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

Phase Linear 400 Complete Restoration

You have checked out the 400 according to my instructions and have determined that it is electronically sound. For the most part, it works, but that is not good enough. The object here is to milk it for the best possible performance and reliability, but in a stock sort of way. Modifications and such are the subject of yet another look at it. Let's do the easiest thing first, let's clean and deoxidize the volume controls (series II).....of course you skip this step on the series I as there are no volume controls. This is very easy. The body of the potentiometer is at the end of the shaft. They have holes in them and you can spray the chemicals right in. First off, use a solvent to get the funk out. These are often labled "tuner cleaner" or some such. You want a cleaner that does not include a lubricant. After doing this, you follow up with a shot of deox. The deox I prefer is made by Caig, and is called Deoxit. The basic is D-5, and they have several others. Any of these will assist in good control function. Ok, that part is done. Next, lets look again at the output transistors. Let me emphasize that the outputs MUST be on the approved list. Phase Linear was very clear about this. They will not perform well nor be stable unless the right outputs are in there.

output banks

Another matter about the outputs is that these were made with rubber insulators on each of the outputs. These are not desirable after these many years. It is necessary to pull each one out and replace them with mica or plastic insulators. When doing so, the output transistor should have a thin coat of heatsink compound, and then another coat once the insulator is in place. The transistor is then reinstalled and torqued down. Don't use a big gob of heatsink compound.....the right amount is more effective that too much, besides it is messy and not pretty to have this stuff all over the place. If you find that the insulators have already been replaced, check the mounting screws to make sure they are nice and tight. You want good heat transfer to the heat sinks. When all of this is done, you wind up with the correct outputs, correctly mounted. Very good, let's go on.

The next thing to do is to replace all of the electrolytic capacitors on the driver board. On the series II there are four of them. On the series one, there are more. Note their values and polarities and replace them all. Just remember, capacitors do not age all that well, and the unit is definitely more stable with new ones.

they sound as good as they look

Next, look at the input devices. The series II's will have the FET op amps, originally LF 356, and later LF 351. These guys are so critical to so many functions, and very slight variations have profound results, so replace them with LF 351. These do not cost much so consider it a "must do". When using the LF 351, the supply resistors (left side of the board) should be 7.5k. Speaking of these resistors, the originals were nasty old sandbox types. Coated wirewounds are very pretty, and I suspect very long term, so replace them. On the series I's the input devices are individual transistors. I like to replace them as well. Use the same numbers or a judicious sub, and be very careful of your basing.

multiple phases

Before regarding the driver board as being done, do a very careful visual of the various solder joints. These are not prone to getting iffy joints, but it is nonetheless a good idea to be certain of it and to resolder anything that is even slightly questionable. At this point, also carefully check all of the wires going to the driver board as when these get flexed enough, they get a little tired, and sometimes break off.....we don't want that. So now the driver board is, for practical purposes complete. You should now have a look at the filter caps. These don't fail a lot, but what the hell, nobody ever went to jail for replacing capacitors. All sorts of capacitors are available out there in the world. Electrically, you want the same value or close to it. A little greater capacitance is no problem, and the rated voltage must be at least the same as what you started out with. Higher voltage is ok. Lower voltage is not. I am fond of using a cap of similar value to the originals, though much smaller. They work very well.

filter caps and driver board, series one

The practical trick is then how to mount them, and how to hook them up. On the mounting, when there is a bracket there, it should be sized and there you are. On some, there is no bracket so they get glued in. The original electrical hookup used screws. If your cap has screws then hot damn.....if not, then solder all of the connections. Double check your polarity and hookup as you don't want to make a mistake here. Once this is done, take a break, go outside and smoke a good cigar, or have a beer or whatever. As long as you didn't screw up you have made some serious improvement in your Phase.

sunset in Dripping Springs, TX

If you now simply plug the baby in, after doing all of that stuff, I will disown you as you have not paid attention. Go back to my article: "Phase Linear 400, Condition Checkout" and run through that entire procedure. Never do significant work on anything without using a variac to make sure you got it right. Mistakes can occur and they can create much damage. After this work is done, it is necessary to confirm the offset voltage, to set the bias and see that it is stable, to to the current sharing test (in the service manual), and to run it for a while with the cover off, just to make sure it isn't getting hot or doing any other evil stuff. Ultimately the best idea is to marvel at how cool the whole process is, and then to have me do it for you. I'm used to the pitfalls and traps, you are not. The bottom line is that the unit will be a joy to everyone who hears it for years to come.


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