Re: Test instrument accuracy vs stage testing during kit builds

James Compton

Thanks for the reply!  I'll check the Q2  collector voltage in the morning.  Thanks for the tip!
Jim C.

-----Original Message-----
From: "WA0ITP wa0itp@... [4sqrp]" <4sqrp@...>
Sent: Jun 21, 2014 6:43 PM
To: 4sqrp@..., jdcompton@...
Subject: Re: [4sqrp] Test instrument accuracy vs stage testing during kit builds

GE Jim,

It's easy to get caught up searching for in the "exact" number, digital
displays do that to us for sure. Your Fluke should be plenty accurate
enough for the job. I always clip the DMM's gnd lead to the circuits
ground, and make voltage readings on the board, not relying on the power
supplies setting or display. If your ps is adjustable, set it at 12
volts at the input to the board. What is the voltage on the collector
of Q2? Mine is 7.90 - remember that JP2 must be shorted to turn on Q2.
If you have a reading near that, continue on, all is well.

btw I loved those old Simpson 260's. I

On 6/21/2014 4:46 PM, jdcompton@... [4sqrp] wrote:
> I may be over-thinking a problem I'm observing regarding some of my
> test equipment (specifically voltmeters) which I use during
> construction of kits and home-brew projects. I just completed the
> SS-40HT and started the SS-40TX yesterday. I had a few bogus step
> test readings when I was building the HT and now am "stuck" at the
> Step 4 Test in the TX.
> I sent an email to Jim, but in the Step 4 test, I'm not getting a
> voltage drop. The Step 2 & Step 3 test were satisfactory.....I think.....
> Here's the rub. I have several DVMs, including:
> Data Precision 1450 bench DVM
> Wavetek BDM 35 bench DVM
> 2 Fluke 77 hand-held DVMs
> Ideal 61-361 hand-held DVM
> a couple of Simpson analog meters which belonged to my father.
> My first problem was with the SS-40TX Step 3 Test where a voltage
> reading is supposed to be 0.2V or less than the power supply voltage.
> Initially, it looked like the test was a success....until I started
> checking the output voltage of the "regulated" variable bench DC power
> supply.
> Most of my voltmeters indicate that the actual output voltage is
> between .2 - .4 volts higher that the LED readout on the power supply
> itself. (Example, when the bench supply shows 12.0 volts, the voltage
> at the tip of the cable measures 12.1 +/-
> When I used a measured PCB input voltage instead of the bench supply
> voltage readout and conducted the Step 3 Test, I got 0.25 V
> difference.....which is above the recommended voltage difference.
> Now.... was the test a PASS or FAIL? Or is the 0.05 v excess
> voltage critical? And if the test FAILED, what am I supposed to do
> about it, assuming the build is correct at that point?
> In the Step 4 TEST, I'm getting a 0.01 volt drop....supposed to be 0.2
> volts. Double and triple checked all components, solder joints,
> everything.....Test FAIL... Now I'm stuck until I hear from Jim.
> Looks like a problem with Q2....maybe?
> Then, just because things were getting muddy, I compared the voltage
> readings of all 5 voltmeters using various DC power sources
> (batteries, bench supply, etc.). The comparitive readings are
> inconsistent, but the best I'm seeing is an average of 0.03 volts
> difference of 4 and as much as 0.45 volts difference, usually from the
> Data Precision 1450.....BUT, it's not consistently the "black sheep"
> in the out-of-the average readings.
> I'm getting paranoid. I don't know how to test a "known" voltage
> source to determine which, if any, of these DVMs are accurate at any
> given reading, since I have no way of knowing exactly what the
> voltage of any voltage source is, precisely. And I don't know the
> degree of precision required when performing tests at various stages
> of building a kit..
> It's very frustrating having so much money tied up in test equipment
> and having such inconsistent readings from multiple supposedly
> "precision" instruments. And I just don't have the knowledge to
> calibrate the equipment or financial resources to have all the various
> pieces of test gear professionally calibrated.
> Any comments, words of wisdom or advice?
> Thanks. Sorry for the long post!
> Jim Compton, K4JDC

I love this radio stuff.

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