This meter pegs but amp works fine

[srinath]

Hi all,

The circuit for the meter is attached.

This is what this amp does - and its a nad 3080 integrated.
The right power meter pegs as soon as its powered up. Then a few sec later the safety clicks and it functions as it were normal. The right and left are equal, though the right meter never gets off the right end peg. Then when you turn it off, it takes 5 mins and slowly gets to 0.

With the amp unplugged - I've measured Dc across the meter terminals and its like 1.3 or so and it slowly drops to 0. Shorting the 2 posts causes the thing to temporarily swing toward the 0 but when you let go it usually returns to where it was

Anyway hope someone can help, I'm afraid to run the amp fearing it may blow.

Thanks.
Srinath.

 

[Mooly]

Remove the 10uf cap. The meter should not move. If it doesn't then check the cap. If that's OK then the amp is oscillating at HF.

If that is all OK then measure the voltage across the diode. Is it around 0.7 volts (should be identical to the other one) ? If that's OK then either a resistor is faulty or the transistor is leaky.

 

[srinath]

OK let me re word it so I understand - C202 get removed. If meter is dead, bad cap. If meter is not dead amp oscillates @ HF. What does that mean ? Sounds bad - but both channels sound fine @ all times.

Now after replacing c202 with a good part, voltage cross d204 - with it in or out ? should be .7v. If it is, then a resistor is bad or the transistor is bad.

Cool.
Srinath.

 

[Mooly]

1) Take the cap out. If the meter now reads zero it means either the cap is bad or more likely, the amp is feeding HF (that you can not hear) into the meter circuit.

2) If the meter is still faulty with the cap out then you have a simple issue with one of the components in the meter drive circuit.

 

[srinath]

Ok let me see if I can get to it in a bit.
Thanks.
Srinath.

 

[srinath]

The meter pegs with c202 removed.
So the rest of the components - Let me see what the resistors look like outside and ow the measure up.
This is my kind of analysis - removing components till the problem stops.
Cool.
Srinath.

 

[FoMoCo]

I'm thinking a shorted transistor...

 

[srinath]

Ding ding ding ... removing the bc546b gets that meter to sit still.
Swapping the transistor from the other channel makes it work fine (I think) cos I have removed the 620 ohm resistor and fiddled the trim pot vr202 on the bad channel. It seems to slowly climb to 0.5 watts but sit there and not climb further. I will replace the transistor and the resistor and see where that gets me. Thanks guys.
Cool.
Srinath.

 

[Mooly]

My money is on the transistor or the diode. I'd be very surprised if a resistor were out of spec.

Have you measured them on a meter ?

 

[srinath]

I pulled that resistor - in a misguided attempt to see if it was going to make a diff - and that destroyed it.

Cool.
Srinath.

 

[Mooly]

There's no answer to that

 

[srinath]

OK I got the resistor in, and that meter behaves.
Cool.
Srinath.

 

[srinath]

New transistor is in and it works ... whooo hoooo.
Cool.
Srinath.

 

[Mooly]

Good stuff