Test Equipment Restorations

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This project has been shelved many a year so after 10 I thought it was about time to blow the dust off and “throw” it up on the bench.  I also wanted to add a piece of equipment to the bench which would allow me to do stage gain measurements.  If you are viewing this magnificent piece of post WWII test-o-mania test gear, you already know what it is and undoubtedly, how to use it or at least have a reasonable idea particularly if like me you do a great deal of antique radio restoration.  Therefore, I am not going to delve into it’s history, specifications or functions. The manual is available at http://bama.edebris.com/manuals/hickok/156/, and there is much to be researched about the 156 on-line.  Mine has an inspection date of 12/31/46 and was probably one of the last 156’s to come out of the Hickok factory on DuPont in Cleveland.  The 156A had just become available.  I don’t have it, yet, but recently purchased the 155 which preceded the 156 and was available around 1940.

Instead, I am going to endeavor to take you step-by-step through the restoration of this “beast” and hopefully keep you from making a couple of mistakes that I made throughout the 84 plus hours I have in the 156 restoration. I may digress a bit but bare with me!


Your first “tools” NEED to be a legal pad and a camera.  Make lots of notes from the time you remove the chassis until you make your last calibration measurment of the instrument.  You will also be making notes when you place your newly restored Hickok into service troubleshooting a vintage radio. I snapped a plethora of detailed pics, a sampling you see here.  A switch with half a dozen components connected to it looks simple until you remove two and forget which goes where.  The first switch I removed was the Audio Input switch with at least a half dozen components attached.  In a bit of a hurry and relying on what I believe to be a fairly good dementia-free memory (so far), removed three resisters, and oh boy, where the heck do the replacements go! I had failed to take a detailed pic of the switch so wasted an hour tracing circuits. It’s kind of like boating and not taking note of landmarks.  You might have a frustrating journey back where you started from.  Have some quart size storage bags available, too, for storing parts as you remove them from each section this includes switches, tubes and hardware.  BTW, folder all of your pic files (as you create them) and send to Dropbox.


Now you’re ready to begin, and more than likely the old non-polarized line cord is in place (or “frayed” itself to death and is no longer a subject of this restoration) so slice it off (or it’s remains) at the cabinet back.  You will replace it with a nice heavy guage three-conductor power cord at the completion of the restoration.  With the cord cut and the cabinet on it’s back, remove the 10 cap nuts, gently lift on one side with a small flat blade screwdriver placing fingers with your free hand under the lifted edge.  Lifting enough, you should now be able to place fingers (or fingernails) from your other hand under the other side or somewhere on the tester front to get a firm grip lifing the chassis straight from the cabinet.  Stash the cabinet.  You won’t be seeing it for awhile.

With the chassis on the bench, prop with small blocks the rear of the chassis so it’s sitting level (you will do this each time you are doing any work with the chassis tube-side up).  Sitting at an angle (as it likes to do), will place weight on the delicate wafers that you will note are on the back of the respective switches found on each side.  Now take a brush and remove all excess dust.  A light air blow will remove accummulated dust from the RF/IF Low Frequency variable condenser and High Frequency Oscillator variable condenser. You may have rust, oxidation and general corrossive “crapola” to remove, too.  Sorry, can’t help with that.  I was fortunate that my 156 had been well-cared for since new with only a light accummulation of dust to remove. Even the cabinet was scratch-free as was the front panel. However, I have read “horror”stories!

Remove all tubes and check each.  Discard any that are below spec. My tester of choice is the Hickok 538A which lists the spec on the roll chart.  I stickered each tube with a small tag which gave the reading vs. the tube spec.  All but one, the 6J5 (DC Voltmeter circuit), were above spec.  When you pulled the chassis from the cabinet, perhaps a few old batteries were left behind.  These are “bucking” batteries which are used in the meter circuit allowing the meter to ZERO as you rotate the controls on the front panel.  Most of the time they are badly corroded leaving corrosion in the plastic tube mounted under the chassis.  They can leave a wicked nasty mess that has to be dealt with particularly if it has spread throughout the chassis.  Again, I count my blessings that I didn’t have to deal with that!  If the battery tube is badly corroded and/or NEGATIVE contact, you might as well deal with it now.  Clean the contact. Carefully turn the chassis over, prop one side with one of the “prop blocks” which will keep the chassis from leaning to one side giving you a firm surface to work on and remove the plastic tube.  When you have nearly completed your restoration, replace with with a comparably sized piece of PVC, or, maybe you can restore the original tube.  Leave it out for now as it will be in the way when you remove and replace the RF coil assembly (hey, that’s where the fun begins!), and you WILL do it!


