Sansui AU-555A Post Restoration bias problem

[Assil]

Hello everyone I am currently restoring an AU-555A and facing an issue with the left channel of F1266 power board. In brief I have replaced all electrolytic caps, all transistors except the C1030, still good. I have individually checked all resistors, pots and conductivity of all copper traces.

The problem I am facing is that as soon as I power up the amp, the DBT goes bright, thus showing an overcurrent. I have removed the power transistors and carried out voltage measurements on all remaining transistors on the left channel as well as on the healthy right channel. See ATTACHED.

I traced problem to around the bias section and saw that TR805 Vce was exceeding 4 volts while the bias pot VR813 was set to a minimum. I noted that varying the pot would not affect the value of the Vce. Note that VR813 potentiometer is rated 1k and good.

The voltages on TR807 were also not normal and I found the transistor heating a bit. see voltage levels in blue in the attached photo.

For info, on the left channel, i replaced the new C984 bias transistor with a C1815 transistors. No change was found. Same issue.

Could you please advise on the matter

Regards,

Assil

 

[gslikker]

You have 29 V at the emitter of TR807 and 26.2 at the other side of R837.

Probably the path R841-TR811-R845 is open circuit. Then the TR811 does nothing and Tr807 takes the heat.

Also try again whether TR805 is open circuit, since if base is 26.6 then its emitter can never be 25.7

 

[Assil]

You have 29 V at the emitter of TR807 and 26.2 at the other side of R837.

Probably the path R841-TR811-R845 is open circuit. Then the TR811 does nothing and Tr807 takes the heat.

Also try again whether TR805 is open circuit, since if base is 26.6 then its emitter can never be 25.7

Hi
Thanks for your reply.
In fact all measurements have been done with TR 811-814 removed from sockets. The reason for that being that the DBT will light on and will not allow the voltage measurements to be properly taken.

TR805 is not open circuit. I checked again and again. I crosschecked associated resistor values and found them ok

 

[Assil]

@Leestereo
Any suggestion on this strange issue please

 

[Leestereo]

What were the replacement transistors and was the amplifier working properly before component replacement.

 

[Assil]

The amp worked fine till around Dec 2022 as stated by the owner. It belongs to a friend and was having the original components, that is never repaired. Board is F1266.

The replacement transistors are:
TR801 & 802: 2SA992E (NEC)
TR803, 804, 807 & 808: ON KSC2690AYS
TR 809 & 810: ON KSA1220AYS
TR 805 & 806: 2SC984
TR 811-814: original C1030 tested.
.

I have been using the exact same set of transistors and other components for another AU-555A and same runs excellent, to date.

For the current AU-555A I have even tried other transistors in the left channel to see the difference:

TR801: KSA992FB (ON)
TR803 & 807: 2SC2383-O (Toshiba)
TR 809 : 2SA1013-O ( Toshiba)
TR 805: 2SC1815 (Toshiba)
TR 811, 813: 2N3773 (Motorola) old stock

No change.