Wednesday, 12 September 2012

Citation II Rebuild

Gentlepeople,


Welcome to the wonderful world of Jackson-May.  In this issue we'll rebuild a  Harman Kardon classic:  a Citation II.  For those unfamiliar with this particular unit, it's an audio amplifier.  But what an amplifier!  Designed in the late 60's as the ultimate expression in Audio amplification, this 70lb wonder made best use of the technology available at the time to create what was then, and arguably now, one of the best stereo amplifiers in the world:


This particular unit came from the University of Alberta audio lab.  It was purchased in the late 60's by the University in kit form and remained in the lab until it was given to Mr May (that's me) when I was an undergraduate.  I'm ashamed to say that it remained dormant on my shelf for many years until I assembled the parts and wherewithal to completely restore it.

Being that the unit was used for research purposes most of it's life, I did not know what to expect when I opened it up.  There could be anything in there!  Here's what I saw when I removed the bottom cover:

Don't make them like that anymore
The unit had obviously suffered a lot of heat damage.  In addition, several electrolytic capacitors had leaked all over the interior of the unit.  Many of the wires in the original harness had been so badly damaged that they original cloth insulation had become charcoal; perhaps not the best insulator.

At this point We (that's the Royal we) decided that only a full re-build was appropriate.

Let's Gut It

The first step was to, well, take it apart!  The first thing to remove was the capacitor plate.  This holds all of the decoupling capacitors for the multi-tap B+ supply.  The capacitors are all Mallory FT style twist lock and all were junk.  The 40+ year old diodes were also attached to this plate.  Here's the plate after removal of components and cleaned up:


Once that was out, I decided to attack one side, then the next.  This amp was made using 'turret board' construction techniques, which makes it very robust and fairly simple to work on.  Here's one channel pulled out:
Look at the left hand side above the vent holes.  Something got pretty hot in the past!

You can really see the heat damage around the output tube sockets in the above photo.

Here are the top and bottom views of that turret board.  We've got Carbon comp resistors, ceramic caps,

Some ancient sprague "black beauty" caps, and another electrolytic hidden in there!
They may be black beauty, but the're gonna go in the black bin

Because of the extensive heat damage, it was decided that no components would remain.  We didn't want to be in a situation where the unit might become flakey, and we didn't want to deal with out of spec components screwing up performance of the amplifier.  So the next step is to gut the board.

Special, high tech, low impedance board holding jigs were created:
The ultimate audio hack jig: the 2x4

Because I'm lazy and not great on maintenance I decided to replace the hidden electrolytic (used for decoupling) with a nice solen fast cap.  This cap won't dry out over time and allow me to use the amp to my hearts content without needing to replace it.  It's also the only new component that's larger than the original:
That's a mighty fine cap you have there sir!

Some time with a soldering iron and some pliers removed the old components.  Here is the gutted and cleaned turret board.  silk screening was fairly fragile, so I didn't go crazy getting every spec of dirt off:

All Cleaned up and ready for new components

Don't shed a tear over the old parts

 Component Selection

We here at Jackson-May Audio Hacks are, of course, highly opinionated.  One of the things we are the most opinionated about is components.  We do not believe in the mid-range nuances afforded by oxygen free copper.  We don't groove on oil and film caps blessed by chicken bone welding witch doctors.  We don't get excited about impossible to qualify sonic benefits of silver hookup wire running only a few inches.  No, I'm happy to say we do not practice Audio Cork Siffery.  Except when it's amusing to do so.

And this brings us to the components selected for this project.  We're going to use high quality, but still fairly mundane parts, and generally not deviate too much from design intent.  Resistors will be good quality metal film types.  Signal caps will be either silver mica or poly film, depending on values required.  Filter caps are going to be old-style electrolytic cans.  Diodes will be good quality, but we won't go overboard.  In short, we're going to make a very good amp with some good parts.

After some selecting, we placed our order.  A few days later, this box appeared on our doorstep:

Presents!  And it's not even Christmas!

Let's Get'er done

With parts in one hand, and a soldering iron in the other, it's time to rebuild the old girl.  First go on the caps on the bottom:

The new caps are drastically smaller than the original caps.  Note the addition of heatshrink on the component leads to act as insulators.  One issue became immediately apparent: the leads on the caps were too short!  A little splicing was required to put things right.

