The 2009 Radio Workshop
The 2009 Radio Workshop was held February 8th at the Pavek Museum of Broadcasting as an official event of the Northland Antique Radio Club. Originally, we were going to do a presentation using downloaded freeware that would turn our computer into various test instruments. Instead, we wound up using an interface called a PicoScope model 3224 USB oscilloscope for the presentation. Their URL is www.picotech.com
The PicoScope gets its power from, and sends its signal through the USB port on a PC.
The device turns the computer into an oscilloscope, distortion analyzer, frequency counter, spectrum analyzer, and voltmeter. System requirements include Pentium or newer with at least 32 MB of Ram and 10 MB of disc space.
The first test was made on a Hewlett Packard 200CD audio oscillator. The distortion level of the HP 200 CD never exceeded 0.08%.
Next, we looked at the audio portion of a rebuilt RCA model 143. Well-designed and versatile, the 8 tube 143 chassis was used in five different models in 1934 and 1935.
We connected the output of the H.P. 200CD audio oscillator through a .25 mfd capacitor to the top of the 143's volume control. All tests were made at full volume.
We had previously increased C52 from its original 200 pf to 0.0068 mfd. Since the "Fidelity Link" was only open on floor models, we left it shorted for this test. We disconnected the voice coil from the 3.2 ohm secondary of the output transformer and put in a D.P.D.T. switch that allowed us to switch back and forth between the voice coil and a 4 ohm non-inductive resistor.
As a precaution against damaging the PicoScope, we shunted the primary of an ADC #114A 600 ohm matching transformer across the 4 ohm load resistor and took all of our “scope” readings from the secondary of the transformer. A General Radio model 1800A RMS AC VTVM was connected across the 4 ohm resistor. We looked at the distortion as we increased the signal level to the radio. The analog G.R. followed the output voltage much smoother than the digital interface. It also helped illustrate to the audience the relationship between the RMS AC voltage and output power.
Normal listening level was reached at only 0.34 volts RMS, or 0.03 watt. Distortion at that level was 0.5% THD.
At 0.25 watt the distortion ranged from 1.5% to 3.5% depending on the setting of the tone control.
Already, the volume was getting louder than normal listening levels.
At 1 watt the distortion ranged from 3% to 10%, with full “Treble” position yielding the highest distortion.
At 2 watts it was so loud that we quit switching back and forth between the load and the speaker.
Distortion ranged from 7% to 17%.
At 3.5 watts the poor little output transformer on the 143 was totally saturated and wouldn't go any louder.
Distortion was around 35%.
At one point we added an 80 mfd capacitor to the power supply of the 143 and saw a 10 db drop in 120 cps hum, but no change in the distortion readings. 1
Then we looked at the output of a “stock-as-a-rock” Dyna ST-70.
At 1 watt the distortion was an admirable 0.1%.
At 30 watts, the distortion was 0.28%; not bad for an un-modified Dyna.
Terry Schwartz followed with a quick demo using shoe polish to clean up a Bakelite cabinet; it worked remarkably well.
Then, Matt Hyman finished up with a comparison between five modern table radios, including a Bose Wave Radio, a Cambridge Audio model 88, a Boston Acoustics “Solo,” a Tivoli, and the Emerson knock-off of the Tivoli. Almost everyone thought the Bose sounded the best, but the mono “Solo” was surprisingly clean, loud, and easy to listen to.
It was a good show.
Everything worked the way it was supposed to.
Steve Raymer
1A few weeks after this demo, we took another look at the audio section of the 143 and discovered that there was 50% more signal on one half of the secondary of the driver transformer than on the other. Obviously, this was the cause of the distortion. We checked the DC resistance of the secondary and both halves were nearly the same. We replaced the output tubes and that didn't change a thing. We have yet to replace the driver transformer. |
