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Old 11th August 2005, 07:55 PM   #1
Buhl is offline Buhl  Denmark
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Default Help needed repairing HK Citation 12

I have a Citation 12 Deluxe standing on my desk - with a faulty channel. The Faulty channel measures 42 volts over the output terminals (about the supply voltage) and visiually the PC board has a power resistor that shows signs of very intense heat.
I suspect that the resistor is the 10 ohm 1 watt resistor, named R15 in the attached schematic. The schematic is for the "normal" citation 12, but is corrosponds very well to the amplifier in front of me. The transistor names seems to be different though.
I have very little (almost none) experience with repairing amps, but I suspect that the power devices have blown.

Can any of you help me with: where to measure what, in order to determine what else have blown ?

How to calculate the bias value again if I change output devices ?

Which devices, available today can be used ?

Thanx in advance !

Cheers !
Hans
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Old 11th August 2005, 08:32 PM   #2
anatech is offline anatech  Canada
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Hi Hans,
Check or just replace Q4, 5 and 8. Q3 may go bad due to age. Check R5 and R14 while you are there.

Bias current is measured across R16 and R17. Make it the same as the good channel for now. C4 should be replaced in both channels once you get it working. When bad, it will affect bass response and possibly the DC offset voltage. After replacing C4, if your DC offset is still high, match a pair of transistors and replace Q1 and Q2 - these must be matched.

The outputs (Q6 & 7) should be replaced, don't even bother checking them, same for the emitter resistors (R16 & 17). New heatsink grease and insulators. MJ15023 or equiv. should work fine. The faster devices may need more compensation in the event they oscillate. Once it's running, change the other electrolytic caps. That alone may solve an oscillation problem.

-Chris
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Old 11th August 2005, 09:20 PM   #3
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You also may want to chack the dual transistor at the input. The earlier units had a dual transistor that failed quite often. Later on HK used selected single transistors.
Altough not confirmed, nor proven...it is my suspicion that a near failing input transistor can cause the driver transistor to fail as a result of DC instability if the amp is driven close to the limits.

Suggestion: Try the mods documented at Nelson Pass...It is quite an improvement over the original I'm afraid.

Goog luck!
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Old 12th August 2005, 01:04 AM   #4
Buhl is offline Buhl  Denmark
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Anatech and tarasque - thank so much for your replys !
I will change output devices and emitters - I will also change the working channel, when the first is working, dont you think that would be best to ge the same sound i both channels ?
Electrolytics have been changed recently, but what can I use for the drivers Q4 and 5 ?
and what about the Q3, 5 and 8 ?

Q4 and 5 says: 4219 - RCA - CXP 514 and 4218 - RCA - CKU 440

What about 3, 5 and 8 - BC 550 ?

Thnx a bunch !

Cheers !
/Hans
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Old 12th August 2005, 02:17 AM   #5
anatech is offline anatech  Canada
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Hi Hans,
Listen first to see if you can hear the difference between the channels before changing anything on the good channel. Try to match the case styles with something current that Motorola (ON Semi) carries. Don't try to look up house numbers. Possibly a member already has a list of subs worked up?

I would just hit the transistor selector based on case style and go with their recommended devices. MJ15015 are not very good BTW. Cheap parts never pay.

-Chris
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Old 12th August 2005, 02:47 AM   #6
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I'll be of no help to your problem here.
But I used to have a Citation16 I used in a sound studio situation. It did the same thing, spit out DC voltage and blew up one of my Yamaha NS10 monitors.
I wasn't even pushing the amp very hard a the time. I heard the channel cut out and figured it was wiring. As I went to inspect the cabling to the amp I smelled the speaker and then it began to plume smoke out. You could see the coil glowing, then it caught fire!
All this happened in the matter of 1-2min.
Now I have an Alesis RA-500 that sounds better and has protection circuitry.
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Old 12th August 2005, 09:19 AM   #7
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"MJ15015 are not very good BTW"

"MJ15023 or equiv. should work fine. "

The MJ15015 will work better in this amplifier than the MJ15023 will. The MJ15023 is a PNP.

MJ15003
MJ15015 (marginal Beta into 4 ohm loads)
MJ15022/24
MJ21194
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Old 12th August 2005, 09:24 PM   #8
Buhl is offline Buhl  Denmark
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What about MJ15003 - I have read good things about it in the JLH Class A amp - and its only 2mhz, and its also a NPN ?

What to use for driver - TIP29 ?

What to use for the rest - Q3 and 8 ??

Cheers !
Hans
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Old 12th August 2005, 09:55 PM   #9
anatech is offline anatech  Canada
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Hi djk,
You got me on that one. Brain f a rt . I did intend the NPN. Your list is something I'll totally agree with. Parts with a lower fT may be better in that circuit.

I'd use something of higher quality than a TIP part unless it's made by Motorola. MJ15030 are pretty good to, again the selector guide is something I'd refer you to.

-Chris
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Old 3rd September 2005, 11:03 AM   #10
Buhl is offline Buhl  Denmark
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Default Still putting grey hairs in my head.....

Ok, so I allied me with a friend with more test gear, and more knowledge about electronics.
We took out all the transistors in the faulty channel, and the only one that was cooked was the BIAS transistor Q8 - even the output was OK, despite we have 40 volts on the output.....
We changed the cooked device, but still 40 volts on the output.
We cheked all diodes, and caps on the mainboard - still problem....
We hooked up a 3 khz sinus on the input, but after Q1 the scope shows thats its heavely distorted - so something happens between Q 1 and the output - thats all we know - ant thats not enough sadley.......

Besides Q8 we changed Q1 and 2, and also put some MJ15015 in the output.

Q 6 is getting hot, and also the driver Q 5 - R15 cooks like hell - of course we use a variac now when testing........

Anyone have som good ideas of what to try next ? - or should I consider it gone ???

I would appreciate any help, suggestions or directions to go - anyone has a servicemanual lying around ??

Cheers !
Hans
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