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Dennis Feucht

Build Your Own Curve Tracer, Part 1: Introduction

Dennis Feucht
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D Feucht
D Feucht
1/22/2017 10:26:05 PM
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Re: another project!
Hi Jim,

I can relate to your list of broken instruments. I have my own pile of them.

"Tweaked it back to where the paint lines matched up on the trimpot, and now it's good to go."

It is always a real boost to have a broken instrument working again. I've fixed my ESI 253 RLC meter about 3 times, here in the humid air of the Belize jungle. (Each time, it was a CMOS 405x part; the glass passivation layer must not have held out the moisture.)

Your Iwatsu scope: if you run into a roadblock on it, send me a copy of the circuit diagram for the horizontal amplifier and we'll figure it out. As for the 7A26 and 7B85, the designers of both of those plug-ins are friends of mine. Electromechanical parts - even Tek ones - are prone to have a higher failure rate than electronic parts - something we have to live with for now.

I have tried to minimize the electromechanics of the TPA202, though without a microcomputer, it has more manual switches than otherwise. I too did not think I would use it much, but it turns out to be a much more versatile instrument than originally supposed. Programmable supplies are always in demand on the bench.

1/20/2017 4:25:37 AM
User Rank
another project!
Darn it, Dennis, now you've prompted me to undertake yet another project!  At first I was like, pshaw, curve tracer, shmerve tracer, who needs one?!  I haven't used one for 30 years.  But then I started thinking, gee, I'm interested in power amplifiers (PA's), both for audio and radio, so a curve tracer could come in really handy, duh!

Not like I need another project!  I'm between jobs for about another week and a half now, having been laid off from Broadcom Ltd last November and starting at Raytheon January 30th, and there's no shortage of stuff left to do during this "retirement preview".  Let's see, I've been concentrating on diagnosing and repairing faulty equipment in my garage lab, while off-and-on working on my homebrew 70 cm band ham radio receiver and streamlining my 25-6000 MHz RF synthesizer box.

I have a Wavetek model 166 function generator that used to flatline the squarewave, pulse train, and TTL outputs after a few minutes.  I opened it up and found the +5.2 V supply dropped down to about 0.4 V coincident with the flatlines.  But after probing on the +5.2 V rail for a while, it has behaved itself since.  But in the process of troubleshooting the flatline problem, the triangle and sawtooth waveforms got goofed up to close to squarewaves!  I tracked that down to a pot that I must have bumped that shorted the collectors of a diff pair used for injecting triangle or sawtooth current into the preamp.  Tweaked it back to where the paint lines matched up on the trimpot, and now it's good to go.

And there's my TLA711 logic analyzer that wouldn't even boot up because of a dead battery in the realtime clock module.  Unfortunately that RTC module is obsolete and unavailable, but there is a cross to it, without the battery and 32.768 kHz crystal.  So I made my own, and jsut by chance, a battery holder fit perfectly between the backup power pin and the VSS (GND) pin on the rather large 24-pin DIP.  Unfortunately, it still complains about not having a hard disk connected, so I have to open it up again.

Then there's my Iwatsu SS-5710D 60 MHz scope with the horizontal circuit problem.  It only sweeps about 90% of the way across the screen and puts a bright dot at the end of the trace if I keep rotating the Horizontal Position knob clockwise.  I got a schematic off the Internet and tracked it down to the horizontal sweep signals saturating at about 100 V differential, but I still don't know why.  I'll have to ask for help on one of the forums.

And in troubleshooting the Iwatsu, my go-to scope, a Tek 7904 with a 7A26 vertical amp and 7B85 timebase, now shows some funniness.  Channel 1 of the 7A26 goofs up a few minutes after power-up when volts per div is greater than 50 mV.  The gain goes way high and the edges of the calibrator squarewave round to about 100 us transitions instead of <1 us.  From the 7A26 schematic (Internet, of course!), this leads me to suspect the second 10X attenuator from the input, as that one is clicked in for >50 mV/div and out for 50 mV and less.  The trace is unstable after the first few minutes of working properly, so I suspect a poor connection in the contacts clicking the 10X attenuator in and out, but they look so delicate that I don't want to touch them.  Again, I'm appealing to those knowledgeable folks on the Internet to help me out when I've reached the end of my knowledge.

The synthesizer works OK, but it leaves a few things to be desired.  I dropped +15 V from a wall-wart power supply to +5.5 V to go to the Hittite/ADI HMC833 synthesizer evaluation board and post-amplifier board through an old-time LM317 linear regulator, so it warms up the whole aluminum box!  I'd like to build a PCB with a switching regulator and the power-on reset circuitry to clean that all up.  And the output amplitude drops like a rock up at high frequencies because both the HMC833 synthesizer chip itself and the post-amplifier roll off, so I'd like to add an RF PCB to level the output.  And in order to get minimum attenuation in a PIN diode attenuator in the leveling circuit, I need to bring in greater than the +15 V that the 10 MHz reference OCXO needs, so I plan to make it work with a ubiquitous +19.5 V laptop computer power supply.

Beyond that, I have a couple of semi-working computer monitors that somebody in my neighborhood left at the curb around Christmastime.  One appeared to be working, but now that I have it all set up in my computer-music-scrapbooking room, of course it has crapped out!  I suspect electrolytic caps, as we had numerous monitors at Broadcom die due to nothing but failing electrolytics.  Wish me luck!

Sorry for the long post, but that's just me.  Have a good one!


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