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.
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.
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.
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.
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.
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.
Enjoy!
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