blown output transistors

As I noted in my post, I'm willing to spend some time on this and would love to learn something.

I've checked all of the driver transistors inasmuch as is possible using the diode check function of a DMM. The ones that didn't pass were replaced. I've checked all of the diodes and resistors in-circuit, everything passed, resistors all "in the zone", i.e. within 40% of their stated values. My multimeter also has a transistor check but it's pretty flaky especially with small transistors.... I've tested known bad transistors and the meter happily gives me an hfe value for them.

I can power up the amp with output transistors removed, as soon as they go back in the two in-line 5A fuses on the transformer secondaries blow. there are no dead shorts across the outputs. I get proper voltages at the inputs to my IC's (really the only values I can check as I have no service manual or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

The new output transistors are NOT fried, they're rated at 10A peak and the new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of instantaneous current but they're brand new and likely can handle it a few times. ALL but two (drivers) of the transistors in the amp circuits are

Almost makes you think there's a short before the amp section if the fuses blow but not the drivers/outputs... but the thing runs fine with outputs transistors removed.

I'm going to do something I should have done a few posts ago... install the outputs one pair at a time which will at least localize my problem to one of the three channels.

I know there's no substitute for experience, but does anyone know if any books or online publications that cover DC coupled (audio in particular) amplifiers and the troubleshooting thereof? I know you're all busy and probably tend to lose interest when you see posts and think "this guy ain't never gonna' fix this" but I'm a relatively smart guy with a couple of years of Electrical Engineering schooling (although that was more than 20 years ago we were still in the transistor age).

Dave

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40% needs replacing.

then why do you say those trs are bad?

a fault of any kind anywhere in the power amp that results in the output trs being turned on.

very unlikely. If they handle it once you've been lucky.

so its your output trs that short. Or the speaker wiring.

OK first thing you need to do is put current limiting Rs onto the collectors and bases of all these big expensive trs. This will stop them dying instantly, and enable you to measure whats going on. Your psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the collectors. You can make those out of a reel of resistance wire, or buy high power Rs if you've money to waste.

Base R values will depend on hfe of the TIPs.

Next thing you need is a circuit diagram. Without that its a waste of time. Then you should be able to follow the cct through, check voltages all over and narrow down where its going wrong.

NT

I mean that when I test them in circuit I never get the exact correct value. I figured that if I was looking for a gross over-current-causing resistor fault, it'd likely be open or shorted. or close.

Because they don't work in circuit. Case in point I replaced a PS transistor in a tuner awhile back which was putting out completely out-to-lunch voltages (I had a circuit diagram for this one). Problem solved. Stuck transistor into my DMM transistor tester and it gave it an hfe which was in the range of what the data sheet might lead one to expect.

These are 6A continuous rated. Why shouldn't they be protected behind a 5A fast-blow fuse? Or isn't "fast-blow" fast enough?

Damn, probably used up a bunch of luck on those trannies, then. I have a feeling I still am going to be in need of some down the road on this one...

The amp was powered up with no load (no speakers connected). So unless I got some bad brand-new TIP's I don't know how they could be shorting.

Can I cheat and use a 25W light bulb?

hfe at 3A is

min=15 max=75.

Do I use avg=45? 1.7A / 45 = 38mA, 15V / 0.038A = 400 ohm 1/2W.

I've got it mostly drawn out... another hour or two and I should be there with a schematic.

measuring them in circuit is going to give you all sorts of out there readings. And resistors dont normally short, so I'm not sure if you'll find much that way.

perhaps the meter just reads dc collector current, and on faulty goods its reading leakage? Try measuring a resistor from C to E connections on the meter, see if it gives an hfe reading.

Everyone asks this one.

1. Fuses dont limit current, so way above 5A flows when a 5A fuse blows to clear a short
2. Fast fuses are very slow compared to what a tr can survive. Fuses can not protect trs to a significant extent.
3. Fuses dont blow at 5A, 5A is the current they'll pass for decades. Blowing one takes much more.

Look up fuse curves to see what it takes to pop one.

Yabbut theres wiring from the pcb to the external connectors, plus maybe a headphone jack etc. Remove the audio output connection at the pcb, if convenient, to isolate all that crp.

Filament bulbs have a power on surge of 8-10x run current, their cold resistance is about a tenth of hot resistance. If you take this into account you can use a bulb with the right cold resistance. 10 ohms cold is 100 ohm hot. If you're on 110v thats a 100watt bulb.

If you use average hfe, half your trs wont get enough base drive to deliver the required collector i. So use min hfe of 15. If you want to run 1.5A thru collector, you need at least 1.5/15 = 0.1A into the base.

cool

NT

This is basic and may have already been covered, but DO check the DC resistance between the collector of the outputs and the heat sink. Should be open-circuit, an insulator could be damaged or missing. Also check resistance between each emitter resistor and both the + and - supplies. Should not be a short - should show open or cap charging. If you show a short here, then an output transistor is probably bad, despite earlier testing.

The hFe showing on a leaky transistor is common with these checkers. Lift the base lead. If the transistor still conducts with only the E-C connected, it's bad.

Mark Z.