"M.Joshi" wrote in message
...

M.Joshi;2716810 Wrote:
Hello,

I am trying to diagnose a fault with an Aiwa P30 DC Stereo Power Amp.
It is part of a mini separates system consisting of a separate tuner and
pre-amp.

The power amplifier does not output any audio to the speakers from
either channels. I have checked that the startup relay engages and both
the positive and negative power rails seem to be OK?

Any other pointers would be useful.

Thank you.

I have investigated further - the gain sections appear to be working
fine. If I inject a 1KHz signal at the input on the RCA jacks, an
amplified version is present on one side of the protection relay. The
other side feeds the speaker output terminals. The relay is just not
switching for some reason? I have tested the relay out of circuit and
it does work.

The protection circuitry is built around a TA7137P IC.

I also found a zener diode which is measuring short circuit. The part
number is D162 - I cannot seem to find any data on this?

Thanks.




--
M.Joshi

In what part of the circuit is this zener ? Are you sure it's a zener and
not just an ordinary diode ? Have you checked it out of circuit to make sure
that you are not reading across some parallel device ?

The relay might not be closing for one of two reasons. First, that it's
doing its job, and protecting your speakers from a DC offset at the midpoint
of one of the amplifier channels. The first thing that you must do is to
determine if this is the case, by measuring with a multimeter set to a DC
range, at the input side of the relay switching contacts, for each channel.
You should do this with no signal present, and not expect to see more than a
few mV - probably 15 mV maximum.

If there is no offset, then the chances are that the problem is around the
7317 (that's 7317, not 7137 as you stated) protection IC. In my experience,
it is seldom the IC itself which is faulty. Most often, it is one of the
components that defines the delay time before activation of the relay -
about four to six seconds for most amplifiers. The two components involved
are a small electrolytic, and a high-ish value resistor feeding it with
charge current. Either of these components, when faulty, can cause the IC to
malfunction in the way you are experiencing.

see

http://pdf1.alldatasheet.com/datashe...A/TA7317P.html

and take a look at the test / application circuit. Look at the components at
pin 8 for instance.

Arfa