Well, here is a very nice looking amplifier from Harman/Kardon.
I have one here that has a problem which is the same in both channels. When powered up, both channels are swung hard to the positive supply rail even though none of the outputs appear to be bad. The transistors on the driver boards are good as well.
I'm wondering if this is a good chance at being the chip and if so, is it a common problem?
I have one here that has a problem which is the same in both channels. When powered up, both channels are swung hard to the positive supply rail even though none of the outputs appear to be bad. The transistors on the driver boards are good as well.
I'm wondering if this is a good chance at being the chip and if so, is it a common problem?
Sorry, it's the negative rail that the amp is stuck on...
Does anyone have any clues?
I'm beginning to believe that it's the ua739 opamp in the input stage that is causing the problem.
I measured about 2.5V between the inverting and non-inverting inputs, so yeah, it's not working properly in that respect.
Does anyone have any clues?
I'm beginning to believe that it's the ua739 opamp in the input stage that is causing the problem.
I measured about 2.5V between the inverting and non-inverting inputs, so yeah, it's not working properly in that respect.
Hi Duo,
The chip may not be able to correct for the fault and be just fine. Since both channels are affected, look in the power supply or common bias circuit first. I have worked on these eons ago, quite frankly, I forget the configuration.
Above all, keep an open mind.
-Chris
The chip may not be able to correct for the fault and be just fine. Since both channels are affected, look in the power supply or common bias circuit first. I have worked on these eons ago, quite frankly, I forget the configuration.
Above all, keep an open mind.
-Chris
Thanks for the reply.
Well, I have looked at the power supplies and they are doing about +-68V which appears to be just fine. There's no unnecessary current draw.
I'm not sure what you mean by the common bias circuit? On the input? Gainstage? Output? Which part?
Well, I have looked at the power supplies and they are doing about +-68V which appears to be just fine. There's no unnecessary current draw.
I'm not sure what you mean by the common bias circuit? On the input? Gainstage? Output? Which part?
Yup, that's one place.
If we had the schematic, we could help you in more detail. Look for anything that is shared by both channels. Op amp supplies, bias supply. Do you have any bias current in the output stage?
-Chris
If we had the schematic, we could help you in more detail. Look for anything that is shared by both channels. Op amp supplies, bias supply. Do you have any bias current in the output stage?
-Chris
Well, I checked the opamp power. It is fine. +-10V on each opamp.
There is nothing shared between either channel besides the chassis, power cord, power switch, and ground. Very separated design, I like that.
Stupid of me that I didn't think to test the output current.
There is no output current flow at all. Well, I'll certainly be looking into that.
There is nothing shared between either channel besides the chassis, power cord, power switch, and ground. Very separated design, I like that.
Stupid of me that I didn't think to test the output current.
There is no output current flow at all. Well, I'll certainly be looking into that.
Worked on a few of them.. Even have a schmatic for it. Not here but at the shop. 🙂
There is a bunch of info available for mods on this amp also.. I believe Nelson Pass wrote an article on making this amp into a mosfet output...
I'll locate schmatic.. and my notes on repairing amp.
There is a bunch of info available for mods on this amp also.. I believe Nelson Pass wrote an article on making this amp into a mosfet output...
I'll locate schmatic.. and my notes on repairing amp.
Ah, thankyou very much.
Yeah, I've already seen some things I'd like to modify in this amplifier. It has real potential
Yeah, I've already seen some things I'd like to modify in this amplifier. It has real potential
Current source(s) are down
Hi Duo,
No bias normally means the current sources are down too. That's why the 739 is railed. If I recall, there was the uA739 and uA749. One had a resistor built in, the other used an external current source (or resistor) for pull down (?). Marantz 3800's used them as well.
That chip has selectable fixed gain if I remember correctly.
Lubbie, would you mind sending me a copy too please?
Thanks, Chris
Hi Duo,
No bias normally means the current sources are down too. That's why the 739 is railed. If I recall, there was the uA739 and uA749. One had a resistor built in, the other used an external current source (or resistor) for pull down (?). Marantz 3800's used them as well.
That chip has selectable fixed gain if I remember correctly.
Lubbie, would you mind sending me a copy too please?
Thanks, Chris
Well, after observing the opamp rails, which are regulated by zener diodes, I have found that the opamp is swinging its output so hard that the positive rail is being pulled down through it's bias resistor. So much so that the resistor is getting warmer than it should. I measured 78V across the positive bias resistor, and 39V across the negative one, so the opamp is obviousely drawing enormous current.
Does anyone here have the schematic for this amplifier handy?
Does anyone here have the schematic for this amplifier handy?
The grounding throughout this amplifier is completely intact to my observation.
Both channels interestingly are doing exactly the same thing and it makes no difference whether the input jacks are connected to the boards or not.
Both channels interestingly are doing exactly the same thing and it makes no difference whether the input jacks are connected to the boards or not.
With 78V on one bias resistor you have lost a ground.
78V + 39V = 117V
The zeners are around 10V (if my memory is correct).
Normal is ±67V or so.
So it should be +67 (57 across resistor), +10, 0, -10, -67 (57 across resistor).
Follow the input grounds through the PC board to the power transformer.
78V + 39V = 117V
The zeners are around 10V (if my memory is correct).
Normal is ±67V or so.
So it should be +67 (57 across resistor), +10, 0, -10, -67 (57 across resistor).
Follow the input grounds through the PC board to the power transformer.
My apologies Ralph, but I cannot read these images, they have been compressed too much.
Could you possible email me full sized copies?
Could you possible email me full sized copies?
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