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Yes, in this case, "better" was the enemy of "best", where "best" was what solved the problem the soonest and with stuff in my junk box. I wouldn't trust the ohmmeter to hold tight tolerances over a wide temperature range (or ever) with this circuit, but it never got too hot or cold in San Diego. Worked well enough to fix TVs and radios. And the VTVM was thankfully hard to kill, even when probing a horizontal output tube's plate voltage. Love them toobz!
Sent from my iThing, so please forgive the terseness and typos.
On Dec 9, 2020, at 13:14, "Jim Ford" <email@example.com> wrote:
No need to overthink it. If it works, go with it. The engineer in me wants to make sure it has some margin, though. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Tom Lee <firstname.lastname@example.org> Date: 12/9/20 1:05 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] OT - VTVM’s are awesome I didn’t like having a battery in my Heathkit VTVM, either. Having lost countless flashlights and toys to leakage was enough of a lesson. It was in the days before the LM317 (maybe Dobkin was in the process of designing it), so that wasn’t an option. I just put two diodes in series, powered through another rectifier off the filament supply (I think), with a big old ‘lytic across the series diode pair. Not elegant, but as a junior high school student, I was very pleased to have it work. When I revisited the meter after many years in storage, I thought briefly about upgrading it with a more modern implementation. But it still worked well, so I let laziness make the decision.TomSent from my iThing, so please forgive typos and brevity.> On Dec 9, 2020, at 12:47 PM, Jean-Paul <email@example.com> wrote:> > Hi: The Heathkit, Eico, Paco and Knight (Allied) VTVMs all had bannana jacks and probe cables of rubber, that cracked over the years. I recall one meter with the old microphone screw coax perhaps for AC input shield. > > The VT bridge circuit seems to limit meter movement (200 uA?) overload, I never had damage to the movements even with extreme overload. > > IM-28 were the newer series but basically same as the VT-7.> > Hope that someone can please post a link to the Zener or LM317 battery substitue circuit? > > Jon> > > > > > > >