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 Post subject: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Mon 19, 2018 8:40 pm 
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Location: Marquette, Michigan, USA
I have an old Heathkit signal tracer on my test bench, it works fine, except it is always noisy, not really 60 cycle hum, like a bad filter, but a broadband noise, no matter where the volume control is set. I have heard guys say the input stage is run full on, for maximum sensitivity, but shouldn't there be a way to quiet this beast? Any suggestions, aside from getting a better one are helpful, thanks.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Mon 19, 2018 9:13 pm 
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There is a Mr. Carlson's Lab Youtube video on the signal tracer; it describes how to make a simple modification to make it quieter. Yours may have a problem though, but the video should help.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Mon 19, 2018 10:19 pm 
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Location: Marquette, Michigan, USA
My buddy mentioned something about changing as plate resistor to a metal oxide type instead of the carbon one. Does that make sense?


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Mon 19, 2018 11:46 pm 
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Bob S wrote:
... a broadband noise, no matter where the volume control is set.

Changing plate resistor type or modifying the input stage circuit isn't going to fix your problem if the noise doesn't vary in volume with the volume control. That means that the fault is in the output stages.

Have you replaced the electrolytic and paper capacitors? Paper caps can generate noise if they are breaking down and on their way to shorting. Any broken resistors? Probe around with a chopstick and prod the resistors and solder joints - does the noise change when you poke something? A bad resistor or solder joint can generate noise.

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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Tue 20, 2018 4:00 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Marquette, Michigan, USA
I haven't really troubleshot the tracer yet. Your hints are a great starting point. I have never considered checking resistors for being noisy, thinking they were either good or out of spec value wise. I will give the tracer a good going through at first opportunity.
One more question, could a bad tube cause noisy operation ? I have heard that some tubes are noisier even micriphonic, so maybe that could be a source of trouble too.
Thanks for the help, I really appreciate it.

Bob.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Tue 20, 2018 4:02 am 
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A tube is the first thing I would look at, they can indeed become noisy. That won't show up on a tube tester either, the only way to find it is to substitute another tube that you know to be good.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Wed 21, 2018 11:21 pm 
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Location: Marquette, Michigan, USA
Point of reference, my signal tracer is an IT-12. I watched the Dr Carlson video and that was useful, even though it isn't the same device. We just got home from vacation so it will be on the list of things to do, after unpacking, groceries paying bills etc, you know the drill.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Thu 22, 2018 1:39 am 
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Location: Beaver Falls, PA. USA
If it is an older model, replace the selenium rectifier; it's in a little potted block mounted on the top of the chassis. A silicon diode can be mounted on a terminal strip in its place. The next suspect is the .022 MFD coupling capacitor which feeds the 12CA5 grid. If you still have problems, tube substitution is next. Also, check all of the solder joints and ground connections.

I have owned one since the early 1960s, and have kept it going all those years! It's still on my bench.

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Last edited by Tim Tress on Feb Thu 22, 2018 1:46 am, edited 1 time in total.

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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Thu 22, 2018 1:44 am 
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Location: Mpls, Minnesota
The selenium will not produce any noise but the silicon diode will.

Dave


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Sat 24, 2018 4:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Marquette, Michigan, USA
Ok, so I have done some work on the signal tracer. It has a silicon diode rectifier, and ceramic, not paper capacitors in it. I poked around, and found that tapping on C4, the coupling capacitor between the first stage and second stage made noise. I changed it, and C3, the input to the first stage coupling capacitor. The noise persists, but is reduced somewhat. It is a hiss, and there is just a little ac hum component too. The 12AX7 is microphonic, tapping on the tube, or anywhere around it makes a clanging noise, I will have to dig up another one, and change it for sure.
Following Mr. Carlson' troubleshooting procedure, I did what he did, and shorted the plate connections on the 12AX7 tube to ground, independently. In both cases the amp went dead quiet when I did that. I am going to make the assumption that the culprit is the first stage, and work from there. Anyone have any ideas on how to proceed at this juncture?

