NEC Amplifier Repair

[Steve Mackie]

-NEC AUA-7000E Amplifier.
-No speaker output.
-Problem is intermittent.

See http://www3.ns.sympatico.ca/steve.mackie/nec1.jpg and
http://www3.ns.sympatico.ca/steve.mackie/nec2.jpg for reference pictures.

Circled in red is a 24V relay that turns the left and right outputs to the
speaker selector switch (circled in green). When the system is working fine
I am reading ~26.4V to the relay coil, then the system is not working
properly, there is no voltage present. The relay does seem to be operating
properly. I couldn't read the ohm rating on the bobbin to see if it's
correct, but I am getting continuity. Circled in blue is the DC input from
the transformer circuit, I am measuring ~79V.

The pink circle is pointing out the area where I am hearing a buzz/arcing
sound when the system is not working properly. No buzz when everything is
fine. None of the wires in that area seem to be shorting or bare. I haven't
checked the blue transistor. I left the system on (while it was working
fine) for about 10 minutes and the relay clicked off and on at the same time
a buzz came from this area. While the system was not working properly, I
just started tapping a few components. When I tapped the resistor (I think)
circled in yellow the relay clicked on and stayed on. I left the amp on for
about an hour after that and it worked fine, but the relay did click off and
on once since then. This may have been a fluke, I will turn the amp back on
tomorrow to see.

Steve

 

[Alex]

checked for obvious things I hope, like dry joints.

 

[Ben]

When the amp is in fault condition, check that there isn't any DC voltage on
the output of either channel.

 

[Steve Mackie]

Acutally, when the system is in fault I am getting no DC voltage at the
either output. But when it's working properly, I'm getting ~3.2V out of the
right channel and none out of the left.

Steve

 

[David]

Sounds like you are looking mostly in the wrong circuit, the protect
circuit.
You need to troubleshoot the audio outputs as one of the channels is going
into a fault condition, probably an excessive dc offset, which causes the
relay to shut off.

If you really have 3.2v DC on one of the channels even when the unit is
working, you clearly need to troubleshoot that channel for the problem.
Usually the protect circuit kicks on somewhere around 3 volts on many older
amplifiers so your intermittent might just be due to the close tolerance of
the dc already present on the one channel output. You should under proper
operating conditions have generally less than 100mV or 0.1 volts on the
speaker outputs. Make sure you do measure the amplifier to speaker outputs
before the relay when it does kick off as once it is off you will not
measure any dc on the speaker terminals.

Since direct drive audio amplifiers are very easy to troubleshoot when you
have two identical circuits, one that is working, you should have no trouble
making voltage measurements and comparing between the two channels to find
the problem.

David

 

[Steve Mackie]

Makes sense. When it's not working and the relay is "off", I am reading
between 4V and 13V in the right channel circuit going into the relay. When
the relay clicks "on", I am reading less than 3V. When the relay clicks off,
I don't get any voltage on the speaker terminals. Which is obvious because
"Terminals---->Selector---->Relay----->Circuitry".

The four transistors on the heatsink are all reading 39V in the centre (C).
The left channel is reading about .5V on one side of the transistor (B) and
0V on the other (E). The right channel is reading anywhere from 3V-12V on
both sides of the transistors (B and E), depending on how long the amp has
been turned on.

Steve

 

[David]

A few special notes after looking again at the pictures.
1. those output transistors are for a long time not available. The very few
places that have them charge a LOT for them. Take extra care not to blow
them and hope they are not bad.
2. Those white ceramic resistors are the emitter resistors and any slight
change in value WILL produce a DC offset. These are usually precision type
2%, but not always. A poor solder connection will on one of these can cause
a problem.
3. Due to the age the electrolytic capacitors will need checked for both
high esr and dc leakage.
4. The multi-legged transistors are known to go leaky from age and could be
the trouble.
5. The easiest way to troubleshoot one of these is to remove the output
transistors and insert 150 ohm 2 watt 1% resistors in place of the B-E for
the transistor AND powering the unit up on a variac to limit the power
supply voltages (works well on a stereo unit with one good channel). You
need only make sure you have enough voltage to bias all the transistors in
the output stage. Then you can make all voltage and current measurements
without fear of blowing the output transistors.
6. The Adjustment potentiometers if it has TWO per channel, one will be for
output bias current and one will be for dc offset fine adjust usually. If
it has ONE per channel it will be for output bias offset.
7. Almost all Audio Amplifiers of discrete design are very similar in layout
and function. Go find some basic amplifier schematics that are similar and
you will have a good idea what to expect.
http://sound.westhost.com/project03.htm

David

 

[Steve Mackie]

Question: Can you check a resistor without removing from the board?

 

[Mark D. Zacharias]

Yes, with some qualifications. Parallel circuit resistance may make the
reading lower, and if you're using a digital mutimeter, residual circuit
voltage may make the reading higher, especially with resistance over about
5-10 kOhms. I usually check higher value resistors out-of-circuit.

Mark Z.

 

[Sofie]

Steve Makie:

Yes..... and No.
Low ohm resistors can usually be confidently checked in-circuit, but many
times anything higher that a couple hundred ohms may fool you and your meter
because of the surrounding circuitry. It is a good idea to measure
in-circuit resistors 2 times..... reversing the red and black test leads
for the 2nd test..... if you get a big variation in readings then it would
be best to unsolder and lift one leg of the resistor to make an accurate
test possible.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair

 

[Asimov]

to "All" (12 Aug 03 22:35:26)
--- on the topic of "Re: NEC Amplifier Repair"

SM> Reply-To: "Steve Mackie" <[email protected]>
SM> From: "Steve Mackie" <[email protected]>

SM> Question: Can you check a resistor without removing from the board?

I do it all the time with an ordinary digital multi-meter. But there are
more than one way to measure in-circuit. The simplest is with the
circuit operating and then checking for the proper voltage drop.

Another is to lift just 1 leg but I guess technically it is removing it
even if just partially. Still another is to cut the resistor leg at the
corner of the 90 degree bend but I guess this is also technically a
little bit like removing it and it's not practical with smt's but then
cutting a trace would do the same thing.

One more way is to inject a very tiny known current and measure the
resulting voltage drop with a sensitive meter. As long as no forward
bias is applied to a semiconductor or a coil isn't in parallel shorting
the resistor then the measurement should be pretty much correct, barring
other resistors in the circuit. That is where theory comes in handy
using Norton and Thevenin by shorting one end then the other, etc.

Finally the really fancy way is to use a ground synthesizing circuit.
Basically it applies a low impedance on either side of the resistor to
isolate it from the circuit and then measures the floating resistance
like an ohm meter. It's a bit cumbersome since it requires 3 or 4 wires
and it may damage the circuit if there's too much current injected into
it, plus the extra expense of the specialized equipment.

There are probably many more ways to test resistors in-circuit but I'm
hoping someone more experienced jumps in with their perspective.


.... Real techs don't lick nine-volt batteries!