Humble Electronics #009 - PAD - 234 Analog/Digital Trainer 01.1 (Summer 2023)

I'm going to go through the operator's manual before I get into demonstrating the specific outputs of the PAD-234. The manual is 11 pages long including the cover page and schematics. It is a set of stapled together printed/copied pages. The schematics are attributed to R.S.R. Electronics Inc. and drawn by Bruckner in 1994 and 1995.

The Operator's manual begins with the general procedures and practical advice, that I will admit I don't always follow, on page 1. The five points are summarized (emphasis mine): 

• 1) Read manual
• 2) Build circuit with unit OFF
• 3) Double check your wiring BEFORE turning unit ON  
• 4) If a problem happens, TURN OFF unit and UNPLUG
  
• 5) If unit needs repair don't do it yourself.

Since I'm not using this unit in a classroom environment, I own it, and the schematics are included, I will fix it myself if I damage it. They sell a build your own unit so I think that was a move to limit liability.

Page 2 says it is about maintenance and covers changing the MDL 0.5A fuse. It includes 3 warnings:

 

Page 3 covers some of the specifications. For the power supplies, it lists the short circuit current each supply can handle. The +-12V supply is regulated with 0.5A short circuit protection. The +5V supply is regulated with 1A short circuit protection. The +-15V (lowest is 1.5V on each) variable supply has 1.5A short circuit protection. The AC supply lists no short circuit protection. I guess the main 0.5A fuse covers that one. It says the function generator produces the 3 wave forms described in the previous post from 1Hz to 100 kHz and 0 to 15V peak to peak. The clock is controlled by the coarse and fine frequency knobs with a rise and fall time of 400ns, very slow by today's standards (I worked with a BBAI where the timing on the PRU had a maximum 5ns execution, there was not a modern timer for it at the time). The LED indicators are TTL compatible and buffered. The switches were talked about in the previous post but this page adds that the pulse switches are de-bounced, nice. The breadboard has 840 tie points. Power consumption is 117V at 0.5A. Additionally it comes in a high impact plastic case with a grounded plug and a fuse protected AC line.

Page 4 is titled Operation. It covers the obvious, but first time user friendly, connections like the load should be connected between the +5V and Ground binding posts and where the knobs are to control the variable supply. To use the AC supply as 6.3V AC connect the load between a 6.3V tie block and the 0 tie block next to it. To use it as 12.6V AC connect the load between the two 6.3V tie blocks. Then it describes the function generator biding post location and the knobs for adjusting them at the end of the page.

Page 5 continues describing the operation of the function generator. It suggests to use an oscilloscope to view the output, how to hook it up and vary the output in order to get familiar with it. I wish we had done that in my EE labs. (I am grateful we did have labs as I've heard some students never have them.)

Page 6 covers the digital section, which I covered in the last post. It suggests to become familiar with this area by connecting wires from the toggle switch and pulse switch tie blocks to the LED tie blocks and flip the switches. (Another school aside: my prof thought lighting an LED was not that big of a deal. I think he lost sight of where beginners come from and for the layman and starting student lighting an LED is akin to magic. I hope to never forget that.)

Page 7 describes the extender area of the board. It describes the jack as a 16-pin Insulation Displacement Connector that you can use to connect to PCs and add on boards. It has an illustration of the connector and tie points, that reflects the board, and a table listing the functions of each connection pin. the A-H tie blocks and pins are uncommitted bi-directional lines that the user can connect to the breadboard, LEDs and switches. The section below, on the X and Y inputs, I don't understand so I guess I'll have to look that up and figure it out. That is the last page of operations.

Pages 8 - 10 are the schematics for the function generator, digital section and power supply section respectively. The function generator uses a chip with the number 2206 on it and a chip for the clock numbered 40106B on the schematic. I will probably open the cover and see if they are labeled clearly.  There is an unfamiliar gap on the schematic between the knobs and the ICs. I've only seen simple schematics so its not surprising I don't understand everything. The digital section shows the LEDs connected to a 74HC04, with a couple of unused ones also listed (I think I used that IC in a lab once). The pulse switches are connected with NAND gates (74LS00) in a set-reset latch. 

The power supply schematic I find most interesting and is likely fairly common and basic. It is a linear power supply using a transformer to step down the voltages into three windings, one for the 12.6V AC supply, one for the +5V DC supply, and a third for all the +-12V DC supplies, regulated, variable, and function generator. Aside from the AC supply, the other two windings lead to  Lxxxxx linear regulators. Switch mode power supplies are so much more complicated than that.

I find the +-12V supply the most interesting. It rectifies the AC and uses the + output for the +12V supply and the positive rail variable supply, normal use case. Instead of going to ground, the negative output goes to a 7912 regulator, for -12V, and a LM337 regulator, for the negative variable rail. I did not know that's how you get negative voltages but I may have seen that in a video. I need to put those on my to get list of components. If I find this is a common use of voltage regulators and it's in the datasheet, I will post the schematic. Otherwise I feel uncomfortable sharing someone else's work unless I can link to it.

That's all of the operating manual in a nutshell. Next time I'll show some of the outputs on the oscilloscope. Until then...

Happy Making!

Michelle