Sansui AU717 HFE match

[akabek]

I am currently in the process of refurbishing a Sansui AU-717. I was looking over some previous rebuild threads and I see comments around hFE matching. Below is an example:

TR01-TR06 – Original 2SA726 (PNP) ECB / Now KSA992FB (HFE Match) TR1/TR5 & TR2/TR6

I just want to make sure I understand what is required. For the example above do I hFE match the 4 transistors? Is there a range to stay within?.

 

[Hyperion]

I just want to make sure I understand what is required. For the example above do I hFE match the 4 transistors? Is there a range to stay within?.

Generally you will want the highest gain (Hfe) pairs you can find, but their matching is far more important than the absolute figure. Also, I like to find matched pairs that are the same for positions in both channels, so if I am replacing 4 transistors in 2 matched pairs (2 transistors per channel) - I find 4 with the same Hfe.

For the AU-717 you want matched pairs for TR01 & TR05 and TR02 & TR06 on F-2723, and also TR01 & TR05 and TR02 & TR06 on F-2720. You need matched pairs for these because of the way they are used in the circuit, there is a balance to be preserved and matching the transistors achieves this.

One last thing, the temperature of the transistor makes a difference to the Hfe you measure - so don't hold the transistors while you are measuring them - your body heat will make a mockery of any matching done while holding the transistors.

 

[akabek]

Thank you for the response. The information was exactly what I was looking for.