[Boatanchors] VTVM QUESTION

[Dave Brown]

OK Dan
 Well if the VTVM has separate inputs for AC/ohms and DC then the AC 
probe has nothing inside it. Basically just a bit of wire. The DC 
probe has a series resistor in it-probably 1 Meg as others have said.
 Try measuring a known AC voltage-say a 6.3 volt filament line, with 
the VTVM switched to AC, and you should see it reads just fine with a 
staright-through probe on the AC input.
But you'll need a high frequency rectifier probe on the DC input 
instead of a probe with the 1 meg series resistor, if you want to read 
low level RF voltages- say at frequencies above a few hundred kHz- 
where the basic VTVM AC measurement cct response drops off.
73
 Dave, ZL3FJ

----- Original Message ----- 
From: "k0dan" <k0dan at comcast.net>
To: "Dave Brown" <tractorb at ihug.co.nz>; <boatanchors at mailman.qth.net>
Sent: Sunday, February 05, 2006 8:02 AM
Subject: Re: [Boatanchors] VTVM QUESTION


> Hi Dave...
>
> Thanks for your reply. Well, I don't know much about this meter 
> other than
> that it has 3 probes: ground, DC, and AC (the AC probe is also used 
> for
> ohms). So, I was gonna just put a diode (not too big, not too small) 
> inside
> a probe handle and build my own. But I'm not sure how critical the 
> specs are
> in order to have an accurate reading on the meter. To me, a diode is 
> a
> diode, as long as current doesn't burn it out. I realize this is 
> heresy for
> to the engineers and lab dudes, but that's why I posted my question! 
> ;-)
>
> Thanks and 73
>
> Dan
> K0DAN
>
>
>
> ----- Original Message -----
> From: "Dave Brown" <tractorb at ihug.co.nz>
> To: <boatanchors at mailman.qth.net>
> Sent: Saturday, February 04, 2006 1:10 AM
> Subject: Re: [Boatanchors] VTVM QUESTION
>
>
>> Dan
>> I don't know the WV77E but a lot of those early VTVMs didn't have 
>> an
>> external recitifier type AC probe- some of the RCA ones had a 
>> combined
>> AC/DC probe with a slide switch on it to short the internal 
>> resistor
>> on the AC ranges.
>> There was an add-on high frequency diode rectifier for this type of
>> probe - it was in a housing that slipped over the front of the 
>> probe
>> for RF work.
>>
>> But if the WV77E does use a separate AC rectifier type probe, I see 
>> no
>> reason at all why a homebrew diode rectifier replacement probe
>> shouldn't work-as long as you use a similar type of diode to the
>> original so the existing AC meter scales can be used.
>>
>> 73
>> Dave, ZL3FJ
>>
>> ----- Original Message -----
>> From: "k0dan" <k0dan at comcast.net>
>> To: <boatanchors at mailman.qth.net>
>> Sent: Saturday, February 04, 2006 4:37 PM
>> Subject: [Boatanchors] VTVM QUESTION
>>
>>
>> > This might be a tad off topic, and if so I apologize in advance.
>> >
>> > I picked up an RCA Volt Ohmyst (WV77E) VTVM. No probes. 
>> > Apparently
>> > the
>> > original AC probe has a diode built in but it's not spec'd in the
>> > schematic
>> > or manual I got off BAMA.
>> >
>> > Is there any reason I shouldn't build myself a probe with a 
>> > junkbox
>> > bullet
>> > diode (1N5401, 1N4816, etc.)? If this requires a more 
>> > spec-focused
>> > approach,
>> > what needs to be inside the probe?
>> >
>> > Thanks and 73
>> >
>> > Dan
>> > K0DAN
>> >
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