This article will discuss Mercury tube testers. The Mercury Electronics Corporation of Mineola NY produced a number of tube testers in the 1960s.  All models are small, portable, and lightweight.  Popular models include 990, 1000, 1100, 1100A, 1100C, 1101.  Models and comments will be added as I find time.   Electrical voltages are present; repairs should only be attempted by a qualified technician.

Mercury model 990 and 1100C

Mercury model 990

Mercury 1100C

Models 990 and 1100C are electrically similar and differ mostly by cosmetic design.  Model 990 will be the example photographed and discussed.  These models are an example of a nice portable tube tester that packs a lot of punch into a very small package. It is Made in USA.  Model 990 measures only 7.5 x 7 x 4 inches and weighing only 2.6-lbs, it is one of the smallest tube testers that you can buy.  Model 1100C is also slightly bigger, and the size difference is because model 1100C has a wood case with lid.  The portable size offers easy mobility for remote service calls.  The instruction manual / tube setup chart is combined in one book, and neatly tucks away inside the unit.  It has a carry handle for easy mobility.

Model 990 uses a traditional Cathode Emission test circuit with four selectable loads, and also checks for Leakage and Shorts. The leakage circuit is factory-set at 1-MΩ (this is the 1-MΩ resistor across the neon lamp), which means it will detect inter-element leakage from (solid short) up to 1 MΩ.

The 990 has eight test sockets: 12-pin Compactron, Nuvistor, 7-pin miniature, Octal, Decal, Magnoval, Novar, 9-pin miniature. This socket configuration is suited for people working on post-1950s tube equipment, including guitar amps, HAM radio rigs, 1950-1960’s jukeboxes, etc. Since the tester does not have the antique sockets (such as 4-pin, 5-pin, etc.), this model is not suitable for early antique radio servicing (1920’s to 1940’s).

Servicing

Clean the unit, and inspect the line cord (replace if necessary.)  Replace the 0.01 µF capacitor.  The parts list shows a 100µF electrolytic capacitor, although it does not appear on the schematic.  (This is the case with every Mercury emission tester.)  This capacitor (100µF-6V) is not installed from the factory on every unit, and goes across the meter terminals (+ to +, – to -).  Treat all switches and tube sockets with Deoxit.  (This includes all rotary switches and all slide switches; work each switch vigorously after applying Deoxit. The top sides of the rotary switches can be polished by hand for even better cleaning.)  Use a multimeter to check continuity on both sides of the 12 slide switches: from Open-to-Test position, then Open-to-Norm position.  Check continuity of all 12 circuit leads from each tube socket pin down to its corresponding slide switch.  The top cap has continuity to slide switch 11.  Check all resistors for values close to schematic. (R4 will be tweaked later).

Emission Calibration:

To begin, I must point out that some of the instruction manuals supplied with model 990 contained a schematic with errors. (For example, Chart 990 #69730). Load switch Z is drawn completely wrong on the bad schematics.  If your schematic shows Z switch position #1 connecting to 10k resistor, then you have a bad schematic. Position #1 should have load resistor R4 ( 330Ω ), Position #2 (R5, 820 Ω), Position #3 (R6, 1500 Ω), Position #4 (R7, 10KΩ resistor in series with R6).  The other side of this switch is drawn wrong also, but is not relevant to this discussion.  The manual and tube data chart that I sell in my storefront has the correct schematic.

A quick glance through the Model 990 tube data chart will show that approx 95% of the tubes test with load #1.  Therefore, Load #1 will be calibrated.  The “Good” range of the primary scale on this tester goes from 50 to 100, and in this configuration a “standard” new tube will test approx 75.  A new 6L6 tube will be used as a reference standard.  If you do not have a “standard” 6L6 tube that you use for this purpose, you should acquire several new uniform 6L6/5881 tubes (same brand, same production lot).  Adjust the value of R4 (the 330Ω resistor) until 6L6 tube tests at approx 75 — repeat the test with the remainder of the new 6L6 tubes.  (The purpose of repeating the test with several factory uniform tubes is to make sure that you did not receive one tube that was “out of spec”. )   On the last Model 990 that I repaired, R4 value of 390Ω was optimal, but each unit should be checked individually.

Testing 6L6

Testing 12AX7A

Testing weak 6N6

The remaining three loads do not require tweaking because (1) the factory defaults are reasonable; (2) they are very seldom used (95%+ of the tubes use Load #1); and (3) some of these loads focus more on the lower (diode) good-bad scale and not the upper primary scale.

