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 Post subject: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 2:09 am 
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Location: Wake County NC
Hi, I have a Tenma signal tracer that I've been trying to check out and make operational. I've only used Sig Tracers in the AF mode so I'm not sure how they are supposed to work with RF signals as compared to the AF. I know this one has a germanium diode (1N60) and a few caps that somehow convert AF signals into something that can be heard (by the human ear) out of the speaker on the signal tracer. Also, I'v established that this tracer works for AF signals in AF mode. But I can't get it to do anything in the RF mode on any dB setting. I've tried hooking a signal generator directly to the input in the RF mode and scanned from about 100 KHz to 60MHz and I hear nothing from the speaker with the vol and dB set to express the weakest signal. I've looked at the flow of the signal through the front end tracer input and, say, 160KHz gets to the Germanium diode, present on one side, yet no signal out the other side. Do these diodes go bad over time? This information says to me that the diode is bad, but it reads like a typical diode with my Fluke. 325ohm one way and zero the other. I don't understand how a diode and a few caps is going to convert a whole range of RF signals into something that a Signal tracer detect and convert to AF, in RF mode. At this point I am willing to accept that I'm using the device wrong. Again, it works as expected in AF mode.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 2:54 am 
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Location: Stone Mountain, GA
Did you mean OL instead of zero when testing the diode?
In diode test mode the Fluke shows voltage, not ohms. 0.325V is about right for a germanium diode.

The diode rectifies the RF and generates a voltage proportional to the RF level. This will produce a signal similar to the modulation of the RF signal. If your gen is unmodulated, you will hear nothing.

That is an odd way to do the diode usually it is through ac cap the diode to ground.
Check the RF switch for bad connections or is dirty.

Note: the gates on the injector oscillator look to be drawn backwards. Some other things on the schematic look odd. You may need to see what you have rather than rely on the schematic.

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 2:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Minneapolis, MN USA
Does the RF generator have a modulation control or switch ?

To trace RF, the signal needs to be modulated so there is something for the tracer to convert to an audio signal for you to hear.
With a functional radio, the modulation is done by the radio station, by the music or voice they are transmitting.
With an RF generator, there is an option to turn on audio modulation, an audio signal around 400 hertz or so.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 2:59 am 
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KE4CDR wrote:
I've only used Sig Tracers in the AF mode so I'm not sure how they are supposed to work with RF signals as compared to the AF. I know this one has a germanium diode (1N60) and a few caps that somehow convert AF signals into something that can be heard (by the human ear) out of the speaker on the signal tracer.

Well no, that's not correct.

If you'll notice on the print, the double pole switch -which shows connection for audio frequencies- is displaying that connection, with the (audio) input going directly to the amplifier circuit.

Throwing the switch to the RF input injects the Diode into the mix. The Diode rectifies the incoming signal to AF. Ideally you should be able to put the hot (ungrounded) probe on various points of a receivers front end, antenna, control grid (or base if a Transistor circuit), sweep the tuner, and hear broadcasts from the tracer. I highly doubt the Diode is defective.

Edit, I see younger, swifter fingers have already answered. :wink:

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 10:16 pm 
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I would be a little concerned about the lack of DC blocking on the input.

John


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 10:21 pm 
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KX5JSC wrote:
I would be a little concerned about the lack of DC blocking on the input.

John

There appear to be capacitors on both ends of the Diode.

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 11:31 pm 
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Location: Montreal, Quebec
It won't do a thing unless the signal generator is modulated. The capacitors ensure that.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Thu 24, 2021 11:54 pm 
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KE4CDR wrote:
...... I know this one has a germanium diode (1N60) and a few caps that somehow convert AF signals into something that can be heard (by the human ear) out of the speaker on the signal tracer. .......


AF signals are by definition frequencies that can be heard by the human ear (20Hz to 20kHz) and don't need to be "converted" to be heard. The signal tracer only amplifies the signal as needed. RF signals are well outside the hearing range of the human ear and cannot be heard unless they are modulated by a lower frequency signal (usually 400 Hz or 1kHz in most signal tracers). The diode just demodulates ("converts") the signal before sending it on to the amplifier. If the RF signal is not modulated, there is nothing for the diode to demodulate and hence you will not hear anything if you are feeding an unmodulated signal (as mentioned by others in previous posts)


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 12:12 am 
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It's not hard to understand how a diode functions as a detector. The RF signal is at the carrier frequency or the IF frequency in a radio; the modulation consists of amplitude variations in the RF waveform. The amplitude variations occur at the audio rate.

