Active8 wrote:
In article ,
says...

Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

In article , lid
says...
Active8 wrote:
In article ,
says...

Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.

oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

In article , lid
says...
Active8 wrote:
In article ,
says...

Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.

oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter
capacitor would get hot and the receiver wouldn't work.

As others have pointed out, an electrolytic capacitor with reverse
voltage applied can catch fire or explode. Be careful when
troubleshooting while the capacitor is unhappy. When you do find the
problem, it would be a good idea to replace the capacitor.

Roy Lewallen, W7EL

Active8 wrote:
In article , lid
says...

Active8 wrote:

In article ,
says...


Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.


oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter
capacitor would get hot and the receiver wouldn't work.

As others have pointed out, an electrolytic capacitor with reverse
voltage applied can catch fire or explode. Be careful when
troubleshooting while the capacitor is unhappy. When you do find the
problem, it would be a good idea to replace the capacitor.

Roy Lewallen, W7EL

Active8 wrote:
In article , lid
says...

Active8 wrote:

In article ,
says...


Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.


oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

In article , says...
If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter
capacitor would get hot and the receiver wouldn't work.

As others have pointed out, an electrolytic capacitor with reverse
voltage applied can catch fire or explode. Be careful when
troubleshooting while the capacitor is unhappy. When you do find the
problem, it would be a good idea to replace the capacitor.

i've had a couple explode, usually from overvoltage from a shorted
series pass reg, i think. nice thing about (some? most?) electrolytics
is that X that's scored into the top so it blows straight up. i'm glad
my face wasn't over top of it, though. even with safety glasses, it's
not something i'd like to experience.

mike c

Roy Lewallen, W7EL

Active8 wrote:
In article , lid
says...

Active8 wrote:

In article ,
says...


Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.


oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

In article , says...
If you're using a diode bridge like most people do these days, and the
diode bridge output terminals (+ and -) are reversed, the filter
capacitor would get hot and the receiver wouldn't work.

As others have pointed out, an electrolytic capacitor with reverse
voltage applied can catch fire or explode. Be careful when
troubleshooting while the capacitor is unhappy. When you do find the
problem, it would be a good idea to replace the capacitor.

i've had a couple explode, usually from overvoltage from a shorted
series pass reg, i think. nice thing about (some? most?) electrolytics
is that X that's scored into the top so it blows straight up. i'm glad
my face wasn't over top of it, though. even with safety glasses, it's
not something i'd like to experience.

mike c

Roy Lewallen, W7EL

Active8 wrote:
In article , lid
says...

Active8 wrote:

In article ,
says...


Tom:

The hot cap has two likely possibilities: either the cap is shorted
internally (an ohmeter check will show leakage) or you have bad diode(s) so
AC is being applied to the cap. An ohmeter will show the diodes as bad.


a backward diode will supply AC to the cap, also

No, it won't.


oops. right. 1 diode backwards in a full wave (not a bridge) rectifier
would short the xfrmr.

It will however supply the wrong polarity of DC, which could indeed
cause these symptoms.

I would think some other semiconductors might be running rather hot too
if that were the case.

Everyone:
Thank you all for some great ideas! I will have more time tomorrow and see
if I can make 'er go.
This thing looks sooo easy, maybe that is why I am so frustrated with it!!
I DO have schematics at my work, if anyone wants I could post tomorrow
night......if it would help.
best wishes
Tom

Everyone:
Thank you all for some great ideas! I will have more time tomorrow and see
if I can make 'er go.
This thing looks sooo easy, maybe that is why I am so frustrated with it!!
I DO have schematics at my work, if anyone wants I could post tomorrow
night......if it would help.
best wishes
Tom

TC Dufresne wrote:

Hi:
I am having a problem with a "Simple Receiver for Beginners". It is from a
design that was featured in an old (1981) ARRL handbook. It is a DC
receiver, looks like alot of fun, but it isn't fun now.
Here is the problem.

1) No oscillation! I can't find it (the VFO) on my DX-398 receiver at all,
or on my Radio Shack freq counter.
2) The power (+ side) electrolytic is supposed to be a 500uF, 16 volt. I am
using a 470uF 35 volt. It gets really hot!! Short somewhere?!?!?!

The VFO is REALLY simple, it has one transistor and about 7 other parts. I
can't seem to see what is wrong, and I have tried.

I would sure appreciate some Elmer'ing here!
ANY help would be much appreciated. I have the above mentioned tools at my
command, a multimeter, and an old Heathkit "scope. (which I really don't
know how to use.

If you happened to insert the diodes in the fullwave rectifier reversed, you
will have negative voltage at the input of your filter instead of positive
voltage, and the input filter cap will conduct, and get hot (in a fullwave
rectifier supply). If that happens you hope the filter cap DID short the supply
to ground instead of passing the reverse polarized voltage on to other solid
state devices! In time this will be rough on your transformer, too.

You might want to recheck all the polarities of the diodes.

Dick