Balder lightning guitar amp schematic please!

[nickb]

I don't think oscillation is your problem, at least not primarily. I'd say idle current is too high. This brings me to something I had wondered about from looking at the pictures. In order to balance the output transistors it is usual to put small value resistors in the emitters. The problem is I don't see any. There is a row of four resistors that seem to be the same value on the top left but I can't make out the value and they seem a bit wimpy for the purpose. You should be able to determine if they are going to the four emitters with a continuity check. The whole point of this is that you would normally measure the idle current by looking a the voltage drop across those resistors.

I see there is trimmer pot between what look like two driver transistors on black heatsinks. I suspect this is idle current adjustment. You can note the position and set then it to one end then the other with the power off and see what effect it has on the point at which the idle current shoots up. It might simply be a case of a bad trimmer. Failing that I don't think there are any great choices. My preference would be to lift the PCB and trace out the power amp section and draw a schematic of it.

[J M Fahey]

Were you using a speaker or t was unloded?
Try both ways.
20mV at the speaker out is *nothing*, it does not justify the 1.5A mains current jump which amounts to over 150W mains consumption.
Where would/could that current be going?
Place a scope probe *across* an emitter resistor (ground clip to one end, hot clip to the other, you might have to disable mains grounding on the scope) to check transistor current consumption.

[The Dude]

Were you using a speaker or t was unloded?......

That's a good question. If you are bringing the amp up slowly on a variac with a speaker attached, you could have problems. Often, power supplies do not come up in a linear fashion when you bring them up slowly. This causes DC on the output until the amp rails balance and, of course, lots of current. So, rule of thumb, don't use a load when bringing an amp up slowly on a variac.

[nickb]

I understood he did it with no load, see post #15. :

scope on the output, no load.

[The Dude]

I understood he did it with no load, see post #15. :

Oops! Apologies, I missed that part. Carry on.

[bigdrums925]

Here's a look at the other side of the board. These are probably the resistors you spoke of. Except there are twice as many resistors as there should be (1.3 ohm)

Yes I thought it was idle current problem too at first. And assumed that trim pot would be for idle current.
At the point when the current shoots up the drop across one of the resistors is 1 volt.
again this is at half power on the variac, and more interesting is I get a 16 v p-p ac sine wave at the speaker output.

[The Dude]

On a side note, whatever that part is that's had it's solder pads lifted off (near the emitter resistors) does not look like it's very well attached. I'd unsolder it, clean the traces a bit and solder it in there better. Looks like it's barely (if at all) tacked in there.

[nosaj]

On a side note, whatever that part is that's had it's solder pads lifted off (near the emitter resistors) does not look like it's very well attached. I'd unsolder it, clean the traces a bit and solder it in there better. Looks like it's barely (if at all) tacked in there.

I understand at this point you all are trying to figure out whats wrong. I haven't seen it mentioned yet. The power transformer what kind of voltages is it giving out disconnected from the PCB? With the transformer disconnected from the circuit and a load put on the PT will it still give a high amp draw? just trying to start from the beginning of the circuit.

nosaj

[bigdrums925]

I understand. Does it help that the rail voltages are +- 46v and +- 12.6volts without disconnecting the trans?
If not I can do that.

[J M Fahey]

Yes I thought it was idle current problem too at first. And assumed that trim pot would be for idle current.

And it can still be an idle current problem: who says the trimmer or the circuit associated to it is working properly?
In fact I see the area where the Bias trimmer and bias sensing transistor should live, with signs of broken tracks and poor repairs.
That alone is an alarm siren going full blast.

At the point when the current shoots up the drop across one of the resistors is 1 volt.
again this is at half power on the variac,

Another red flag: 1V means >1.5A per transistor, or over 3A , at idle and with only half voltage applied or less.

and more interesting is I get a 16 v p-p ac sine wave at the speaker output.

What frequency sinewave?
Are you injecting a tone while you do this test? You shouldn´t.
And no load connected.

To be able to suggest tests, we need that output stage schematic, specifically so you can follow instructions and we all speak the same, so grab pencil and paper and draw it.

Not all of the amp (hopefully) but at least the power rails, ground, main filter caps, speaker out, Zobel network, grounding, power transistors and mainly the bias circuit.

[bigdrums925]

Curiously just before the amp meter swings full on, a 600Hz signal @ 14Vp-p appears at the output with no input signal.
The sine wave output remains after the meter swings full on and I see no oscillation. However the speaker plug was only half way in.
So it seems possible that the 600Hz is on the ground plane perhaps. When the speaker plug is in I see the oscillation and no 600 Hz.
Now can't recreate the 600Hz at all or the oscillation but the meter still swings hard 1/2 way on. I ramped up the variac all the way momentarily and measured 3.4 volts drop across the emitter resistor.

The schematic for the output will probably take a little time because I've never drawn one before and following the traces on this amp is a nightmare. But I will try in the next day or so. I don't know what a Zobel network is, can you explain?

[nickb]

The 1.3 ohm resistors may be in four parallel pairs giving 0.65 ohms. That makes your idle current huge so there is no question there is a problem there. It might help to look at other solid state power amp schematics to get a feel of what's there as there are often great similarities between designs.

A Zobel network is a capacitor and resistor connected across the speaker. It's there to improved the amplifier's stability especially at higher frequencies. Zobel networks

[MarkusBass]

It's clear that the amp failed in the past and the question is whether it was fixed correctly, or not. You can easily see that several components were replaced. The bias can be altered with the upper trimmer (2k2). It works together with the little transistor on the top of the board. So I would check the trimmer and the transistor. Also, what are the voltages on collector and emitter of the transistor. It seems to me that there is no other way to fix the amp then drawing its schematic and verifying whether it is correct. This shouldn't be to difficult since I think that this is typical solid state amp.

Mark

[J M Fahey]

The schematic for the output will probably take a little time because I've never drawn one before and following the traces on this amp is a nightmare. But I will try in the next day or so. I don't know what a Zobel network is, can you explain?

We don´t need the full schematic, just the power transistors, what´s directly connected to them, and mainly
the bias circuit , which basically is a transistor, a trimmer and 4 or 5 resistors.
To be able to suggest what to measure and how to adjust it, we need at least that, specially so we all talk the same.
Draw what you can and if we find something missing, we´ll ask you to add it ... but we have to start with *something*

This is the area we are interested in:

[nickb]

Attempt:



Did you acquire the amp in this condition?