[volt-nuts] Replace neons with LEDs in Fluke 845AB

[Dallas Smith]

Hi David,

That's just what I was thinking.  As well I couldn't find any data on the GA2817's.
Do you think the transistors are actually bad?

What is the status of the new modern chopper design talked about in earlier post?
Is there a PCB design now for it?

Dallas


> From: davidwhess at gmail.com
> To: volt-nuts at febo.com
> Date: Tue, 7 Jun 2016 11:18:56 -0500
> Subject: Re: [volt-nuts] Replace neons with LEDs in Fluke 845AB
> 
> Silicon replacements should work fine; I would try the TIP30 or other
> low current and slow power transistors like the BD234/BD236/BD238. The
> only thing I would worry about is oscillation during switching if the
> replacements are too fast and that is something I would check with an
> oscilloscope by monitoring the collector waveform.
> 
> The transistors operate in saturation so the bias change will be
> insignificant; the higher Vbe of silicon will lower the frequency by
> about 3%.
> 
> I could not find any data on the GA2817 shown in the parts list.  The
> minimum Vce in the circuit is double the supply voltage.  I think Ic
> is below 100 milliamps.
> 
> On Mon, 6 Jun 2016 12:25:32 -0400, you wrote:
> 
> >Hi Lou,
> >
> >It's normal for some leakage in germanium transistors. Do you have a 
> >curve tracer?
> >If you change them to Si you will have to re-bias them (i.e. R207_8).
> >The 84 Hz may change also, might have to adjust C203.
> >But should be possible.
> >
> >Dallas
> >
> >
> >On 6/6/2016 7:33 AM, Lou Amadio wrote:
> >> For Dallas Smith
> >> Thanks for the info. It is in keeping with the original design.
> >> My 845AB 15v supplies are currently down and I suspect the germanium
> >> transistors around T202 may be leaky?.
> >> Am thinking of replacing them with Si transistors. Your thoughts?
> >> Regards, Lou
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