LOOKS GOOD SOUNDS AWFUL?
No matter how good your set looks after youíve solved its various picture problems, missing, poor, weak or distorted sound really does spoil the effect. So lets look at the sound stages in a Dual Standard Set. I am using a Dual Standard Set as this allows me to demonstrate AM and FM sound all in one set! The set weíll use for most of this is the Bush TV135. Just for show, hereís a picture of one from the front of the manual.
Incidentally has anybody got one?
405 LINE SOUND & 625 LINE INTERCARRIER AMPS.
On 405 Lines, the sound is AM. The stages shown here act as IF amplifiers on 405 and Intercarrier amps on 625. Switch 2S1D changes the input to the first stage 2VT5 from the 405 Line IF to the 625 Line Intercarrier (6Mhz) beat note. It is shown here in the 625 Line Position.Stages 2VT5 and 2VT6 amplify the IF or Intercarrier beat depending on the system being used.
405 AM DEMODULATION.
The 405 Line Demodulator is fed from the collector of 2VT6 via 2R38. Sound is demodulated by detector diode 2MR5 and noise is limited by a rate-of-change noise limiter diode 2M86. When 405 Lines is selected the sound is fed to the audio stages via the system switch and 2C62.
625 FM DISCRIMINATOR CIRCUIT.
Fed via 2C49 from the base of second Intercarrier and Sound Amp stage 2VT6. This is a ratio detector circuit. There is little to go wrong here.
EH90 DISCRIMINATOR STAGE.
A diversion from the Bush TV135 to look at this common arrangement.
Here, the incoming 6 Mhz intercarrier beat, frequency modulated with the transmitted sound, is applied to the control grid. This induces a similar signal on the suppresor grid, this signal lags behind the original 6Mhz intercarrier signal by 90 degrees.This oscillation is locked to the original and any frequency deviation results in a change in the 90 degree phase difference between the two signals. The anode voltage is a function of the phase differnce of the two signals, an AF waveform results here.
Any radio restorer should see straight away that this is very similar indeed to what they are used to when restoring radios and has exactly the same weaknesses. Note the Anode Coupling Capcitor as I have referred to in the past as THAT capacitor. If you are met with distorted audio, look on the control grid of the pentode stage of the PCL82 (Pin 3) and if its positive, go straight for 3C56!
WHAT GOES WRONG?
Well in most cases very little. Fault finding on the audio output stage is very similar to radio fault finding. Perhaps the system switch isnít making contact or maybe even has a wiring fault?
Loss of sound on one standard points to either the system switch or the AM Demodulator / FM discriminator stage. Distortion, if not down to the AF stages, is often due to misalignment.
Itís worth mentioning that when the set has an EH90 stage, the system switch sometimes changes this stage from a Discriminator on 625 lines to an AF amplifier on 405 lines.
SOLID STATE AUDIO STAGES.
Again, these are just about the same in a TV as in a radio or any audio device and can for the most part be treated in the same way. Hereís an example of a push-pull sound stage. Look familiar? This is from the Murphy V1400 single standard monochrome portable from the early 1970s and here,the sound IF and demodulator is in an I.C.
I have declined to spend too much time on sound problems as they are often traced in a similar way to in Vintage Radio.
Intercarrier buzz problems are usually down to IF alignment and I will be dealing with this problem there.