A problem with a tube receiver

[gaetan8888]

Hello

I have a Pioneer SX-34 tube receiver.

I have recap some parts of the amp and put new output tubes, and the amp section work perfect and sound very good.

The AM tuner work good.

But the FM tuner don't work (except a small white noise), I've just test all the tuner tubes and they are fair to good.

I really don't see why the FM tuner don't work, (even using it with a good antenna).

Any hint ?

Here is the schematic;

AK Database PDF Gallery - Pioneer Receivers/Pioneer SX34 Schematic

Thank

Bye

gaetan

[TheGimp]

Test all the tubes in the FM section first. Coupling caps could be bad (electrically leaky) shifting the bias as well.

[gaetan8888]

Hello

I've allready test all the FM tuner tubes and they are testing fair to good.

Thank

Bye

gaetan

[TheGimp]

The schematic shows a lot of the bias conditions for the FM circuit. Check all the voltages and see if you can find one that is out from expected values.

If you have an exceptionally strong station in your area, tune to that approximate frequency based on the dial indicator and look to see if any of the bias voltages shift as you tune in that area.

[multisync]

Am, Fm radios, tv tuners and many other high frequency tuners generate a frequency higher/lower than the tuned frequency. Both of these frequencies, the tuned frequency of the radio station and the higher "local oscillator" frequency are mixed together in a non linear fashion in a device called a "mixer" to produce difference frequency. The mixer also produces a sum. These sum and differences are commonly called inter-modulation distortion when produced at audio frequencies. At RF frequencies the difference is what we want and is called the IF or intermediate frequency. It is always lower than the radio station frequency and is easier to amplify. In AM radios the IF is 455Khz, in FM it is 10.7 Mhz. If the local oscillator or mixer is not working you won't get any FM radio. Quite often the local oscillator and mixer is one tube. Another problem you may have but less likely is the AFC control. Automatic Frequency Control. The local oscillator drifts so a correction voltage from the FM decoder is sent to the oscillator to keep it stable. I am surprised that I don't see a switch to turn off the AFC. Normally you tune in a station, then turn on the afc to lock it in. If one of the IF stages is dead it is possible that you will hear nothing, but most likely you may hear a strong local station faintly. Did you try the FM selector(mono). The FM MPX selector requires that the FM stereo decoder is working.

[gaetan8888]

Hello

In both FM STEREO and FM MONO switch positions, the FM tuner don't work.

Thank

Bye

Gaetan

[gaetan8888]

Originally Posted by TheGimp

Test all the tubes in the FM section first. Coupling caps could be bad (electrically leaky) shifting the bias as well.

Hello

I forgot to ask.

Which part numbers those coupling (bias) capacitors have in the schematic ?

Thank

Bye

Gaetan

[DF96]

If you have another FM receiver, tune the Pioneer to near the centre of the band. Then on the second receiver tune to 10.7MHz above and below the Pioneer (you will have to move the Pioneer tuning a little too). The aim is to see whether the local oscillator is working.

For example, Pioneer on 95MHz. Try other on 105.7MHz. Put Pioneer on 100MHz. Try other on 89.3MHz. You should hear a plain carrier on one of these positions, but not both.

Fixing an FM unit requires some knowledge and some RF test equipment, except for the simplest faults.

[multisync]

is there a signal strength meter / tuning indicator on the receiver? Does it change when you turn the tuning knob up/down?

[Wavebourn]

For the beginning please touch grids (pin 1) of V9, V8, V7 by a screwdriver tip and tell if you hear some noise.