Issues resolved, main board checks out

AFTER THE THOROUGH CHECK OF EVERY RESISTOR, MISTAKE WAS LIMITED

I found no errors at all on the front panel or front board. The main board problem was restricted to eight resistors which were intended to be 100K ohms but were installed with 1K ohm parts. This was likely an error I made when sorting the parts and assigning each bag to a list of resistor locations; I must have misread the bag. Alternatively it might be a sorting error at the distributor where what I received were different value resistors than listed on the bag. 

I removed the incorrect resistors and soldered in the correct 100K ohm parts, then repeated the testing of the triangle wave generator, which performed as expected now. The rest of the procedure alternates between testing of sections and installation of the final two transformers which generate the higher voltages to drive the device under test - from 35V to 200V in three ranges. 

MODIFIED TRANSFORMER TO ALLOW RELAY SWITCHING OF VOLTAGES

One of the transformers was a center tapped unit producing 28VAC on each leg, 56VAC across the two ends. The design of this tester requires that the secondary windings be separated so that relays can either put the two in parallel or series to generate the raw voltages that result in 35VDC and 70VDC maximums after some regulation. 

Since the center tap terminal on the transformer had wires from each winding soldered to it, it was easy to unsolder and separate the wires to isolate the two secondary windings. I then had four wires from the secondaries to the terminal block on the main board. 

The tester uses two relays, one to control the parallel or series wiring of the modified transformer and another to switch on the high voltage transformer if the 200V range is selected. In all, the tester has three transformers, one that generates the 5V, 7.5V, -15V and +15V rails for the logic, another to produce the 35 or 70V we just discussed, and a third to delivery the 200V for higher voltage device testing. 

VERIFIED ALL THE VOLTAGES AND RELAY OPERATION

In addition to verifying that the relays switched properly and delivered the max voltages for the three ranges, I wired up a potentiometer to verify a circuit which adjusts the actual delivered voltage between 0 and 100% of the maximum for each range. This worked as well, thus the main board functionality is verified.

MAIN BOARD AND 35/70VAC TRANSFORMERS MOUNTED IN ENCLOSURE

The transformers for the device under test voltages must be able to source a sufficient maximum current to properly test beefier devices such as transistors and diodes. The 200V supply only provides a maximum of 100ma of current, allowing testing of up to 20W components at that range. The two lower ranges are designed to supply a max of 2A at 35V and 1A at 70V, thus handling up to 70W components. 

To deliver the higher wattage, the transformer for the two lower voltage ranges is bulky and has to be bolted to the enclosure separately. The other transformers are smaller and mounted directly on the main board PCB. After bolting the transformer in place, I wired it up to the terminal block on the main board.

Transformers

The board was screwed into place. My final work is on the front panel/front board sandwich and the cabling between the various boards. Since I have three components for the front board and the male connector parts for the cables still coming from Digikey, it will be a few days until I wrap this up.

VIEWS OF THE ALMOST ASSEMBLED TESTER

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Front panel before knobs added

Rear panel

Temporary insertion of front sandwich

Front board waiting for last components


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