At this point I would like to stress and not enough to TAKE YOUR TIME.  Frustration can begin to build so take a break, watch a movie, toss the frisbee with the dog or jam to some tunes (don’t complicate things with alcohol consumption…until you’ve called it a day…or night!).  BTW, the 84 or so hours it took me to complete my project did NOT include walk-away time.  This is labor intensive and concentration provoking WORK, but we will have fun!  No words can express the feeling of accomplishement you will have when your efforts are rewarded with something that WORKS correctly right off the bench, and you see those meters come alive with normal readings.

There’s the old saying, “there’s a right way, a wrong way and MY way”.  I won’t be so bold as to say that my way IS the right way, but my method worked.  Feel free to deviate, but if you stick somewhat closely to my outline, you should have a hassel-free restoration.


With the schemataic in front of you, remove the AUDIO INPUT switch and INPUT jack.  Snap that close-up pic detailing components and connections.  Replace the .05 cap with a .047.  I did not use any precision part replacements  throughout this restoration.  I did adhere as closely as possible to original parts specs replacing any resisters which were more than 20% out of tolerance.  In some instances, I did have to combine resisters to make the correct value.  Digressing a bit here, Hickok DID make engineering changes during the build of the 156 so you may see different value components in your rig which do not match with those on the schematic. DO NOT  deviate from the values of those components used in your TRACEOMETER.  As an example, in the RF section of my 156, the 6SK7 screen loading resisters are 100K.  My schematic is showing 82K.  Hickok engineers knew more than you and I combined (seemingly) so those changes were made for a reason.  With that said, check the values of the 7 resisters which comprise the switch replacing any which are more than 20% out of tolerance (I know, I repeat myself sometimes…for a reason!).  Once you have rebuilt the control, switch through it a few times being certain that A, the control is making contact at each point and B, that your resister values are correct for each contact point. C40 is part of the MONITOR circuit so replace it at this time.  Satisfied that you have correctly rebuilt the AUDIO INPUT control, “bag” it with the INPUT jack and related hardware.  Mark the bag accordingly.  CONGRATULATIONS, you have made your FIRST of many individual rebuilds in your 156 restoration so we are ready to progress.

Aside note, I clean all contact controls and the ZERO control pots with 3M NOVEC (about 40 bucks for a 12oz can by the time you pay for the product, plus shipping PLUS HAZ MAT charge). A little goes a long way. It is a cleaner/lubricant and available from DigiKey.  I did NOT clean and lubricate pots used for calibration which were done at the Hickok.  After 70 years, each may be slightly off, but I preferred to discover how far off once I had the 156 up and running.  As it was, as close as I could tell, those pots were fine.

Now, move to the DC VOLTMETER INPUT control and repeat the steps above.  Similarly, do exactly the same with the OSCILLATOR INPUT and RF INPUT switches.  I did NOT remove and check the micas on the switch(es) nor should you.  Leave ’em ALONE! This holds true for ALL micas in your rig.


The time has arrived to begin the rebuilding of the under carriage of the chassis.  I began at the POWER SUPPLY by checking the secondary of the power transformer.  With the 5Y3 pulled, connect the PRIMARY to a variable AC source.  Connect the SECONDARY to your AC VOM (VTVM) and gradually bring up the voltage.  Dementia has kicked in here as I don’t recall the reading across the secondary precisely, but somewhere in the neighborhood of 450 VAC PLUS seems about right…guess I should have noted it on that legal pad as you are doing….RIGHT?  Do the same with the 5 VAC and 6.3 VAC secondaries. These will be slightly higher than the spec noted.  If seemingly you have abnormally low voltages on either of these three measurement points, it is time to stop and begin searching for another power transformer.

Assuming all went well with your power transformer check, measure the choke (L13).  It should not read ZERO or INFINITE.  Assuming that that is OK, begin replacing the filters, ALL “waxies” (non-polarized capacitors) and check all resisters, replacing those which are more than 20% out of tolerance.  If you get seemingly “odd” value readings on resisters, there are a few that I had to disconnect one end and take a reading.  As I recall, it was necessary to disconnect one end of each of the 2200 ohm individual meter loading resisters.  Speaking of meters, there should not be any maintenance you need to perform with these.  DO NOT attempt to take a resistance reading or place any voltage directly across the terminals.  If one or more fail to display a reading at the completion of your project, you will have to go back and troubleshoot the circuit, the probe assembly OR find a replacement via some helpful soul (there are one or two) on ARF (Antique Radio Forum).

