This is one of many examples where tube testing requires experience to properly evaluate any tube.  In this situation, using a tube tester would yield a completely wrong answer.

12AX7A tubes - incorrectly factory marked as E283CC

These tubes were factory marked as E283CC, but are actually 12AX7A tubes.  Hence, if you tested them in your tube tester using E283CC settings, you would have concluded that the tubes were shorted and defective, and you would have discarded valuable NOS 12AX7A audio tubes.

The fact that these were not E283CC was readily apparent to anyone with experience.  E283CC is a “special quality version 12AX7 for audiophile” with three distinguishing characteristics: (1) different pinout than 12AX7, (2) only a 6.3v heater, (3) a shield between triode sections that terminates at pin 7.

A shield between triode sections would be easily visible, and these tubes do not have a shield.  Hence, they are not E283CC.

Since E283CC is a “special quality version of 12AX7″, that was a logical place to start.  Experience shows that they look like Amperex 12AX7A tubes, and testing them as 12AX7A verified that premise.  In fact, each tube is well balanced between its triode sections, and they are quality audiophile 12AX7A examples.  (Tube #1: Triode #1 = 36, Triode #2 = 36.   Tube #2: Triode #1 = 31, Triode #2 = 30.  Test scores from my professionally calibrated B&K 707 mutual conductance tube tester, and also without shorts or leakage.  For 12AX7, scores of 22+ good, with scores in 33 range considered typical new).

Lessons: (1) experience matters, (2) tube tester results MUST be interpreted based upon experience, and not blindly accepted as gospel, (3) you should have the experience to recognize when a tube does not look correct as marked, because blindly inserting a wrong tube into your equipment may cause serious damage and/or fireworks.

Roller-Smith bridge ohmmeter

There is nothing digital about a vintage Roller-Smith bridge ohmmeter.  It can measure from 0.5 ohms to 50,000 ohms using 4 ranges.  This is a beautiful antique test instrument.  It has an attractive hardwood case, leather handle, and quality construction.  

The meter operates on 3 vdc, which can be supplied using two D-cell batteries or a portable external dc  power supply connected directly to the battery terminals.

Usage is very simple.  Connect the resistance to be tested to the terminal posts, select the appropriate range via plug socket connection, push the Test button and rotate dial until meter nulls at zero.  At this point, the bridge is balanced, and ohms is read by multiplying the dial scale reading by the plug-socket range.

Bridge-type equipment has always been very accurate, and this is no exception.

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by Bob Putnak, ©2010.

Of all the subjects that I have discussed, I receive the most inquiries about my HP Officejet “scanner system failure” repair article.  That article also fondly mentions the old-school Laserjet’s, and I had several inquiries about repairing them.  These old dinosaurs are still useful to any small business that prints traditional business text documents or for printing schoolwork.   So here is an article about how I have repaired and refurbished these Laserjet 4L and 4P printers in the past (not so much anymore!)  Do not attempt if you are not qualified to perform such repairs or do not want to risk further damage or inability to reassemble your printer.  The basic concept is similar with other models not shown here, although disassembly will be somewhat different.

HP Laserjet 4L

These printers were built like a tank and page-feed problems are the most common complaint.  A general overhaul will take care of most of these issues.  In summary: the printer is torn down, thoroughly cleaned with compressed air (including the 4 optical sensors on the mainboard), rubber rollers are treated with rubber rejuvenator, the sticky-stuff is cleaned off of the relay coil, the relay coil arm is wrapped once with thin black electrical tape.  If you have a dead printer, you will also inspect the power supply fuses, although a blown fuse is usually the symptom of a real power supply defect.

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This article discusses repair and calibration of the Hickok 6000 tube tester. Models 6000A and 6005 are the same with only minor feature differences that are not relevant to tube test calibration, therefore the discussion is applicable to those models as well. Serious electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted by Bob Putnak, all rights reserved.

