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Luxman L-58a / L-580 (70's) phono problem

rbphoto

rbphoto

Member
Hi all,

I'm slowly working my way through a Luxman L-58a imported from Japan as a non-working unit. So far I have managed to work my way through the various issues and the amp is working well, adjusted to service manual specs and sounding sweet, except for the phono input.

The issue is that the phono stage appears to have no gain, and basically no low end response. Both channels have the same fault. With the volume knob at max, the MM input sounds like it's being played through mobile phone speakers. I'm so far unable to test the MC functions. Line level inputs are as expected.

So far I have established that:
  • It's not related to input switches, wiring or connectors before the phono stage, or after it.
  • Power supply voltages are good and present at the phono board.
  • The issue presents across a few different vintage MM cartridges I could test with.
  • There is no apparent distortion - just very little gain and no bass.
  • Both channels present the same.
I have already done the following to attempt a fix:
  • Check all resistor values of one channel against schematic by lifting legs, no obvious issues found
  • Replace all double diodes (precautionary)
  • Replaced all 2SA836 transistors with KSA992FTA (precautionary)
  • Replaced two 2SK163 FETs at Q214a/b (precautionary)
  • Cursory checked values of electrolytic and film caps, no issues found
If anyone could from experience or intuition point me to where to look next, I'd be really appreciative!

Attached the relevant part of the schematic - equalizer board PB-1272

Thanks, Rick

luxman-l58a-phono-schematic.jpg
 
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Well, problem solved. Turns out I am just a klutz and had replaced two of the 2SA836's at Q209a/b with incorrect parts. Fitted KSA992 there as and all is well!

I had not been able to check the phono output before I elected to start replacing transistors to solve problems I didn't know existed - there's the lesson.
 
Glad you found that issue. It would have been difficult to diagnose from a distance.
 
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