Valve AM tuner fault

[Pio1980]

Is there somewhere in the tuner that I can see the oscillator? I have a good scope.

Perhaps, at the cathode of the 6BE6 where the probe would less likely to quench the LO. The handheld AM radio is a simpler way to determine if it's working at all.

 

[QSilver]

I've got a waveform on the cathode (Pin 2) Scope is saying its around 1.05MHz Switching to LW makes it disappear.

It's also appearing on Pin 1 but much larger. Turning the tuning knob adjustings the frequency up and down but no lower than 1MHz.

There is a small amount of crackle and a hum on the right channel that comes and goes. The rest is silence.

 

[Pio1980]

I've got a waveform on the cathode (Pin 2) Scope is saying its around 1.05MHz Switching to LW makes it disappear.

Does it vary with tuning?

 

[QSilver]

Yes

 

[Pio1980]

Yes

I think LO is working.

 

[QSilver]

Ok, where next?

 

[Pio1980]

Ok, where next?

Have you tried a replacement 6BE6?

 

[QSilver]

No, I don't have one to hand. But if its worth trying I can get one.

 

[Pio1980]

Look at output at T1 while tuning the receiver on AM.

 

[QSilver]

T1 Pin 1?

 

[Pio1980]

T1 Pin 1?

Yes.

 

[QSilver]

I'm getting the same signal present on Pin 1 of the 6BE6 but a lot weaker. But it is there and varies with the other frequency as i adjust the tuning dial.

 

[Pio1980]

I'm getting the same signal present on Pin 1 of the 6BE6 but a lot weaker. But it is there and varies with the other frequency as i adjust the tuning dial.

You should see some IF freq peaking with modulation as stations are tuned if the AM front end is working properly.

 

[QSilver]

I don't think I'm seeing that. Even if I feed an AM signal into the AM aerial post.

I've also noticed, that if I connect to T1 Pin 1 first - there is no signal. Only when I then connect another probe to either Pin 1 or Pin 2 of the 6BE6 do I get a waveform at both positons.

Also the waveform at T1 is unstable. Sometimes its strong and other times its weak with another higher speed waveform on it too.

 

[Pio1980]

Is the meter still pegging?

 

[QSilver]

Meter is unchanged.

 

[Pio1980]

Simple things first then, another 6BE6.

 

[QSilver]

I'll get on it and report back.

 

[gadget73]

i had another random thought. Does the signal meter sit at full high when the power is turned off?

 

[QSilver]

Only if the AM or LW is selected and it fades down as the power drains.

When I power it up, the meter almost immediately goes up to near maximum long before the valves have warmed up.