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Old 08-11-2020, 04:23 PM   #1
JimBanville
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Default Strange malfunctioning LED audio peak level meter problem

So, I just got this old Harmon Kardon TD292 cassette player. It plays, but there are two groups of three LED's, one on each channel of the peak level meter, that aren't lighting up. I've done some minor electronic repair, but I can' figure this one out. I've taken it apart, but I don't see anything out of the ordinary. I don't know what "drives" these groups of LED's. These LED's light up individually, but I'm assuming there's something "assigned" to drive them in "three's", yes? The housing the LED's are in is mounted to a small board in a way that you can't see the individual LED's nor their connections to the little board they're on. Any ideas where to start?
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File Type: jpg Clipboard01.jpg (288.7 KB, 15 views)
File Type: jpg meter.jpg (80.5 KB, 19 views)

Last edited by JimBanville; 08-11-2020 at 04:26 PM..
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Old 08-11-2020, 04:58 PM   #2
petehall347
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Default Re: Strange malfunctioning LED audio peak level meter problem

check the soldering around both ic chips
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Old 08-11-2020, 05:19 PM   #3
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Default Re: Strange malfunctioning LED audio peak level meter problem

Quote:
Originally Posted by petehall347 View Post
check the soldering around both ic chips
Thanks for the reply, but which IC chips? I mean, do you know where they're located? This is a pic of the main board. The only things on the meter's tiny board are couple caps and resistors. There's a small rectangular box that houses the LED's. Are the chips inside there? I don't see a way to open it up. There is nothing on the opposite side of the board, under the
"box".
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Last edited by JimBanville; 08-11-2020 at 05:22 PM..
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Old 08-11-2020, 05:21 PM   #4
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Default Re: Strange malfunctioning LED audio peak level meter problem

the 2 biggest ones .
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Old 08-11-2020, 05:33 PM   #5
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Default Re: Strange malfunctioning LED audio peak level meter problem

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Originally Posted by petehall347 View Post
the 2 biggest ones .
Ok. The biggest ones are pair of DOLBY chips (one for each chanel I guess).

From tha manual...
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Old 08-11-2020, 05:42 PM   #6
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Default Re: Strange malfunctioning LED audio peak level meter problem

Show us good clear pictures of the inside of the unit and the boards.
The LED meter assembly should have the LED driver IC's on that board, so check for bad the solder joints first.
Can you upload the service manual?
Never mind, I found the service manual.
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File Type: pdf Harman-Kardon-TD-292-Service-Manual.pdf (3.55 MB, 5 views)
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Old 08-11-2020, 05:47 PM   #7
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Default Re: Strange malfunctioning LED audio peak level meter problem

Quote:
Originally Posted by JimBanville View Post
Thanks for the reply, but which IC chips? I mean, do you know where they're located? This is a pic of the main board. The only things on the meter's tiny board are couple caps and resistors. There's a small rectangular box that houses the LED's. Are the chips inside there? I don't see a way to open it up. There is nothing on the opposite side of the board, under the
"box".
Judging by the schematic you provided in your startpost, the 2 driver IC's are indeed located on the PCB holding the LED's. This PCB is connected to the mainboard through a 8 wire flatcable of some sorts. The connector is the one on the lower side of the mainboard shown on the photo in post 3, just below the text 'Meter Level Adj'.

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Old 08-11-2020, 06:10 PM   #8
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Default Re: Strange malfunctioning LED audio peak level meter problem

The VU meter LED driver IC has them in groups of three LED's.
If one LED goes open, the set of three goes dead. Put your multimeter on diode-test and see if the LED's are ok. Or cold solder joints. What is the IC's part number?
A similar one is the UAA180 but I'll bet your HK uses Hitachi parts. Or maybe the red LED's died, I have seen that too.
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Old 08-11-2020, 06:46 PM   #9
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Default Re: Strange malfunctioning LED audio peak level meter problem

