Re: Infamous D180 is it good or bad.


Eric
 

Well given that the TD is good That means that the 284 has a bias or connection issue. I pulled apart the assembly and I have and R180 measures at 746.1 close enough for me for a spec of 750. However R 184 B + C measure 723 Ohms when I expect 660, R184 A was not involved in the measurement as the resister was removed from the air line at the time of measure so the 50 Ohm R184A was open on one side.

But a 15V pulse in to 746 Ohms would lead to 20.11mA of current in to the front of the TD unless I am missing something?

On 6/8/2020 4:34 PM, Dennis Tillman W7pF wrote:
Hi Albert,
The TD's negative resistance creates instability in the curve tracer power supply.
The curve tracer supply needs to respond to slowly changing demands for current or power. A TD can't wait to oscillate at hundreds of MH at the drop of a hat when you apply power to it. When that happens on the curve tracer the oscillations are so fast the curve tracer doesn't even notice it. If you don't see something you can't fix it.

Years ago I came across a simple circuit that prevented the oscillations when testing the TD on a curve tracer. It worked but it was more trouble that it was worth to have to use it. I probably still have it somewhere. There are a few other ways to dampen this oscillation:
* Inserting a few ferrite beads in each of the EBC jacks helps.
* Adding 20 to 50 ohms in series with the TD swamps out the negative resistance but it distorts the curve on the curve tracer CRT.
It is very easy to ignore the oscillation when it appears because the TD curves are still very visible so that’s the solution I use.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Monday, June 08, 2020 11:18 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Infamous D180 is it good or bad.

I don't understand why you (Albert and Dennis), are surprised by the
clarity of the curves (Eric's and mine).
Hi Raymond,

It is what I thought to remember. Perhaps that was more related to displaying the negative resistance part of the IV-curve.
Today I also did some 576 measurements and these gave nice results like in your pictures. One measurement was with the 284, P181 and P184 disconnected. Pulse Out fed from the 576, range 15V / 650R. This way of course the resistance of R184A (54R nominally) largely determined the slope of the ascending and descending parts, but reading Ip was no problem (also somewhat below 20 mA.) The other measurement was with a TD716, 4.7 mA type. Mounted directly in a 576 diode adapter. Again a perfect loop and a very clean trace.

Albert




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