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!
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!
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Did you test the unit before removing any parts?
What was its exact initial operating condition?
What was its exact initial operating condition?
Research what you can before making any more decisions. I jump ahead sometimes and cause unexpected problems.
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
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).
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).
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?
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.
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.
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 !
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 !
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
but on the IC itself it’s (pa739pc) ? Is that the same? Maybe I should post a pic
Changed all output resistors
Changed zener diodes
Changed Resistor R9 and R10 (infamous) it seems
And the left channel has a stable Idle current. But the right is so strange… when I turn it on. The idle current is about 300mw minus 😐 and the longer it stays on. It crawls down in voltage. And eventually it gets positive…
I haven’t changed other resistors about 15k ones. That seems also (infamous) they are just 10k so maybe bad. But they are the same on both channels. But the left one work so.. I don’t know 😖
I appreciate all answers and help. And sorry for any misspelling
The ua739pc is a dual op amp that was made by Fairchild. The pa739pc is a clone from a different manufacturer.
Check the connectors: clean the copper fingers on the driver boards and reseat them.
You could try swapping the driver boards between channels, since they should be identical.
Check the connectors: clean the copper fingers on the driver boards and reseat them.
You could try swapping the driver boards between channels, since they should be identical.
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It seems that I found the problem. I switched the A-B cards. And the problem moved with it. And then I switched the IC 739 and the problem moved with it. So the IC is probably on the right channel is the problem. Guess I have to order one of those.
Don't belike and order a single part, and then two days later remember that I forgot to another part that I needed. Besides shipping will be the same for multiple chips. I get what you mean about cost though. Makes it tougher to enjoy this hobby.