Introduction

Hickok 6000

Hickok 6000A

Hickok 6000 series is a compact mutual conductance tube tester. Most Hickok tube testers fall in two categories: those that use a 5 vac signal voltage and those that use a 2.5 vac signal voltage. Otherwise, most Hickok’s are remarkably alike (except for the really expensive models such as 539C or 752…). The 6000 series in the the 2.5 vac signal voltage group. The compact size of the 6000 is a welcome asset to any technician’s workbench because space is always at a premium. The 6000 series also has a replaceable socket panel.  One panel has an older compliment of sockets (4-pin through 9-pin-miniature sockets); the other panel has a newer socket compliment (compactron & novar sockets instead of the antique sockets).  This system allowed easy replacement of worn out sockets — the customer simply purchased a new panel from Hickok.

The setup chart configuration for the Shunt control gives “Good-Bad” readings, which were previously called the “English” readings in older Hickok testers.  In fact, the Shunt control was named “English” on older models.  The chart also provides a micromhos score for tubes that have transconductance (ie – not rectifiers, not diodes, not thyratrons…), and the Shunt control must be repositioned to the red dot on the Shunt control that is appropriate for that micromhos reading.  For example, if the chart says “2000″ and you had some reason to benefit from knowing the micromhos score (which is seldom the situation), you would ignore the chart’s shunt number and instead set the shunt control to the red dot near “73″ which is the 3000 micromhos range.  For most testing needs of a technician, the good/bad scale is convenient and appropriate.

Hickok 6000 paperwork package - $9.99 free ship USA

Need paperwork for your Hickok 6000? I offer a remastered paperwork package for Hickok 6000 — manual, schematic, remastered factory calibration document, and obsolete tube chart data supplement.  $9.99 free ship USA.


Vintage is approx 1957. The 6000 sold for $182 in 1958. Dimensions: 17 x 12 x 8 (inches) and weight 16-lbs. Model 6005 is the same tube tester but integrates multimeter functions. Model 6000A has basic transistor testing capability, which I doubt that anyone uses today. Most (not all) of the 6000A that I have seen have the newer socket panel with compactron and novar sockets. Which socket panel that is best for you will depend mostly on what types of tubes you test.

As a sidenote, this socket panel makes it very difficult to install socket-savers if you still want to close the case lid.  It can be done, but it requires a lot of work and creative thinking.  See my article HERE.

Repair and Calibration

A thorough explanation of the Hickok test method, calibration voltages, and the equipment that was used to make those voltage measurements is in a United States military document TM 11-6625-274-35 for military TV-7 series of tube testers.  This information is applicable to most Hickok tube testers (not all), with the caveat that some models use 2.5 vac signal voltage instead of 5.0 vac.  It explains the voltages for the plate, screen, grid, signal and the method used to obtain those measurements. A factory calibration document for the 6000 series also exists (and has one typo mistake) and closely follows the voltages in the military TM document.

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Today was my annual warehouse spring cleaning day, so it was a good opportunity to take a “family photo” of my tube tester collection.    My collection changes all the time, sometimes daily.    I did include all of the models that I use everyday for tube sales and bench servicing, and also models that I “rotate in” regularly to keep familiar with them.  Many of these are a permanent part of my collection, others will be repaired/calibrated and sold to customers who want a quality tester that works great.

TubeSound tester collection as of April 2010

Columns are numbered left to right and models listed from top to bottom:

