Sony V-Fet Amp TA-N7B problem

[Bernhard]

Hi,

my new vintage amp got two problems:

Sometimes on one channel the output relais does not switch on.
If the channel goes on, the sound is distorted if the volume is more than room level.

So far I have compared DC voltages on the small board that is attached to each heatsink.
There are TO220 Transistors, and one leg has 0,7V each transistor on both pcbs, but another leg got -0,5 on each leg on the working channel but it is also 0,7V on each leg on the the defective channel.

Does that point to a bias problem located on the main driver board ?
To me it looks like distortion starts when bias current is exceeded by signal current, just a guess...
There is offset 80mV both channels.

I did not measure anything in condition when relais did not switch after pushing power button.

Both heatsinks stay cold.

Any fast & useful hints ?

Thanks, Bernhard

[anatech]

Hi Bernhard,
An oscilloscope would answer a number of questions for you. Is there DC offset when the relay does not switch on?
You may have poor solder connections or a cracked joint on the PCB. More info would be helpful.
-Chris

[Bernhard]

Ok, looks like it is unstable. Even before distortion is noticeable it starts to show oszillation packages. (?)

[Bernhard]

And while increasing volume, oszillation packages divide in two and move , after distortion is noticeable, relais switches off, and it looks like this: Permanent oszillation (?) Bad capacitor ?

[janneman]

Originally posted by Bernhard
Ok, looks like it is unstable. Even before distortion is noticeable it starts to show oszillation packages. (?)

Bernhard,

The oscillations stop at the point where the other half takes over. Could it be that the bias is too low, that it is in class B instead of class AB? The oscillations could trigger the protection circuit and/or show up as offset on the DC meter. The difference in Vbe also points that way.

You verify the correct bias setting by measuring between 30 and 70 mV across two Re's in series, like R140-143, or R141-1444, or R142-145, other channel: R240-243, R241-244, R242-245.

You can measure that on each power stage sub board: put the amp front of you so that the sub-board looks like a 3-legged bridge, the "legs" connecting to the three individual power transistor sub-sub boards. The bias test points are the top left two pads of the "bridge", where no wires are connected.

Jan Didden

To adjust the bias, turn RV202 (R-channel) or RV102 (left channel).

[Bernhard]

Jan,

thanks, I measure 6,2mV across the pads of the working channel, and 3,6mV on the bad channel.

Are those pads equal substitutes for those resistors in series you mentioned ?

[janneman]

If you measured correctly, they are BOTH much too low, the bad channel the most. Recheck it with the attached picture where to measure Vbias.

jan Didden

[Bernhard]

Something is wrong.

Turning the bad channel bias pot does not change much, also it is adjusted to max.
Min gives 3,3mV.

Both cchannels were adjusted to max by factory, on the bad channel the white seal was broken or there was not enough white material to bridge from pot to pcb.

[Bernhard]

Turning to Min on the good channel pot gives 2,8mV

[janneman]

Originally posted by Bernhard
[snip]on the bad channel the white seal was broken or there was not enough white material to bridge from pot to pcb.

.... meaning somebody "improved" the amp after it left the factory. Can you adjust the good channel to 30-70 mV?

Jan Didden

Edit: With this info I think that one power transistor on the bad channel is (partially) shorted, the one that measured the lower Vbe. Can you isolate that sub-sub board, take off the wires to it, and see it the rest now recovers? Don't forget to turn the bad channel bias to min first!