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Old 29th August 2014, 12:58 PM   #1
seta1 is offline seta1  United States
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Default Citation 12 differential pair

I have an HK Citation 12 power amplifier and the differential pair in one channel is not up to spec. I have a bit of noise in that channel and an offset voltage of .5 volts. When I compare voltage readings to the other channel, everything leads to this pair of transistors. But, the numbers in the parts list and on the transistors are no where to be found in the real world. What number transistors can be used to replace these? This is the amp that uses two separate transistors as opposed to one case with dual matched transistors inside.

These are called Q701 and the number on the transistors is: M412 27722

I have recapped the circuit board and that did help the good channel (which has an offset voltage of 20 mv) but the bad channel, this had no effect. I also replaced all out of spec resistors. Again no effect on the bad channel.
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Old 29th August 2014, 01:55 PM   #2
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What are the + and - power supply rail voltages at the difference amp? You will need a matched pair of transistors. If you match them yourself you will probably need to buy 100 transistors to find two matched pairs, unless you can buy them already matched from a service man that has them already on hand. You will also need to match them at the voltage and current they are going to be operated in the circuit. They have to be matched for equal hfe. There are some pre-matched pairs avaliable like a MAT02, LM394 etc which saves a lot of grief. You really need to find someone who repairs these amps and has a parts cache on hand. GRH
" "The fool doth think he is wise, but the wise man knows himself to be nothing but a fool." Will Shakespeare
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Old 29th August 2014, 02:12 PM   #3
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A pair of 2N5087s will work, just be sure to get the leads in the right holes and then place the pair so that they touch. A drop of glue will help. They are very similar to what was used in later versions that did not have the dual transistor.
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Old 30th August 2014, 07:46 AM   #4
djk is offline djk
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"A pair of 2N5087s will work"

Candidates for the Darwin Award should not read this author.
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