Assembling and Wiring the Chassis
Here are the construction steps:
- It’s assumed that you’ve already:
- Built and tested the main board and the heater regulator board (HRB).
- Applied the vinyl art to the panels and done the machining operations.
- Install the chassis parts, including: (Ref gallery slides 17, 31.)
- Rear panel:
- (Leave the AC line cord and AC adapter until later.)
- Install 4-position AC and 3-position DC terminal strips.
- Fuse holder with a 3A slow fuse installed
- (Leave the DB-9 connector until later.)
- Top panel:
- Four tube sockets (7-pin mini, 8-pin octal, 9-pin mini, 12-pin compactron)
- Four 1.5-in long standoffs
- Front panel:
- 12 DC jacks in the pin connector row
- 12 banana jacks in the test jack row
- Instrument row: power switch, green power LED, 9 DC jacks, SP3T toggle switch.
- 6 DC jacks in the storage group
- Red HV LED and black banana jack for ground
- Four 1/4”-long standoffs for the HRB. Use #4 x 3/16” screws and put a lockwasher between the standoff and the panel.
- Wire the banana jacks (except ground) to the sense terminals of the row of 12 DC jacks. The inner conductors go to the sense terminals. Ref slides 18, 19.
- Wire the socket field according to the schematic. Keep wires short and direct but try not to block fasteners or solder lugs. See the notes on the µTracer Schematic for details on the beads. Ref slide 20.
- To install the AC adapter:
- Orient it as shown in slide 21 to check positioning. The prongs for AC should be on the left and the DC pigtail is on the right.
- Mark the edges of the adapter on the inside of the rear and top panels to guide application of contact cement and installation.
- Remove the AC adapter, noting which sides were in contact with the panels. Apply contact cement to those sides and to the enclosure panels which make contact with them. Allow 20 minutes for the cement to dry tacky. (Contact cement works by instant fusion between two such tacky surfaces. It cannot be repositioned after first contact.)
- Carefully bring the adapter in close to the panels, in alignment with the marks you made, but do not let it touch until you... Press it into place, making contact with both panels simultaneously. Apply heavy pressure for several seconds.
- Install the AC line cord with a strain relief.
- Wire the components on the rear panel as shown in slide 21, the schematic and “uTracer-HR wiring plan--assembly.pdf”. This includes choke L201 connected to the DC terminal strip and secured to the AC adapter with a small dab of construction adhesive. (Too much is shown in slides 21, 23, making it harder than necessary to remove.)
- Connect the AC power switch with a twisted wire pair as shown in slide 23. As seen there, I secured the wire pair with a small cable tie at the standoff near the AC adapter and with a solder eyelet held by a screw in the 12-pin socket.
- I designated the center lug of the DC terminal strip as a single-point ground for the system. Thus, you may notice four green wires going from there to: (1) the ground lug of the AC terminal strip, (2) DC- of the main board, (3) Gnd of the HRB (heater reg board), (4) front panel ground jack. The AC adapter DC- connection also goes to the single-point ground. The yellow wire in that group is +19V before the choke going to the HRB and the red is +19V after the choke going to the main board.
- To protect people from the AC power switch terminals, I used a piece of heatshrink, heated, folded over and secured with a small cable tie.
- Installing the HRB:
- Mount the HRB on the 1/4” standoffs using #4 x 3/16 screws. The longer header strip should be towards the rear panel.
- In general, route the wiring as shown in slide 24. From the HRB, each wire should exit from the header and run along the long edge of the HRB for some distance before turning towards its destination. The idea is that when the board is pivoted up around that edge, the wires can twist in torsion instead of bending. Might not work completely that way but it helps.
- Connect wires from the rear panel for: Ground to P201 and +19V before the choke to P202. DC terminals are identified in the slide 21 caption. (-15V from the main board is covered below.)
- Connect wires from the three-position toggle switch but first, solder a jumper on the switch as shown at right. (Click for higher resolution.)
- Add wires from P206 (by the SRM), and P205, P205, P207 to the Htr and HtrG connectors. Recall that sense lines (HtrSen, HGSen) go to the inner conductors and drive lines (HtrDrv, HGDrv) go the outer conductors. From the HRB terminals, these wires pass under the board going to the pivoting edge, then travel along that edge to their destination connectors.
- Installing the main board:
- To allow the main board to be pivoted up for service, all connections need to be routed to it from the side closest to the HRB. Hence, wires coming from the rear panel run under the board and up, as seen in slides 20, 24, 27 and 28. For the RS-232 cable this means increased length but that’s no problem for those signals.
