Harmon Kardon Citation Sixteen

[lowell]

This amp has a dead left channel. The 120ohm (not in schematic) current sensing resistor from the 60v filter cap on the positive side was fried open. I replaced it and the new one burned in about 10 seconds. So we have a moderate current draw happening here. I disconnected R9 and all is good. So it's something to do with the 10v supply. I checked Cr1, C25, C9 and they seem fine: there's no short to ground. I'm suspecting the IC, which is apparently obsolete. I need help with finding a replacement.

http://manuals.harman.com/hk/Service...xteen%20sm.pdf

[lowell]

Nevermind yall... found one on google shopping.

[lowell]

Ok so I replaced the IC in here and it's still burning up the 120ohm resistor. I pulled the IC again to make sure nothing else was causing this and it was fine without the IC in there. I even adjusted VR1 per the manual and it didn't help the issue. I'm hoping someone can help me with this. The +-10v supplies seem fine. Of note is that I cannot get VR2 to adjust to 10mv across the emitter resistors. If there's an issue with a power transistor in the power amp would this cause the 120ohm resistor in series w/ the +10v supply to burn up? I don't see how this could be, but maybe someone can clear this up for me.

[J M Fahey]

Find hard to speak about a part that is not shown in the schematic .

[lowell]

Yeah it's odd. However, the 120ohm resistor is a 1/2watt and it's in series between the main filter caps and the rest of the supply rails. It seems like it's there to act like a fuse, if I were to guess.

[Enzo]

So they added a 120 ohm series resistor from +60 to the driver circuits. And it burns up. You lift R9 and it does not burn up. R9 is also in series with the 10v zener. It is 3900 ohms. If the zener was a dead short, that would put the 60v across the 3.9k resistor. I get 15ma for that worst case. And 15ma flowing through a 120 ohm resistor is maybe 28mw. SO any dead zener or other dire load on the +10v would not burn up the 120 ohm resistor. You may well have a +10 problem, but it is not directly drawing current.

If I have to guess, I'd say something about the IC is trying to turn on the power circuits with something open and all the output stage current then tries to flow through the resistor ans drivers. Is +60 on all those output transistor collectors?

And yes, the bias adjuster that doesn't adjust bias is a clue.

[J M Fahey]

I had made the exact same calculations Enzo did, obviously with the same results, and also found impossible what was being *verbally* described, that's why I wrote:

Find hard to speak about a part that is not shown in the schematic .

*Please*: post the schematic showing how and where that 120 ohm resistor is connected.
Just saying "it goes from a to B" is not enough.
Why do I insist on this?
Because a little voice keeps telling me it's *not* connected as you say.
A 120 ohm resistor just doesn't burn if it's in series with a 3900 ohm one as described.
.
EDIT: I also don't understand:

it's in series between the main filter caps and the rest of the supply rails.

What does it mean?
Please hand draw a little schematic showing this.

[MarkusBass]

However, the 120ohm resistor is a 1/2watt and it's in series between the main filter caps and the rest of the supply rails. It seems like it's there to act like a fuse, if I were to guess.

Lowell,

As J M Fahey says, you are not reporting the problem correctly, nor precisely. We need precise information where exactly the resistor is connected.
Isn't it just a resistor between the ground of the circuit and the enclosure? Is the enclosure connected in any way to the ground of the power supply?
I think that the amp is quite similar to the famous Fender 212 that you were fixing once. I suggest to understand the schematic and use some logic. Disconnect the speaker, check the output transistors (whether they are shorted). If there is not current flowing through them, check why - most probably there is no sufficient voltage on their bases. Check drivers, check voltage on R17,VR2, R19. Maybe Q16 (thermal sensor) is shorted.
Please read the manual especially the part about bridging the amp. Maybe someone tried to bridge it and he made some short circuit.
Is the other channel working correctly? So you have correct voltages to compare with.

Mark

[lowell]

Thanks for chiming in everyone! I don't have a scanner, but this should suffice. The 120ohm 1/2watt resistor is electronically in series with J1 and P1 which is labelled "15" at the top of the schematic where the +60v comes into the driver board. Sorry for the bad initial description.

I'm in the process of the checking the output transistors collector voltages and seeing also if any power transistors are bad. Will report back.

[J M Fahey]

The 120ohm 1/2watt resistor is electronically in series with J1 and P1 which is labelled "15" at the top of the schematic where the +60v comes into the driver board.

I'd love to see a drawing of that.
What I imagine maybe different from what others might, and still different from what you mean, so I'll wait a little more for the schematic.
The little voice that whispers on my left ear now tells me that Enzo was even closer than he thought.
It also suggests me that that 120 ohm resistor is not original.
Does it sit properly on the PCB?
*How* is it mounted? A picture would be useful.
Do you have another 120 ohm resistor on the -60V line?
Just to save some time check Q9 to Q21 for shorts and get ready your series lamp.

[lowell]

Not sure if this schematic is much help but hope it is. Also here's a picture. The resistor is mounted on a terminal strip. The left side is wired to the + terminal of the 10,000uf filter cap AND the power transistor collectors, and the right side where the zip-tie is is wired to the "15" juntion on the driver board. Also yes all four power supply rails have this exact arrangement. The resistor color codes look wrong because it's been overheated. Finally I pulled the power transistors and tested their junctions... no shorts.

[Enzo]

I have attached a PV CS800 schemo, obviously very different from this, but look at the driver card. Just inboard from the output stages, the V+ and V- rails have a series 4.7 ohm resistor decoup;ling the driver stages. I THINK that is what his 120 ohm resistors are doing, and might have been added to a later revison of his amp model than the one in the schematic.

My working hypothesis is that the amp is trying to turn on the outputs, but a pathway is open, so the oputput current is trying to flow through the system via the drivers, hence burning the 120. The 10v circuit itself is not what is wrong, nor the failure current path. Though a lack of 10v could be involved in making the rest of the circuit misbehave.

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.

[J M Fahey]

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.

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?

[MarkusBass]

Thanks for chiming in everyone! I don't have a scanner, but this should suffice. The 120ohm 1/2watt resistor is electronically in series with J1 and P1 which is labelled "15" at the top of the schematic where the +60v comes into the driver board. Sorry for the bad initial description.

So the resistor is between +60V rail on the power transistors' board and the +60V rail of the rest of the power amp (top of page 6 on the schematic).
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'm in the process of the checking the output transistors collector voltages and seeing also if any power transistors are bad. Will report back.

I wonder why you haven't check it until now . But I think that a shorted transistor is rather on the power amp board (maybe drivers are shorted). You could try to disconnect the power transistors (but leave the temperature sensor Q16) - both rails +60V a -60V. You can even add a new 120R resistor and quicky measure the voltage drop after you switch on the amp (to see what is the current draw).
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.

Mark

[MarkusBass]

Of note is that I cannot get VR2 to adjust to 10mv across the emitter resistors.

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