Help Please -- very strange oscillator problem

Could some electronics guru please help ? I have two identical(same resistor, capacitor values) implementations of an astable multivibrator. I am using the 2N3055 and each transistor drives a coil each of a bifilar coil, whose common terminal is connected to a 12 Volt 2A(max) DC power supply. The coils is air core.

The problem is that one implementation works as designed, the other does not. To check if the oscillator is working, a pickup coil is placed in the vicinity and the output of this pickup coil is monitored via an oscilloscope.

I have observed that the base voltage on one of the two transistors is getting 10 V pulses, while the the voltage on the other transistor base is 0.2 V. I have un-soldered and checked the resistors, but they are fine. I have replaced the two capacitors, with two new identical ones but no luck.

Any hints/suggestions would be of immense help. Thanks in advance.

Reply to
dakupoto
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On Friday, September 15, 2017 at 2:12:54 PM UTC+10, snipped-for-privacy@gmail.com wrote :

You may need a faster oscilloscope. Many weird oscillator problems turn out to be an oscillation at a higher frequency than you can see.

And note that not all 2N3055s were created equal. The RCA originals had con tained a lot of silicon and were slow and hard to blow up. Other manufactur ers worked out how to fit the data sheet with less silicon, and their parts were faster and easier to blow up.

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Bill Sloman, Sydney
Reply to
bill.sloman

0.2 V on the base sound like a blown transistor, have you tried replacing them?

George H.

Reply to
George Herold

A schematic would be helpful. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Schematic?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

The 2N3055 is a power transistor, typically takes milliamps of base current and can deliver amps of emitter current, but at frequencies where air core coils work best, it's low in gain. It could be as simple as transistor high-frequency gain variation.

Reply to
whit3rd

You should be able to check most components without unsoldering. At least I suspect that is true. Without a schematic , it is really a guess.

My guess is one of the transistors is not working. So how much trouble is it to swap the transistors and see if the problem follows the transistor?

Dan

Reply to
dcaster

please show schematic here in ascii art or some website.

Reply to
M Philbrook

It's probably a variation on this circuit:

which in my experience is a PITA to get to work correctly at the best of times. It's bad don't use it

Reply to
bitrex

I would be happy to supply a schematic, but then would need an email address. This Google group does not allow attachments.

Reply to
dakupoto

Don't the coils need protection diodes across them?

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John Devereux
Reply to
John Devereux

Go to my website, click on the envelope icon. Do NOT attach anything, just send an E-mail. I will reply with an E-mail address you can use to send the schematic. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

We use something like Dropbox, or a photo posting site. Or make an LT Spice schematic and include the .ASC text in a post here.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Sounds like a blocking oscillator. this circuit?

formatting link

12V kind of high for that circuit, it generates lots of reverse base voltage which will damage transistors.

this might be worth a look

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This email has not been checked by half-arsed antivirus software
Reply to
Jasen Betts

Thanks, yes I have did have in mind a Royer oscillator in mind in the past, but shelved it after I could not find a straightforward way to implement the transformer with the feedback coils.

My existing problem is that one implementation of my oscillator workd fine, but the identical copy does not.

Reply to
dakupoto

No, my circuit is is a loosely related to the last one at :

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Reply to
dakupoto

I did all that, but when trying to send the email with the schematic as attachment, I got an error message that the mail server was not responding.

Please take a look at a fairly close relative of my oscillator, the last one at :

formatting link

Reply to
dakupoto

[snip]

I never got you initial E-mail (without any attachment) ??

That would be _your_ E-mail server.

Check to see if you have the diodes reversed. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

The circuit shown does have a stable state: both transistors saturated and nothing going on.

But it's impossible to analyze a circuit defined as loosely related to some other circuit.

There is an interesting, recurring psychological effect in our business: people describe the way a circuit can work, and ignore the many ways that it might not work.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin
[snip]

Indeed! I have a client right now arguing with me about such a "state machine". ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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