Dave wrote:
wrote in message
ps.com...

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

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.

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.


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.

then why do you say those trs are bad?

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.

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.


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

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.


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.

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

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.

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.


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.

Can I cheat and use a 25W light bulb?

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.


Base R values will depend on hfe of the TIPs.

hfe at 3A is

min=15
max=75.

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

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.


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.

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

cool


NT