Need help with transistor match

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Need help with transistor match

Post by RobLewis » Sun Feb 21, 2021 10:22 am

Hi ,I'm repairing a mini contour / keyboard pcb.
Q9 was snapped off , so I've decided to replace the pair --Q9 & Q11 with 2n3904/2n3906 pair.
The footnotes on the schematic state that the PNP should be matched to 10-20mv lower than the NPN @ 200MA.
With the transistor matching cct provided , how do we do that?

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Re: Need help with transistor match

Post by mafalda55 » Thu May 13, 2021 5:21 am

No answer? I also have the same problem. If you have the solution, please share. thanks!
jigsaw puzzle

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Re: Need help with transistor match

Post by analogmonster » Fri May 14, 2021 1:57 am

Some thoughts:

If I understand the circuit correctly, Q9 and Q11 just form a current source for the kbd resistor string. The result should be 8.33 mA. If I am right, you just have to ensure to provide 8.33 mA. This can be done by any current > 8.33 mA and an addtional trim. So I am not sure that matching is that necessary. I would just solder transistor sockets and try it out. And if Q11 is still alive, just replace Q9.

And even with a current <> 8.33 mA the Mini should work if you just adapt the oscillator scale.

Or am I missing something?

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Re: Need help with transistor match

Post by SelmaAdam » Mon May 17, 2021 4:02 am

The simplest way of matching transistors is to use a fancy active current source to drive a stable current through the devices one at a time and to record the individual Vbe(voltage) values. Then matched devices are selected based on the recorded values.

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