audiolab 8000p has no output

[moss99]

Hi all , was wondering if you guys could help me out with your expertise ,. 😀

I have an audiolab 8000P for my rear speakers (so never really driven hard) a few weeks ago i noticed the power led was out and found the fuse was blown , i replaced the fuse , power light came back on and i didnt think any more of it until i watched a movie ,,.. !! .. no sound from rear channels 😱.
anyway i have taken the covers off and all looks ok ive taken some pics and some voltage readings from the main output transformers ,, again see photo (sorry about the crudeness of paint , lol ) anyway, not too sure where to go from here ,, ?







any help appreciated ..
thanks
mark.

 

[moss99]

these are the voltages

 

[stoN_Cold]

Check SM: Audiolab 8000P Manual - Stereo Power Amplifier - HiFi Engine

 

[moss99]

ok . i have schematic file . but not too sure what i should be checking

 

[stoN_Cold]

First check DC voltage on output stage.

 

[RM]

Most of the problems with this amp is the fact that the two transistors responsible for the voltage amplification is NOT fittet with a heatsink. As a result they run VERY HOT, and eventually break down. I can`t see the number, but one is a 2SA1209 and the other 2SC2911. Replace with 2SA1358/2SC3421, AND add heatsink.

regards
Roar

 

[moss99]

do you mean at the speaker terminals ? there is .112 dc on right channel and .351 on the left

 

[AndrewT]

I'm fairly sure the 8000p has a DC servo to control the output offset.
That tl071 is probably the servo opamp. check the schematic.

0.112Vdc and 0.351Vdc is far too much from a servo.
How are you measuring the output offset. There should be just one voltage from a mono amplifier.
There must be a fault.

 

[RM]

As it is a servocontrolled amplifier. it should be very close to 0VDC.

 

[moss99]

is it fixable ?

 

[moss99]

ive just stuck the test probes in the speaker terminal

 

[RM]

Try touching one of the transistors I mentioned while monitoring the offset. If it changes considerably, say several 10 of mVs, the replace the transistors. From time to time the servo IC, normally a 741 goes unstable and need replacing. Peplace ONLY with the same type, as the circuit is prone to oscillation if a "fast" IC is used.

 

[stoN_Cold]

All are fixable.

 

[RM]

On very rare occasions I have had to change the input FET, a 2SK389, but I think only twice in fifty amps.

 

[moss99]

ok , touching those transistors doesn't change the output voltage

 

[moss99]

is it worth taking those transistors out to test them ?

 

[moss99]

i have to go back to work for a few hours now so will pick this up later .. all help appreciated , thanks

 

[AndrewT]

ive just stuck the test probes in the speaker terminal

you should be measuring the voltage difference between the two speaker terminals.

Set the voltmeter to 2.000Vdc insert the red probe into one terminal and insert the black probe into the other terminal. Read the voltage.

 

[vzaichenko]

The amp has got a speaker protection, so, if there is some considerable DC offset at the amplifier output, the speaker terminals will be disconnected from the amplifier.

I would start from measuring some simple basic things:
1) Rail voltages (+/-), measured at the amplifier PCBs - what are they?
2) DC offset at the output, measured at the PCB (not the output terminals) - what is it?

Then, having all these values, we'll see, where to go next.

Cheers,
Valery

 

[moss99]

Ok ,here are the speaker output voltages , im not too sure where to check the rail voltages though .