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|
At this point We (that's the Royal we) decided that only a full re-build was appropriate.
Let's Gut ItThe 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 SelectionWe 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 doneWith 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 SupplyWith 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
|Always use protection on your diodes, kids|
|The diode on the right is for the bias supply. Happy caps will soon occupy the holes|
|No Timmy, Carbon is not a good insulator|
|What did we say about protection kids?|
|Caps are on the bottom. Yes, I picked up some hookup wire at the same time.|
|New output tube sockets and AC bias pots were installed as well. All neatly soldered to the original wiring harness|
|It's Alive!! Aliiiive!!!!|
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|
RedemptionNow 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:
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|