capacitor value without unit in schematic

[okbro]

hello,
I have the following oscillator schematic but the capacitor unit are not indicated. Are those microFarad(uF)?

thank you

 

[JimB]

All the capacitors except the one which I have indicated in the red circle are micro-farad values.

The capacitor in the red circle is 100pF I think.

JimB

 

[okbro]

All the capacitors except the one which I have indicated in the red circle are micro-farad values.

View attachment 136564

The capacitor in the red circle is 100pF I think.

JimB

hmm....yes makes sense and i think it could be replaced by variable capacitor

 

[Nigel Goodwin]

Must admit, I'm a a bit dubious about the 100pF?, it seems rather pointless in the circuit, and it's low value could upset the operation - normally it would be just a piece of wire.

 

[upand_at_them]

I think, as JimB pointed out, that the convention in older circuits was that capacitor values with a decimal were uF and values without a decimal were pF.

 

[Nigel Goodwin]

I think, as JimB pointed out, that the convention in older circuits was that capacitor values with a decimal were uF and values without a decimal were pF.

I'm perfectly fine with the values, and totally agree with JimB - it's the actual purpose of the 100pF that I'm dubious about, it's not something normally fitted in that oscillator configuration.

 

[JimB]

Well it is funny that you say that, because JimB himself thinks that the 100pF capacitor in that position is a bit odd.
It could be that the idea was to create a capacitive divider so that only a small signal was coupled out from the oscillator to the following amplifier stage.

JimB

 

[Nigel Goodwin]

Well it is funny that you say that, because JimB himself thinks that the 100pF capacitor in that position is a bit odd.
It could be that the idea was to create a capacitive divider so that only a small signal was coupled out from the oscillator to the following amplifier stage.

JimB

That was my thought as well, but it's seriously messing up the ratio of the two capacitors forming the oscillator along with the crystal. Normally you'd take the output directly from the emitter to the buffer, and use an attenuator between the two if needed. Crystal oscillators can be quite tricky to get running, and I suspect the spurious 100pF may make this one even more so.