I didn't expect the Tolex to be perfect as a piece of test gear so I was OK with the condition. Though there were issue of concern that did need addressing. Seems two of the corners took a bad hit. So i begin by disassembling as much as possible so that necessary repairs can begin.
Luckily, nothing on any of the electronics broke and the wood pieces were repairable. Nails went back into original holes with epoxy and it was like nothing was damaged. The corners were trickier but were also repaired to satisfaction.
Because I did not recognize the Island for this model I did want to check on the back of the island to ensure that this unit want modified. Sure enough this island is legit. It could be a rare interface, i could be wrong.
So using the newly repaired case as a stand I immediately identify caps to be replaced and also the vacuum tubes. But first I remove the paper roll so I don't damage it while working. One tube was loose in its socket so i added super glue to fix it.
I ended up having to fix another tube tester just to check the tubes for this unit. the tubes tested ok. So I swapped out the caps. Ended up having to go to the century tube tester for the 5Y3 because the tester still needs more repairs.
I then get the the deoxit phase, my leased favorite part, that includes every tube socket, also re-tensioning tube sockets that need it. Also the two sockets for the testers own tubes were done.... and EEEEEEVERY switch and knob on the whole test set.... I did not do the calibration pots though, they get done during calibration. Also, the fuse bulb and socket, island and its socket, were lightly cleaned with deoxit, and then all resistors were checked. The device was turned back over and left to drain for a few hours as it was laid back in the wooden box.
The island was screwed in. And a power on test is ready for this unit. Before turning on it was found that the physical 0 of the bias knob and the static zero of the meter needed adjustment. The power line adjust test is done as part of the smoke test through the Variac.
The smoke test looks good, and the basic turn on operation seems to be good to proceed. Tune in for the next video in this project for more.
Video 1 - Repair and Restoration
Having finished all necessary repairs and a good smoke test, its time to go through all of the functions of the unit to see if further repairs are needed, precision is not yet required, ballpark is sufficient. For this test a 12AX7 will be used in the demonstration for a myriad of reasons mentioned.
The tests conducted will be right out of the book as shown in the video. The test set is configured for one of the 12AX7 triodes. Incidentally the idle test set sits at 300ma. The short light illumination is the first test and shows everything to be good.
As there are two possibilities, Setting shunt to the prescribed value will return a GOOD/BAD. Setting it to the range of the expected value will actually return the MICROMHOS.
The next is a gas test which becomes a 100ma current meter and should not deflect more than 2 divisions. After this a tube life test is attempted as explained in the video.
That pretty much covers the basic functionality, onto the next video, calibration.
Video 2 - Post Repair Functional Demonstration
This video starts the two parts series in the bigger Hickok 6000 restoration project. Having repaired the unit and conducted a functional demonstration we begin with the calibration procedure. This test will be conducted in accordance with the procedure as shown in https://tubesound.com/hickok-6000-calibration/
The first order of business will be the turn-up of the unit and allowing the tubes to come to temperature. and conduct the short lights test, which i find somewhat limited, but continued on anyway. This is only done with the cathode and suppressor.
The Second test will be the voltage test of the meter at half deflection, ensuring that 50% shows 58.25 by way of the top adjustment. This pot required cleaning and was not in a good way. A stand was assembled during this test to ensure that gravity would not effect the reading.
The next test determines the 2.5vac given the needle exactly on line test as shown on pins 5 and 8. All configs will be done with a config for a 6L6 so it will be config used as such. The second pot is adjusted to bring this into spec.
For the next tests we need to adjust for not having 1000ohm/V volt meter so there will be different shunts for different tests. Some, more creative than others.
The next test will be connected across pins 3 and 8 for the plate voltage for a value of 150v +/- 3V This only works with the shunt in place as specified in the video. The high screen test is done next on pins 4 and 8. This though produce 135v +/- 3V.
The rest of the calibration is in part 2
Video 3 - Calibration Part 1
This next test though not explicitly mentioned in the manual is conducted with an oscilloscope to check the rectifier tube balances. This is an observation without adjustment, conducted in two separate locations on the test set as to check both tubes. In this case, both tubes look healthy.
The next test, using a 50K shunt is a grid bias test that should show 40v at 100 on the knob, further demonstrating what should be demonstrated if it was out of calibration.
The next test, using a 10k shunt is just the same with the bias at 22 looking for 3VDC. The next two tests with 50K shunts should see 13.4 with the bias knob at 50 and 25.8 with the bias knob at 75. I'd found that the slightest rotation of the knob 2 dashes would have brought this all into order.
Next is the low screen voltage with a 120k shunt, looking for 56.
The bridge balance test required some creativity. shunting to a 10K 10W resistor for the test I used the gauge for this test and not my meter. When hitting test and having the meter fall to 0, were looking for minimum deflection as the meter moves from middle of shunt to top and middle of shunt to bottom. The balance looks good except right at the very outer edges it starts to move a little.
For the next test I use a 6V6, a 6L6 is specified in the manual, this is the gas test. Clipped across the tube is a setup to short the two specified pins with a 1mohm resistor during the gas test to bring it up exactly 1 division.
That concludes all of the calibration tests, next is post calibration tube testing demonstrations.
Video 4 - Calibration Part 2
Having finished calibration of the unit we begin the last video in the restoration series. This begins with the re-installation of the tube chart back into the unit.
As a matter of observation I've learned that there are just a few things missing from this unit that separates this consumer grade unit from a professional grade unit, the first of which is the shunt control. The separate positions used for the Micromhos ranges should have been defined ranges on a separate multi range switch to fixed resistors and not on the shunt knob. This would have significantly increased the precision of the unit. While the shunt knob is just fine for pass/fail where precision is not required, it is not suitable for the precision measurement of Mmhos as desired this is worsened as each range is increased towards the highest range where resolution is lost..
Another issue is with the bias knob which is measured in 0 to 100. again fine for the pass/fail, however it is extremely sensitive. What they should have done is have the bias voltages on the chart and a second meter that shows the bias voltage that could be dialed in. This would have allowed how much greater precision.
In looking at these two issues there is a discussion to be had with regard to other mitigating factors that can be said to further effect precision. Does other claims hold water when it can be argued when something shifts a value 10 micromhos, when repeatability is at best around as little as 100 micromhos?
When a test is done, simply making adjustments during a test will demonstrate sensitivity. First is shunt. It would seem that the shunt dash thickness deflects the needle the same thickness. However, the bias deflection is a factor of 4, so much so, that any other issue such as differences between 83 tubes vs. solid state are completely masked.
Further the design is such that given the specified shunt values, there would have been an actual point on the meter for 100% good when setting the actual shunt as specified for consistency, but there is not.
With various tubes, we can see how the current draw loads down the tester, especially on heat-up, the EL34's demonstrate this, the GZ-34 as well. The Rectifiers however are only PASS/FAIL.
Further demonstrations of the bias in the 6000 range show just how much the bias knob is the weakest link in this test set. Hands down.
A cool discovery is that on magic eye tubes, you can do a test to open and close the eye. This was not possible on the Century tester.
This concludes the Repair and restoration of the Hickok 6000 tube tester.
Video 5 - Hickok 6000 End of Restoration - Applications, Testing and Analysis
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