Harman Kardon citation 16 Issues

[Badula]

Hi there!

I have a citation 16 with huuuuuge problem it seems. Picked it up cheep. And works che thereafter unfortunately

first the usual problems with these amps it seems. The variable resistors were bad. Replaced them and recapped the hole machine, But after it seems that I shortened an output transistor 😑 it is at least my suspicion because I have around 30 volt on the output rails 🥺???? And cannot change the idle on Channel A

channel B is another story. There I can change the idle current. Problem is that as hotter it gets. The lower the idle current is!!?. Why is that? And soon as I cool the heatsink on channel B, let’s say I just blow on it a little, the idle current rises?!? Is it the emitter resistors they gone bad? And they change value with temperature ?

Another suspect is the IC/Opamp ua739pc…. Wherever I read about the Citation 16 it seems that these ICs seems bad. Anyone here that have successfully replaced them? Or know where one can get original?

Thanks!

 

[rayma]

Did you test the unit before removing any parts?
What was its exact initial operating condition?

 

[Ixnay]

Research what you can before making any more decisions. I jump ahead sometimes and cause unexpected problems.

 

[rayma]

Most people do. Not a good way to do things.

 

[Badula]

Did you test the unit before removing any parts?
What was its exact initial operating condition?

i Did not… just started it. I knew then it was something strange with channel A because the light on A was flickering. But just thought then it needed a recap

 

[rayma]

First evaluate and diagnose, then repair. Shotgunning seldom works out very well.

Variac, dim bulb tester, or current limiting lab supply all can help initial start up testing.
Then check in-circuit PN junctions and electrolytic capacitors.
The soldering iron should not be used until you have a pretty good idea of the problem(s).

 

[Badula]

I thank you for your knowledge. I guess I have to measure all of the emitter resistors and the output transistors… what a bore 🤮 but still? The IC 739… Is that something that could cause idle problems?

 

[rayma]

Yes, certainly measure in-circuit the emitter resistors as well. They dissipate more power than the other resistors do.
Sometimes their heat can even damage their own solder joints or the pcb underneath.

You can find bad parts most efficiently by the process of deduction from the observed and measured symptoms.
That's how these threads can be most helpful, to facilitate this process. Replacing parts en masse to avoid this
process can cause even more, and more serious, problems.

Until the 739 is proven to actually have a problem, it should be assumed ok. They are available surplus, if needed. https://www.westfloridacomponents.com/T581PF01/UA739DC+Dual+Op+Amp+Ceramic+IC+Fairchild®+UA739.html

Such a complex design can oscillate and/or exhibit high DC output offset in case of certain parts failure or degrading.

 

[Badula]

You found the IC !?! 🤩 wow your knowledge is incredible!
sir you have ignited a spark of hope and enthusiasm! Thank you so much.
I will get to work and measure the transistors and resistors !

 

[Badula]

But!… I just want to say. In the service manual It says the IC I named (ua739pc)
but on the IC itself it’s (pa739pc) ? Is that the same? Maybe I should post a pic

 

[rayma]

Yes, photos will help.

 

[Badula]

Well I have at least tested all of the Transistors on channel A. They seem to be all ok strange enough 🙃