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 Post subject: LED to replace panel light in Sansui A-900 vintage amplifier
PostPosted: Nov Sun 17, 2019 10:53 am 
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Joined: Nov Thu 20, 2014 4:44 pm
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Location: Heliopolis, Egypt
I own a Sansui A-900 amplifier since 1986. It has a nice 3-D display that is lit with 12 volt 0.15A blue T 1 3/4 bulb soldered to a large circuit board behind the display. The board also carries a similar transparent (not blue) bulb that illuminates the slider volume control. For some time both lamps were not working. After consulting the service manual and schematics, and some testing on the board, I found out both are supplied from 17 volt winding on the power transformer with 33 ohm resistors in series.
It will be impossible to get similar lamps where I live.
I Did some trials with a blue and a white LED of unknown specs, with 420 and 460 ohm resistors soldered in series respectively. Each one draws slightly less than 20 mAmps. There was a flicker apparent on the white LED but not on the blue. Then I did some web research, which suggest another diode is needed to protect the LED from reverse voltage, when AC supplied.
The limiting factor is space. The blue bulb fits in a recess in a plexi-glass angled piece that spreads the light to the 3-D graphic power display. The white has slightly more space as it fits inside a metal shroud that reflects its light towards another plexi-glass piece above the volume slider potentiometer.
In order to accommodate the resistors I have "shaved" the LEDs with a file to keep their height above the board close to the original bulb.
My question is: does it make a difference how I solder the diode? to the anode or to the cathode of the LED? May be I should solder the diode to the anode and the resistor to the cathode? Or should I solder the diode antiparallel to the LED? I don't think I will have enough space, unless I use SMD parts which are difficult to procure and solder.
Will the diode eliminate the flicker?
Any ideas? Mind you, I do not want to cut the trace on the board to install a diode and cap which is another solution for the flicker that came to my mind.
Photos are attached for better explanation.


Attachments:
File comment: Top view of amplifier innards.
Amp_top.JPG
Amp_top.JPG [ 752.99 KiB | Viewed 290 times ]
File comment: Blue bulb with the board freed from retaining clips to be able to view the bulbs
Amp_blu2_lmp.JPG
Amp_blu2_lmp.JPG [ 1.61 MiB | Viewed 290 times ]
File comment: White bulb inside reflector.
Amp_wht_lmp.JPG
Amp_wht_lmp.JPG [ 2.01 MiB | Viewed 290 times ]
File comment: The two LEDs with resistors installed and tops "shaved". I did not solder them to the board, just used jumpers for testing purposes.
Amp_LED_surg.JPG
Amp_LED_surg.JPG [ 1.45 MiB | Viewed 290 times ]


Last edited by etherman on Nov Sun 17, 2019 4:19 pm, edited 1 time in total.
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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Sun 17, 2019 1:43 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5361
Location: Sunbury, Ohio 43074
Either a diode or a capacitor should probably stop the blinking you are seeing, which I'd guess is 50 or 60Hz depending on your power frequency there. Just try the diode one way... if it doesn't work, reverse it. Won't hurt anything if it's reversed but I'd imagine it may work either way.

If it still blinks at all, and it may since you're going to now be feeding it pulsating DC rather than AC ..... which is still AC as far as the diode is concerned, you will have to add a capacitor.

Another solution is to just hook the LED's up to the lowest DC voltage available in the Sansui, and use the appropriate dropping resistor or resistors.

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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Sun 17, 2019 5:46 pm 
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Joined: Dec Wed 23, 2015 3:03 am
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Location: Northwest Indiana 46350
I make these little rectifier boards tapped into the low voltage secondary. Just a bridge rectifier, 470uF/16V cap and the dropping resistor of choice.

Attachment:
IMG_2106.JPG
IMG_2106.JPG [ 121.2 KiB | Viewed 250 times ]

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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Sun 17, 2019 5:53 pm 
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Joined: Nov Thu 20, 2014 4:44 pm
Posts: 5
Location: Heliopolis, Egypt
tbone8 wrote:
I make these little rectifier boards tapped into the low voltage secondary. Just a bridge rectifier, 470uF/16V cap and the dropping resistor of choice.