- Mount the main board on the 1.5” standoffs using #4 x 1/4” screws. The side with Power and Heater connections should be towards the HRB.
- To connect -15V from the main board to the HRB, we need to locate the via (plated-through hole) and solder a short length of #24 wire there as a -15V terminal. As seen at right, the via is close to R20, which has beads on it. Run a wire from it to P203 on the HRB, as seen in slide 24. This allows each board to pivot for service.
- Other connections on the main board:
- Terminal labeled “Power” and “+” TO +19V on the DC terminal strip on the rear panel, after the choke. That’s the side NOT connected to the fuse.
- The other Power terminal TO single-point ground on the DC terminal strip.
- Terminals labeled “Heater”: Left one (closest to large cap, C6) TO (Programmed) Htr+ (J206) on the front panel. The right terminal goes to Htr-, J205. Connect only to the outer shell lugs (drive), leaving the inner pin lugs (sense) open.
- If you stuffed the two main board LEDs, those should be replaced with 2-pin headers to serve as solder terminals. The sides labeled with “A” are positive. Connect the “PWR” LED terminals to the front panel Power LED and the “HV ON!” terminals to the front panel HV LED, observing polarity.
- Tube connections, A, S, C, G, go to the front panel connectors for Plate (J202), Screen (J203), Cathode (J207) and Grid (J204), respectively. Connect only to the outer shell lugs (drive). Also, solder a wire between the drive lugs of Cathode (J207) and Supp (J208). (Supp is short for suppressor grid.)
- For the RS232 connections, I recommend using a shielded cable with two conductors plus ground. Mine appears as the black cable in slides 23, 24 and 27. I used red for the transmit line and black for the receive. Of course, the shield is ground. The pic at right shows the connections needed. Click for a higher resolution view. If you’ve already mounted the DB-9 connector on the rear panel, you will need to remove it to wire the jumpers. Having it out will also make pre-wiring the cable easier.
In the upper left corner of the pic, you can read the pin numbers on the unwired connector. (It’s shown upside down because that’s what you see when it’s mounted on the rear panel and you have the unit open for service. At the lower left is the wired connector, with these connections:
- Jumper pin-7 to pin-8.
- Jumper pins 6, 1 and 4 together.
- RS232 ground to pin-5.
- Transmit data to pin-2.
- Receive data to pin-3.
On the right side of the pic, the µTracer connections are shown, top to bottom:
- Transmit data (marked with left arrow ←)
- Receive data (marked with right arrow →)
- Ground (marked with upside-down ‘T’)
As seen in slide 23, the DB9 connector is mounted on the rear panel and its cable is routed past the AC terminal strip, down along the side of the AC adapter, secured with a small cable tie to the standoff near the AC adapter, past the compactron socket, secured with a small cable tie to the nearby standoff. From there, as seen in slide 27, it will run under the main board until it gets to the RS232 corner of the board, where it curls around the side to the RS232 terminals on top.
- That completes installation of the main board, which in turn completes assembly and wiring of the µTracer/HR chassis—congratulations! All that remains is to attach the bottom cover using the four sheet metal screws supplied with the enclosure. You will also want to affix (4) rubber bumpers to the bottom. Small ones are supplied with the enclosure but I substituted larger ones.
- Oops, there is one other thing: There was a small problem with the USB/RS232 adapter cable which plugs in to the DB9 on the rear panel. The cable had threaded sockets which interfered with the screws holding the female DB9 connector on the panel. The male adapter expects the female connector to have captive thumb screws which hold it in place. My solution was to cut off the threaded sockets of the adapter using a Dremel cutoff blade.
Making the Patch Cords and Cap Connector
As shown in the schematic, seven patch cords are needed to connect the instrument-row of jacks with the pin-row of jacks. These are two-conductor cables, carrying the drive and sense lines. I recommend using the flexible, 600V, silicone wire listed in the BOM (but I forgot I had it and made them with small zip cord). Mine are about 11 inches long, which has worked well. Notice that the front panel is designed so the cables can always be left plugged into the instrument row. Unused ends may be plugged into the Storage jacks. Thus, the cables can stay with the unit, reducing the risk of losing them. When making the cables, be careful to connect inner conductors together and the same with outer conductors.
Also shown in the schematic is the plate/grid cap connector, used for tubes which have a metal cap connection on top. Be aware that tube caps were made in multiple sizes. The drive and sense lines should come together at the cap.
