Colpitts oscillator problem 400hz

Thread Starter

davidjohnhills

Joined Oct 8, 2017
61
Good evening

I have a 400hz colpitts oscillator made from a PNP audio transistor(MF400 1 can’t finf this part number anywhere!!) 2 x 0.01uf caps

And a undefined audio choke(maybe a round 10 henries)

Which is not working, the dc resistance of the audio choke is 3.6 mega ohm’s

This seems unreasonable high, could this be the problem



Thanks

David H
 

MisterBill2

Joined Jan 23, 2018
11,556
Indeed that audio choke is open circuit. A repair may be possible, though, depending on where in the choke the break is located.
As for the MF400 transistor, that seems more like a part number than a transistor model number. If you can provide a circuit along with the resistor values we should be able to suggest a currently available replacement device.
 

Thread Starter

davidjohnhills

Joined Oct 8, 2017
61
Hi thanks for your replies
have attached circuit diagram schematic for Nombrex 31 signal generator.
can anyone explain how the af trans is PNP and the rf trans bc183k is NPN
why did they build the circuit like this?
It all looks original and no parts seem to have been changed.
the way the battery is connect is the opposite to the schematic

the supply voltage postive is going to ground (pp3 9v)

thanks

David
 

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dendad

Joined Feb 20, 2016
4,109
Both transistors are PNP in the circuit.
It looks like the -ve is ground, at least if you go from the AF and RF output connectors.
But hat is not a direct connection as it goes via R10.
Is there an actual connection to the chassis?
If the battery is the other way, it is possible the transistors have been changed to NPN types, and the battery polarity swapped.
Otr is the top libe in the circuit the ground?
 

Thread Starter

davidjohnhills

Joined Oct 8, 2017
61
Both transistors are PNP in the circuit.
It looks like the -ve is ground, at least if you go from the AF and RF output connectors.
But hat is not a direct connection as it goes via R10.
Is there an actual connection to the chassis?
If the battery is the other way, it is possible the transistors have been changed to NPN types, and the battery polarity swapped.
Otr is the top libe in the circuit the ground?
 

Thread Starter

davidjohnhills

Joined Oct 8, 2017
61
Hi Dendad
thanks for your answer.

the rf trans is definitely marked bc183k which is a NPN .
I looks like it inserted upside down so to speak with emitter going to the -9v rail.
(Remember the battery is in backwards so what marked as + on the circuit board is the neg side of the battery and the
+ side of the battery is going to the ground rail)
the rf part of the circuit works fine.
It does not look like any components have been changes, I think it was built like this.
I wonder why?

thanks

David(
 

Thread Starter

davidjohnhills

Joined Oct 8, 2017
61
Hi Dendad
thanks for your answer.

the rf trans is definitely marked bc183k which is a NPN .
I looks like it inserted upside down so to speak with emitter going to the -9v rail.
(Remember the battery is in backwards so what marked as + on the circuit board is the neg side of the battery and the
+ side of the battery is going to the ground rail)
the rf part of the circuit works fine.
It does not look like any components have been changes, I think it was built like this.
I wonder why?

thanks

David(
the schematic shows the battery + going to the + rail (top of the diagram and the on/off switch)This is not how it's wired up,
in the actual circuit the neg side of the battery is going to the top diagram and the on/off switch, and it does work
thanks
 

dendad

Joined Feb 20, 2016
4,109
What type is the AF transistor?
The circuit would work quite well if NPN transistors are used instead of PNP versions, and the supply swapped.
There may be some resistor changes to alter the bias too.
It does look like the device has been modified to suit new transistors.
The original use of PNP types could indicate it is pretty old, and they may have failed at some time, or that the model was upgraded.
But, unless the circuit is changed from the original, both transistors should be the same type. Even saying that, the audio section may have had its connections swapped to use a PNP still.
 

MisterBill2

Joined Jan 23, 2018
11,556
The resistor R10 is used for modulation in a very strange manner in that the AF transistor sort of bypasses it a bit. And certainly the thing would work if TR1 were an NPN with the collector and emitter reversed. I suggest that you trace out the actual circuit on the board as it may be that some resistor values do not match the part list.
My guess is that a component shortage lead to a design revision to use an available transistor, but that the circuit drawing never was revised.
 
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