Harmon Kardon Citation Sixteen

[lowell]

In my opinion there should not be any resistor. Someone was trying to fix the amp, he noticed that there is an excessive current draw and put the resistor (which did not help at all). I'm sure there is a shorted transistor on the amp board.
I wonder why you haven't check it until now .

All 4 voltage rails for the left and right power amps have these resistors and are mounted the same: on terminal strips. It'd make more sense what Enzo said, that this was a revision on later models. I have not checked the power transistors because w/ R9 disconnected the 120 resistor did not burn up, so I then eliminated the power transistors as suspects.

But frankly speaking you should check for shorted transistors first. Then, maybe this resistor will not be necessary.
EDIT: I've just noticed the photo of the resistor. Looks original - but as Enzo says, a small value resistor is used also in other amps. It is not critical. Just check for shorted transistors or other short curcuit on the power amp board.

I'll check the drivers for shorts. The resistor is not original, I replaced it after I replaced the IC.

Agree and add: leave Q9 still connected through the 120 ohm resistor, I think its current is the one overheating it.
We must find why you have so much current there: is Q9 shorted? is it being overdriven?
I would recheck it for shorts in all 6 possibilities (all 3 junctions, both ways) and also its Vbe.
Under or above 650 mV?

Ok I'll check this out tomorrow... thanks!

[lowell]

Lowell,

I wonder where you found this adjustment procedure (for this amp). I'm not sure whether 0.6V on bases of the output transistors is enough to bias them. Lately I was fixing a similar amp (Ampeg SVT III) and the output transistors were not biased. I would rather set 0.6V on emitter of Q11.

Mark

This procedure is in the schematic notes.

[lowell]

On understanding this amp, correct me if I'm wrong:

-Q16 keeps Q9/Q10 bases 2.7v apart (why 2.7v?)
-VR2 controls how "on" Q16 is
-Q7/Q8 are limiters
-Q9/Q10 are predrivers
-Q1/Q2/Q5/Q6 are VAS
-Q3/Q4 are current sources

[MarkusBass]

All 4 voltage rails for the left and right power amps have these resistors and are mounted the same: on terminal strips.

I think that this is the first time you mention that there are 4 factory assembled resistors. It would have helped if you'd say this from the very beginning .
So these are factory assembled resistors and they provide a filtering for the amp board and there is nothing wrong with them.
It means that you don't have to look at the resistor but rather to a short circuit on the power amp board.

On which page of the schematic there is this biasing procedure - I cannot find it?

Mark

[MarkusBass]

On understanding this amp, correct me if I'm wrong:

-Q16 keeps Q9/Q10 bases 2.7v apart (why 2.7v?)

Yes. Please note that the ouput stage uses only NPN transistors - it is not symetrical. And the current through Q16 changes with the temperature of the output transistors.

-VR2 controls how "on" Q16 is

Yes. It just changes the current through Q11 and Q12.

-Q7/Q8 are limiters
-Q9/Q10 are predrivers
-Q1/Q2/Q5/Q6 are VAS

Yes.

-Q3/Q4 are current sources

Not exactly. They are current mirrors. Current source provides a constant current, while current mirror provides variable current, which "mirrors" the current in the other branch.

Mark

[lowell]

I think that this is the first time you mention that there are 4 factory assembled resistors. It would have helped if you'd say this from the very beginning .

I stated this in post #11.

On which page of the schematic there is this biasing procedure - I cannot find it?

Mark

At the bottom of pg. 15. "Alignment Procedure Adjustments"

[lowell]

I almost suggested reconnecting the 3.9k resistors to the +60 side of the 120 ohm resistors to see if the 10v held up while the rest of the circuit failed.

Will this not damage the circuit? It seems the 120 ohm is acting as a fuse somewhat, being that it burns open and stops the amp from drawing all that current.

[lowell]

Maybe I should use a series lamp? Should I use a 60w bulb in series w/ the mains or use a lesser wattage bulb in series w/ the +60v supply? Wouldn't I need a 1/2w bulb in the latter case?

[Enzo]

The 120 may be acting like a fuse, but the current is not coming through that 3.9k resistor. The current burning up the 120 is coming from the driver circuitry.

[lowell]

Ok... but will the amp be further damaged if I test it the way you say? Or do I need to limit current somehow?

[Enzo]

I can't imagine it will change the operation at all, but it might help isolate the problem.

And someone else may have a better idea.

Think about what you would be doing. Right now you have a 3.9k and a 120 ohm resistor in series between 60v and a zener. 3.9k is 3900 ohms. The difference between wiring arrangements is that one way there is 4020 ohm and the other 3900. About a 3% change.

If it makes you uncomfortable, then don;t do it. There are always other ways to troubleshoot.

[J M Fahey]

I think the bulb lamp limiter, with a 60W lamp, will let you measure *something* without nuking your house.
I'm not kidding, or making fun out of you, that's what the current limiter is for.
The main suspect is still Q9.
*Do* turn the amp on, through the lamp limiter, and measure Q9's Vce and Vbe.
Vce won't be 60V, in fact your B+ will not be 60V (thanks Limiter), post what you measure.
Also post Vbe.
Maybe you burn *another* 120 ohm resistor. So what? Buy a dozen, or 20.
It's a symptom, not the cause of the problem.
Good luck.
PS: I have a zillion other questions, but please answer this one first.

[lowell]

Alrighty, with the limiter:

Q9
Vce: 28v
Vbe: 600mv

Doesn't strike me as being off.

[J M Fahey]

No, those values look quite normal.
I forgot to ask you the actual B+ voltage now.
More precisely, I want to know the voltage drop across the 120 ohm resistor, which will tell me what it's dissipating.
I'd also like to know what voltage does Q9 emitter have with respect to ground.
If all normal, *maybe* you can plug straight into the wall without limiter.

[lowell]

With limiter.

B+: 26.72v
Voltage across 120ohm: 8v (voltage across 120ohm on other channel is only 800mv)
8v across 120ohm is .53 watts, and that's with the limiter. Makes sense that it's burning up.
Q9 Ve: 400mv (low compared with stated 1.5vdc on schematic but maybe because of limiter)

Not sure what to make of this.