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Phase Linear
Evaluation Process: Phase 400
Restoration Process: Phase 400

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.

The early ones looked like this 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.

Front panel down Front panel down, check the new filter caps!

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.

The driver board is out of the way 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.

Algernon thinks you need a short diversion 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.

turkey vulture at dawn 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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