.5V, and keep eyes and face away from the PCB just in case!