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khstudio

How to find the bad output Transistor in Power Amp???
« on: August 09, 2009, 05:12:02 AM »
I had a gig with my band tonight & one of my main Power Amps went down.  :(
It has protection circuitry to disable the whole amp & to stop further destruction.

I just opened it up & narrowed it down to the Left channel.
I'm now able to get the Amp to turn on by disconnecting the power to the Left ch. outputs.

The Amps are Sankin 2SA1216 & 2SC2922 (4 of each)

I doubt they all blew. This happened before & I had it repaired but there is currently no local techs.
I think I can do it with a little advise.  ;D

What is the best way to find the Bad Amps ???

There's no way to turn on the amp with the Left channel connected because the protection circuitry will kick in, so... what to do?
Thanks in advance.

Kevin


mac

Re: How to find the bad output Transistor in Power Amp???
« Reply #1 on: August 09, 2009, 05:26:46 AM »
Hey Kevin,
I am not familiar with this amp so my advice may not be helpful, but i recently had this issue with a couple of amps. One power amp and also a hi fi amp.

First base for me was to figure how to bypass the protection circuitry, to allow signal through to the output stages, and then swap out the transistors (left to right - or in pairs if they can be measured this way), to test each transistor individually across the outputs to weed out the bad eggs.

Also look around for any other obvious suspect looking parts - I had a couple of burnt looking resistors (which were suprisingly still within tollerance once i tested them off the board - but I changed these anyway), and anything else obvious like sus looking caps etc.

all the best with it, Mac.
Barclaycon: The client said 'That sounds great, what are you using on it?'
The engineer's reply was: '30 years of experience'.

khstudio

Re: How to find the bad output Transistor in Power Amp???
« Reply #2 on: August 09, 2009, 05:42:54 AM »
Thanks mac.

Disabling the Protection doesn't seem like a good idea, I'll explain:
I can actually hold the reset button in to keep it on but it blows an "External" Fuse or breaker... I also started smelling stuff burning when I tried this.

I see no burnt resistors or puff'd out caps, etc...

MagnetoSound

Re: How to find the bad output Transistor in Power Amp???
« Reply #3 on: August 09, 2009, 06:32:38 AM »

You need to make sure that you will not cause further damage. The following procedure will help you to achieve this:


Remove all the power transistors one at a time and check across the pins with a meter for low resistance/shorts.

Then do the same for the driver transistors (the ones that drive the power transistors).

Then check all diodes and low-ohm resistors in the same way.

Replace all bad components with like parts. Use heatsink compound where you need it.

Do a final idiot-check for open bias resistors, burnt PCB traces etc, before you apply power.


If, at any of the following stages, excessive current is drawn or smoke appears, you will need to fix the problem before you proceed further.


Apply power with no load connected. Check current draw on each rail.

If nothing burns or smells bad, apply input signal and check for signal at the output. If clean signal appears, connect load and gradually increase input signal from zero to 0dB, or until distortion or clipping appears. Measure output power using Ohm's Law.

If no problems are evident, soak test for 45 minutes at half of rated output until you're satisfied that all is well.

« Last Edit: August 09, 2009, 07:27:05 AM by MagnetoSound »
Dan

I don't think people realize what an embarrassment of riches this place is. - Paul Gold

khstudio

Re: How to find the bad output Transistor in Power Amp???
« Reply #4 on: August 09, 2009, 06:49:23 AM »
Thank you!
I will pull one Transistor at a time... I thought I might have to do it that way.
They are easy to get to.

Can I power it up with ANY of the transistors pulled?
I remember years ago, when I had a local tech work on one of my Power amps he temporarily removed a few & told me I could run it like that but said it would have less power. I'm assuming he removed them in pairs but can't remember?

I have another question:
Can I put a fuse or circuit breaker on the output rail for protection after I fix it?
I don't think that the + & - shorted together to cause it to blow this time.
I'm using "Speakon" connectors that are just about impossible to short but I will double check the cables, etc...

Sorry, what is a soak test?



MagnetoSound

Re: How to find the bad output Transistor in Power Amp???
« Reply #5 on: August 09, 2009, 07:20:39 AM »

Sorry, I didn't need to say one at a time. Just remove them, throw the bad ones away and put the good ones back.


Can I power it up with ANY of the transistors pulled?

Yes, but leave in at least one pair at a time. Lopsided (2:1, 3:2) is OK for quick tests, but not one-sided.


Can I put a fuse or circuit breaker on the output rail for protection after I fix it?

It is tricky. You need to break the circuit quickly to protect your transistors in the event of a short, but not blow it open on transient peaks. Calculate max current at your intended load and over-rate by a small margin. Quick-blow is better protection but more likely to blow spuriously. Slow-blow is not much help, the devices will die before the fuse. A DC crowbar circuit is better, but this type of output protection is usually designed to protect the speakers in the event of a faulty amp, not the other way around.


Sorry, what is a soak test?

Current soak into a known load, over a period of time. Allows the amp to reach working temperature and prove itself in a simulation of working conditions. A dummy load is preferable to a steady tone through your PA speakers.  ;)

« Last Edit: August 09, 2009, 07:38:03 AM by MagnetoSound »
Dan

I don't think people realize what an embarrassment of riches this place is. - Paul Gold

khstudio

Re: How to find the bad output Transistor in Power Amp???
« Reply #6 on: August 09, 2009, 07:55:32 AM »
Great info, thanks so much. ;D

JohnRoberts

Re: How to find the bad output Transistor in Power Amp???
« Reply #7 on: August 09, 2009, 10:28:27 AM »
Yes, typically the power devices will fail as a short circuit, collector to emitter. Sometimes it is a good idea to just replace all the power devices, since when one fails as a short to the supply, the opposing devices are severely overloaded.

After making repairs, amp techs often use a variac to bring the amp up to voltage slowly while looking for problems. Another old amp trick is to use a 100W light bulb in series with the amp mains power to current limit the amp at roughly 1A, until you are confident it is working properly.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

bruno2000

Re: How to find the bad output Transistor in Power Amp???
« Reply #8 on: August 09, 2009, 11:56:33 AM »
I second the notion of replacing them all.  You've already gone to the trouble of getting into the unit, and if your time is worth much, it's just more economical.  That way you are sure that you have not left a "stressed" component in the circuit.
Best,
Bruno2000

Gus

Re: How to find the bad output Transistor in Power Amp???
« Reply #9 on: August 09, 2009, 12:51:53 PM »
As posted by others.  Remove and test each transistor power then drivers.  I like to start at the outputs.  Google etc the transistors some power devices can have a added diode or might be a darlington so you need to make note of that if you are using a ohm meter checking the junctions.

A BJT should look like two diodes base to collector and base to emitter using a meter.

Sometimes the tricky thing it to find why a device(s) shorted opened etc in an output stage if the unit has a protection circuit built in.


khstudio

Re: How to find the bad output Transistor in Power Amp???
« Reply #10 on: August 09, 2009, 09:21:01 PM »
Thanks everybody.

The protection isn't actually on the output + & - legs... it seems to just kick in when a problem arises to save the unit from a complete meltdown.

The Power-Amp is a Fender PA13000. They were blowing them out real cheap at SamAsh about 7-10 years ago.
I have to say that this amp sounds great, even better than my brand new QSC, etc...
I don't think I shorted the outputs this time but I have twice in the past... I think it just failed. I may have turned it off too soon after the last gig. Probably should have let the fans cool it down for a few.



 

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