When an RF or IF signal is applied to a diode, it does just what a power rectifier would do: it conducts when its anode is more positive than its cathode, and it cuts off when the anode is more negative than the cathode. With a small capacitor connected across whichever side is used for the output (both positive and negative going detectors are possible), the cap will charge to the peak voltage when the diode conducts and the capacitor is prevented from being discharged by the signal source when the diode is not conducting. Since the capacitor then has to discharge through the load resistor connected to it, the voltage across the load resistor will follow the amplitude and frequency variations of the audio modulation. It can then be connected to the input of an amplifier and the audio will be heard in a speaker. If there is no modulation the output of the detector will be a DC voltage which will not produce sound when amplified.

Now looking at the schematic, we see that a suitable load resistor is only present when the range switch is set to 0 dB. Try touching the input terminal with your finger, you should get a loud if somewhat distorted hum out of it. If the hum is weak or you don't hear it at all, it is possible the diode is damaged; even though it checks okay on the meter it may not have much sensitivity if it was previously overloaded by application of excessive voltage. Other things to check include the switches and the capacitors.

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 12:56 am 
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On May 11th, someone was asking about a schematic for a Tenma signal tracer/injector. I suspect this schematic would be a good start.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 5:27 am 
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Location: Corinth, TX
fifties wrote:
KX5JSC wrote:
I would be a little concerned about the lack of DC blocking on the input.

John

There appear to be capacitors on both ends of the Diode.


<Hangs his head>
I SAW the caps. I meant "inadequate". Alas, the only way to know is to look at the voltage ratings of those caps.

John


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 6:32 am 
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KX5JSC wrote:
fifties wrote:
KX5JSC wrote:
I would be a little concerned about the lack of DC blocking on the input.

John

There appear to be capacitors on both ends of the Diode.


<Hangs his head>
I SAW the caps. I meant "inadequate". Alas, the only way to know is to look at the voltage ratings of those caps.

John

Don't you think the maker of the tracer, Tenma, made sure there was adequate blocking for the DC voltages found in plate circuits?

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 4:00 pm 
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If I'm not mistaken Tenma hasn't been in existance a very long time. Are you sure this tracer was intended to be used with tube electronics?
In any case it doesn't seem very well documented in general.
There doesn't seem to be any DC block in the audio mode.

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 5:14 pm 
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Location: Montreal, Quebec
There is a blocking capacitor at the input of the first transistor.

But there are protection diodes before that, and that could route B+ onto the tracer's power supply line.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 7:56 pm 
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Location: Corinth, TX
fifties wrote:
Don't you think the maker of the tracer, Tenma, made sure there was adequate blocking for the DC voltages found in plate circuits?


One would think, or rather, hope. But did they? I checked the BAMA site. They don't have one listed. They do have the 72-585 solid state RF signal generator (no parts list), though. For THAT, the manual says to make sure "there is no high voltage present". If there is, use a blocking cap. That alone suggests to me that, at least for the signal generator, they designed for solid state gear.

Tenma is budget, solid state gear. Who knows what corners they cut? I don't.

John


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 8:24 pm 
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Location: SoCal, 91387
KX5JSC wrote:
fifties wrote:
Don't you think the maker of the tracer, Tenma, made sure there was adequate blocking for the DC voltages found in plate circuits?


One would think, or rather, hope. But did they? I checked the BAMA site. They don't have one listed. They do have the 72-585 solid state RF signal generator (no parts list), though. For THAT, the manual says to make sure "there is no high voltage present". If there is, use a blocking cap. That alone suggests to me that, at least for the signal generator, they designed for solid state gear.

Tenma is budget, solid state gear. Who knows what corners they cut? I don't.

John

Well you did bring up a good point, and I guess with solid state tracer gear, it might just be good practice to incorporate a capacitor -new ones are now rated at 630 volts- in series before the hot probe.

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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 10:48 pm 
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Location: Melbourne, Florida
Take a look at the signal injector circuit.

RRM


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Fri 25, 2021 11:53 pm 
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Location: Montreal, Quebec
I looked at that.logic section, and it didn't quite look right. I know the earlier thread had a tracer/injector, but I thought this didn't have an injector. The circuit is a little off, I'd hope just error in drawing theschematic rather than error in design.


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Sat 26, 2021 4:24 am 
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Location: Corinth, TX
If you flip just the inverters AS DRAWN, the circuit will work. Pinouts in that case are good.

It's a common circuit. I suspect the draftsman, not the engineer who designed the circuit, messed up. OTOH, a proofreader somewhere in the chain should have caught the mistake.

John


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 Post subject: Re: question about rf signal tracer
PostPosted: Jun Sat 26, 2021 5:26 am 
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The problem is that the triangle ought to indicate the size of the signal. So the input should be at the small end. But that's not how it's done. Back in the day when this stuff was new, I can see why that sort of error got through.


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