• Column 1: B&K 707. Jackson 648 early tweed case version, Precision 620, Simpson 555, EMC 206, Sencore Continental MU140, Jackson 648R, Simpson 330, B&K 700
• Column 2: Hickok 532, Precise 111 Mutual Conductance, Knight 600B, Hickok 800, Sencore TC28 Hybrider, B&K 550, B&K 747, Hickok 800, Simpson 1000, Hickok 533
• Column 3: US military I-177-B, US military I-177, Hickok 6000A, Precision 10-40, Knight 600C, Triplett 3423, Jackson 648S, B&K 707, Jackson 648A, B&K 707
• Column 4: US military I-177-B, US military I-177-B, US military TV-7D/U, B&K 700, B&K 700, B&K 500, B&K 700, Precision 640, Heathkit TC1, US military I-177-B, Jackson 637
• Column 5: Mercury 1000, Eico 625, Heathkit IT-21, Sylvania 620, B&K 550, B&K 700, B&K 700, Jackson 648-S, mint Western Electric KS-15560-L2, Heathkit IT-17, Supreme 550
• Column 6: Eico 666, Precision 612, Eico 667, Precision 10-12, Eico 666, Precision 10-12, Jackson 598, Philco 9100
• Column 7: B&K 747B, Sencore TC28 Hybrider, Sylvania 220, Accurate Instruments 151, Jackson 648, Jackson 648-S, Hickok 533A
• Column 8: Hickok 6000A, US military TV-10D/U, US military TV-7A/U, Hickok 6000, B&K 707, NRI Professional 70, Jackson 561, NOS NIB Sencore Continental II MU150
• Column 9: B&K 707, Hickok 533, Hickok 533, Hickok 539A, Hickok 752


One nice thing about having a large personal collection is that it makes easier troubleshooting of strange wiring problems in a customer’s tester that they have sent for repair. Being able to quickly examine another unit is often much faster than tracing the circuit. It is also nice to have another unit to compare, especially if I suspect that a component may have been replaced many years ago with a non-factory part.

I am frequently asked to explain what all of the “letters and numbers” mean in a vacuum tube number.  In the early days of radio, tube numbers were haphazardly assigned to new tube types.  As more vacuum tube types were developed, it became more difficult for technicians to recognize any fundamental characteristics about a tube.  To alleviate this confusion, the Radio Manufacturers Association (RMA) developed a tube numbering standard.  The standard allows a technician to have at least a basic understanding of the construction and purpose of a tube.

Notes: Early vacuum tubes with 2-digits (such as #50) and 3-digits (such as #485) predate the RMA standard.  Similarly, 4-digit industrial/commercial numbers are not part of this standard.  The standard consists of a first numeral(s), followed by a letter or two letters, a last numeral, and optionally letter suffix(es).

Also, there are tube numbers that look like they were part of the numbering standard, but in fact they were not.  An example is 2D21.

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How to Use a Tube Tester… and Tube Issues (in General)

by Bob Putnak, ©2010.  All rights reserved.

Your tube tester is often the first test instrument that you reach for when diagnosing problems in tube gear.  All tube testers are a compromise in functionality, compromise in accuracy, and none are perfect.  No models test for all tube characteristics.  Not even the tube manufacturers themselves had test equipment that could detect all bad tubes, or verify all good tubes, or test for all tube characteristics.  A 100% accurate tube tester is not even possible because a tube can work excellent in one circuit and not work at all in another circuit.

Tube testers were designed as a tool for repairmen to detect “bad” tubes …tubes that could be causing a problem in the item that the technician was repairing.  Tube testers do a very good job in this role.  However, many people do not use a tube tester properly, do not interpret the results correctly, or have unreasonable expectations of them.  Other people have developed poor workflow habits that need improved.  While each tube tester has different capabilities and therefore different operating instructions, here are my thoughts regarding tubes, testing, and all tube testers in general.

Table of Contents (summary of what is discussed below):

• Use a calibrated or professionally serviced tube tester.
• Do not test tubes with dirty or rusty pins.
• Tubes with multiple sections, each section must be individually tested.
• Configure test settings before inserting the tube.
• Adjust “Line control” both before AND after inserting tube.
• Filament test is always the first test made.
• Shorts Test(s) will be the second test made and must always be performed before any Emission or Gm test.
• Leakage test will be the third test made (if Leakage test is separate from Shorts test in your tube tester).
• The Shorts and Leakage test(s) are the most important tests you will make.
• For any tube, there is no one “TEST SCORE” that is “right”.
• Some tube types do NOT even have a “test score”.
• No tube tester will tell you whether an oscillator tube will work properly in-circuit.
• Now that you have some understanding how LIMITED the usefulness of a “test score” is, you can proceed to make your Emission/Gm test.
• Do NOT perform the Emission or Gm test until the tube has had sufficient time to warm up.
• Do NOT perform the Emission or Gm test for ANY LONGER TIME than necessary to get a reading.
• Interpreting Good/Weak emission and Gm test results.
• Repeat Shorts and Leakage tests again
• Know YOUR tube tester
• Noise / Microphonic tube issues

