[TenTec] 255 Power Supply

[Dr. Gerald N. Johnson]

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 at 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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