Hartke HA4000 output blown

[pontiacpete]

enzo "If the amp is in protct mode, the speaker relay will not turn on, so you won't have DC on the speaker terminals, but that doesn't mean the amplifier is not making DC on its output bus before the relay."

Giving this one another try after a long break , so.... ..... there is DC on the middle rail -9.25v.
This amp has been through a lot, the board had been cracked and a number of traces were broke then repaired by someone else. I have been trying to sort out the connections. But still working on that

The posistor , with one leg lifted measures 50k and turning the amp on without it, with limiter, gives the same result , -9.2v on center rail.

the collectors of the power transistors are at +/- 37 but all the base and emitters have the same -8 to -10v.

All 4148 diodes have be checked and/or changed. I changed D 319-320 4006 to 1n4007. I don't see these on the schematic but they are located between all the collectors and ballast resistors.

Don't know what to ask at this point but I will be posting more info

[Jazz P Bass]

You need to find out why the bases are turned on so hard.

The schematic is here:Hartke HA4000

[Enzo]

He doesn't seem to have an excess curent issue, he seems to have an offset issue. That to me suggests the problem is in the left half of the power amp, before the bis curcuit so to speak. In fact the amp has an offset trimmer, I might start there.

[pontiacpete]

Is that vr301?

[pontiacpete]

The voltages at the anode of D301 and the cathode of D302 are -9 and -10v.

[Enzo]

VR301 is offset trimmer, does turning it change the output offset voltage?


So that trimmer has about -9v on it, just like the output? Sounds like the amp wants to work.

Are the main power rails even? Look in the center where the bias control is on the drawing, now at the very top and bottom find the 4.7 ohm resistors. R315 and R323. Are they both OK and close to value? And the voltages on the left ends of those?

You gave voltages at D301, D302, but how about the diodes themselves?


Ooohhhh, wait. By any chance are you operating this power amp module with the little input signal connector unplugged? Can't do that, your input stages get their ground through that connector. You either have to plug that back in or run a clip lead to a good ground from pin 2 of that connector.

[pontiacpete]

Turning that trim pot does very little to nothing, moves one volt. -9 to -10v

Trimmer has -9 on it.

The main rails are +/-38 .. The 4.7 resistors are measuring 4.7 in circuit, there it has 38v on both sides.
The diodes 301 and 302 have -9v or -10 on both ends, anode and cathode. In circuit they are not showing a short.
Could they be open? maybe need to lift one leg.

Pin 2? are you referring to the AN301? I'm having a heck of a time finding this connector. There was one, AN112 that was disconnected/broke but now it's not. This didn't do anything. All other connectors are connected while working on the underside of the board.

[Enzo]

Yes, AN301. it ABSOLUTELY must be connected to at least a ground, otherwise the input section has no ground, and your amp floats to whatever voltage it feels like.


I just know it is not connected, you repost -9v on BOTH ends of D301 and D302. Look at the schematic, each of those has one end to ground!!! So there must not be a ground present, ie the connector is off.


Your trimmer works, it moves the output a volt or so. That is usually all it needs to trim away a 200mv offset.

AN301 will be a small two pin connector on the end of a thin little shielded wire, typically on the end of the board.

[pontiacpete]

you are right but this connector was in the wrong spot on the pre board, and it is labeled AN112 just to make things confusing.
I fired it up and the -9v on the center rail was down to 0v
Took it off the limiter and it blew the fuse, a lower rated one, 5A not the required 8A fuse.
unfortunately there was a slight poof of smoke. I turned it on again with the limiter, there's +/-30v on the collectors of the power resistors.
D301/2 now have .5v. Still haven't pulled on leg to check those.

[pontiacpete]

Tried turning it on again it looks like R355 is the resistor that smokes before the fuse goes.
STill measures 4.7 R.
With a 3A fuse, collector voltages fluctuate between 42 - 50v limiter light pulsing. Fuse blows without limiter.
D301-2 have +/- .6v.
.4v on base and emitter of Q314-21


I changed out D301-2

[Enzo]

The amp is unstable, it is oscillating at RF, that is what burns up that resistor.

[pontiacpete]

really?
of course I want to know how that happens..what kind of part goes bad or parts
can it be fixed?

[Enzo]

Of course it is fixable, sometimes the resistor fails on its own leaving the amp unstable. That resistor and cap are called a "zobel network" or zobel circuit. You should be able to look it up for a better explanation than I can give.

Now if your amp is prone to oscillation to a greater extent than normal, it is hard to say why from here. Loss of a ground connection somewhere or loss of decoupling somewhere come to mind.

[pontiacpete]

Ok,,,,,not really sure where to go from here.
The zobel cap and the resistor test good in circuit. I guess since this amp as been worked on quite a bit,
I will continue checking for bad parts or connections. I'm troubled knowing that the schematic isn't accurate. Hopefully it's accurate enough.
Any other suggestions?

[Jazz P Bass]

I would change out that zobel cap.
Just to be sure.