[R-390] OT: Transistor Amplifier Question

[Charles Steinmetz]

Barry wrote:

> These are across some of the 2SC1000s (HT310002As) as well as some of the 2SC458s (HT304580Rs).  I'm using KSC1845FTAs to replace both   *  *  *
>
> The other 2SC1000s are on the normal preamp board.  The schematic doesn't show those caps on that board but they're were installed there - at least for the first transistor in signal path.   *  *  *
>
> Unfortunately, the schematics do not entirely agree with the boards (particularly on the phono board) so I'm having to go by what's there instead of entirely what's on the schematics.   *  *  *

These capacitors were most likely added in a feeble (that is, mostly 
ineffectual) effort to provide RF suppression for signals coming in from 
the outside world (which is why you find them on the input transistors). 
  None of the Japanese hi-fi companies knew anything about RF 
suppression, so they did anything suggested by folk wisdom whether it 
helped or not.  Those capacitors helped some, but are really not the 
right way to handle the problem.  However, do re-install (or replace) 
them when you replace the transistors.

KSC1845FTA is simply Fairchild's part number for the medium-beta 
selection of the very common 2SC1845.  They should work fine to replace 
both 2SC1000s and 2SC458s.  Note that the Fairchild parts are somewhat 
noisier than the original NEC parts.

No surprise that the samples you have do not match the schematic(s) you 
have.  Marantz was famous for this.  If you look closely, you will 
undoubtedly find other examples of parts kludged onto the foil side of 
the boards, and perhaps even traces cut where they changed the circuit 
after the boards were made.

Now, the real question -- why are you replacing multiple transistors in 
the first place???  It's not like they get tired and need to be 
refreshed.  You're not replacing every transistor in sight instead of 
properly troubleshooting it so you know what is wrong, are you?

Note that the 1060 is chock full of small-value electrolytic coupling 
and bypass capacitors.  These will account for most of the problems in 
the preamp sections.

Best regards,

Charles