Heathkit IT-12 Probe theory

[G4_Pete]

Hi,
I have just started to look at a Heathkit IT-12 that I have but I am struggling to understand probe theory when set to RF and probing an AM signal. With two blocking capacitors C1 and C2 in series with the diode how does this circuit demodulate?

Obviously this is a tried and tested design but I would have expected to see a DC discharge path. I am just going out to have another look at it but right now it is a bit confusing.

Pete

[David G4EBT]

The RF probe looks to be a fairly standard circuit typically used with signal tracers. As I understand it Pete - just as in the detector stage of a radio - the diode will detect and pick off the AF element of the RF/IF signal and pass it to the AF amplifier in the IT-12. The capacitors, C1 & C2 will block any DC from the circuit under test. After the detector stage in a radio, for audio signal tracing, the diode in the probe is switched out.

As to AF tracing (which isn't the subject of your query but might be of interest), the Heathkit manual states:

Audio Signal Tracing:

After detection has taken place in the receiver, the use of the RF probe is not required and the switch in the probe should be switched to the audio position. Here again the same general conditions previously described apply to the use of the indicator tube and the speaker. From the detector stage (pin 5 and 6 of tube C) the signal can be traced through the various coupling circuits, through the audio and output stage (tube C and E) and the output transformer. These tests can be made quickly and easily and any circuit fault would be indicated by a loss of gain or signal in succeeding stages.

Intermittent or faulty volume controls and coupling condensers can also be quickly spotted.

When using the tracer keep in mind that the Signal Tracer will detect the presence as well as the absence of signal. For example, it is a common practice to use a high capacity by-pass condenser in the cathode circuit of the output tube (pin 8 of tube E). In event the condenser is open there will be a definite indication of a signal at the cathode, whereas a good condenser would by-pass the signal to ground in the desired manner. As a signal is traced through the audio system the signal level will be extremely high, and it will be necessary to reduce the setting of the signal tracer gain control. A definite reduction in gain will be noted as the probe is moved from the primary to the secondary of the output transformer. This is normal and is due to difference in the turns ratio and impedance of the windings in the transformer.

A very nice tracer that looks the part - expensive in its day, with a twin triode, a pentode and magic eye.

Nearly enough bits to build a TRF radio!

[Chris55000]

Hi!

Full bookwork for the curious:–

https://www.google.com/url?sa=t&sour...Cuwx-Us-53UFLo

Chris Williams

[ms660]

The diode's reverse leakage is all I can think of.

Lawrence.

[PJL]

Germanium point contact diode so plenty of leakage current.

[G4_Pete]

Thanks all for the comments,

I also now think it must be using the leakage as the "Drain" because as a sanity check I set up a simulated test using a low leakage silicon diode. With a good drive to overcome the volts drop it hardly demodulates, the capacitor just charges to peak as expected, however with a point contact diode in the same test circuit there is some demodulation albeit inefficient. I think that with such a high gain amp they were not worried about efficiency which could be traded off against probe component count.

I had a good search but none of the independant write ups mention the probe opperation.

At any other time I might have just accepted that it works!

Pete

[G4_Pete]

Thinking about this a bit more the user is free to probe about on all sorts of high voltage and also can set the probe to inject 100 volts or so. This inject setting is not interlocked to the RF/Audio switch allowing incorrect user settings. This simple design is robust to all those situations as found on a normal broadcast radio.
Pete

[avocollector]

To add to G4 Pete's posting it's the 'noise' switch that puts 90 volts DC on the probe tip to find noisy components - mine was faulty and did not work - most of the time. Guess how I found out it DID work sometimes

[avocollector]

Originally Posted by Chris55000

Hi!

Full bookwork for the curious:–

https://www.google.com/url?sa=t&sour...Cuwx-Us-53UFLo

Chris Williams

Thanks so much Chris - greatly appreciated.

[Radio1950]

Re original question.
I think I would like to see a 10M resistor from detector diode anode to ground.
This shouldn't affect the Noise Function much, and should provide some detection efficiency improvement, although Heathkit may be just using leakage for the DC path?
Maybe I am missing something here?
Heathkit normally get it correct; simply, but correct?
.

[Chris55000]

Hi!

All you need to know about probes and add–ons:–

http://www.tubebooks.org/Books/intro_Zucconi_probes.pdf

Chris Williams

[Chris55000]

Hi!

The above I posted was just contents list, full book:–

http://www.tubebooks.org/Books/Zucconi_probes.pdf

Chris Williams

[G4_Pete]

Chris,
Thanks for posting that, Apart from being very comprehensive it is a real reminder of the huge amount of development work that went into getting the most out of availiable test equipment in the early days and of not affecting what you are measuring by the use of the wrong type of probe.
Pete

[DMcMahon]

For some reason I could not get the second link to the full document to work but found it elsewhere and downloaded, even though a large file it was a very slow download.

Excellent informative and useful document.

David