Aris Silzars Display Technology Consulting



The Modern Day Vacuum Tube?…

Do you still remember television sets with vacuum tubes?  Of course the CRT is a large vacuum tube and is only now disappearing from the scene.  But that is not the kind of vacuum tube that I have in mind at the moment.  I’m thinking of the ones that were replaced by transistors and then integrated circuits. 

As television became popular in the 1950’s – with most programs in “black and white” -- and then into the 60’s and 70’s with color sets taking over, the typical console had at least a dozen vacuum tubes.  These tubes had standardized numbers such as 12AX7, 6AU6A that served the various functions of demodulating the transmitted signal, amplification, horizontal and vertical scan generation, and so forth.   These tubes, as thermionic devices, had a limited life of perhaps a few thousand hours.  The typical failure mode in a television set was a “burned out” tube.  This might be as obvious as a failed filament or it could be that the cathode emission had dropped and the tube could no longer provide the desired output (transconductance). 

For most consumers the electronics inside a television set were pretty much a mystery.  When it quit working properly, a service technician would come to the home, diagnose the problem, and in almost all cases replace a tube to get the set back into operation.   A few braver or more knowledgeable souls would pull suspected tubes and go to the local electronics store and use a “tube tester” to see if a faulty tube could be found.  These tube testers would check for continuity of the filament and for cathode emission.  The cathode emission was typically shown on an analog meter with the proper settings entered based on the tube type. 

Life was simple and predictable.  Television sets were purchased with the expectation that they would last for at least 10 years and typically much longer.  Even with relatively high failure rates, the use of standardized and easily replaceable components made this long service life possible.

Then in the late 50’s something came along that gave birth to a different world.  While I was still in grade school, one day a student brought in what appeared to be an impossibly small radio.  He called it a “transistor radio” and with an earphone he could get reception of the local radio stations.  The radio worked from a small battery without having to be plugged into a wall socket!  Wow, this was dramatically different than the briefcase size portable radios that used miniaturized vacuum tubes.  Soon I was reading about this marvelous new technology in Popular Science magazine and ordering my own rudimentary transistor radio experimenter’s kit.  

So here we are today with television sets, appliances, cars, tools, telephones, computers, e-readers and every other conceivable gadget – all containing complex electronics that provide wonderful “features” whether we want them or not.  Given all they do, the reliability is really quite good.  But certainly not perfect.  Important devices such as cars and appliances do occasionally have a failure in one or more of these electronic modules. 

When that happens how do we repair them?  Do we replace the defective components?   Unfortunately, no longer.  With all the high pin-count surface-mount packages that are soldered onto the boards, field replacement is nearly impossible – and in most cases the custom chips are not available.  So we have had to resort to replacing entire boards – i.e. “board swapping”.

Given the manufacturing cost of most of these boards, perhaps that is not such a bad alternative.  After all, vacuum tubes were not cheap in the 50’s and 60’s.   A $50 circuit board is not all that different in today’s dollars from a $5 vacuum tube in the late 50’s.   Unfortunately, we cannot go to a local electronics shop and buy most of these circuit boards as we were able to do with vacuum tubes.  Each board is a specially designed part and only available from the manufacturer’s designated supplier(s).  And this availability may be for a limited time -- based on the good will of the manufacturer.  There are seldom second sources for these boards.

The limited and controlled sourcing can lead to behaviors that are not always in the best interest of the consumer.  Recently, I had such an experience.  Our gas-fired furnace quit working and after doing some preliminary diagnostics on my own, it became clear that there was a problem that would require replacing a component.  I just didn’t know which one.  Beyond the normal burner components, this furnace has a logic circuit board that controls the starting sequence.  The furnace repairman, who had been sent out in response to my urgent request, concluded that it was this control board that needed replacing. 

Now, this control logic board consists of one large IC DIP package, a few small relays, a 24-volt power supply, and a few multi-pin plastic connector sockets.  The manufacturing cost of this board is most likely less than $10.  So I figured that even with generous mark-ups along the way, I would be told that a replacement board would be roughly $100.  But what I heard instead from this technician was  “Unfortunately these circuit boards are really expensive.  The price for this one is going to be $525.”  So here we were on a cool fall day without a working furnace being given a price that is clearly beyond anything even remotely reasonable.   What to do?   These folks were obviously trying to take advantage of a difficult situation. 

Now, for the good news!  The technician didn’t have a replacement board on his truck and said he would have to order one.  That meant being without heat for one more day, but it also gave me some time to investigate further.  In short order, I was able to find a replacement board on the Internet for $135.  It gave me great satisfaction to be able to call up this repair shop and tell them what I thought of their prices and that I will never again be doing business with them.  Realistically, I don’t think they cared.  I’m in that tiny engineering minority of customers who have some idea of what this part should actually cost.  Virtually all of their other customers will most likely just swallow hard and pay the exorbitant price.   Why not make an additional $400 in pure profit if the customer doesn’t know any better?  Unfortunately, without standardized components like we had in the days of vacuum tubes, it is all too easy to encounter these situations where the price is set based on greed and circumstances rather than on actual product value.    

The really interesting epilogue to this story is that the furnace didn’t even need a new logic board.  Another – and honest – repair technician quickly diagnosed the problem as a failed igniter and for many fewer dollars my life is once again back to normal. 

It sure is pleasant to be writing this column from a warm home instead of one that is kind-of-warm from a couple of gas-fed fireplaces.  If you have had some similarly interesting experiences that you would like to share, you may contact me directly from this site, by e-mail at, or by telephone at 425-898-9117.