The top of the board progressed much in the same way, with the only serious issue being lead length.

Power Supply

With One channel finished, it's time to get the power supply up.  I have a feeling that it might be easier to test the unit if we had a supply voltage.  The first item on the list was the rectifying diodes.  This amp was created at the dawn of silicon devices, which were very primitive.  As a result 120Hz hum was a constant companion.  We have no such limitation these days, so we went with a nice selection of Hexfreds:

Sidney Harman wished he had these

With two 'freds in series on each leg of the transformer B+ secondary, we'll have a nice rectified supply to feed the decoupling caps.  The terminal strip is not stock, but will do the job handily.  One issue with the 'freds is that the exposed metal tab is at anode voltage.  Or is that Cathode voltage?  No matter - it will be many hundreds of volts about ground; a scary proposition for something floating in the breeze like that.  The solution?  Heat Shrink!

Always use protection on your diodes, kids
Each tab was covered in two layers of heat shrink.  Then the freds were soldered into place.  The original amp used the capacitor mounting plate as the ground.  I ran 14 gauge copper bus wire (straight from Home Depot!) to act as a ground instead.  No it's not oxygen free copper.
The diode on the right is for the bias supply.  Happy caps will soon occupy the holes
Once that was done, it's time to address the heat issues on the wires.  The wiring harness didn't look too bad, but the wires from the output transformers were completely carbonized:
No Timmy, Carbon is not a good insulator
 The only way forward was to trim the leads, splice solder on replacements, and
 Heatshrink the bejeasus out of them!
What did we say about protection kids?
New manufacture FT style can caps were chosen for decoupling.  IMHO, caps meant for PC boards have no place in an amp like this because of the diffuculty mounting them.  These replacements by CE Manufacturing use the original tooling that Mallory employed back in the day, but with modern dialectics.  They are great caps and are period correct.

Caps are on the bottom.  Yes, I picked up some hookup wire at the same time.
 Once the caps were installed on the plate, the plate was installed back into the amp, and the power transformer was hooked up.

New output tube sockets and AC bias pots were installed as well.  All neatly soldered to the original wiring harness
 Then I connected the (new) three pin plug to my variable voltage, current limited mains supply (essentially the 110V mains run through a light bulb), and ...
turned ...
it ...
on!

It's Alive!! Aliiiive!!!!
Yep, 500VDC seems like a lot.  Note that the voltage will drop once a load is applied.

First steps - then Disaster!

From that point on it should be an easy matter of re-installing the turret boards, hooking things up to the harness, installing the tubes, and passing some audio into a dummy load.  I had acquired a matched quad of winged C 6550s for the output section and a set of NOS preamp tubes which were installed.  The power switch was thrown (again with current limiting mains supply) and fingers were crossed.

Unfortunately things did not work in our favor.  After warmup, the mains current draw started going up.  Then up.  And up again.  Then the smoke came.  Time to shut it off.

Some analysis showed that the point of failure was the old wiring harness.  Crumbling insulation was the culprit.   Yes, the one component that was foolishly not replaced - the wiring harness.  And even after the shenanigans with the output transformer leads I did not suspect that the harness would cause trouble.  But it did.
Out with the old
In with the new
Replacing a harness in a half build amp is rather a big deal.  But I managed it.  And I only learned to swear in two new languages.

Redemption

Now we are ready.  With a dummy load hooked up and a signal generator attached, we could see if things passed through.  Here's what we found:

Input on the bottom, output on the top.  A bit of phase distortion is visible. but wait... what frequency are we running at?
Yep, I can live with the phase distortion at 30KHz.  At full power no less.  I ran it from 20 to 50KHz and didn't see any power loss; it was pushing over 50W over that range.  Interestingly, the original published 3db points for this amp are 10Hz and 100KHz.  After a bit of playing around, I believe it.  No wonder that original Citation II output transformer sets are so desirable.

But of course the ultimate test is listening.  For this I used only the finest source, and the finest speakers possible:

What? You don't like iPhones?
Yep, nothing like using a old PA monitor and guitar amp cabinet to test your vintage tube stereo amp
  Even using these crude tools it was apparent that the amp was very very good.  It sounded very linear - not really like a tube amp.  More like a very high quality transistor amp.  At this stage of the game it's not possible to talk about sound staging and the like, but I was very hopeful that it would perform well.



No comments:

Post a Comment