HK Citation 12 Woes

markd51

Nyuk Nyuk
Hello all,
I have a good friend who owns a pair of Citation 12 deluxe since new.

As I understand it, he's having problems with one of them, and set out to re-cap both, but has a problem with one Amp's Driver Board that he cannot correctly cure.

I guess what he seeks is a new Driver Board, or another Amp in which he can swap out the Board?

I'm no electronic guru, but understand there's matched output devices on the Board, and he's trying to avoid some makeshift repair with non original parts.

Does any know of any sources for the parts, or perhaps someone who has a trashed amp that these parts might still be good?

I've given him Harmon Kardon's number, but I sort of doubt they'll have parts lying around for a 37 year old Amp. Mark
 
Unless there has been some modifications, the outputs do not have to be matched. The only matched transistors are the input pair (MD8003?) which are in a single "can" package but can be replaced with a matched pair (get a meter with HFE measuring circuit) Onsemi MJ21194 should work well as replacement devices - perhaps someone who has done one recently will chime in.
FWIW - many people were modding them to MOSFET outputs - the problem now is availability of suitable MOSFETs in TO-3 cases. Unless the board is hosed, it will problably be easier to repair it than to find a replacement. Either way - good luck.
 
I just recently recapped one if he needs some help PM me .Use matched BC556B or 2SA1175 for input transistors .

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After
 
Thank you Batt, I think this is what he's looking for.

I know he spoke with Terry Dewick about this, and Terry had said there was no way he could just rebuild this board, without having the rest of the Amp on hand.

We probably cannot argue, that Terry of course knows his beeswax, that after this Board is rebuilt, then the amp must also must be biased? This is what we gather. Mark
 
We probably cannot argue, that Terry of course knows his beeswax, that after this Board is rebuilt, then the amp must also must be biased?

Quite true. When amps are partitioned this way, it's always necessary to tweak bias after the driver board is installed. That will be true even if you're just swapping boards between channels in the same chassis, with nominally identical output transistors. Furthermore, there's no reasonable way to insure that a repaired driver board is fully functional, short of mounting it in an amp chassis and testing the whole assembly. One could fabricate a test fixture to emulate the amp chassis, but that's way more effort than most folks are willing to pay for.
 
Rebuilding the boards is 90% percent of this amp , there are four power supply caps , two input caps , and the four output transistors in the main chassis .The bias would need to be checked and or set after the board was rebuilt .

Parts for rebuilding two amps would be about than $90 .

One of the input pairs was shorted on my amp ,and matched new pairs of
of BC556b
 
Quite true. When amps are partitioned this way, it's always necessary to tweak bias after the driver board is installed. That will be true even if you're just swapping boards between channels in the same chassis, with nominally identical output transistors. Furthermore, there's no reasonable way to insure that a repaired driver board is fully functional, short of mounting it in an amp chassis and testing the whole assembly. One could fabricate a test fixture to emulate the amp chassis, but that's way more effort than most folks are willing to pay for.


Rebuilding the board would be an option if the person that did it has an amp , to test it in and then send it back , but the outputs transistors and power supply would have to be good . The filter caps in the PS are 35+ years old
 
I've got a Citation 12 with similar issues and I've finally decided to just apply the Nelson Pass modifications to the board. Have picked up most of the parts now. Just need to find time to put it all together.

I can see why this amp was so popular as a base for mods.

Cheers,

David
 
Gentelmen,
I do deeply thank you all for the information that has so far come forward. I've been trying to help my friend, and have literally combed the web for information over the past few days, and found very little to nothing about this Amp.

Please excuse my electronic ignorance, but I think I have read about some type of MOSFET modifications for this Amp, but aren't these types of mods more pertaining to the output stages of the Amp?

I was not aware of any other "Nelson Pass" mods, such as you mention David, that is done to this board?

Would certainly like learning more.

As I understand it, this is one heck of a fine Amp. That it was in its day, and still is a great Amp till this day. Mark
 
Beyond the Nelson Pass mods which came out back in 1981, here's an Asian vendor offering an updated complete replacement circuit based on the Pass design but using newer and currently-avaialable parts: http://kikitronic.blogspot.com/2009/11/citation-12-mosfet-power-amplifier.html.

In addition, there have been off-and-on discussions about the Pass design including from Nelson himself: http://www.diyaudio.com/forums/pass-labs/160736-citation-12-schematic.html.

You might want to spend some time over there searching for Cit 12 discussions over there. They're rather technical but you'll get more insight into the attitude about spending a lot of time trying to fix the original circuit. This is what swayed me to go with the Pass mod approach. Beware that the mod is significant in that you strip off the old components and use the board, along with new output devices. This pretty much means that you're just using the old chassis and power supply components from the original.

My Cit 12 project awaits on the bench as I write this. It will complement the Cit 11 and 17 preamps along with its bigger brother, the mighty Cit 16 power amp. Should be fun!

Hope that helps.

Cheers,

David
 
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