[ARC5] [Milsurplus] Dead BC-348-R

[JAMES FALLS]

1- No ESR meter, but I have an Eico 950B that I reworked a of couple years ago (new caps, resistors). It applies applies around 250 VDC to caps under test and appears to measure leakage well. 

2- Should I leave the burnt resistor in place at that 4-way junction for the leakage test, or lift one end of it? (I have replaced cap 11-9 with a modern unit.)

3- I will re-check the 1st IF tube (6K7). Be sure I got the settings correct 😀. My Hickok 600 was happy with it. No shorts indicated, well above the mutual conductance listed. 

This is great information ! I NEVER heard of that Chernobyl resistor! 

Jim K6FWT 

> On Aug 9, 2022, at 19:24, David Stinson <arc5 at ix.netcom.com> wrote:
> 
> P.P.S.
> If you have an "active" cap checker like the ZM-11
> which can ready leakage current, connect it
> (with 57-3 out of the circuit and cap 11-9 either
> double-checked or disconnected)
> between 1st IF pin 3 and ground.
> If the meter shows any leakage current to ground, the
> first thing I would suspect would be that IF can wafer.
> D.S.
> 
>> On 8/9/2022 9:07 PM, David Stinson wrote:
>> Resistor 57-5 is the Plate B+ resistor for
>>  the 1st IF.
>> There are two ways to burn-up this resistor.
>> Either there is a short somewhere between it
>> and the plate of 1st IF, or the resistor failed
>> by drifting lower, thus drawing higher and higher
>> current until it burns up, like the infamous
>> "Chernobyl Resistor" of Johnson Ranger fame.
>> The most likely suspect is indeed capacitor 11-9.
>> Is your tester passive or does it apply 200+ Volts
>> to check it for leakage?
>> Put your Ohm meter from the junction of 547-5, 11-9,
>> 35-1 and coil 119, and the other lead to ground.
>> This should read a fully open circuit. If there is
>> leakage to ground, you need to find it.
>> If you don't find anything that would leak the B+
>> to ground, I would double-check the first IF tube
>> to see if it is weak.  If so, the resistor could
>> have drifted low, which would raise plate current to
>> the point the tube gives up.
>> If the tube is good and you don't find a short
>> ground, the next suspect would be capacitor 35-1
>> shorting and also raising the tube current, enough
>> to slowly roast the resistor.
>> If you find no leakage to ground as above, replace
>> resistor 57-5 and watch it for overheating.
>> GL OM ES 73 DE Dave AB5S
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