• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Harman Kardon Citation II

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A friend of mine just dug one of these beasts out of his garage and asked me to revive it. I have read some posts how to restore it. Takes $$ and time. I looked under the hood and saw no visible signs of previous failure (no burned components, etc). Would it worth while to power it up with a variac at all? Is it possible that it would function?

Thanks!

Gabor
 
The power supply capacitors and coupling caps have to be replaced; they're over 50 years old and leaky. There's a selinium rectifier also that needs to be replaced. That's quite a few things that HAVE to be replaced, so might as well go through the whole thing.
Just my opinion.... Hope it helps.
 
I'd say that you should change out the selenium rectifier with a modern silicon diode. I expect that is in the bias supply so it is non-critical, a 1N4006 will be more than sufficient.

Then fire it up with a variac, and only the rectifier tubes, bring it up just high enough to first see that the bias supply works with minimum ripple, then a bit higher to see that the rectifier tubes conduct - that will be about 50-60volts AC from the line... let that sit for some time, like hours. Check the power supply voltages on the filter caps immediately, and watch it over time... they should be proportionally lower than the operating voltages. Vdrop on the resistors that separate the cap sections that shows significant current probably highlights a leaky cap.

But the best way is to temporarily substitute silicon diodes for the tube rectifiers, and very very slowly bring up the AC mains voltage checking for voltage and ripple on each power supply cap section... a rate of 5-10Vac increase every 15-20mins ought to be about right... excessive ripple means a faulty/bad cap section... one can disconnect a single bad cap section or cap and sub in an appropriate single value cap under the chassis...

assuming you can bring the voltages up to *normal DC values* with no tubes in, which is a LOWER AC voltage than normal, and have no serious ripple, and the caps don't outgass, get hot, or swell, ur home free. Next put in the tubes, after lowering the voltages and letting them drain down, bring the voltage up slowly, to the point the tubes start to light and conduct, look for reasonable current draw on the cathodes of the output tubes (you can do one channel at a time), and then bring the AC up to nearly full, watch the bias voltages and the current. If that is ok, try the amp.

Of course it is best to do this if you have a variac, DVM and scope and know how to use them...

They do usually have some leaky caps, but not always...

If you don't know what I just said, then best to keep ur hands OUT, and hire someone who does...

Look for cracked or heated carbon composition resistors... frequent source of failure.

_-_-bear
 
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I might be completely wrong but I thought the Citation II does not have tube rectifiers.

Please clarify.
You are correct, there are no rectifier tubes. There are two plug in diodes (perhaps soldered in) that operate in a doubler circuit. Do what bear suggests without any other tubes in place. Connect a voltemeter to the output of the doubler and slowly advance the variac to get about fifty volts. Then leave it sit at that level for a while. Then advance the power to one hundred volts and let that sit. Check the filter capacitors for heating very carefully. If they stay cool, keep advancing the variac 50 volts at a time incrementally until you can make 450 volts. I have done this several times in the past with Citation IIs without problems.
 
The OEM configuration is 2X series connected pairs of "top hat" diodes wired as a "full wave" doubler. The diodes are mounted in the clips of a fuse block.

Part of the McShane "treatment" is replacing the noisy OEM diodes with very quiet 3 A./600 PIV parts. I've uploaded a "snip" photo of the PSU bracket. The diodes are "circled" in red.
 

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  • Cit. 2 Partial PSU.JPG
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I have some Citation II information here:
Harman Kardon Citation II Amplifier

I use a PCB for power supply parts, which I like better than individual clamps for caps. You don't have to drill the chassis, and you've got a nice selection of PCB mount caps out there these days, including high temperature long life and low ESR caps.

I just rebuilt a fourth Citation II. It started with un-potting and repairing a transformer:
Sheldon’s World Blog Archive Citation II Amplifier Output Transformer Repair

Then I put in improved input and output connectors:
Sheldon’s World Blog Archive Citation II Input/Output Connector Improvements

Then I improved the bias circuit:
Sheldon’s World Blog Archive Harman Kardon Citation II Bias Meter Modification

Then I shoehorned a switch on the back:
Sheldon’s World Blog Archive A proper switch for a proper amp


Sheldon
 
The Citation II is a great electrical design, but there are some quirks that show that it was made by an engineer and not thought through for the home user.

An example of that is the bias pots; They are setup such that turning "up" (clockwise) turns the bias voltage "up" which reduces the bias current. The net effect is that the bias meter and the bias pot work in opposite directions. IE, you want the meter to move to the right, you turn to the left. Dumb... I swap the wiring on the pots to make the meter and the pot rotation match.

On the top of the amp, are big holes next to the attachment points for the cage. The big holes are for the screw heads that attach the other side of the attachment boss. So rather than recess the screws, or using flat-head screws, they drilled big holes in the chassis.

Sheldon
 
So far so good. I did exactly what was suggested in post #7 of this thread. The unit held 450V well, caps looked fine, cool, etc.

Once the tubes arrive, I will proceed and post the results here. Thank all of you for the help!

I'm just going from memory, but with no load on the high voltage, you could be exceeding the voltage rating of the power supply caps. Keep an eye on that as you raise the variac level. The b+ will be a lot higher without any plate current.


Sheldon
 
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