Power Circuit Board Mystery, Harman Kardon HKTS220 SUB/230

Yoake10

New Member
Here is a power circuit board mystery to solve!

For 3 months now I have been trying to repair and error search Harman Kardon 220 sub power circuit board without any luck at all. Therefore I need some more electric minds and brains to solve this problem. I will attach the schematic below with the replaced components and voltage measurements.

Anyone have any idea whats going on?
The problem started with R23 burning out and recitifer bridge BR1 broken. It should be 35 VDC at DN4 and 3,3V DC at CN2.



 
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@rjenkinsgb

What is the next step? Any more ideas or you know anyone who have more knowledge?
I tried to switch out the TL432 or a TL431, it should have the same electrical specifications. When TL431 is connected it gives me 6V fluttering on CN2 Pin 1 (CN2 not connected o main circuit board) and around 33V on 15VS (D4). I find it strange. Could the TVS diode have anything to to with the fluttering? Messing with the feedback?

And when CN2 is connected i get fluttering 4,8V at CN2 pin 1 and 40-47 Volt at CN4. The volt at CN4 goes up to 47 then down again to 40, when it reaches 40 something starts (electric sound) and when reaches 47 it stops, and it repeats that way.

I have repaced C13 and C15 aswell because it could mess upp the feedback.
 
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Can you unplug CN2? - that will take the load off the standby PSU, and see what the voltages are like then - and of course put the TL432 back so the voltage regulation is correct.

I presume the main reservoir capacitors are OK?, if those are O/C or low capacitance, then strange things can happen.
 
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Thank you for more help.

More info above and I think i have that readings in previous pictures, but here is answer to your question:

Without CN2 connected:
Around 6,5 V fluttering with TL431
2,5-3,1V fluttering with TL432

With CN2 connected:
Around 4,8V fluttering with TL431 (And I acutally get 47V at CN4 but not constant, more info above)
0,5-1,8V fluttering with TL432



I am not familiar with the expression main reservoir capacitors, could you give me their names from the schematics?
 
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Okey so your hypothesis is that PGND is unstable and therefore interfere with Feedback on U1? I will take a closer look at the R1/R4 aswell, if they really do 1Mohm in resistance. and get back to you.
 
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Yoake10 said:
Okey so your hypothesis is that PGND is unstable and therefore interfere with Feedback on U1? I will take a closer look at the R1/R4 aswell, if they really do 1Mohm in resistance. and get back to you.
Click to expand...

No? - R1/R4 are just to balance the voltage across the series connected reservoir capacitors.

If those capacitors were O/C or very low, it could cause your issue.

However, I'm a bit concerned about R23 having burnt out, that's a component which never fails - unless perhaps C30 is S/C. Obviously though, it's not part of the standby circuit, and shouldn't affect the standby supply.
 
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Okey.

O/C = Open Circuit?
S/C = Short circuit?

So what is left to look at the stanby supply, almost everything has been replaced with original parts.
The T1 transformer?
 
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Yoake10 said:
Okey.

O/C = Open Circuit?
S/C = Short circuit?

So what is left to look at the stanby supply, almost everything has been replaced with original parts.
The T1 transformer?
Click to expand...
Transformers only VERY, VERY rarely fail - and when they do it's usually only one specific type in one specific unit, and it would be unlikely to cause the supply to pulse.
 
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I'd suggest a new TL432 to rule that out - and do not connect CN2 if the "3.3V" supply is anything more than a few percent above that, as you may blow the main processor, which that supply feeds!
 
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rjenkinsgb said:
I'd suggest a new TL432 to rule that out - and do not connect CN2 if the "3.3V" supply is anything more than a few percent above that, as you may blow the main processor, which that supply feeds!
Click to expand...
Yes I will do that, to see.

It seems like the "old" TL432A T0-92 is hard to buy/doesnt exist any more. Because if you read datasheet you will get the information I mentioned before about TL431 and TL432 has the same spec (Vref 2,5 V).
--> https://datasheet.octopart.com/TL431CLP-Texas-Instruments-datasheet-58987.pdf


But I think the "old" TL432A (Vref 1,24V) I have is this one:

TL432 Datasheet(PDF)

Part #: TL432. Download. File Size: 611Kbytes. Page: 9 Pages. Description: Low Voltage Adjustable Precision Shunt Regulator. Manufacturer: IK Semicon Co., Ltd.
www.alldatasheet.com

This has a Vref at 1,25 Volt, which I have measured at the base pin.

Could this product be a replacement for the "old" TL432A?
 
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Nigel Goodwin said:
I've repaired many thousands of switch-mode PSU's, and I used to keep spares in stock for all the popular faults - and I only ever kept two types of transformers in stock, and they were the only two types I ever had fail.
Click to expand...
Thanks, I'm new to this kind of electrics, I have some education in automation engineering but thats it.
 
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This is my initial scan of this thread. It appears that 3.3VSB/0.1A either is greatly exceeding 0.1A load or else there is a faulty rectifier component between T1-5 and CN2-1. This is because the 15VS is reading 24Vdc meaning that the TL432 must be working and the SB switcher is working almost twice as hard to reach 3.3V but cannot. The most likely candidates are D1, C5 if the CN2 is disconnected,

T1 is a highly coupled transformer that like in all PCs relies on the constant ratio of all output voltages within the design expectations for load currents. Only the primary 3.3VSB is sensed for feedback -3.3 and + 15VS must be lower currents and thus their voltage will indicate an imbalance of loads or a faulty part. This design is good but relies on low cross-load regulation error by tight mutual coupling in T1.

My guess is that from R3 bleeder cap failure, this supply failed from a lightning or grid transient or excess %RH & dust causing surface voltage breakdown of R3 with consequences throughout. It must also be rated for 1kV like C2 and not the standard 0.3" body of a 1/4 W 250~300Vac rated Resistor.
 
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Hi Tony, thanks for more input!

All components between T1-5 and CN2-1 are replaced.

Not really sure what I will do with this information: "My guess is that from R3 bleeder cap failure, this supply failed from a lightning or grid transient or excess %RH & dust causing surface voltage breakdown of R3 with consequences throughout. It must also be rated for 1kV like C2 and not the standard 0.3" body of a 1/4 W 250~300Vac rated Resistor." This is just a cause hypothesis? That the resistor R3 couldn't handle the voltage? That the Harman Kardon havent fitted a 1KV resistor, just a normal 250 one?
What I can read R3 and C2 seems fine atm.
 
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It seems you need at least a USB scope to observe charge / discharge effects to isolate if it is insufficient /or/ excessive on 3.3VSB/0.1A. The 24V on +15 indicates it is certainly trying more than it should to reach 3.3V and that DC readings thus far point in this direction. Is there excessive voltage drop on the diode or excessive load on the 3.3 somewhere by observing the shape/ slope of the voltage on the storage cap. .. failures might be a fractured solder joints but the AC waveform will give more clues.
 
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