928K

Philco, Philadelphia Stg. Batt. Co.; USA

Data
Forum
Pictures
Collections

  • Year
  • 1938
  • Category
  • Car Radio, perhaps also + sound player/recorder
  • Radiomuseum.org ID
  • 51240

Click on the schematic thumbnail to request the schematic as a free document.

 Technical Specifications

  • Number of Tubes
  • 7
  • Main principle
  • Superhet with RF-stage; ZF/IF 260 kHz
  • Wave bands
  • Broadcast only (MW).
  • Power type and voltage
  • Storage Battery for all (e.g. for car radios and amateur radios)
  • Loudspeaker
  • Electro Magnetic Dynamic LS (moving-coil with field excitation coil)
  • from Radiomuseum.org
  • Model: 928K - Philco, Philadelphia Stg. Batt
  • Notes
  • This model is equipped with a vibrator for the generation of B+.
  • External source of data
  • Ernst Erb
  • Circuit diagram reference
  • Rider's Perpetual, Volume 9 = 1938 and before

 Collections | Museums | Literature

 Forum

Forum contributions about this model: Philco, Philadelphia: 928K

Threads: 1 | Posts: 1

Fellow Radiophiles,

In a recent email exchange with Ed Lyon (editor of Radio Age) about the application of negative feedback (NFB) from the speaker output to the cathode of a detector-diode/triode in an AM radio, Ed recalled his experience with this type of circuit in a PHILCO 928k from his repair notes.

The email exchange was, in part, related to some experimentation I reported here with this type of feedback a couple of years ago. For an explanation of how the same feedback path is negative for audio, but positive for detection, refer back to Detection and AGC in a 5 tube AC/DC radio.

Two email messages from Ed:

February 27th 2010
Joe:
Interesting analysis of the detector-plus-audio circuit that you noted about a week ago.  I had been busy with snow and the weather, and had a good chance to look over the circuit only recently.  It rang a bell.  A long time ago, when I was in college, a classmate had a car, I believe it was a Nash, that had a Philco radio in it.  He needed it serviced, and I worked on it. My old notes say it was a Model 928-K.  It had a negative feedback circuit around the audio stages, from voice coil back to detector cathode, just like your example.  This was a 1938 model.  I just looked up the schematic in Rider's, and sure enough, it has a tone control that adjusts the frequency response of the feedback circuit, and it does go the way you said.  This radio's symptoms were that it seemed to blast on nearby stations, so I figured it had a shorted AGC bypass cap, and that was the problem.  When I replaced tha AGV bypass caps, and a couple others that seemed leaky, I found that the radio already had some AGC action with the AGC bus shorted to ground, but not with a CW signal generator signal.  It only had AGC action when the generator was audio modulated.  It was that positive feedback loop you pointed out that improved the detector gain at low levels, but only when there was an audio signal feeding back to the diode cathode.  So the radio exhibited exaggerated gain for low level broadcast signals, further improved by reconnecting the AGC loop.
 
Ed
---------------------------
March 4th 2010
...on re-reading what I wrote, you might add to my article that I used to check radios for alignment and AGC function by placing a fixed negative 1.5 volts to the AGC bus, using a flashlight battery with clip-leads on it, leading to the AGC and B-.  My notes stating that the gain for weak signals was further improved by re-connecting the AGC meant that when reconnected, the -1.5V bias was first removed.
 
Ed
 
Ed Lyon refers to the following schematic. I highlighted the feedback path in blue.
 

Another noteworthy aspect of this radio is the Push-Pull output stage that requires no additional phase inverter. The inverted signal for the lower tube is collected from the screen of the upper tube. Prof Rudolph has posted an overview of Phase-Inverters in English and Phasenumkehrstufen in German.

Regards,

-Joe

Joe Sousa, 05.Mar.10

Weitere Posts (1) zu diesem Thema.