Where to start? Or: cool these plates down

[koolatron]

Hi guys,

Yesterday morning, I went to a hamfest out in the boonies of North Carolina and picked up a Harman Kardon Citation II power amp. The guy selling it made a big point of telling me that it was AS-IS, did not work, and that it'd need repair before it'd be usable as an audio amplifier.

The story was this: he picked it up at an estate sale, and it didn't work for him. He didn't know how to troubleshoot it, so he brought it to a local tech who declared the power transformer dead. The amp apparently blew its line fuse on powerup.

Given that his asking price was worth it even if all the iron was trash, I cut him a check and schlepped it home.

First thing I noticed: line fuse wasn't blown. Odd. Maybe he replaced it so I could blow it myself. Plugged it into the variac and brought it up slowly. Tubes glowed. Looks like the power transformer wasn't bad after all. After a little while (5-7 minutes), however, the output tubes' plates began to glow glow a dull red. I switched things off immediately and let things cool off overnight. Then I sat down to write this post.

So, my question is: where to start fixing this thing up? Obvious answers first: replace the coupling and filter caps. Will that possibly fix the red plating issue? I'm guessing that a faulty coupling cap or something is causing B+ to leak into the bias circuitry. But since I don't have a schematic for the Citation II, I don' t even know whether this is possible.

Can anyone suggest filter and coupling caps that will do this work of art of an amp justice?

--k

[Sal Brisindi]

Jim McShane is the man you want to talk to, his website is http://pages.prodigy.net/jimmcshane/

Regards,
Sal

[astouffer]

So, my question is: where to start fixing this thing up? Obvious answers first: replace the coupling and filter caps. Will that possibly fix the red plating issue?

Get to know your ohm meter very well. Find a schematic and check all the resistors for drift or going open. Losing bias will make the tubes draw too much current and turn red. Replace all the electrolytic caps and the rectifier for the bias. It may be an old selenium rectifier and they go bad after a while. Replace all the coupling caps and anything else out of spec. The output tubes are questionable so unless you have access to a tube tester invest in some new ones. Put them on ebay "as is" and they will probably fund the restoration

After replacing the components the amp will need to be re-biased for the new tubes. Now sit back and enjoy your new find.

[kevinkr]

Most likely the bias supply has failed. I have repaired many of these and modded a more than a few as well. (See some of my old OTS articles for further details.)

Jim McShane is the man as far as Citation II goes these days.

Schematics and other information here: http://pages.prodigy.net/jimmcshane/specs.htm

Note that the bias rectifier is a half wave selenium and this is probably bad. Should be replaced.

Before you run it you really ought to replace all of the power supply capacitors. Most people running these around here use modern snap in types and generally add a lot of additional capacitance. Stack them in series with voltage sharing resistors, (post choke) I typically use 220uF/400V types with 100K 2W resistors in parallel. Change the stock silicon rectifiers when you do this. (BIG INRUSH CURRENT!)
(I used roughly twice the stock values in the doubler - use high ripple current tolerant types here, and about the equivalent of 330uF - 470uF @ 700 to 800V after the choke.)

Lots of other parts need to be checked and potentially replaced. Just take your time, you have a great amplifier and a worthwhile project. This is the only vintage amplifier I actually like other than the Marantz 9.

Modern KT88 will probably not survive long at the stock 100mA plate current, hopefully you have the originals. (IIRC Genalex KT88) FWIW I have had pretty good luck with current KT88EH in these amplifiers at somewhat lower currents. Check with Jim - he can better advise.

[koolatron]

Modern KT88 will probably not survive long at the stock 100mA plate current, hopefully you have the originals. (IIRC Genalex KT88) FWIW I have had pretty good luck with current KT88EH in these amplifiers at somewhat lower currents. Check with Jim - he can better advise.

I do have the original KT88s in place. As well as a few contemporary KT88s that were given to me with the amplifier as replacements. The owner claimed that the tubes tested good and even went so far as to supply me with numbers, but he didn't tell me the type of tester he used or what a 'good' rating consisted of. All that on top of his telling me the power trafo was bad doesn't make me confident, so I'll have to get them re-tested locally.

The side getters on two of the tubes have turned chocolate brown, but the top getters are still silver. I initially took this to mean that the tubes were gassy, but after a bit of research I'm not sure. If there were gas, I would think that all of the getters'd turn.

As far as restoring this goes, I'm now in the process of checking out the page of this Jim McShane guy, and figuring out precisely what I need to do. You guys have been super helpful. I'll keep ya'll posted with my progress.

--k

[kevinkr]

Originally posted by koolatron


I do have the original KT88s in place. As well as a few contemporary KT88s that were given to me with the amplifier as replacements. The owner claimed that the tubes tested good and even went so far as to supply me with numbers, but he didn't tell me the type of tester he used or what a 'good' rating consisted of. All that on top of his telling me the power trafo was bad doesn't make me confident, so I'll have to get them re-tested locally.

The side getters on two of the tubes have turned chocolate brown, but the top getters are still silver. I initially took this to mean that the tubes were gassy, but after a bit of research I'm not sure. If there were gas, I would think that all of the getters'd turn.

As far as restoring this goes, I'm now in the process of checking out the page of this Jim McShane guy, and figuring out precisely what I need to do. You guys have been super helpful. I'll keep ya'll posted with my progress.

--k

Not gassy, but certainly relatively high hours at high dissipation. Most Genalex KT88 I have run across were still good with many thousands of hours of use. I suspect that will probably prove to be the case here..

His seeming misfortune is your good luck, presumably because he thought the power transformer was toast you got a reasonably good deal..

[koolatron]

His misfortune was in trusting a local tech who didn't know what he was doing to troubleshoot the thing.

Obviously this guy didn't possess a variac to test with?!

[Serge66]

Hi Koolatron,

Check the HV supplies on the tubes. If possible - with all safety requirements in place - check with an oscilloscope.

I had this problem once with a kit. It was due to bad PSU design. In fact, it was motorboating. No decoupling, no choke between pre-amp and power stages.
Maybe your HV caps have dried out.
You may be able to measure the ac component on the HV supply with a good multimeter.
Good luck.

Serge

[koolatron]

Serge,

The HV caps have almost certainly dried out. In all honesty, I am quite lucky that far worse than a bit of red-plating didn't happen when I plugged the thing in in the first place. I probably owe that to the robustness of the Citation II design and the fact that I brought it up on a variac as opposed to just plugging it on in.

I contacted Jim and should be getting a big fistful of capacitors here soon - a few nights' work to get it running, and then I'll be able to give this baby the listening it so sorely deserves.

--k

[TubeHead Johnny]

This is the kind of amp that collectors absolutely drool over! Once you get it fixed and restored you're in for a treat. A properly set up Citation II sounds incredible! Just check out how much they want for these amps on eBay.....CRAZY!

http://cgi.ebay.com/Harman-Kardon-Ci...QQcmdZViewItem