Model 1100C is easily calibrated by adjusting R8 meter shunt while testing a new 6L6 for test score in the 75 range.

Shorts and Leakage Calibration:

The 1-MΩ leakage sensitivity is standard.  Check the value of R1, which is the 1-MΩ resistor across the neon lamp.  This resistor should be 1-MΩ or slightly higher.

Leakage and Shorts can be checked by using a Test Socket, a pair of jumper wires, and a 1-MΩ resistor.

Mercury 990 leakage detection

In the photo above, I am using an Octal test socket. A pair of jumper wires connect a 1-MΩ resistor between pins 3 and 6. You will see a moderate glow on the Leak lamp. If you use a resistor of lower resistance, the glow will be brighter.

Similarly, if you jumper those pins together (instead of connecting them thru a resistor), you can simulate an internal tube short as follows:

Mercury 990 shorts detection

(Notice the brighter glow of the neon lamp when testing a Short vs. Leakage).

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Mercury model 1101

Mercury 1101

Mercury model 1101 is another lightweight American-made portable tube tester from Mercury.  The manual has a copyright date of 1967.  Cosmetically, it resembles a Sencore Mighty Mite TC 154 in some ways – it has a large number of pre-configured sockets (16 of them), it has 3 control knobs at the bottom, the Shorts/Leakage test focuses on cathode-to-control grid and cathode-to-heater, and the 1101  is similar physical size (although lighter weight).  It measures a small 12 x 9 x 4 inches and weighs only 4.1 lbs.

The 1101 tester that you see in these photos looks brand new.  If not NOS, then it was seldom used.

Mercury 1101 case

One very nice feature – It has a complete set of five Pin Straighteners built into the unit, which allows easy pin straightening of all small-pin tubes (everything from nuvistor to compactron).  It has a hard plastic case, a large easy-to-read meter, and metal carry handle for easy portability.

16 pre-configured sockets cover “all heater pin arrangements in use today” (as quoted from Mercury 1101 manual).  Socket types include 7-pin, 9-pin, and 10-pin miniatures, Octal, Loctal, 5 & 7 pin nuvistor, Novar, 12-pin Compactron, Magnoval, and Decal.  The unit also ships with a television picture tube test cable.

Electrically, Model 1101 uses the same test circuit as Model 990 (described in detail above), which is Dynamic Cathode Emission, Shorts Test, and Leakage Test.  The neon lamp is sensitive to 2 MΩ, although the tester is factory set at the standard 1 MΩ leakage sensitivity.  You could change R5 resistor for up to 2 MΩ sensitivity if desired.

Servicing

Use the same basic procedure as described for Model 990.  Of course, the components are obviously designated differently. Here is a brief summary:

• For Emission calibration, the loads are found in Switch W, with Position ‘A’ (R6) being your primary focus, and calibrated using uniform 6L6 tubes.
• Switch X connects (in most tubes) the cathode to the meter for testing.  In full-wave rectifiers, the plate is connected here.
• Switch Y sets the heater voltage.
• Switch Z connects (in most tubes) the Control Grid to the return circuit. In diodes (not to be confused with full-wave rectifiers), the Plate is connected here.  Examples are 6H6 and 6Q7.
• The Shorts/Leakage test is automatic on this model.  Once you setup controls X,Y,Z as indicated on the tube chart, and then insert your tube, the Shorts/Leakage test is automatically engaged.  You do not need to engage any buttons/switches/knobs.  If the Leakage lamp does not light after tube warmup, you do not have K-G1 or K-H leakage within the given sensitivity, and can proceed to the Emission test.
• Shorts Test functionality can be quickly checked for all pin positions: when Switches X and Z are in the same numbered positions (1-1, 2-2, etc…except for 12-12), the Shorts light should be bright, otherwise you have an open in the circuit.  The exception is 12-12, which does not light.

Testing 6L6

Testing 12AX7A

Testing weak 6N6



Simulating (and testing) Tube Leakage and Tube Shorts:

Mercury 1101 Shorts Test

Mercury 1101 Leakage Test

Configure the tester to test a 6L6 tube, insert an octal test socket in Socket #4.  Verify a bright Shorts light if you jumper together 2 & 8, …then 5&8, …then 7&8.

To check for proper Leakage detection, connect a 1-MΩ resistor between pins 2 & 8, …then 5&8, …then 7&8.  At each of these three tests, the Leakage lamp should glow dimly.  If it does not, reduce the value of R5 until the 1-MΩ leakage is detected.


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