Hopefully by now you have compiled a stack of 30 PLUS photos of your work and used up an entire legal pad of notes.   NO, well good luck!  BTW, it is a pain, but when you replace any circuit component, go back and check for continuity in its respective circuit.  You may have a cold joint, forgotten to solder or God be with you, made the wrong connection.  It bares repeating, TAKE YOUR TIME, TAKE A BREAK and STOP WHEN YOU ARE TIRED! We are about to move to the RF sub assembly so if you haven’t been diligent about it up to now, start taking those detailed pics and comprehensive notes.  Once you are done with this challenge, you DON’T repeat DON’T want to re-do ANY part of this rebuild.  Not to discourage you, but this is a rebuild unto itself and like trying to climb out of a pit of rattlesnakes if you get into trouble. If you make a mistake, you will probably have to go through the entire process again.  Also and hopefully, you have been familiarizing yourself with the manual as you progress with your restoration.  If not, begin to do so NOW.  Understanding it’s operation and applications will be of help to you when you fire up the 156 for the first time.  I have had mine up and running a couple of weeks and yet don’t fully understand all that it is able to do.  Hickok was “long” on “user” manual detail but “short” on specifications, particularly voltage readings and trimmer adjustments as you have probably noted.  The management at Hickok was interested in profit wanting the owner’s “coin of the realm” thus preferring the non-working instrument be sent to Cleveland for “technical support” as opposed to “do-it-yourself” technology.


Be well-rested AND sober before proceeding!

Remove the bolts, washers and nuts which attach the metal cover to the chassis and place in one of those bags which you will mark “SUB ASSEMBLY”.  When you were rebuilding the under carriage of the chassis, you should have checked out the oscillator coil for continuity.  (Sorry I didn’t mention it earlier.  If any of the coils are open, you will have to search for another assembly).  I mention this because before you go to the work of re-moving the RF sub, check ALL the RF coils for continuity.  There are 9 in total.  These are well-constructed unlike those found in many old radios so unless some idiot has put HVDC to one, all should be good.  If one is defective, you will still have to remove the sub in order to remove the coil and replace it or have it re-wound.  Simply take an ohm meter and measure point-to-point across each coil.  Like a “hand-wringing old woman” ever cautious, I probably re-checked each coil as many times as there are coils during the sub restoration.

This is where my “guideing” hand needs to be watched closely. Take as many detailed pics as possible probing deeply as possible  into the bowels of the sub.  You will see input and output connections which are the interconnects between the 3 RF stages and IF (6SQ7) stage.  These interconnects MUST be removed from their respective connections in the sub or “she” ain’t about to come out.  Directly on top of the sub are the three (C31-C32) 6SK7 SCREEN to CATHODE bypass caps.  Mine were marked “SOLAR” (generally a cap with longevity) and I clipped them clean thinking they were leaky (my second mistake in the project).  They weren’t.  All three tested as NEW.  Therefore, carefully remove each and test it.  If one or all are good, you will re-connect at the completion of the installation of the sub assembly. They must be removed before you can detach one wire in the sub.  It is nearly impossible to get a soldering iron down deep without smoking a coil and an impossiblity to remove the closely crimped ends of the respective wires from their respective connections on the band switch. C7 is not possible to remove without a wire cutter.  (This is the only mica I replaced in the entire rebuild).  The only solution is to use a sharp and long side cutter, cutting directly at the end of the wire where it connects to the band switch in order to remove it.  Once you feel that you have successfully removed all wires AND CAREFULLY NOTED WHERE EACH GOES, remove the sheet metal screws which hold the sub in place and bag with the cover hold-downs.  You may find one connection from the L13 choke will have to be removed.  Note its routing.  I wasted 20 minutes (relied on brain not a pic) trying to figure the correct route in order to re-attach the metal shield.  Don’t e-longate it!  It will fit in the proper place.

Hopefully your labotamy of the sub was successful and you are now ready to remove the “waxies” as well as check the circuit components as you did when you overhauled the undercarriage.  One of my pics displays the RF section before and another shows how the section looked after the overhaul.  Study closely.  Begin with V1 and progress in reverse toward the 6SQ7 (V4) with your sub rebuild.  One restorer on-line got the hee-bee-gee-bees and didn’t remove the sub assembly leaving 7 capacitors in place.  I believe the 156 sports 13 “waxies” not including the Solars already noted above and 2 tested OK.  Only one good cap was found in the RF/IF section.  It is absolutely imperative when doing a rebuild of vintage electronics, particularly vintage test gear, that ALL “waxies” be “shucked”.  Before removing one of the old “waxy” caps, make an outline of it (like a chalk mark of a deceased body). If you don’t place the replacement in the EXACT place as the original, you will not be able to place the sub assembly correctly, and you will have mess!  Replace the connecting wires that you nipped with new and very slightly longer replacement wires of the same color for uniformity and originality. Strip the ends ahead of time.  I realize that this may seem very elementary, but hey, we do sometimes overlook the obvious, and this is no area for time consuming needless mistakes.  A new C7 should also be installed at this time.  This will take a little figuring on your part in order to determine the best way to dress the leads for proper fit.  When  you have completed a proper rebuild of the RF/IF section, it should look like the one pictured….NO “waxy” in sight.


