[William Sommerwerck]

I do sympathise old boy. In years past, I never gave up on anything
either,
but I must admit that these days, sometimes I do, out of both a need to be
commercial and keep the wife off my back for not earning any money, and
out of sheer frustration. I think that latter condition is something that
creeps
up on you with age. Suddenly, some things are more important than not
letting some inanimate object beat you ...

That said, I reckon that this baby has got to be worth one more go if -
and
*only* if - you can lay hands on a Variac, so you can wind up the input
power to make the rails just high enough to take sensible measurements,
but low enough to stop the ouput trannies releasing their magic smoke ...

As William says, give it a break over the weekend, then come back to it
with fresh eyes (and a Variac !!) on Monday. If you do, carry on keeping
us updated. It's an interesting saga, if a little frustrating for you.

One of the reasons I've enjoyed helping is that you've explained exactly
what you've done, which makes the process fun -- and educational, too!

PS: I have several working Advents. Let me know if you want me to check the
voltages on a "good" one. I'm willing to do it "live", in real time, if we
can find a time suitable for both of us.

[Chris Hornbeck]

On Thu, 11 Sep 2008 17:51:45 -0700, Tiger Luck
wrote:


Spoke too soon. Three of the output trannies fried and the fourth probably would
have if I had left the power on longer.

Even a variac doesn't save output transistors in some cases
(but helps a lot in others).

An old-fashioned solution is to wire an incandescent
120 volt good old Edison lamp in series with the power
transformer's primary. Folks would wire up a contraption
with a lamp socket and male and female AC plugs/jacks.

The series lamp has two beauties: its cold resistance
is high enough to save some situations but its hot
resistance is higher, and can sometimes save others.
And, it gives a visual indication of current drain!


The other (and essential!) ingredient of amplifier trouble-
shooting is a current-vs.-voltage curve-tracer. This is
made with a single resistor, two test probes, a (usually 6 volts
or so) AC transformer as a signal source, and a dual-trace
X-Y-able scope.

The scope can be made from a computer sound card. Works
just fine at line frequency, and common grounds are not
an issue for this use.

With an I/V curve tracer, you simply walk your two test
probes through the (unpowered) circuit, observing and
comparing to experience and/or a known-good circuit.

Resistors look like a straight like of slope 1/R; diodes
(and transistors) look like an "L"; capacitors look like
elipses; etc. When you see something strange, you're in the
ballpark. Very fast and non-invasive.

I always include a couple of different resistor values,
switch selected for best inpedance match to the local
circuit. A K Ohm is a good middle value to begin with.


All good fortune, and don't give up. You have several
problems, probably lightning (overvoltage) related, but
they're all solvable.

Chris Hornbeck

[Geoff]

wrote:
Tiger Luck wrote:
snip
The grounds are intact. By that I mean to say that point in the CB
where
electrolytic caps C312 and C313 are grounded is continuous with the
ground at the
power supply filter caps. Chassis is the same. All points which are
supposed to be
grounded are continuous as read by an ohm meter.

Those little 100uF caps are _not_ the main 'lytics. The mains would
likely be 1000uF to 10000uF (or possibly even more) and be located
near the main bridge rectifier - which is on the _rest_ of the
schematic along with the power transformer.. I think you'll find the
main negative bus 'lytic to be open or dried out so that you're
putting LOTS of ripple current on those littlle 100uF caps. Those
100uF caps are just local 'clean up' caps and can't handle the load.


Also could be crook bridge rectifier, and the caps are being pumped up by
half-wave surges rather that continuous full wave ones, so the average
charging current is higher.

geoff

[Chuck]

On Fri, 12 Sep 2008 02:36:23 +0100, "Arfa Daily"
wrote:


snip


Output trannies are back in. Speakers hooked up. Tuner tuned in.

Fingers crossed.

And here goes... power up... power down, got smoke.

Output trannies on the left side got hot, but held.

The drama continues. Will pull output trannies on left side and check
right side for okayness.

Spoke too soon. Three of the output trannies fried and the fourth probably
would have if I had left the power on longer.

I have never given up on a piece of gear until now. It almost seems like
the power supply might be the problem seeing as how the outputs on both
channels fried, but who knows. It reads okay.

I've been at this thing for about four days straight now and I am totally
fried behind it. I've resurrected stuff the local vintage stereo store has
given up on, but this took me to my limits and left me out to dry.

If anyone wants this pup, with schematics, minus a couple of output
trannies, for the price of postage, email me at jack6128 at gmail dot
com

Je suis fini.


--
Einstein forgot to carry the two

I do sympathise old boy. In years past, I never gave up on anything either,
but I must admit that these days, sometimes I do, out of both a need to be
commercial and keep the wife off my back for not earning any money, and out
of sheer frustration. I think that latter condition is something that creeps
up on you with age. Suddenly, some things are more important than not
letting some inanimate object beat you ...

That said, I reckon that this baby has got to be worth one more go if - and
*only* if - you can lay hands on a variac, so you can wind up the input
power to make the rails just high enough to take sensible measurements, but
low enough to stop the ouput trannies releasing their magic smoke ...

As William says, give it a break over the weekend, then come back to it with
fresh eyes (and a variac !!) on Monday. If you do, carry on keeping us
updated. It's an interesting saga, if a little frustrating for you.

Arfa



If you don't have a variac, put a 60 watt light bulb in series with
the hot side of the AC in. If the light glows brightly, your amps are
still drawing too much current or there is a power supply fault. Once
you find the problem, the light will hardly glow after the initial
currrent rush and the bulb will usually keep the outputs from blowing
if there is still a problem. Chuck

[Ancient_Hacker]

if the -22 supply has failed, due to a blown diode or fuse, and if the
output transistors have shorted, then the -22 rail will be pulled
positive and that will blow the capacitor.

Also note that the schematic has errors on it, for instance the output
transistors have emitter and collectors reversed!

Hope you didnt wire the new ones in like the schematic!!

[[email protected]]

Ancient_Hacker wrote:
if the -22 supply has failed, due to a blown diode or fuse, and if
the
output transistors have shorted, then the -22 rail will be pulled
positive and that will blow the capacitor.

Also note that the schematic has errors on it, for instance the
output
transistors have emitter and collectors reversed!

Hope you didnt wire the new ones in like the schematic!!

I thought that at first too but if you zoom to 300% you'll see the
emitter marks where they belong and the 'squiggles' on the collectors
may be a 1970's version of heatsink. You'll see the same marks on the
collector of Q312 indicating it is thermally coupled to the heatsink
with the outputs.

Way back in the days when I worked on stereo gear I had a unit with
very bizarre distortion. It turned out to be a transistor with a
'leak' which on a curve tracer showed up as a transistor in parallel
with a resistor from E to C. If Tiger has a similar quirky part it may
explain why he's having such a bad time with that amp - which is
pretty straightforward and decent for it's day.

G²