The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will also
get warm but it's still safe. The other post that advised the use of freeze
spray was a good one too. I have used both...Eddie
"Henry Kolesnik" wrote in message
...
See my previous post about finding the culprit but I had another problem
and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek came
to
life but not great. I measured the 15 volt terminal and it was 23 volts
so
I may not have much left. I have question on your method. For a shorted
component with very low resistance, 0.2 I can't see much heat created
since
P=I squared R. A short of 0.2 ohms wouldn't dissapate much power and all
you'll do is heat the traces, other components and perhaps blow short if
you're lucky!
73
hank wd5jfr
"Eddie Haskel" wrote in message
m...
Henry, dont laugh at this method..it works....read it thru before nixing
it.
I have had this happen before too. simply put about 5 volts at 500 mils
on
the b+ line, regulated at the 500 mils. Let it sit for a few minutes and
then go looking for the part to be running warm. The part will be
dissipating 5*.5= 2.5 watts of heat. sooner or later the bad part WILL
get
warm. It will NOT lift traces unless they are VERY small.
If this approach fails, the next thing I do is go in with a new(sharp)
razorblade and start as far away from the power supply and cut B+ traces
one
at a time until the short goes away. This isolates the short to a
smaller
area.
Suspect Tantalum caps as they usually fail in this mode of low ohms
shorts...let us know when you find it...Eddie

"Henry Kolesnik" wrote in message
...
Sometime ago I think I recall someone posted or wrote an article on a
neat
way to isolate a shorted component on a pcb using common test eqpt but
I
can't recall the methodology. I'm trying to find a shorted component
on
a
Wavetek 188-S-1257 signal generator. The B+ line reads about 0.4 ohms
and
I'm not having much luck disconnecting componets. I don't have a
schematic
and my eyes ain't what they used to be for tracing and I want to
minimize
the unsoldering. Does anyone recall the article or have a good way?
tnx
hank wd5jfr

"Eddie Haskel" wrote in message
.. .
The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will also
get warm but it's still safe. The other post that advised the use of
freeze
spray was a good one too. I have used both...Eddie
"Henry Kolesnik" wrote in message
...
See my previous post about finding the culprit but I had another problem
and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek
came
to
life but not great. I measured the 15 volt terminal and it was 23 volts
so
I may not have much left. I have question on your method. For a
shorted

I think he was referring to a 15 volt rgeulator that has 23 volts on the
output indicating it was bad and the overvoltage caused other components to
go bad.

23 voolts on output!
later
"Ralph Mowery" wrote in message
...

"Eddie Haskel" wrote in message
.. .
The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will
also
get warm but it's still safe. The other post that advised the use of
freeze
spray was a good one too. I have used both...Eddie
"Henry Kolesnik" wrote in message
...
See my previous post about finding the culprit but I had another
problem
and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek
came
to
life but not great. I measured the 15 volt terminal and it was 23
volts
so
I may not have much left. I have question on your method. For a
shorted

I think he was referring to a 15 volt rgeulator that has 23 volts on the
output indicating it was bad and the overvoltage caused other components
to
go bad.

"Henry Kolesnik" wrote in message
...
23 voolts on output!
later


The way I read it , the 15 volt regulator had gone bad and was passing a
much higher voltage as it was not regulating . That was giving 23 volts on
the output instead of the 15 volts it was suspose to.

When I got the 188 the 7815 output was nearly zero and I assume the shorted
tantalum caused the low voltage. After I found the bad tantalum and
replaced it the 7815 voltage was still very and the output pin to ground
mesured 12 ohms. I had a NOS Radio Shack 7815 still sealed in its package
so Iused it and that's when pin 3 went to 23 volts and that nearly made a
grown man cry because I thought I fried addtional components. The PCB was
running too hot. After getting a known good 7815 the B+ stayed at 15 volts
and the unit runs cooler and seems to function except that the dial
calibration is off. I'm trying to find a manual.
73
hank wd5jfr
"Ralph Mowery" wrote in message
...

"Henry Kolesnik" wrote in message
...
23 voolts on output!
later


The way I read it , the 15 volt regulator had gone bad and was passing a
much higher voltage as it was not regulating . That was giving 23 volts
on
the output instead of the 15 volts it was suspose to.

I found a good 7815 and installed it and except for the frequency dial being
off the thing seems to work. If I find a manual I should be able to cal it
or find out if the cal is off because of a failed componet because of being
overstressed to 23 volts when it was designed to operate at 15 volts. The
output remains flat from 0 to 4 MHz but the when the dial reads 4 MHz the
counter shows 3.5 MHz. At 1MHz on the dial the counter reads 0.570MHz and
similar readings on at audio frequencies.
hank wd5jfr
"Ralph Mowery" wrote in message
...

"Eddie Haskel" wrote in message
.. .
The 7815 needs only 3 volts of headroom, so 23 volts is plenty. Any more
than that will be dissapated as heat. Your right that the traces will
also
get warm but it's still safe. The other post that advised the use of
freeze
spray was a good one too. I have used both...Eddie
"Henry Kolesnik" wrote in message
...
See my previous post about finding the culprit but I had another
problem
and
it was the 7815 3 term reg which I replaced with NOS and the Wavetek
came
to
life but not great. I measured the 15 volt terminal and it was 23
volts
so
I may not have much left. I have question on your method. For a
shorted

I think he was referring to a 15 volt rgeulator that has 23 volts on the
output indicating it was bad and the overvoltage caused other components
to
go bad.