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Here is a truly historical item — a LEEDS & NORTHRUP CO. Wheatstone bridge. It has Western Electric K.S.3011 designation.  I originally called the item a mirror galvanometer but it is more accurately termed a Wheatstone bridge.^[1]

The craftsmanship and build quality of this device is truly remarkable.  I cannot fathom how much money it would cost to build this same device today using identical components.  The unit consists of a large magnet, floating mirror, bulb, magnifier, panel scale, brass and silver switches with no measurable resistance, and a large number of precision resistors that were hand-wound and perfectly balanced using the finest balance bridge equipment available at that time. Truly the highest laboratory-grade accuracy that was possible — BEST of best.  Resistance accuracy should approach 0.1%.  For this reason, the unit makes a fabulous (true) precision resistance box for checking calibration.  This gives it a modern “use” instead of sitting on a shelf as a discussion piece.

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This article discusses repair and calibration of the B&K 600 / 606  tube testers.  Both models can share the same tube setup data book, and therefore both models are functionally equivalent.    Model 606 will be example shown in this article.  Electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted 2010 TubeSound, all rights reserved.

Cosmetically, these models look like a little brother to their larger siblings — B&K 700 and B&K 707. Model 600 shares the same color scheme with Model 700, and model 606 shares the same color scheme as model 707. The wood case construction is similar also.

Electrically, models 600 and 606 are very similar (though not identical) to the “Switch” section (bottom panel) of the 700 / 707, employing the same grid leakage test and very similar tests for Emission Quality and Shorts.

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Sprague Tel-Ohmike TO-3

This article discusses repair and calibration of the Sprague model TO-3 Tel-Ohmike Resistor-Condenser Analyzer.    Serious electrical voltages are present, repairs should only be attempted by a qualified technician. Copyrighted by Bob Putnak, all rights reserved.

Introduction to model TO-3

Model TO-3 Tel-Ohmike “Capacitor Tester” is based on the older Sprague design.  It is a balanced-bridge design with the following specifications:

• Capacitance testing from 10 pf to 2000 uf, spread across 4 ranges
• Resistance testing from 2.5 ohms to 25 meg, spread across 3 ranges
• Capacitor Insulation Resistance test
• Capacitor leakage test using a true load from 0 to approx 660 V (factory design). Note that a good argument can be made to drop the overall polarizing voltage down to 500 vdc (as discussed below).
• Meter reads in DC Volts (up to 750), DC milliamps (up to 75 ma), or Megohms (up to 2.5 gigohms).

Repair and Calibration of Model TO-3

Sprague TO-3 precision cap set - $43 free ship USA

NEED PRECISION REFERENCE CAPACITORS? — For those of you doing your own Tel-Ohmike repair, I sell a set of 5 precision reference capacitors for $43 free ship USA/CAN.   First, you get 2 precision reference caps (1 of 2.0 µf, 1 of 0.02 µf.)  Your original 200-pf cap is left installed.  Next, you get a complete set of 3 precision caps (1 of  2.0 µf, 1 of 0.02 µf, and 1 of 200-pf) for use in testing and calibrating your machine.  All 5 caps guaranteed accuracy 1% or better.

Sprague TO-3 remastered manual & updated schematic - $8 free ship USA

Need a remastered manual and updated schematic for Sprague TO-3 Telohmike? $8 free ship USA/CAN.

The original schematic for TO-3 has several mistakes and ambiguities.  Therefore, I have revised the schematic, corrected errors that I have found, and added notes to make the schematic more user-friendly.  The schematic is available as part of my TO-3 paperwork package. To elaborate: I corrected significant drawing mistakes at switch 2F, added labels to define the functionality of range-standard resistors, added the trimmer pots to the bridge pot, added clarification to the installed values of several resistors, added a ‘Notes’ section for better overall understanding, and digitally cleaned-up the schematic with photoshop for excellent readability.

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