Forums

Strange LO Oscillator won't oscillate

Started by Bill Bowden June 4, 2015
It's a Autodyne converter in a small AM Radio. DC voltages look ok but 
doesn't oscillate. Originally, the problem was a weak transistor with a hFE 
of 4. I replaced the tranny with a 2N2222A and it worked fine for a couple 
weeks and then died. Now the DC voltages look ok with about 1.5 volts on the 
emitter. Usually, this type of oscillator has the coil tap connected to the 
emitter through a cap with no bypass to ground. But this one has the tap 
connected through a 100 ohm resistor in series with a 3.3nF cap and then 
through the secondary of the antenna loopstick to the base of the tranny. 
This radio worked pretty well with only one IF stage, so I'm wondering if 
this configuration provides additional signal gain from the converter 
section? I plan to play around with the tranny bias to see if it might 
oscillate with a little more or less current.
. .


         +V                     +V
          |                      |
          |                IF Stage winding
         /                       |
         \                       |
         / 150K            Feedback winding
         \                       |
          |                      |
          |                     /c
            Antenna winding-----b|
          |                     \e
         /                       }
         \                   ___ |
         / 100              |   /
         \                  |   \
          |                ---  / 2.2K
         --- 3.3NF         ---  \
         ---                |    |
          |                 |    |
          |                GND  GND
         Tap
    |- Primary winding-|
    |                  |
    |-------||----------
    |
    |
   GND




--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Bill Bowden wrote:
> > It's a Autodyne converter in a small AM Radio. DC voltages look ok but > doesn't oscillate. Originally, the problem was a weak transistor with a hFE > of 4. I replaced the tranny with a 2N2222A and it worked fine for a couple > weeks and then died.
Was the original transistor silicon or germanium?
> Now the DC voltages look ok with about 1.5 volts on the > emitter. Usually, this type of oscillator has the coil tap connected to the > emitter through a cap with no bypass to ground. But this one has the tap > connected through a 100 ohm resistor in series with a 3.3nF cap and then > through the secondary of the antenna loopstick to the base of the tranny. > This radio worked pretty well with only one IF stage, so I'm wondering if > this configuration provides additional signal gain from the converter > section? I plan to play around with the tranny bias to see if it might > oscillate with a little more or less current. > . . > > +V +V > | | > | IF Stage winding > / | > \ | > / 150K Feedback winding > \ | > | | > | /c > Antenna winding-----b| > | \e > / } > \ ___ | > / 100 | / > \ | \ > | --- / 2.2K > --- 3.3NF --- \ > --- | | > | | | > | GND GND > Tap > |- Primary winding-| > | | > |-------||---------- > | > | > GND
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message 
news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com...
> > Bill Bowden wrote: >> >> It's a Autodyne converter in a small AM Radio. DC voltages look ok but >> doesn't oscillate. Originally, the problem was a weak transistor with a >> hFE >> of 4. I replaced the tranny with a 2N2222A and it worked fine for a >> couple >> weeks and then died. > > > Was the original transistor silicon or germanium? >
I believe it was silicon since the e/b voltage read about 0.5 as I remember. But the new 2N2222A hasn't died (I don't think) since the emitter current is about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or germanium? It did work for 2 weeks. .
> >> Now the DC voltages look ok with about 1.5 volts on the >> emitter. Usually, this type of oscillator has the coil tap connected to >> the >> emitter through a cap with no bypass to ground. But this one has the tap >> connected through a 100 ohm resistor in series with a 3.3nF cap and then >> through the secondary of the antenna loopstick to the base of the tranny. >> This radio worked pretty well with only one IF stage, so I'm wondering if >> this configuration provides additional signal gain from the converter >> section? I plan to play around with the tranny bias to see if it might >> oscillate with a little more or less current. >> . . >> >> +V +V >> | | >> | IF Stage winding >> / | >> \ | >> / 150K Feedback winding >> \ | >> | | >> | /c >> Antenna winding-----b| >> | \e >> / } >> \ ___ | >> / 100 | / >> \ | \ >> | --- / 2.2K >> --- 3.3NF --- \ >> --- | | >> | | | >> | GND GND >> Tap >> |- Primary winding-| >> | | >> |-------||---------- >> | >> | >> GND
--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Bill Bowden wrote:
> > "Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message > news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com... > > > > Bill Bowden wrote: > >> > >> It's a Autodyne converter in a small AM Radio. DC voltages look ok but > >> doesn't oscillate. Originally, the problem was a weak transistor with a > >> hFE > >> of 4. I replaced the tranny with a 2N2222A and it worked fine for a > >> couple > >> weeks and then died. > > > > > > Was the original transistor silicon or germanium? > > > > I believe it was silicon since the e/b voltage read about 0.5 as I remember. > But the new 2N2222A hasn't died (I don't think) since the emitter current is > about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or > germanium? It did work for 2 weeks.
They require different biasing. .5 v is low for silicon, but high for Germanium. A failing Germanium transistor often had an open base, which would read higher. Some were so leaky that they were self biasing. GE's early transistor radios had to have bias resistors added when you replaced a bad transistor, because they depended on the leakage to make it operate.
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message 
news:BrqdnXocHooeN-zInZ2dnUU7-N-dnZ2d@earthlink.com...
> > Bill Bowden wrote: >> >> "Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message >> news:686dnYoytaIcn-zInZ2dnUU7-U-dnZ2d@earthlink.com... >> > >> > Bill Bowden wrote: >> >> >> >> It's a Autodyne converter in a small AM Radio. DC voltages look ok but >> >> doesn't oscillate. Originally, the problem was a weak transistor with >> >> a >> >> hFE >> >> of 4. I replaced the tranny with a 2N2222A and it worked fine for a >> >> couple >> >> weeks and then died. >> > >> > >> > Was the original transistor silicon or germanium? >> > >> >> I believe it was silicon since the e/b voltage read about 0.5 as I >> remember. >> But the new 2N2222A hasn't died (I don't think) since the emitter current >> is >> about 1.5/2.2K= 0.68mA. Why would it matter if the tranny was silicon or >> germanium? It did work for 2 weeks. > > > They require different biasing. .5 v is low for silicon, but high for > Germanium. A failing Germanium transistor often had an open base, which > would read higher. Some were so leaky that they were self biasing. GE's > early transistor radios had to have bias resistors added when you > replaced a bad transistor, because they depended on the leakage to make > it operate.
There is only one bias resistor (150K) so it must be silicon. As I remember, germanium required a low resistance (maybe 300 ohms) between emitter and base to reduce leakage. My first transistor radio (Zenith 1957) cost $50 and had germanium transistors in sockets. My dad's radio (GE table model) used 5 "D" cells. with a tap on one cell used for negative 1.5 volt bias. I think they used the negative bias to reduce leakage. . Anyway, I got it working. Looks like something is wrong with the oscillator coil. I changed the emitter resistor to 1.1K to increase the DC current to about 1mA and soldered a resistor to the tap on the coil and it worked for about 5 minutes. I figured the heat from the iron had something to do with it so I heated the coil connections again and the thing ran all day. If it dies again, I'll rewind the coil. --- news://freenews.netfront.net/ - complaints: news@netfront.net ---