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My first attempt at SMD repair. How would you do it?
My first attempt at SMD repair. How would you do it?
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Old 19th September 2019, 09:28 PM   #1
StewartH83 is offline StewartH83  England
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Default My first attempt at SMD repair. How would you do it?

Hello people.

I decided to look at a Pioneer A-109 for a neighbour which won't power on, it's not something I'd like to own yet he likes it and with he being a vet, I'd love to help him out

The problem is 4-6 resistors are burnt out in the speaker protection circuit under the board.

Would you replace the resistors with SMD?
I was thinking it's better to replace with normal fixed resistors as there's room.

Also how would you ground them? Clean the original ground points or re-route?

Here's some pics and photo of the printed schematic
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Old 19th September 2019, 09:31 PM   #2
StewartH83 is offline StewartH83  England
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Couldn't upload on original post:
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Old 19th September 2019, 09:46 PM   #3
kevinaston1 is offline kevinaston1  United Kingdom
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From the look of the circuit, this would appear to be the output Zobel network. The burnt resistors mean they have been passing a lot of current.

If you are lucky, the Zobel capacitors could be leaky, but it is much more likely that the output is oscillating. If this is the case, the new resistors will cook until you clear the fault causing the oscillation.

Alternatively, does the owner have the volume welded at 11? if so, you will never cure the cooking resistors, it is just the amplifier clipping and oscillating.

Kevin
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Old 19th September 2019, 09:55 PM   #4
StewartH83 is offline StewartH83  England
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Hi Kevin, thanks for the reply, very interesting.

All he said was the tone controls was not working, he got someone to look at it and now don't power on when it's returned.

I had no continuity on the small caps on the other side of the board 0.22uFs I was going to replace.

I feel the tone control problem was a still button as it's locked in place and will clean with Servisol or replace

Thanks again
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Old 20th September 2019, 12:13 AM   #5
jaycee is offline jaycee  United Kingdom
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AS the pads are clean burnt off the board, you would be best to replace with a through-hole part. However as kevinaston1 says, if those resistors burnt like that it is a symptom of a deeper problem
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Old 20th September 2019, 01:06 PM   #6
StewartH83 is offline StewartH83  England
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Thanks for your input too jaycee, well appreciated

I was guessing the owner driven the low powered amp hard and/or with 4 ohms speakers which is not stated to handle.

All other components appear fine, maybe a dry SMD cap on the tone control board needs reflowing. I have a feeling this IC near the direct switch is faulty.

I have thousands of new cheap resistors but no 1.5 ohms haha. I'll order some through hole 1/4W or 1/2W. Replace the mylar 0.22uf caps I have spares of and hope that will just be it. Not worthy shotgunning more parts I feel.
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Old 20th September 2019, 05:43 PM   #7
Ylli is offline Ylli  United States
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Replace with a single 4.7 ohm 1W.
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Old 20th September 2019, 05:52 PM   #8
Mooly is offline Mooly  United Kingdom
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My first attempt at SMD repair. How would you do it?
I wouldn't invest much time/expense at this stage...

Those burnt resistors look very bad news to me. Anything in the 3 to 10 ohm ballpark should be fine to keep an otherwise OK amp stable.

You say it doesn't power on which suggests something could have failed short circuit and taken out any fuses that might be in there. First check for me would be to see if the output transistors are short circuited.
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Old 20th September 2019, 06:39 PM   #9
jaycee is offline jaycee  United Kingdom
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That IC near the direct switch looks like the system microcontroller. If that's dead, game over.
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Old 20th September 2019, 08:33 PM   #10
StewartH83 is offline StewartH83  England
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Thanks Illi, Mooly and again Jaycee

It's definitely time I invested in a new multimeter as my 20 year old Fluke finally packed in last night, the screen is dead, could only get the beeps from continuity. I'll get a decent one ordered. The scope will have to wait haha

I'm thinking the output transistor test will need to test voltages not just continuity. Very likely they are gone.

How will I test the microprocessor chip?

4.7 ohm @ 1 watt seems easier and makes sense

Great help guys, I'm sure the vet down the road will be happy with you helping me out
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