Resistance Check Help

[rdonahue]

I completed the resistance check on my Crack build and am getting two numbers that aren't right. Terminal 13 I am getting 0Ω and on terminal 22 I am getting anywhere from 9 to 35Ω.


Bottlehead Crack by Ryan_D, on Flickr


Bottlehead Crack by Ryan_D, on Flickr

 

[fullheadofnothing]

You are missing the nuts on your transformer. You also appear to have prematurely installed the tubes, which could influence your resistance readings. Fix these issues and remeasure.

 

[rdonahue]

Okay, I've removed the tubes and installed the nuts. Now I am getting 0Ω at 22 which is good. Terminal 13 is odd though. When I first measured it it was reading around 12MΩ and went down to about 8.8MΩ. I removed the probe, waited a few seconds and tested again. It started around 8.2MΩ and went down to the low 7's. Then I took the probe off, waited again and re-tested and it was reading 6.9MΩ and continued to decrease.

 

[fullheadofnothing]

The values signified with a * are going to vary from ohmmeter to ohmmeter because these terminals are connected to the filter capacitors, which try to charge themselves off the battery in the meter, causing a fluctuating reading. If the circuit is connected properly these readings will wander in the tens or hundreds of Kohms or higher range. What you want to watch out for is a zero reading at one of these terminals, which would indicate that something is mis-wired.

 

[rdonahue]

I figured this might be the case, I was just unsure since the manual states that the value of this terminal will "climb slowly towards 270k ohms" and I seem to be getting the opposite of that.

 

[Doc B]

Someday we will have to change the manual to say

If it's not 0, it's the correct value.

 

[rdonahue]

Thanks for the replies Joshua and Doc. I just wanted to confirm before I go ahead with the voltage checks. This being my first high voltage project I wanted to make sure everything was good before I proceed.

 

[fullheadofnothing]

Someday was several months ago. This is an old Crack with the old manual.

 

[rdonahue]

Well, that was exciting. I began the voltage check but had a nice little fireworks display when checking T1. I'm thinking that my hand slipped from T1 to T2 as I looked over at the meter. A large spark and a blown resistor but I'm still alive and kicking (new pants may be required however  )

I'm going to place an order w/ Mouser for a new resistor and pick up some extras of other common resistors, capacitors, LEDs, and such while I'm at it.


Blown Resistor by Ryan_D, on Flickr

 

[rdonahue]

Can I get a quick confirmation to make sure that this resistor will work  as a replacement that I burned up at 1U / 2U. The only difference being that it is 1W instead of 3/4W.

Metal Film, 22.1 kohm, 500 V, Axial Leaded, 1 W, ± 1%

 

[fullheadofnothing]

Have you measured the resistor to confirm that it's damaged?

 

[rdonahue]

I'll check this evening but since it has a large black burn mark (you should be able to make it out in the picture) on it where the spark came from, it's probably wise to replace it.

 

[Paul Birkeland]

That resistor will work fine as a substitute.

 

[rdonahue]

That resistor will work fine as a substitute.

Thanks, I'll go ahead and order some as well as some other common value components to have as spaires.

As per Joshua's suggestion I checked the resistor and it is still reading 22.2kΩ. Would you guys suggest re-doing the voltage check or is there something I should be looking for that would have caused the spark?

 

[Deluk]

The soot on the resistor is a by product of the spark and may wipe off.. I would suspect that there is no actual electrical damage. I would just, CAREFULLY, recheck the voltages. There is some quite bad heat damage to a wire close by but again probably OK as there is no wire showing.