Signal tracer help needed

[Radio Scotland]

Originally Posted by ms660

Probably, the diode you are using is a Schottky diode, it's not the best choice.

As well as in the Heathkit IT-12 (in the link I posted earlier) the same trick was also used in the Heathkit T-1 and T-4 and IT- 5283 at least and Eico also used the same trick in at least one of their signal tracers, the Eico147a, they did that because it worked.

Lawrence.

Cheers for the info lawrence


Jay

[ms660]

Originally Posted by Radio Scotland

Cheers for the info lawrence

No problem, remember that all circuits are potential dividers.

Lawrence.

[Radio Scotland]

Originally Posted by ms660

No problem, remember that all circuits are potential dividers.

Lawrence.

Youtube video of the tracer build and testing on a valve set for anyone interested


https://www.youtube.com/watch?v=PbsnI0uM5BM

[ms660]

Originally Posted by Radio Wrangler

Try adding a pair of 10k resistors as a guessed starting value. One from anode to earth, one from cathode to earth.

If you do make sure the diode's cathode/resistor junction is preceded by a capacitor

Lawrence.

[Radio Wrangler]

Originally Posted by ms660

If you do make sure the diode's cathode/resistor junction is preceded by a capacitor

Lawrence.

Important advice, especially ifyou go probing around in valve radios.

There are other RF probe circuits which are more economical on capacitors etc. Have a look at some of the Marconi and HP valve voltmeters (there's one in the AVO CT38 for example...) an input coupling capacitor, then a shunt diode to ground and then a load resistance to ground. The coupling capacitor and the dc path of the circuit under test acts as the filter capacitor as well.

David

[Radio Scotland]

All very useful & helpful info fellas, But i'm still wondering why the guy in the video had good results with his version of the circuit without any of the modifications mentioned in this thread ?


jay

[ms660]

Try the right type of diode, the parts list calls for a 1N34a type if I read it right.

Lawrence.

[Radio Scotland]

Originally Posted by ms660

Try the right type of diode, the parts list calls for a 1N34a type if I read it right.

Lawrence.

Thats what i was thinking lawrence, I'll need to order some. Beginning to think it would have been easier to buy the velleman kit tracer lol. Commercial tracers seem to be far and few between iv'e noticed. But theres always a certain satisfaction to be had from building your own gear (When it works)


Cheers
Jay

[Keith956]

Originally Posted by Radio Scotland

All very useful & helpful info fellas, But i'm still wondering why the guy in the video had good results with his version of the circuit without any of the modifications mentioned in this thread ?
jay

Well it's always best to try a circuit out with a sig gen and scope if you're not sure. Here's a quick mockup of the circuit of the probe. It uses an 1N4148 as it was handy. The anode of the diode D1 is driven by the sig gen (and Ch1 of scope). The cathode of the diode goes to a 1000pF cap C1 going to ground. Then a 10nF cap C2 is connected from the cathode of D1, the other end goes to scope probe Ch2 (pic 1)

With the sig gen set up for 1MHz modulated by a 400Hz tone and 0.1Vpp, and the scope set to 50mV/div you can see that little or nothing is present on the output (pic 2). Not really surprising as the diode forward voltage is about 0.6V so it won't conduct.

If we then turn the sig gen up to 1Vpp, there is a little bit of life on the output... but mostly RF still (pic3, ch1 set to 500mV/div)

Adding a 10K resistor in parallel with C1 (pic 4) so there is a DC path to ground you can see a rectified output (pic5).

So the DC path makes a lot of difference.

[ms660]

I think the two threads should be merged or at least the relevant posts in the other one in the link below should be merged with this thread:

https://www.vintage-radio.net/forum/...d.php?t=179602

Lawrence.

[Radio Scotland]

Interesting keith, I Do have a scope and signal generator, But never thought of trying that, I suppose i expect circuits to work straight off, But thats seldom the case. It seems to be working more to my satisfaction now, When i get time i'll mount the circuit in a tin to shield it and feed the probe via a BNC Socket.

Homebrew probe. Using shielded cable, Diode is inside PL259 Plug.

[ms660]

Originally Posted by Radio Scotland

Diode is inside PL259 Plug.

Mistake, it should be a close as possible to the probe.

Lawrence.

[Radio Scotland]

Originally Posted by ms660

Mistake, it should be a close as possible to the probe.

Lawrence.

Why is that lawrence ? It is as close as possible to the probe tip, As mentioned its inside the body of the PL259 to which the probe tip is attached'


Cheers
Jay

[ms660]

Coaxial cable capacitance, that could de-tune a tuned circuit quite far, and all circuits are potential dividers so double whammy.

Lawrence.

[Radio Scotland]

Originally Posted by ms660

Coaxial cable capacitance, that could de-tune a tuned circuit, and that aside all circuits are potential dividers.

Lawrence.

Ok. I'm not really noticing any detuning effects. The probe was quickly knocked up, Can always be changed at a later date,



Jay

[Radio Scotland]

Originally Posted by ms660

I think the two threads should be merged or at least the relevant posts in the other one in the link below should be merged with this thread:

https://www.vintage-radio.net/forum/...d.php?t=179602

Lawrence.

That was a different project that didn't go as planned, But happy for them to be merged,



Jay

[ms660]

Originally Posted by Radio Scotland

Ok. I'm not really noticing any detuning effects. The probe was quickly knocked up, Can always be changed at a later date,

Typical capacitance is approx. 30pF per foot, could be more, could be less.

Lawrence.

[ms660]

Originally Posted by Radio Scotland

It is as close as possible to the probe tip, As mentioned its inside the body of the PL259 to which the probe tip is attached'

Inside the body of the PL259 or the BNC ? The probe tip looks to be attached to a BNC connector.

Lawrence.

[Radio Scotland]

Originally Posted by ms660

or the BNC ?

Lawrence.

Inside the body of the PL259 plug to which the probe is attached, (Plug at the top of the pic,) Other end will attach to the tracer via a BNC socket

[Radio Scotland]

Tip of the probe in pic below. The diode is shielded inside the body of the plug.


Jay