CPR

AS OF APR 12th 2026...

THIS PAGE IS UNDER CONSTRUCTION UNTIL THIS MESSAGE GOES AWAY.

DON'T TRUST ANYTHING YOU READ HERE.

ALSO NEED TO REMOVE THINGS SPECIFIC TO RADIO #5 DEBUG AND MAKE THE TEXT MORE GENERAL IN NATURE.

The intent of this page is to suggest things you can do when you have a radio that's supposed to work but doesn't.

Prerequisites

Make sure that:

  1. You've measured the resistance to ground for every pin on every tube and compared the results to what is contained on pages V1 through V15. It's a long, annoying check but if there's something stupid in play, there's a very good chance this will find it before power is applied.
  2. B+ is 270-305 volts.
  3. Screen is 90-125 volts.
  4. All the tubes get hot.
  5. The speakers are the only form of output the radio has. Make sure they work. Plug them into something else and see if they make noise. If the speaker impedance is 500 or 5,000, you'll need to kludge some sort of matching transformer. Failing that,
  6. You have a signal generator that will produce a 1 mv sine wave of 1,400 kilocycles1.
  7. You have a scope. Doesn't need to be fancy. Beware: those inexpensive scope modules designed to be used with a laptop or PC frequently have an absolute maximum voltage input limit of 35 volts or so. B+ is 280.
  8. You have a tube extender. See the one I built. Other designs will work as well.

Check audio output

There's a phone jack on the rear of the radio ("phono input") that allows you to pipe an external audio signal into the audio output and then to the speakers. An old cell phone with a headphone jack will work. The headphone out from a PC or laptop will work.

Check Standalone Circuits

The radio contains a couple of oscillators that should be producing some kind of signal no matter what the rest of the radio is doing. Checking these signals might uncover some other issue.

Step 1:

Put V3 on an extender and select the AM band. Connect the scope to pin 5. (The pic makes the intensity look brighter than it was. I turned it down even more though.) This is the signal from the cathode of V4, the local oscillator.


Tune the radio a bit and see the frequency change. Tuned to 1,400 kilocycles, the period is about .54 microseconds. Being lazy and sitting in front of a computer, I Googled "period to frequency calculator" and yes, there are several. Plugged in the period and got 1,852 kilocycles. Subtract the 455 kilocycle IF frequency and get 1,397 kilocycles which is close enough to 1,400 for gummit work2.

I put the probe on pin 8 thinking I would see something from the 2nd RF amp but got nothing except the local osc frequency.

I noticed noises that sounded like carriers as I tuned the radio but it turned out to be caused by an ungrounded probe. The selectivity switch had a great impact on this.

Trace the Antenna Signal

The procedure here is to follow the signal from the antenna input to the speakers.

This procedure will be done using Band One, good old AM, 550 - 1,600 kilocycles. The 1st two bands only use the 2nd RF amp so it gets us to where we need to be faster.

Step 1:

Put V2 on an extender and select the AM band. Connect the scope to pin 4. Connect the signal generator (0.01 volt sine) to the antenna inputs. Warning Be sure to set up the signal generator before connecting to the antenna terminals. That silly generator of mine powers on with 5 volts p-p output.

I was not able see anything significant except 40 millivolts of 60 cycle hum. Given that this is one of the tubes most sensitive to being on an extender, I wasn't surprised.

Step 2:

I replaced the 6SK7 with another from my box of tubes and got this:


Generator is producing 1,400 kilocycles at 90 millivolts to the antenna terminals. Scope is at 20 millivolts and .5 microseconds per division. A sound - mostly a hiss - is coming from the speakers. It goes away as I tune off the generator frequency.

Step 3:

I went back to V3 on the extender and didn't see anything different although I know it's there. I suspect my measurement technique isn't up to the grade needed3.

I removed the extender and hooked up an antenna. Nothing except what sounds like carriers here and there.

Step 4:

Decided to cycle through the tube box. V10, V6 & V7 to start. During this test, I had the antenna hooked up and the signal generator, still producing 1,400 kilocycles, clipped to the antenna wire insulation. Output voltage at 250 millivolts. It produces a carrier signal similar to the stuff from the antenna.

V10: Carriers seemed louder
V6: No significant change
V7: #1: Volume significantly reduced. #2: Better but maybe slightly less than the one I took out. Original back in, same as expected.
V9: No effect. None expected really.
V5: No effect.
V10 again: No effect.
V12: As expected, no effect on problem. Volume seems reduced. Replaced original and put the 2nd one in my 'marginal' bag.
V8: Slightly increased carrier sounds.
Was going to try V11 (BFO) but I turned on BFO and it worked as expected on the carriers. Not messing with it.

Footnotes:

  1. When I first got mixed up in electronics and radios, 'kilocycles' was the term. In 1960, it changed to Hertz. Heinrich Hertz is the guy who proved magnetic waves existed in the 1880s. As a tribute to old stuff everywhere, I plan to use kilocycles (etc.) on this page.
  2. I've long since lost track of when I first heard the "Good enough for government work" bit.
  3. Or it's me. For the last 40 years, I've been doing high speed digital circuits and operating system software. Tubes? Radios? Not so much.

Page: /CPR/CPR.shtml

Last modified: Monday, 1 Jun 2026