Whats a good approach to fixing solid state output stages

[mikeydee77]

Hi Guys,

I have a general question about how to approach fault finding solid state amps output stage. I would be interested to hear if anyone is willing to share some hints or tips on their approach to the scenario
where the output stage transistors are fried. For me this has come up in response to a specific amp; a GK MB150 SCHEMATIC.pdf but I guess there is a lot of similarities here with other SS amps.

The scenario is that this amp came to me and had stopped working during a gig when it had been driven too hard.

My general approach is:
Isolate the problem to either power supply, pre amp or power amp. In this case slightly ashamedly I have actually ignored my own approach and leapt in on the power amp based on the apparent state of the amp and the fact that another tech had looked at it and started replacing the output devices but then taken ill and been unable to complete the work.

Assuming that I have identified a power amp problem, I start with the output stage and check the output devices, so working from back to front; all four output devices TIP35C and 36C were short from a quick diode check with a multimeter. On these amps, the output devices are soldered flat on the board (with a rectangular hole in the board for the output device to bolt to a heatsink). This is helpful as you can lift a leg and then make a good "out of circuit" measurement.
Next I check out the driver transistors Q620, Q621 and Q617) with a diode check and in this case I discover Q621 is short and actually a little smokey around the board. The other buzzed out ok on the diode checker but since they do not cost too much I just replace them anyway.
Next I check out the bias circuit and associated small transistors in a similar way to the drivers. I replace a few rather blindly as by this point my understanding of the circuit is getting a little fuzzy to say the least.
Next I check all the diodes and resistors and they check out ok

When I think I have dealt with the fault it is time to test.
My approach is to slowly power the amp up on a variac with a dim light bulb and NO SPEAKER LOAD and measure for any DC on the output.
What I am not sure about is how much voltage I can safely apply with the variac. For example there needs to be enough supply for the various bias conditions to be correct but not so much that any damage can occur. I was taking an approach of allowing the supply to give supply rails of 4.5v which is 10% of the normal operating conditions

does that sound like a reasonable approach?

In the case of this GK amp I am getting a DC voltage roughly in line with the supply volts. Although my current limiter bulb is not lighting up, I feel that the amp must still have a problem because of the DC volts on the output.

Would love to hear any suggestions on alternative approaches, missing steps or logical next steps. And if anyone has any reading recommendations then yes bring em on.

Best wishes to all and hope you can follow my late night post, Mike

[T9c]

Nothing to be ashamed about when you can see obvious damage to go straight to it first. Most of the GK's I've worked on have catastrophic failures like yours did. I would do a precursory diode check on all xistrs and diodes, rectifiers, etc, then check all related circuit resistors. Then I usually just pull all outputs and drivers and check them on the transistor checker, or diode function on the DVM. The burnt spot needs cleaning up, so I'd get on that too. After replacing all the found defective parts I bring up power on the variac and watch my inline ammeter. If it's just a bias problem, the amperage rise will be slower than if shorted. Since you still have rail voltage on your output, something is shorted. I'd disconnect the + & - cd rails from the circuit and bring up power slowly again while watching the ammeter. If there's little or no rise, I'd check the dc rail voltage(s). If they're good, I'd go back to the pwr amp and start pulling the signal transistors, checking them out of circuit.

[Enzo]

Mmmm, DC on output not always shorted parts. Open parts or connections can just as easily put DC on an output.


A variac is half a tool, the other half of it is an ammeter. It is not about how many volts are safe on the variac. it is about turning the voltage up while monitoring current. If the amp is trying to draw 5 amps out of the wall with the voltage only at 30 volts, you ned to back off and stop right there. You light bulb is a sort of curent monitor, but much cruder in its precision.


Your amp will not be stable with 4.5v rails, so circuit measurements taken that way will mean little. If you have the variac up to that point and the current meter is rising fast, THAT part is meaningful, but you cannot take readings at 10% of power voltage and expect them to predict anything at full power conditions.

