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.

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.


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?

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


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.

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

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...

ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

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.


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?

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.


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.