Yamaha AX 392 PREAMP problem

[Lynyrd]

Hello.
Need help for Yamaha AX 392 integrated amplifier.
The power amplifier works fine.
Preamp problem, distorted.
Problem for -15v....only +6.6v.
+15v ..ok.
Where I need veryfied for -15v.........Q128?
Thanks.

SM

https://elektrotanya.com/yamaha_ax-392_sm.pdf/download.html

 

[Lynyrd]

Hello.
Need help for Yamaha AX 392 integrated amplifier.
The power amplifier works fine.
Preamp problem, distorted.
Problem for -15v....only +6.6v.
+15v ..ok.
Where I need veryfied for -15v.....
Thanks.

SM

https://elektrotanya.com/yamaha_ax-392_sm.pdf/download.html

 

[Lynyrd]

?

 

[Osvaldo de Banfield]

One chanel issue or both stereo?

 

[MarcelvdG]

Problem for -15v....only +6.6v.

So the polarity is also wrong: positive instead of negative. Checking the collector, emitter and base voltages (with respect to ground) of Q₁₂₈ would be a logical step.

 

[Mooly]

Duplicate threads merged.

 

[Lynyrd]

On both stereo

 

[Osvaldo de Banfield]

Thus start checking common parts: PSU, biasing networks, multiple units switch(es), etc. May be two simil faults, but more readily that a common fault offending both channels.

 

[Lynyrd]

Q128 was dead.
I replaced but........
Now Amplifier in the protect mode......

 

[Osvaldo de Banfield]

Check surrounding parts. Usualy a semiconductor goes difunct with several other parts: diodes, resistors, and other transistors.

 

[Lynyrd]

Its +40v on collector of q117 & q116.
Diod zener d106....ok,but +40v on both sides.
Resistor R171...ok...681ohm.
On the base and Emitter of q117....q116... negative voltage -2.4v.....

 

[Lynyrd]

I desoldered q118 q119 ,not protect mode, Amplifier works with only one channel.
2 transistors are ok

 

[Lynyrd]

The bias of power transistors is very high.....170mv on both channels

 

[Lynyrd]

?

 

[Lynyrd]

And all power transistors are hot

 

[BSST]

Hi Lynyrd,

Would you post a PDF of your service manual or a link to it? I've downloaded a manual but it must be a different version, as I can't spot the devices you mention in your posts.

Thanks!

 

[Lynyrd]

in First post link of Service manual

 

[BSST]

Thanks. First link I tried, but my access is very erratic. Finally got it!

Have you tried procedure on manual page 4 to reduce idle current?

 

[Lynyrd]

Yes,this procedure not working

 

[Extreme_Boky]

I noticed that in the last month and a half or so, you ask for help with the following audio amps/pre-amps:

Marantz PM7200
Pioneer SA7800
Sansui AU919
Sansui AU2900
Yamaha AX392

Are you collecting them at the garage sale.. or something? What do you do once the gear is fixed...?