Micronta (Tandy) 22-303 Logic Probe

[SiriusHardware]

Just had to repair my old logic probe (cable cores were fractured where the cable meets the strain relief) and I thought a photo might be of interest.

All through-hole soldered discrete components and not a chip in sight despite the unit's main purpose being to diagnose faults on logic circuits.

I bought this from Tandy (Shields Road, Byker) in the late seventies / early eighties and although it can't have been used for more than a few hours in total it rode around in my toolbox for many years when I was a field technician, hence its regrettably shabby condition.

[60 oldjohn]

I have one, only used it a few times. I had no idea of the complex circuitry involved. Todays equivalent I suppose would have just one chip, possibly looking like a blob of plastic on the PCB.

John.

[SiriusHardware]

On a quick count, twelve transistors and numerous passive components. Easily enough to build a radio from.

[Julesomega]

The audible 'display' is very helpful for working on glue logic

[buggies]

I also bought it's mate, the pulser - although it has had very little use in comparison. Unlike the logic probe, it does include a circuit in the tiny (A6) manual.
These days I need a similar probe for 3.3v logic.

[SiriusHardware]

For years, I kept finding a thin black lead with a croc clip on one end and a mini banana plug on the other end and could never work out what it was for... ...now I remember. I might even still have it somewhere.

I never did use it because logic (!) suggests that a logic probe should always be running from the same supply as the circuit under test - if it is, it already has a common GND. The little flying lead would be handy if you chose, for whatever reason, to run the probe from an isolated supply or a battery.

[Mark1960]

I still have the same logic probe from the early 80s. This is first time I realised the crocodile clip lead could be unplugged.

I used to use it in my first job to debug avery weighbridge ticket printers, these had a large pcb full of 74 series ttl. First check was always to make sure every gate output was a correct logic level high or low.

[TonyDuell]

I have the even older version of this logic probe somewhere. I think it's the same circuit but the case is greyish blue. I must have had it about 40 years, I replaced the power lead years ago, and the case is somewhat melted where it came into contact with the spring of an Antex soldering iron stand, but it still works. I used it to debug many homebrew and commercial computers, etc. And yes, the beep and the fact that the LEDs are different colours is very useful as you don't have to look at it too carefully to see what is going on.

Fortunately the 'logic probe' I replaced it with keeps those features. An HP LogicDart.

[SiriusHardware]

Found an online copy of the user manual for the Eighties version Micronta 22-303 :-

http://burningsmell.org/media/micron...obe-22-303.pdf

As buggies said - unlike the Pulser, no circuit diagram.

I've seen a few examples of Tony's blue-grey version as well, that would appear to be cat no. 22-301.

[buggies]

Pic of the input ground lead - and the note about it in the manual.
Can't say I have ever noticed a difference but memory is flaky...

[SiriusHardware]

That's very interesting and in fact I did notice that diode in the main -VE path when I was unsoldering, shortening and reattaching the lead.

So I've been using it wrong all these years. I suppose I could just short across the diode and trust myself never to get the power the wrong way around, or replace it with a Schottky to make the threshold less inaccurate, or put the reverse polarity diode -across- the supply input and add a resettable fuse in series with the +VE power input.

[buggies]

Just an addition to last year's (closed) thread.
I took my probe apart and decided to sketch the circuit - attached in case anyone wants it. No guarantee that it its 100%.

Possible plan was to change the thresholds to suit 3.3 volt circuits but think I might just build one using an ATTINY chip. Should be possible to detect out-of-spec levels with an A-D input and use interupt to show pulses.

[jjl]

That's a remarkably elaborate circuit for a logic probe, but I suppose that's how these things were done in the '70s.

The relevant application note tells me that the ADC of the ATtiny88 can perform 15384 conversions per second with a 200kHz clock. I doubt that would be fast enough unless the logic you wish to probe operates at low speeds.

John

[TonyDuell]

It's a Tandy/Radio Shack product, of course, and I think it dates from the 1980s. I got mine around 1985.

I used it a lot, the only failure was the cable to the power clips broke internally, not a hard fix. The only reason I stopped using it all the time (it does still get used) was that I upgraded to an HP LogicDart.

[cmjones01]

This thread has prompted me to dig out my old Tandy logic probe, which is labelled HYT-07 and also dates from the mid-late 1980s. I used to use it a lot. It doesn't have the TTL/CMOS switch but does have the (very handy) pulse mode, and an entirely unreasonable number of transistors inside. Just out of interest I tested the thresholds.

At 5V supply, LOW is 1.2V and HIGH is 3.4V.
I tried it at 3.3V and it does still work, though the LEDs are rather dim and the beeper somewhat anaemic. The thresholds then are LOW 0.8V HIGH 2.4V which seems pretty reasonable for CMOS logic. The PULSE indicator and beep still works too!

Chris