Making:  The Electronic Detective version of the story was fun to write, but it didn’t get deeply into the  making/fixing part of the linear amplifier project so this morning I wanted to run through the process of restoring an old piece of electronics.

Process, you see, is a HUGE factor in whether a person is successful at anything.  And, just as we have reduced the complexity to building a home to just four basis processes (measure, cut, join, finish) there is a similar series of steps that can be applied to “all things making.”

Those steps are:  Assess, hypothesize, troubleshootize, repair, re-assemble, and test.

(Continues below)



When you use a piece of gear for the first time, you put it through all of its paces and figure out what isn’t working right.  So for the amplifier, here were my observations before putting the unit on the bench:

  1.  Switching was all working normally.
  2.  An odd arcing was sometimes seen, but there was no component smell.
  3.  The amp would not make full power.  Max power out was about 300-350 watts with very little screen current and plate current was only hitting 250 ma. maximum.
  4.  Observationally, the bias voltage regulator tube was not igniting under any of the Mode settings.

There are (sadly) people in the ham radio world who will take their “golden screwdriver” to a perfectly good piece of equipment and just start twisting things (almost like an ape, randomly trying this and that hoping a fault will cure itself).  Surprise:   That won’t!  BUT all that “magic screwdriver” will almost certainly break other things!


I knew that the amplifier SHOULD be making full power output which I would expect to be in the range of 500-800 watts.    I looked at the schematic from the output of the rectifier where the jagged DC is cleaned up:

Starting at the Upper right (-BIAS) I knew this would likely be where my issues were.  I circled the actual faulty component which we will come back to.

The point is that I had hypothesized ahead of time which section of the amplifier was giving me problems before getting the cover off the unit.


Sure enough, measuring things out with an ohm meter, and following the manual’s “typical resistance readings” I measured from pin 2 of J102 to ground.  By the book, this should be either  4,000 ohm or 9600 ohms, depending on mode.

The 4000 ohm resistor had failed open.  So I had my problem solved…or nearly so.

The best news about that resistor being open was that it solved all of my problems in one fell-swoop.  That’s because:

  • When the resistor opened, bias voltage INCREASED.  And, as we all know from Tube Theory 101, too MUCH bias reduces an amplifiers output.  Bingo!
  • Further, we then knew why the 90 Volt regulator tube was no longer working:  When the voltage went up, it went past its design specs and that was the end of the regulator tube.

Since I had figured this might be the case in Hypothesizing, I’d taken the preemptive step of ordering both a new rectifier and a new regulator tube on eBay this week.  Both came to like $11-bucks.


This is where having large hands is not a good thing.

The replaced part is that white boxy-thing (a 10 watt 4.7K ohm [k is 1,000 so 4.7K is 4,700 ohms) resistor plus a couple of other resistors from the junk box).

There’s a good story to the resistor:  All I had in my junk box was the 4.7K which was too big.  And, I didn’t have a large second resistor that would work.  So hitting one of the Online Calculators, I did the substitution using 4.7 K plug (2) 56K ohms in parallel.

This is where the two most important formulas in troubleshooting are useful.

I knew that the likely output voltage of the unloaded power supply might be as much as 150 volts.  Using  Volts divided by resistance to get current (E/R = I) we solved current as 0.03191 amps, or about 32 milliamps.

Using this current, we could then use the voltage times current to tell us how much Power would need to be dissipated.  This is remembered as P=IE.  Power equals (I is current) time (E is voltage).

With the voltage across our resistor network being 150 (we’re guessing), and current of 0.03191, this means about 4.8 watts.  With the 4.7 K being well over, we were safe.  But, what about those two 56K resistors?

150 volts divided  by 56K (56,000) gives us current of 0.00267 amps.  And take that times our 150 volts 0.408 watts.  We’re in luck!  The 56K resistors are 1/2 metal film types!

Repair 2:

The other problem that remained on our list was the mysterious arcing.  You may recall, one of our readers said it might have been a cat hair…

It turned out, however, to be a loose circuit board on the top side of the chassis.  You see, the fan is mounted to this board and as you can see in the picture what happened.

When the board slid to the right, only ONE of the blades on the fan got close enough to arc…which it did in fine fashion.  All the while, though, I was looking to replace the cracked-insulator on the plate RF choke, or tear out the door-knob type high voltage capacitor  (the brown thing left).

Since the arcing was only happening when the board heated…well, it was one of the stranger problems I’ve seen.

Repair of this problem was not fun.  It involved taking out the entire filament transformer to get at the bolts, long-ago covered-up.

Reassemble and Test

With all the faults identified, it was almost heartbreaking to wind up the project.  Repair of tube type radio equipment is incredibly satisfying to me.  Tell me if this looks like something built in 1964 when I got done with it:

Almost hated to plug it in and have it just work.  Oh well, another day, another Making.

When I did plug it in?  My oh my!  Key down the amplifier will put out 750 watts to the antenna.  More cool than the raw power is that the amplifier also gives a reassuring thumping sound (as the big power transformer sucks down the line voltage) when keyed.

Yep…that’s what we love in ham radio:  Something that will dim the lights when keyed.

Totally tactile world-changer for those brave and bright enough to use Morse…the original digital mode.

Write when you get rich,  (de ac7x)

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Numerically Modeling the Future