blackface 1176ln revision F @smoke@

[keeno]

Hello !!

I need some help with my blackface 1176 revision F. Some days ago it started to smoke and stopped working.
I measured the voltage on the 30V zehner and only get 0,8volts. R84 (75ohms/3W ) gets very hot and starts smoking after 30seconds. I replaced Cr5 (the 30v zehner ) Cr7-10 , C19+c20 ( tantalum ) and R84 with no results. The unit has been recapped 4 years ago with Bc138s so I suppose the lytics are ok ... Could anyone give me a hint where to look further ?

thanks a lot !!

 

[sr1200]

Hows your VR?

 

[dustbro]

is your +30V rail shorting to ground? Check that with a meter.

 

[keeno]

Hi !

Thank you so much for your help ! Unfortunately I don`t have the unit right here but I will check it tomorrow.

SR1200 what do you mean with VR ? Do you mean variable resitor or voltage regulator 
The unit doesn't have a voltage regulator but some rusty trim pots... do you think these could short ?

 

[sr1200]

Start at your power supply and work your way back.

 

[keeno]

I checked the 30V+ rail today and it doesn't short to ground ... any ideas ?

thanks - k.

 

[keeno]

Hi !
Sorry for bringing up this old thread, but since my last post 3,5years ago my 1176 is still not working
Like I said, there are no 30volts on this unit and R84 ( 75Ohm/ 3W ) gets VERY hot after 10seconds...
The unit has been recapped and I can't measure any shorts or low resistance above the capacitors or diodes...
Any help would be much appreciated !!

 

[moamps]

If the voltage on the left side of the  resistor R84 is zero, you should check for shorts C23, CR5 (have you place it in correct direction?) and the output transistors in the line output amplifier. 

 

[keeno]

Thanks for your reply !

I have 31V at the right side of R84 but only 0,8V on the left... I checked all the transistors and diodes and they measure fine in circuit - is it possible that these short only under current ?

 

[moamps]

Are you sure that power zener isn't placed backwards?

 

[David Kulka]

Some of those stud mount zeners have opposite polarity compared to standard, strange but true.