Heathkit T3 Visual Aural Signal Tracer - Videokarma.org TV - Video

Tubejunke · #1 03-15-2016, 10:12 PM

I have one of these units and seem to have got it in working condition. By that I mean that I replaced all four electrolytic capacitors that were in a 4 section can and got rid of one heck of a "hummer."

I have the user manual and I am able to use the RF probe to hear signals at different points in a radio through the on-board speaker. My question really is what is an audio probe? The instructions don't picture one. Is it simply a shielded cable that plugs into the headphone jack on the unit and would have alligator clips or something on the end to connect to the secondary of an audio output transformer or is there some greater purpose for this that I am failing to understand?

I would like to know and understand all of the functions of what seems to be a neat piece of vintage equipment. If anyone here has one, please fill me in on the uses of it other than simply using the RF probe to trace signals through stages.

Thanks!

jr_tech · #2 03-16-2016, 12:06 AM

Page 11 of the manual describes how it is made... just a length of shielded cable with a 1/4 " phone plug on one end and a probe tip on the other.
It is used after the detector stage of the radio where audio signals are present, where the rf probe is used to pick up and demodulate signals in the rf and if stages. Useful also for troubleshooting audio amplifiers.
The T-3 can be used as a speaker/output transformer substitute, and can also measure wattage consumed by a device under test. Fun item for the vintage test bench! Eye tube indicators are fun to use.
The T-3 was a handy device in its day, but I suspect that most would just use an oscilloscope now.

jr

Tubejunke · #3 03-16-2016, 05:50 PM

Ok thanks, that takes care of that. Now I have to figure out if something is wrong with the RF probe. When I go to use the probe the unit is quiet. I screw that thing in and all sorts of buzzing goes on and the gain has to be turned almost all the way down or the amplifier is highly over-driven and nasty sounding, but it does pick up signal.

The buzzing is like the 60 cycle hum the unit had before I replaced all four 20mfd caps in the power supply. Before that the thing buzzed or hummed whether the probe was connected or not. Now the thing simply doesn't like the probe being connected or else I just have never used one of these and it should make noise, which I doubt; not this much noise. The probe settles down quite a bit when I grab the ground wire; or if I connect the ground wire to the tip it gets quiet. The instructions simply say that touching the tip should cause a buzz or hum. Sure would be a good time to have a known good probe to compare! Not happening.... I wonder if the cap inside the probe could cause the condition?

Tubejunke · #4 03-16-2016, 08:34 PM

Ok, I remember at some point the amphenol connector from the probe fell apart in my hand. The center conductor was not connected. I thought it was just wear and tear. So I soldered it back and that's when all the noise and having to keep the gain almost off came into play. I put a new .02mfd cap in and nothing changed even though the cap was very leaky on my Heathkit IT-28 at very low DC voltage. Finally I backed off the amphenol connector so that the center conductor wasn't contacting and everything normalized. There must be something wrong in the wiring of the probe. I will have to go through the schematic and see.

Tubejunke · #5 03-18-2016, 10:05 PM

I have looked at a few Webpages relating to these old signal tracers and so many have popped up with missing RF probes that people build them. It's not like I'm dealing with some extremely difficult circuit. Most any unit can have a probe made out of a couple of resistors, a detector diode, and a low value capacitor.

My confusion mainly stems from not really knowing the nature of the instrument, but it's quite obvious that something is wrong if I have to disconnect the center conductor of the probe to get fairly normal, or dare I say reasonable performance out of the thing. So once again what I am doing is loosening the amphenol connector until the center conductor is backed away. So all the noise is much quieter and I am able to trace signals using the gain as needed. If I screw the connector in the noise is just awful and the gain must stay almost all the way down. Strangely what this amounts to is the probe working with only the shielded side of the circuit connected. That really doesn't make sense electrically.

jr_tech · #6 03-18-2016, 10:15 PM

Sounds like a ground problem to me .... have you ohmed out the ground connection from the connector to the probe case? How about to where it connects to the 2 resistors inside the probe?

jr

dieseljeep · #7 03-19-2016, 10:42 AM

Quote:

Originally Posted by jr_tech

Page 11 of the manual describes how it is made... just a length of shielded cable with a 1/4 " phone plug on one end and a probe tip on the other.
It is used after the detector stage of the radio where audio signals are present, where the rf probe is used to pick up and demodulate signals in the rf and if stages. Useful also for troubleshooting audio amplifiers.
The T-3 can be used as a speaker/output transformer substitute, and can also measure wattage consumed by a device under test. Fun item for the vintage test bench! Eye tube indicators are fun to use.
The T-3 was a handy device in its day, but I suspect that most would just use an oscilloscope now.

jr

I see Heath made the same tracer for a few years. The one shown on the manual website is a newer model than mine, but still uses the same circuit. It still uses the JAN war surplus tubes.

maxhifi · #8 03-19-2016, 11:04 AM

Look at the eico manual for the 147A on the BAMA website, it has a good description of the audio probe. Chinese sellers on eBay carry adapters to use banana plugs in pin jacks, it's how I connect things to my eico. You can easily make an audio probe out of stuff you likely already have Laying around, the main thing is it's well shielded, so that it doesn't act as a giant hum antenna.