System Test—Taking ‘er Out for a Spin!
We have assumed that you’ve already built and tested the main µTracer board, so you have installed and tested the GUI software for it, using a USB/RS232 adapter cable. For this example, I’ll use the venerable 6L6GC. The steps for the test are:
Running a Quick Test
- Plug the USB/RS232 adapter cable into the rear of the uTracer/HR and a USB port on your PC. Run the µTracer software.
- Use the patch cords to connect Htr, HtrG, Cath, Grid, Scrn, Plate TO pins 2, 7, 8, 5, 4, 3, respectively. (Supp can go to Storage.) For other tubes, you can use a tube manual or our handy VTA tube settings chart. Make sure the HtrV switch is set to 6.3—its middle setting. Since it’s right next to the Heater jacks it’s good practice to check that switch whenever you make the heater connections.
- Turn on the power switch and check to see that the heater glow looks normal. By the way, it’s normal for the power light to take a second or two to come on. Two cascaded switchers have to get cranked up, while they’re dealing with a cold heater which may have an initial resistance, SIX times lower than when warm!
- In the main screen of the µTracer app, click the Debug button and click the Ping button in the dialog. You should see fields under the button become populated. Close the dialog.
- Back in the main screen, click Heater On twice. The second click stops the ramp. We aren’t using the µTracer heater supply but need to make the button change to “Measure Curve”.
- Click the Quick Test button and that dialog will come up in triode mode. Click the pentode check box to change it to pentode mode.
- In the Quick Test Pentode dialog enter these settings (taken from the VTA settings chart): Va=250, Vs=250, Vg= -14 and click the Test button. The red HV LED should come on.
- Find the gm value on the right side of the dialog. It reads in thousands of µmhos, so the ideal value is 6.00, for 6000µmhos. (My 6L6GC shows 5.59 or 5590µmhos.)
- But since this is a fixed-bias test, it’s also important to check the plate current, Ia, at the bottom of the dialog. Mine shows 60.58mA, which is significantly below the ideal value of 72mA, accounting for the reduced Gm.
- The other number to look for is plate resistance, Ra. My tube reads 38.42K and the ideal value is 22.5K. In the “Gm and Rp Shootout with the VTA” above, this tube read 42.12K on the VTA101, so it is indeed running high. Generally, higher Ra is a good thing though. (Note that the VTA settings chart shows nominal plate conductance, which is 1/Ra, so the 44.4µmho value there is 22.5K in resistance.)
Doing a Sweep Test
- (Continuing from the previous test.) Close the Quick Test Pentode dialog and make these settings in the main screen:
- Select measurement type, “I(Va, Vg) with Vs, Vh constant”. That says to plot current versus Va and step Vg from a list.
- Va: Start=2, Stop=250, Nintervals=30. This will be similar to the 6L6WXT example shown earlier under “Graphing Plate Characteristics” but simplified for brevity.
- Vg: -24 -20 -16 -12 -8 -4 0 Apparently, you must enter the more negative values first.
- Vs: 250
- (Under the graph) On the row starting with “Y1”, in the Style column, the dropdown list of trace markers (normally showing “O”) should be set to the first item, which is a blank. This gets rid of those bothersome markers.
- Click the Measure Curve button and you should get something like the figure at right.
If all that looks okay, it’s a good indication that your µTracer/HR is working properly, excellent! However, I would recommend more extensive tests both to check out your new instrument and to give you some familiarity and confidence with it.
If there are problems, don’t despair; it’s probably something minor, just a matter of finding it. Since the main board and the heater regulator board (HRB) were checked earlier, it’s likely to be something wrong with the system wiring. Go over it carefully, checking against the schematic. You might want to use a yellow highlighter to color the traces on a print of the schematic as you check. If you can’t find it, I will be happy to help—just post a question in the Reader Comments section on the first page of this article. We’ll get to the bottom of it!
I would like to thank Ronald Dekker and his wife, Marie-José, for producing the marvelous µTracer kits with such devotion to detail and support. I continue to receive emails with extensions and tips which improve the usefulness of the instrument. Moreover, the amazingly cost-effective design and the excellent construction manual have made the µTracer an outstanding tool for over 1400 hobbyists, worldwide. Ronald and Marie-José have truly raised the standard for tube testing for so many!
Special thanks go to Arthur Grannell for proofreading and providing valuable feedback.
Next: Appendices for digging deeper...
Copyright © 2019, Stephen H. Lafferty