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Originally Posted by redwire View Post
The VU meter LED driver IC has them in groups of three LED's.
If one LED goes open, the set of three goes dead. Put your multimeter on diode-test and see if the LED's are ok. Or cold solder joints. What is the IC's part number?
A similar one is the UAA180 but I'll bet your HK uses Hitachi parts. Or maybe the red LED's died, I have seen that too.
Ok. If they're on the same board as the LED's I have to figure out how to open up the plastic box they're in.
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Old 08-11-2020, 06:54 PM   #10
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Default Re: Strange malfunctioning LED audio peak level meter problem

are they not through hole devices ?
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Old 08-11-2020, 06:55 PM   #11
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Default Re: Strange malfunctioning LED audio peak level meter problem

pics would be handy
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Old 08-11-2020, 07:07 PM   #12
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Default Re: Strange malfunctioning LED audio peak level meter problem

Quote:
Originally Posted by petehall347 View Post
pics would be handy
Pics of led board
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Old 08-11-2020, 07:29 PM   #13
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Default Re: Strange malfunctioning LED audio peak level meter problem

Yep, one of those encapsulated (Black blob) IC on PCB.
It looks like the LED's pins are mated to the PCB using pressure to make contact with the PCB pads.
If you flex the board you may see the LED come and go.
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Old 08-11-2020, 07:33 PM   #14
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Default Re: Strange malfunctioning LED audio peak level meter problem

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Originally Posted by budm View Post
Yep, one of those encapsulated (Black blob) IC on PCB.
It looks like the LED's pins are mated to the PCB using pressure to make contact with the PCB pads.
If you flex the board you may see the LED come and go.
So what do u think I should do next? IC problem? Dead LED's? Bad led connections?
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Old 08-11-2020, 08:52 PM   #15
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Default Re: Strange malfunctioning LED audio peak level meter problem

Did you try flexing the board while power up with signal so you can see flexing will cause the LED's to come and go?
https://www.badcaps.net/forum/attach...3&d=1597194394
Those white plastic dots look to holding the LED assembly to make contact with the PCB.
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Old 08-11-2020, 10:19 PM   #16
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Default Re: Strange malfunctioning LED audio peak level meter problem

What I have done when desperate to repair vintage gear is VERY CAREFULLY remove the LED diffuser assembly to access the naked LED's on the PC board. This means cutting/remelting or removing the six plastic pins and pulling off the LED diffuser straight up. If you slide it sideways it will kill all LED's by shearing off the bonding wires. I expect raw LED dies on the PC board with a tiny bonding wire.
You could also just carefully peel off the diffuser overlay to see what's inside.

I would solder in 0603 LED's to replace the defective ones.

edit: to get troubleshoot kind of displays to work in the shop we used freeze spray drops on an LED. I think they get a bad connection to the die, the bonding wire doesn't quite work.

Last edited by redwire; 08-11-2020 at 10:23 PM..
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Old 08-11-2020, 11:06 PM   #17
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Default Re: Strange malfunctioning LED audio peak level meter problem

https://www.badcaps.net/forum/attach...1&d=1597194394
I wonder what you will see if you look straight from the side view.

Last edited by budm; 08-11-2020 at 11:07 PM..
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Old 08-12-2020, 05:57 AM   #18
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Default Re: Strange malfunctioning LED audio peak level meter problem

Quote:
Originally Posted by budm View Post
Did you try flexing the board while power up with signal so you can see flexing will cause the LED's to come and go?
https://www.badcaps.net/forum/attach...3&d=1597194394
Those white plastic dots look to holding the LED assembly to make contact with the PCB.

I tried flexing it. No joy.
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Old 08-12-2020, 06:08 AM   #19
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Default Re: Strange malfunctioning LED audio peak level meter problem

Cover came off easily after I used a razor blade to slice the white plastic tabs off the back.
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Old 08-12-2020, 06:26 AM   #20
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Default Re: Strange malfunctioning LED audio peak level meter problem

you can use the testpoints either with another led or a multimeter on diode-test
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