Tube Tracer Build 4 - Solutions and Decisions

Thought process

The tube adapters and the docking bay were by far the biggest  part of my personal contribution in the case design. As soon as I decided to use tube adapters, I had the mechanical principle pretty much fixed.

At first, I wanted to see the adapters inserted in docking rails not unlike in old C-cassette decks. The rails would have a pivot hinge in the rear and some kind of locking mechanism in the front. After a brief design phase I discarded the idea as too complicated.

After about a week of planning, my design finally was simplified to a fixed rail solution. There are two pairs of retaining clips riveted inside the aluminum rails. The adapter PCB is pushed under the retainers until it mates with the D-sub connector.

The PCB outline resembles a very fat letter H. The four protrusions are held in place by the retainers. This way the adapters will always be docked in the same exact position. There is enough area in the center of the adapter to mount tube sockets up to 80 mm in diameter.

The cover plate for the docking bay hole is very simple. However, the need to have it stay in place during transport required some fixing mechanism. I finally came up with a solution, where the rear edge is secured with a small strip of Dilite board superglued under the cover. It protrudes about 3 mm beyond the rear edge and prevents it from popping out from the opening.

The front edge latch was more difficult. Due to esthetical reasons I hoped it to be very invisible. Magnetic locking was my first idea. I was a bit apprehensive to introduce magnetic field so close to a vacuum tube under test. 

Even if this was somewhat unsubstantiated, I eventually decided to construct a mechanical latch from a short piece of strapping steel band. A white plastic piece was superglued on the visible part to act both as actuating trigger and a latch claw.

Cross section of the adapter cover plate as a part of the face plate.
Case front is to the right. Dimensions are not in scale.

The cooling fan may not exactly have been necessary. With hindsight I probably would have needed more research of its actual benefits. However, after having read several stories of the uTracer building projects I seemed to time and time again bump into observations of heat generation in the linear regulators of the PCB. Adding the fan was therefore not a hard decision.

I wanted to see symmetry in the face plate, so I placed the fan in the rear center, behind the tube docking bay. Therefore, the fan fixing screws had to penetrate also the aluminum frame reinforcements. When I planned the final assembly steps, I noted that the fan screws need something to bite into without the possibility to have nuts for the screws. I had a small selection of sheet metal nuts, but I soon found out that they need a relatively even surfaces on both sides. This is unfortunately not the case with fan bodies.

Eventually I filled the fan fixing holes with epoxy putty and drilled small pilot holes for the self-tapping screws. The result was very good and the fan fixing became a breeze.

Wiring the connections

The laptop power supply was stripped from its case to reduce heat build-up. Input and output were connected using screw terminals fixed on small pieces of perf board and glued on the foam rubber.

The 19-volt output was connected to the uTracer board, cooling fan and auxiliary heater supply. The auxiliary heater supply was connected to the heater switch on the face plate.

The uTracer output connections were connected to the D-sub female connector in the tube docking bay. The RFI suppression beads were inserted on each of the four leads close to the D-sub connector. The heater outputs were routed through the heater switch. No RFI suppression beads were used on the heater cables as many users have reported problems in heater control, which were attributed to the RFI beads.

The on-board uTracer indicator LEDs were cut off at the height of about 7 mm. The remaining wires are used as male headers to connect two substitute LEDs on the face plate.

The uTracer final setup without the face plate. The uTracer PCB sits on the left.
Unfortunately it is not quite square with the case. 
The self-adhering feet cannot be moved once they stick with the bottom.
The two indicator LEDs are shown on front left. Green connector for the auxiliary heater supply on front right.