Bob


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Sat 24, 2018 4:37 pm 
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Location: Mpls, Minnesota
As you have found capacitors are microphonic and ceramic's are the worst, most 12AX7 tubes are also microphonic. Signal tracers and other high amplification devices will always have some hiss or white noise. This noise is generated in tubes, and resistors and is normal.

The noises originating in vacuum tubes and the attached circuits are investigated theoretically and experimentally under three headings: (1) shot effect with space charge, (2) thermal agitation of electricity in conductors, (3) noise from ions and secondary electrons produced within the tube.
http://ieeexplore.ieee.org/document/1670580/

Dave


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Sat 24, 2018 6:10 pm 
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Bob S wrote:
I am going to make the assumption that the culprit is the first stage, and work from there. Anyone have any ideas on how to proceed at this juncture?

I just took a look at the schematic for the IT-12, and I see that it doesn't have a preamp/volume control/audio amp design. The volume control is the first thing, then three stages of amplification. OK, so the fact that the noise doesn't vary with the volume control doesn't mean much.

As Dave says, there will always be some level of hiss in a high-gain amplifier. Trying another 12AX7 is easy if you have one on hand. (Otherwise I hear it's painful in the pocketbook these days.) Changing R2, R3, and R4 to be metal film resistors instead of carbon composition might help a little.

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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Sun 25, 2018 10:37 pm 
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Posts: 92
Location: Marquette, Michigan, USA
Ok, so the white noise is essentially a product of the tube itself? Changing the two capacitors did help some. And I didn't have any ceramic ones of that value ( .005 ) so I found some tubular ones, , orange drops, of that value. They are larger, but there is plenty of room under the hood.
No problem obtaining 12AX7's, my best friend collects tubes, and old radio gear, probably has a couple hundred of them ( no joke....he LOVES them ) , so I provably convince him to part with a couple for the cause. Just as long as I don't ask for Miullard or Amperex ones. He'll take those to his grave, really, has no interest in sharing those with anyone.
I might try changing out the resistors too. I tried tapping on them with a chop-stick, and they made some noise, but because the tube is microphonic, everything under there makes racket when you poke it.


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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Feb Sun 25, 2018 11:54 pm 
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Bob S wrote:
Ok, so the white noise is essentially a product of the tube itself?

And the resistors. They both produce thermal noise:
https://en.wikipedia.org/wiki/Noise_(electronics)#Thermal_noise
https://en.wikipedia.org/wiki/Resistor# ... rmal_noise

Tubes also produce shot noise:
https://en.wikipedia.org/wiki/Noise_(electronics)#Shot_noise

Carbon composition resistors also produce flicker noise:
https://en.wikipedia.org/wiki/Flicker_noise

Changing the plate and cathode resistors from carbon composition to metal film will eliminate flicker noise, but the flicker noise might be overwhelmed by the thermal noise. Try a different tube first. If there's no difference, then try replacing the cathode and plate resistors in the first state with metal film. There still might be no change in noise level.

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 Post subject: Re: Quiteing noisy Heathkit signal tracer.
PostPosted: Mar Tue 06, 2018 6:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 92
Location: Marquette, Michigan, USA
Ok, finally some real improvement. I changed out the cathode and plate resistors on the first stage with metal oxide types, and put new coupling capacitors there too. I now only have some ac hum, and mostly when I open up the gain way up. sitting with the volume control fully counterclockwise it is quiet, no annoying hissssss ! I still need to change out the 12AX7. My buddy offered me quieter one with different heaters and more mica insulators that is supposed to be much better at supressing vibration and noise from the a/c heaters. He really knows tubes, so i'll have to take his word for it.

Who knew such a simple device could be that troublesome to get quiet. I am looking forward to using it on my many backlogged projects.

Thanks everybody for all your help and suggestions.


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