TAC Scorpion Meterbridge Telefunken Module (TFK D_634P)

[nashkato]

While waiting for an order for parts i decided to take apart the Telefunken meter module in the Tac Scorpion meterbridge.
I´ve drawn up the schematic , attached .
AML offered a replacement years ago , allegedly this made problems and isn´t produced anymore .

current demand completely lit is about 40mA .
Supply voltage is +15V
not much in there but two drivers and the Leds (are those LEDs at all ? )

are there low current Led´s bright enough with 4mA ?

any chance i would find a replacement for the 2 8pin "SMD" drivers
(no pics , just too small , gotta find the macro first )

also i don´t really understand the schematic .. are these 2 different drivers ?

 

[analogguru]

These are 2 independent chips - probably SMD-version of U257B + U267B - which supply the (normal) LEDs with 20mA:

Gruß aus Wien,
analogguru

 

[PRR]

Yes, today's LEDs can be too-bright with just 4mA.

However in the series-LED connection, you throw a constant 20mA down the string, and short-out the diodes you want dark. That stacks the LED voltages, so ten 2V LEDs won't work with 15V. analogguru's plan (two 20mA strings of five) makes perfect sense.

 

[AW]

Has anyone tried making a replacement bar meter for these?

 

[analogguru]

Why not repair it ?
Probably nothing is wrong except the bonding of the LED's which could be replaced - of course (only) by a skilled technician - with 0603-LED's.

 

[ulisch]

Hi there,
indeed the Telefunken ICs are obsolete for decades, so unless you get another TAC pcb you are likely not able to fix the ICs. At the same time: LEDs can be replaced, and as there is a chain of 5 driven by a single current source you might be lucky, that a single broken LED takes out the chain of 5! Take a DVM and check for all LEDs ( they show as diodes) are healthy one by one.
In case the drive ICs are dead you may rebuild the bar graph with LM3915 devices.
The LM3915 is log scale internally, requires 10...15V  and drives 10 LED bar. No precision rectifier required as the TAC has that circuitry already on the group and mains module.
You'll find ready to go kits for lm3915 circuits on ebay.
There are a few caviats: You have worry about mechanical fixture, the power consumption is a little higher ( the 10 LEDs run in parallel, so 10x7mA e.g. = 70mA. ) .
There are further "tricks" available to cut power by adding a tiny switchmode stepdown converter to run the LEDs off 3V instead. But in case you fix up 1..3 meters only you don't need to worry.
Check out the applications notes for the LM 3914/15/16 on the web!

Hope this helps a bit
Uli
In case you need furthe

 

[analogguru]

If rebuilding, a BA682A will give you better results.

 

[jensenmann]

Does anyone have a datasheet for the obsolete TFK D-634P LED module?

 

[nashkato]

i don’t , sorry . but if you need tfk u257/267 dip i could help you out .
i sold my scorpion without further investigation of the meters .
i still have a soundcraft led meter panel , ts24 iirc , that i planned to replace the original meters with . should be enough space in the meterbridge for these.