Tubejunke · #9 03-19-2016, 11:24 PM

Quote:

Originally Posted by jr_tech

Sounds like a ground problem to me .... have you ohmed out the ground connection from the connector to the probe case? How about to where it connects to the 2 resistors inside the probe?

jr

I have gone over the RF probe connections with a fine toothed comb and cant find anything wrong. There is no short in the cable. There is nothing touching inside where the shield goes to 1 Meg resistor and the center conductor goes to a paralleled 47K resistor. The connections are going to the right resistors and they measure reasonably well. I changed the .02 cap. I looked over the connection point at the unit and other than a slop job of soldering on the center conductor point, I see nothing wrong. I got out the Heathkit Cap checker and load tested the .005 (or so) that comes out of the center conductor point at the case.

The only thing left is the diode. I'm not sure how to check that unless it's like checking any other silicon job. So again, the unit is quiet as a church mouse until that center conductor comes into play when I hook it up. Then it works but I have to keep the gain almost all the way off. Grabbing the ground side of the RF cable makes it quiet.

jr_tech · #10 03-20-2016, 12:25 AM

So how many ohms resistance do you measure between the chassis of the T-3 and the metal body of the probe.... it should be very close to zero ohms if the shield braid is connected correctly and has continuity. Since you can quiet the hum by touching the ground side of the cable (where?) , tells me you still have a ground connection problem.

Does the hum change if you reverse the power plug in the AC socket?

Ohm out the diode as well, if you have the probe body opened again.

jr

Tubejunke · #11 03-20-2016, 01:15 PM

There really is no connection to the metal body of the probe. That stops at the plastic end and is isolated. There is almost no resistance from the chassis to the probe end ground connector. No resistance between the ground side (braided) and the center conductor. No change if I reverse the plug. The unit is quiet until I hook up the probe, so I feel the problem is there. If it hummed as it once did when I got it I would suspect power supply filter caps. 4X 20mfd. and all replaced although not the best job in the world stuffing them under the chassis. I took three of the very leaky caps out of circuit, but left one in jumping across it. I know that is something people do, but is deemed a no no by good techs. Again though, I think if I had a problem there that I would have hum with or without the probe connected, but maybe I'm wrong.

I edited to ad that I can clip the ground lead to the probe tip and quiet the noise. Of course that is shorting the whole probe as far as I know. I have heard of the crystal diode becoming faulty or shorted. I know it's not open. I would think that if the diode were shorted that it wouldn't pass a signal, but it does. I hate being stumped on such an utterly simple circuit. Thanks a ton though for reading in and giving input!

jr_tech · #12 03-20-2016, 01:30 PM

Take a close look at the diagram on page 10 of your manual... see the shield braid that is bent over to the "flat side" of the end cap?. The braid *should* provide a ground connection to the metal probe body. You should have close to 0 ohms betweethe chassis of the T-3 and the probe body.

jr

Tubejunke · #13 03-20-2016, 04:38 PM

Darn, your right. Whoever built it didn't leave enough braid to make that connection, but I can cut it and resolder. Maybe that's it. Don't know if I will do it today or not. Thanks man!

Tubejunke · #14 03-20-2016, 07:59 PM

JR.! Thanks my man! The unit works perfectly!! I have some "egg on face" but that's OK. As stated before, the circuit is very simple, but I have done any troubleshooting nor used a signal tracer, so not knowing the nature of the instrument didn't help. A lot of my checks were good logical troubleshooting, but I failed to hit the mark my misinterpreting the schematic.

Hey, at least I had sense enough to know something was wrong with the thing because again; it did work. But it would have been one NASTY piece of test equipment. Once again this great website and the people here have come through! Thanks again to everyone!

Tubejunke · #15 03-20-2016, 08:01 PM

JR.! Thanks my man! The unit works perfectly!! I have some "egg on face" but that's OK. As stated before, the circuit is very simple, but I have done any troubleshooting nor used a signal tracer, so not knowing the nature of the instrument didn't help. A lot of my checks were good logical troubleshooting, but I failed to hit the mark my misinterpreting the schematic.

Hey, at least I had sense enough to know something was wrong with the thing because again; it did work. But it would have been one NASTY piece of test equipment. Once again this great website and the people here have come through! Thanks again to everyone!