Attachment:
IMG_2106.JPG


Can you please post a schematic?
Regards


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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Mon 18, 2019 1:45 am 
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Location: Northwest Indiana 46350
Attachment:
led ps.PNG
led ps.PNG [ 8.72 KiB | Viewed 213 times ]

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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Mon 18, 2019 7:00 am 
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Joined: Nov Thu 20, 2014 4:44 pm
Posts: 5
Location: Heliopolis, Egypt
tbone8 wrote:
Attachment:
The attachment led ps.PNG is no longer available

Thank you for your schematic. It is the ideal solution. My idea was minimalist, to reduce the number of parts and try to solder them on the original board or on the LED pins if possible. It is represented here in the schematic. I am not very clever with schematic drawing software, so I could not remove the dotted lines. The non-polarized capacitor in parallel with the diode is to reduce switching noise.
What would be the minimum value of electrolytic capacitor that would be adequate for a current draw of about 20 mA?


Attachments:
Rectifier.png
Rectifier.png [ 19.54 KiB | Viewed 201 times ]
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 Post subject: Re: LED to replace panel light in Sansui A-900 vintage ampli
PostPosted: Nov Wed 20, 2019 7:38 am 
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Joined: Nov Thu 20, 2014 4:44 pm
Posts: 5
Location: Heliopolis, Egypt
After I desoldered both lamps, I finally compared their heights to the LED/resistor combinations, and both were OK. (photo).
I fiddled with the white LED to insert it under the reflector which I did not want to remove, since a lot of heat would be required which might endanger the board. I used a small blade to carefully cut one of the tracks leading to the old bulb / now LED, then scratched off the protective layer from the same track at a more proximal point to allow soldering one of the legs of a small diode from the junk box. The distance was estimated by eyeballing while one leg of the diode would be touching the diode anode resistor, then soldered to it. After soldering the diode, and testing for continuity and shorts, I soldered a small 100uF 25V capacitor with + to LED anode resistor and - to cathode. I powered and the LED came without flicker. The voltage between thecathode of the diode/resistor junction and the cathode of the LED was 17.5 volts, probably because the LED draws much less current than the bulb, so the voltage drop across the 33 ohm resistor is much less. However I thought this is good enough. I soldered a small ceramic capacitor across the diode as in the schematic, for the sake of proper design. However when I powered up SMOKE was coming from another board, so I quickly pulled the plug before my sloppy soldering destroyed something. I removed the cap and everything was good again.
Then the blue bulb. After removing it, it was clear that the geography of the tracks was not as simple as the previous one. I was not sure which direction the power was coming or going on one of the tracks. so I decided to not cut any tracks and just solder an anti-parallel diode. But again once I powered up smoke came from another place on the same board like before, so I decided to remove the diode. I tested it outside and was OK. It was close to midnight and I was tired, so I fixed everything back in place and it was good but not perfect.
After the two "smoking" episodes I remembered the old adage "the enemy of good is better".
Some pictures of the events.


Attachments:
Amp_LEDs_ .JPG
Amp_LEDs_ .JPG [ 1.71 MiB | Viewed 134 times ]
File comment: White bulb removed.
Amp_lamp_remov.JPG
Amp_lamp_remov.JPG [ 1.27 MiB | Viewed 134 times ]
File comment: Diode soldered and track cut underneath.
Amp_diode.JPG
Amp_diode.JPG [ 1.74 MiB | Viewed 134 times ]
File comment: This is where the blue bulb used to exist.
Amp_PCB.JPG
Amp_PCB.JPG [ 1.42 MiB | Viewed 134 times ]
File comment: Display lighting after powering the amp.
Amp_display.JPG
Amp_display.JPG [ 575.47 KiB | Viewed 134 times ]
File comment: After work.
Amp_desk.JPG
Amp_desk.JPG [ 648.63 KiB | Viewed 134 times ]
File comment: The display and volume control lights are shown with the tuner on top with its original bulbs, for comparison.
Amp_display_2.JPG
Amp_display_2.JPG [ 563.61 KiB | Viewed 134 times ]
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