Confident that you have made a complete rebuild of the RF/IF section, you are now ready to drop the sub in place and make the connections.  I was unable to use my needle-point HAKKO 888D to re-attach the connections to the sub band switch. Its barrel was too close in proximity to the coils which by now I had checked several times.  I relied on the ancient Weller pistol grip with smaller tip.  It just fit, and I was able to complete a successful installation.  New bypass caps were installed to replace the three .05 Solars which in a stupid move I had sliced out of the picture.  .047’s worked just fine. Now, very carefully attach the sub.  DON’T force anything.  You may have to re-dress a connection and/or ease a cap out of the way upon which the sub frame wants to rusticate.

With the sub in place, carefully guide the metal shield over its top and note that all wires which snake along the chassis and over to the sub are dressed properly such that you aren’t bolting the shield down on top of any.  It ain’t as easy as it looks!  Replace and respectively connect the switches and jacks that you have hopefully individually bagged.  By now all switches and meter ZEROing control pots should have been sprayed with NOVEC.


You aren’t done it!  Replace the battery tube with the negative end of the batteries feeding into the tube first.  The POSITIVE end of the last cell needs to be in contact with the cabinet (POSITIVE ground).  I bridged a piece of brass between the positive end of the battery and cabinet so I could calibrate the TRACEOMETER.  In fact I left the piece of brass in place as the positive end of “D” cell would not make contact with the cabinet.  Apparently the original cells were longer hearing aid batteries or so I have read. Snip a 3-conductor power cord off an old computer you have laying around and attach.  Make sure it’s end will go through the opening in the back of the 156.  Otherwise you will be wasting time removing and replacing the male end which will be a nuisance if you complete your rebuild, put the 156 into service and have a problem having to pull the chassis from the cabinet.  However, you won’t have any problem…..RIGHT!


If you are lucky, the original probes were supplied with your 156.  You will still have to measure for continuity and correct values.  The AUDIO input probe is simply a straight in-line probe.  The High Freq Oscillator probe uses a 1.25uuf capacitor.  The Low Freq/IF probe uses an in-line .85uff cap, and the DC VTVM probe uses an inline 1 meg resister.  It has been suggested for greater sensitivity a 3 meg can be subbed.  The 1 meg input works for me.  If you do not have the original probes, you will have to construct a set.  The values of the caps mentioned must be within 20%.  My probes had the original caps missing.  Fortunately, I was able to construct the needed values.  Instead of placing them in the probe cable or in the connector. I placed the caps between the input jacks and respective inputs to the respective switches.  This seemed to eliminate the capacitance generated by the probe cable, and it worked well.  Burke discusses this technique in his book c. 1950 “Rapid Radio Repair”. This is a great read dowloadable in PDF in which he discusses techniques he used with his “new” Hickok 155 plus his adventures in the early days of radio through Post WWII radio servicing as a shop tech and running his own shop.


The moment of truth is here.  Admitedly, I was skiddish about this being the cautious person I am and made every excuse I could think of to delay putting this beast to the test.  I was confident that my work was correct and complete, but …..ummmmm. Well, I gave it the acid test, and you do this, bring the power up slowly.  Just before the voltage rectifier (V11) fires, the four top meters will peg. DON’T be alarmed, pull the plug and start ripping things apart!!!!!!!!!!!.  Once the VR fires (what a beautiful purpleish hazey glow) the meters will return to normal.  If one or two don’t (my RF voltmeter occasionally sticks) give it a gentle tap, and the needle will drop to normal.  The damping effect of the needles are cool to watch as each return it’s normal resting place.  Try “zeroing” each meter.  If all is well, proceed to calibration.


Unlike one thoughtless soul who posted about his 156 restoration and didn’t disclose the correct CAL. points, I have done this for us and noted them on the sub assembly shield cover as well tagged the High Freq oscillator shield located chassis topside.  One of the 9 trimmers (dementia here…can’t remember which) on the sub is not assessible with any of the many alignment tools I have so it was ignored.  Basically, both sections are aligned as you would any radio using a signal generator.

High FREQUENCY OSC was calibrated with the 25-D signal generator connected directly (low end to chassis) to the HF Osc probe at points 1000 Khz, 3000 Khz and 10 Mhz peaking the OSC/HF voltmeter.  Calibrate adjusting the individual trimmer for each respective band.

Low Frequency/IF was calibrated with the 25-D connected directly to the Low Freq. probe (low end to chassis) at points 1000 Khz, 600 Khz and 180 Khz peaking the RF/Low Freq IF voltmeter. Begin at V1 and progress through V3 similarly calibrating each of the three bands.

Given that this is a 70 year old piece of gear, original readings were extremely spot on.


Now, go to work and later raise a glass of the good grape toasting yourself for a job well done.  Most of all HAVE FUN! Oh, forgot to mention, pull the cabinet out of hibernation and replace the chassis.