Phase Linear 400 Condition Checklist
Like anything else, Phase Linear 400 and 400 series II amplifiers
have failures that run in patterns. I see the same things over and
over again. The purpose of this article is to enumerate the usual
stuff so you may get a sense of what goes into the proper restoration
of these lovely beasts. Note that I do not recommend that you attempt
to do this at home. There is no way that my article will be so
inclusive as to help you to avoid all of the countless ways that you
might screw up and blow the thing up. Having said that, you are on
your own. If you find the information useful, then I am happy. After
all, enough of you will send them in for me to service that I will
surely make my truck payment, and that will suffice.
Ok, here we go. First off, you want to remove the top and bottom
panels. Remove the screws around the perimeter and set the panels
aside. Put the screws into a plastic bag right now, or you will lose
them. Next, the face plate should be detached. On series I, you
simply remove the large bolts, and down it goes. On series II you
take off the handles, then there are screws on the sides that must be
removed. Then down comes the panel.
When the front panel is out of the way, you will see the driver
board. This is the electronic gizmo that interfaces with the power
supply and the outputs to put all of that power into your lovely
speakers. Lots of important stuff happens on that driver board, but it
has to be out of the way to access the business end of the outputs and
the multitude of emitter resistors behind it.
This picture is of a series II. The series I looks much the same.
Note that there were different driver boards over time. Mostly
similar, but very interesting combinations occasionally show up. Now
that the unit is rather undressed, it is time to evaluate its
condition. This is done, initially with no speakers or loads of any
kind. Let's say that I have a Phase that I am evaluating. My first
act will be to check the fuses. I want to be sure that the correct
values are there and that they are not blown. Blown fuses usually
indicate one or more shorted output transistors (usually two to
four). If you have any shorted outputs, the service manual is now
needed. There is a procedure for finding the offenders. It involves
resistance measurement. Next is a most critical consideration. Over
the entire time that 400's were made, Phase Linear used a variety of
output transistors. They put out a service bulletin which detailed
each of these. Under no circumstances may you use anything that is
not on this list. What's more, you cannot do a mix or match within
the list. In most cases a given amp channel must have outputs of one
number. Follow their instructions to the letter, or suffer. For me
it is almost a game....when a Phase amp comes in, I pop off the covers
and see what is there. I frequently find very goofy stuff....so it is
no surprise that it has gone south. Enough said, suppose that the
fuses have not blown, or that you have replaced the offending outputs
with good ones.
You cannot do what I do next unless you have a variac (variable line
transformer) because you must be able to run the voltage up from zero
to line and monitor the current drawn by the amp. So.....the amp is
run up on the variac. If it draws current, then you are screwed and
there is a driver board problem and you sure as hell shouldn't pursue
it. I'll pursue it for you. If your luck holds, and it doesn't draw
current, grab your voltmeter and measure your dc volts at the speaker
terminals. A good gnd is the bar between the filter caps. Your dc
offset voltage should be low, under 10 millivolts is nice. If your
luck is still with you, the next step is to measure the bias voltage
on each channel. The procedure for this is in the manual. It is very
important, and it is often wrong. I usually adjust it, so it cannot
be passed over. The next trick is to use an audio generator and put a
signal into the amplifier.....400hz is nice. Use your oscilloscope and
look at the output terminals. At a low level, you should see a nice
sine wave. It must be pretty, no distortions or any such stuff. Next,
you run up the signal until the amp reaches clipping. You must see a
very clean clip on both channels, at the same level. It must clip on
the top and bottom of the wave in precise and even fashion. If you get
this far, rejoice as you are doing pretty well.
Ok, we have gotten to the point that the unit appears to be
sound....at least enough to hook up a load. Not so fast though, other
pitfalls remain. We are not yet ready to hook it up to speakers.
Instead, a nice set of high power load resistors are the order of the
day. Keep the audio generator hooked up to the input, turned down low
to be sure. Then simply plug the sucker in. If you variac it up with
a load, it will want to draw current and suggest to you that it has a
defect. Sinced it passed all of the former tests, we know better. So
plug it in and then apply the audio generator signal at a rather low
level. Next you wish to use your oscilloscope again to have a look at
the waveform. If you are lucky, it will be a pretty sine wave. If
you have no luck at all, it will be distorted in some way, or it will
have a bit of nasty oscillation showing up most often on the top half
of the wave, but sometimes on top and bottom. When you see this
oscillation, say to yourself "This will sound like hell" and then book
the unit in to send it to me. If you do not have anything ugly to
look at, then you are among the most fortunate of humans. Now you wish
to plug it into speakers, listen to real music for a while......feel
the heat sinks from time to time to make sure it doesn't get
hot. After a while, recheck the bias and offset to make sure it
doesn't drift. Rejoice! Now all you must do is the rest of the
standard overhaul. You could be much worse off though. The standard
overhaul is damn sure easier when it is not associated with rooting
out real problems.
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