Typically the SCR is not give enough heat sink to keep if from heating
up, but its expected to see only a short burst as it blows the fuse. It
should check open anode (postive) to cathode, but with a second ohmmeter
applied to the gate to cathode junction (gate positive) it should
conduct and stay conducting until power is removed from the anode. A
current limited power supply can be used for the test using a resistor
say 10 or 12K to apply a bit of forward bias to the gate to trigger it
into conduction. If overheated, it too can become a short. The crowbar
is dependent on some voltage sensing circuit, usually a zener diode from
the supply output to the SCR gate, generally with a resistor to cathode
(supply negative) and a bypass capacitor to keep RF out. The anode
connection of the SCR can be to the filter capacitor or to the output of
the regulator but the sensing zener is always to the output of the supply.
There are several examples of regulator and crowbars in the 2011 ARRL
Handbook, chapter 7.
73, Jerry, K0CQ
On 3/13/2011 11:02 AM, Jim Anderson wrote:
Is there a way to check the SCR? I pulled the board this morning and upon
closer inspection it is discolored at the base where the leads emerge.
I ordered a couple of the transistors from Mouser and will add the SCR to
the order.
Jim
K6JEA
On Sun, Mar 13, 2011 at 9:08 AM, Dr. Gerald N. Johnson
<geraldj@weather.net>wrote:
The transistors are wired in parallel, but that 0 resistance from base
to one of the connections is a shorted junction in one of them. There's
no repair possible for the transistor, it must be replaced. When you
take them out, you can tell which is bad, but a sturdy pair isn't a bad
investment.
.514 is about right for the forward drop of a silicon junction. OL is
right for open. Which a junction should be one way. .000 for both
polarities of the meter shows the shorted junction.
73, Jerry, K0CQ
On 3/12/2011 10:23 PM, Jim Anderson wrote:
After searching Google on how to test a transistor I realized I was doing
it
wrong.
Here are the new readings...
On the Diode function and the black lead on the base it reads .514 on one
terminal and .000 on the other
With the red lead on the base it reads OL on one terminal and .000 on the
other.
Both transistors give the same results.
On the 2K ohm scale with the red lead on the base it reads 1.2 on one
terminal and .000 on the other
With the black lead on the base it reads 1.1 on one terminal and .000 on
the
other.
Again, both transistors give the same results.
Thanks for sticking with me on this.
Jim
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