Once I have determined the amp is no longer going to blow fuses - ie not going to draw excess current - then I get rid of my variac, it has done its job. Running the amp at reduced voltage will just confuse things, and the bulb will light up if there is a current problem, but you cannot operate the amp with the bulb in place and expect it to function properly.


As to a quick check, yes, I look for shorted outputs and drivers. If i have to replace an output, I generally replace its driver even if it "checks OK." Always look for open resistors associated with any transistors or diodes that have failed or been stressed. Once I have replaced anything that seems to bneed it, I run it up on the variac. There is no target voltage. If I can run it up to 100% without the current meter going nuts, so be it. If it starts ramping up as soon as I leave zero, I stopp right there, even if it is 10 volts. And some circuits might seem to be OK until we get to 80 or 90 volts. In that case, close but no cigar, back off and keep looking.

[Jazz P Bass]

Just to add to Enzo's excellent post, a working solid state amp will generally pull 30 to 50 watts at idle.
And that all depends on the idle bias setting.
I have 4 volt meters, a variac, an ammeter on the mains & a lamp limiter.
The volt meters I will use on the power rails & the output transistor bases. (or the driver bases)
That will pretty much sum up what is happening at turn on.

[mikeydee77]

Thanks for all the great responses. The ammeter point is a good one and easy to do something about.
I will try out the advice this weekend and report back to the forum.

Cheers

[soundguruman]

You turn it on, and see which parts leak smoke.
You can either put the smoke back in, or buy new parts (recommended).

Remember that these appliances are hazardous, and you work on them at your own risk.
High voltages can shock you, even if the amp is disconnected from power.
Or, find a technician (witchdoctor, intern) to help you.

[Jazz P Bass]

Come on SGM.
Lighten up.
The OP is trying to learn something.

[Fragger]

Ammeters have alluded me. The time I picked up a 20A ac guage and hooked it in circuit the internal resistor blew and the smoke excaped. I could try the 10A function on an analog multimeter or try one of the clamp on meter from my electrician days... what do other people use for an ammeter?

[J M Fahey]

I am a minimalist and always use a lightbulb in series, which:
1) is an "automatic variac" by lowering voltage if current is excessive.
Same thing I would have done "by hand", but much faster and trustier than my slow reflexes.
2) same as (1) but in another words: it's a current limiter
3) it's an "optical ammeter" .
I can be looking at a bias trimmer I'm adjusting and still notice when the bulb starts to get brighter/dimmer just using peripheral vision.
No, not Lab precise, but close enough to show trends.
4) is being cheap a "Technical Spec"?
It should.

[loudthud]

Some solid state power amps will have DC on the output if the rails are less than half the nominal voltage. These are usually older or low cost designs without active current sources. They will create a thump on the output when turned on or off even when they are operating properly.

When operating properly you will see a DC voltage on the output as you bring up the power supply with a variac (no speaker or dummy load), but they won't draw abnormal current. The output voltage will suddenly go to zero as you get to about half line voltage.

[mikeydee77]

Just wanted to complete this tread off.

Thanks for all the words of wisdom so far especially from the Forum's big hitters - you know who you are. Hopefully this will be useful for future readers.

I'll just leave you with thought that I have been having about using temperature as a guide; thermal imaging. Right now, for amps suffering with distortion I have found an infra red thermometer very useful for quickly identifying a duff op-amp; and without touching anything. These devices sometimes known as pyro-pens in the UK are used for measuring temperature in food production and chemical plants without contaimination. They are very accurate and what I was wondering was extending this idea to a heat map on an amp would be helpful in narrowing down the crudity revealed by the dim light bulb. Anyway it is just a thought.

PS I fixed my faulty GK MB150S amp for some reason one of the output transistors that I replaced I found was short I guess I must have somehow caused this during the repair. Fortunately the GK service guys were excellent and provided first rate service in terms of supplying diagrams etc. Highly recommended.

[Jazz P Bass]

Yeah, forgot that one.
Heh, heh, heh.
Any & all information is good.
The wisdom of how to use that information is the crux.