LANEY G300 stereo bass amp head schematic ?

[Tilman]

Hi,

I am having DC at the speaker (from 4-16V) output of one channel of this amp.
It is not always there. But when it is : It hums and it burned one of my test speakers already (BEYMA Liberty-8 ).
I have checked ALL active devices (transistors) on the power amp board of that channel. The supply voltage stays at +-45V. I have changed the filter capacitors to new ones.

When the DC is there - the lower driver transistor (quasi comp stage with 2 NPN output transistors) is biased at 10V+ but it should be somewhere at -0,6V or so.

I am afraid a capacitor leaks DC from somewhere to the bias circuit sometimes but without a schematic, it is hard to say which one it could be.

Someone can help ?

Regards!
Tilman

[doctor]

Hi,

I am having DC at the speaker (from 4-16V) output of one channel of this amp.
It is not always there. But when it is : It hums and it burned one of my test speakers already (BEYMA Liberty-8 ).
I have checked ALL active devices (transistors) on the power amp board of that channel. The supply voltage stays at +-45V. I have changed the filter capacitors to new ones.

When the DC is there - the lower driver transistor (quasi comp stage with 2 NPN output transistors) is biased at 10V+ but it should be somewhere at -0,6V or so.

I am afraid a capacitor leaks DC from somewhere to the bias circuit sometimes but without a schematic, it is hard to say which one it could be.

Someone can help ?

Regards!
Tilman

May be G300H schematics will help you

[Enzo]

By "biased" I assume you mean the voltage at its base? And by driver do you mean the TIP32?

SO when you have more or less 10vDC on the output, you also have more or less 10vDC at the base of an output driver. DO you also have about 2 volts more positive at the base of the other driver too?

First, NO SPEAKER LOAD. leave the output unloaded until we have the amp stable with no DC. WHEn amps go DC< the speaker does hum, it is the large ripple current as the poor filter caps try to hold up that output voltage. Not their fault, the amp was not designed for steady DC output.

This is a solid state amp, not tube. Terms like "bias" have a totally different meaning. The whole amp is DC coupled, there are no caps to block any DC. Bias is a measure of how much voltage difference there is between the opposing driver bases. That in turn is a measure of how hard the top and bottom circuits are turned on. Bias is set by TR4. The harder it conducts, the closer together the bases of those drivers get. In fact, shorting across it just binds the bases together, the amp continues to function with a bit more crossover distortion than normal.

Those bases, which sit roughly 2v apart, maintain that voltage spacing all the time, the whole thing moves up and down with the signal, but whatever the voltage might be with respect to ground, those two bases stay the same distance apart.

That is the job of TR7, the voltage amp stage. VAS. It grabs the bias transistor TR4 and shakes it up and down with the signal. The bases of the drivers of course follow, and they make the outputs do the same. If the bias circuit is out of whack, you usually have outputs getting very hot or maybe distortion. But that doesn;t usually put DC on the outs.

No guarantees of course.

I'd watch voltage, or scope the output, to watch for DC. WHen it happens, see what is going on at TR7. Does its base start drooping closer to its emitter?

And one thing to check is upper left, see that 15v zener supply? Is it stable? Does the voltage at the collectors of the differential pair look like that 0.6v?

[Tilman]

Hi Doctor and Enzo,

@doctor : Thank you very much Sir! That will be a great help!

@Enzo

Thanks for your help again! I really appreciate that.

By "biased" I assume you mean the voltage at its base? And by driver do you mean the TIP32?
Yes,yes.

SO when you have more or less 10vDC on the output, you also have more or less 10vDC at the base of an output driver. DO you also have about 2 volts more positive at the base of the other driver too?
I did not measure that during the DC status of the amp. Right now, with no DC at the output, I got about 1,6V over C13.

And one thing to check is upper left, see that 15v zener supply? Is it stable? Does the voltage at the collectors of the differential pair look like that 0.6v?
With no DC at the output 15V is stable. 0,6V is there.

I noticed TR7 is VERY hot. In both channels.

Unplugging the speaker plug somehow "triggers" the DC status. When it happened and I unplugged the speaker plug - it stopped sometimes.
So far, I did not get the thing into DC status again. There is a small transistor between the two output transistors buried in heat transfer paste inside the heat sink.
Somehow I did not find this one on the schematic.

If DC comes and goes - isn't it fair to suspect a capacitor ?

I will come back once I managed to measure something during DC at the output.

Best Regards!
Tilman

[Enzo]

If DC comes and goes - isn't it fair to suspect a capacitor ?

No, not in the way you think. In a tube amp, the coupling capacitors are there to block the different DC levels while passing signal. For example the cap from the phase inverter to the grids of the power tubes. And sure enough THERE if the cap is leaky then unwanted B+ DC from the phase inverter will get on the power tube grid and cause havoc.

SOlid state amps are not tube amps. Forget tube amps while working on this.

Look at the circuit, where do you see caps? Of the very few that are there, which ones could leak DC into the works? Maybe if C8 shorted, but caps are not what I expect to find bad when I have DC on a solid state amp output.

OK, +1.6 at the top of C13, and if I recall about -0.6 at the bottom. (Base of TR2) That is a spread of about 2.2v, not far from the 2v or so that I predicted.

I would assume until I find otherwise that the small transistor buried in goop on the heatsink is TR4. Follow the copper traces from it - do any go to C13? Or to the bias adjust control?

[Tilman]

Hi Enzo,

it is very difficult to fix this amp because it is running for hours or even days without a problem. I play music through the ok channel all day and when it starts to hum (there is no DC at this channel) I run to measure something.
The problem is: Every time I touch TR7 base / R27 with multimeter - the amp goes to ok status again.
This is what I was able to measure during DC condition:

- 15V at D13 is stable
- 0,8V at TR11 / TR12 is stable
- voltage over C13 is stable (about 1,6V)
- the +47V UB breaks down to about 35V or so (that is about the same voltage as at the output)
- UB -47V is stable
- Ub TR7 I measured for a short time >33V+ (from ground!!! - than it jumps back to -46V to result in a Ube of about 0,45V TR7

I have no idea where the high positive DC at the base of TR7 should come from!
Really - no idea!

Regards!
Tilman

[Enzo]

Not many places a +33v can get to the base of TR7 from. A solder bridge somewhere, or through the part itself. Have you replaced TR7? MPSA42 is common enough and cheap. If yours is breaking down, or the emitter leg is broken off, I could see +33 maybe.

[Tilman]

Hi Enzo,

I measured both MPSA 42 (from the two channels). The "good" one has hfe 105, the bad on 85. I don't think that has something to say.
However, I swapped the two transistors from one channel to the other.
The result is that now the other (former good) channel has DC at the output from time to time.
I think this is a clear indication that something with that TR14 is wrong. Very strange - I always thought a transistor is alive or dead - there is no in between - I guess I was wrong.

I will now order a set of new TPSA 42 and that should fix the thing.
ENZO - thanks again for your great help along this way!

Tilman

[Enzo]

A lot of people assume that a part is either good or bad, and there is no reason to make that assumption. ANyone with a crappy guitar cord should know that connections can get intermittant.

Inside that little transistor, the three legs disappear into the plastic body. The insides isn't magic, those legs connect to something by tiny little wire whiskers. If one of those breaks but touches most of the time, then the part will work whenever it touches and won;t when it doesn't. Ever have a light bulb that burns out, but if you shake it just so, you can get the ends of the broken filament inside to touch and the bulb comes on again?

Glad you found the answers.