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Old 24th Feb 2017, 9:52 pm   #1
Neil Purling
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Default Signal Tracer

I want to make a probe that would plug into any amplifier and but the one I have to hand is my Mullard 3-3. Thus I can use it as test equipment.
The total of parts seems to be two capacitors, two resistors, a switch and old meter probes from the junk box. The capacitors will have high voltage ratings as input and output protection for the diode and amplifier. I am thinking of 600- 1.2KV.
The specified diode is a Mullard OA79. I am wondering what the OA79's ratings are in respect of voltages & what frequencies it could deal with. Is there a suitable alternative on the market today?
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Old 27th Feb 2017, 9:39 pm   #2
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Default Re: Signal Tracer

Hi Neil
You can but into the search box at top OA79 this should lead you to a thread dated 27 April 2012. This gives information about OA79 pin contract diode 45v
regard Derrick
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Old 27th Feb 2017, 11:47 pm   #3
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Default Re: Signal Tracer

I am pretty sure that a slightly larger outline audio chip can be got that runs on 1.5 or 3 volts. Then all you have to do is save an old greetings care for the speaker and any cheap hand held gadget with a battery holder and build a bespoke tracer into it.
I was thinking of a nose hair trimmer with the amplifier fitting in the motor space or a cheap E-cig with the amplifier in the "fuel tank".
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Old 28th Feb 2017, 12:45 pm   #4
frankmcvey
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Default Re: Signal Tracer

Hi, Neil,

A Mullard 3-3 on the bench? Man, that's like using a golden mallet to crack a peanut, or using the Bentley to collect the milk!

As Refugee suggested, there are dozens of circuits out there on the net for small audio amplifiers or signal tracers using something like the LM386 amplifier chip, a small strip of veroboard, a PP3 battery, a small loudspeaker from a trashed pocket tranny and half-a-dozen discrete components.

They're versatile, light, handy and don't take a lot of space on the bench or in the drawer; they use a wee battery instead of one of your precious mains sockets; they don't need a floor-standing loudspeaker cab and if you knock one off the bench, it'll probably bounce, rather than making you cry like dropping a Mullard 3-3 would.

And, from a safety point of view, there's one less large lump of earthed metal sitting on your bench waiting to complete the circuit if you brush against something live...

Apart from anything else, it's a rite of passage for all aspiring radio techs to build their own signal tracer, preferably using a 2oz Golden Virginia tobacco tin**. As a rite of passage it's pretty good. For example, it's a lot cheaper than a prom Ball, and certainly doesn't hurt as much as circumcision or a tattoo!

You could also knock up a homebrew signal injector, using something like a 555 IC, then you'd have a matching pair.

Try googling "LM386 signal tracer" or "555 signal injector" The ICs are as cheap as - ermm - chips on Eblag. Hell, PM me your address and I'll stick a pair in the post to you - plus a 2oz tobacco tin!

Cheers,

Frank

** Just because. That's why!

Last edited by frankmcvey; 28th Feb 2017 at 1:08 pm.
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Old 28th Feb 2017, 12:49 pm   #5
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Default Re: Signal Tracer

http://www.pasttimesradio.co.uk/build/diyradio.html

Nice easy one here.

Andrew
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Old 28th Feb 2017, 2:02 pm   #6
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Default Re: Signal Tracer

Quote:
Originally Posted by frankmcvey View Post
................

The ICs are as cheap as - ermm - chips on Ebay. Hell, PM me your address and I'll stick a pair in the post to you - plus a 2oz tobacco tin!

Cheers,

Frank
Given what two ounce tobacco tins seem to go for in bric-a-brac shops these days, that's a very generous offer!

FWIW St Bruno (brown) ones work just as well, or Escudo (white) if you prefer the circular look.
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Old 28th Feb 2017, 2:39 pm   #7
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Default Re: Signal Tracer

Quote:
Given what two ounce tobacco tins seem to go for in bric-a-brac shops these days
Oh, I'm not short of a tobacco tin or two! These were once the small components storage system of an old radio guy I knew, who smoked like an ICI beagle. Given the price of 2oz tins and interest rates nowadays, they now represent my pension pot...

Cheers,

Frank

Note - the more eagle-eyed will have spotted a mixed bag of Jeremy's coils awaiting sorting and storing!
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Old 28th Feb 2017, 3:51 pm   #8
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Default Re: Signal Tracer

An easy approach would be this: http://www.velleman.co.uk/contents/en-uk/p261.html

See here for David's write-up on his build: https://www.vintage-radio.net/forum/...76&postcount=4

That post also includes details of an RF detector probe.

I must admit that for all sorts of reasons (including safety), I agree with the advice to use a stand-alone, battery powered amplifier that isn't connected to mains earth. It wouldn't take long to build the Velleman kit (or equivalent) into a small plastic or wooden box.
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Old 28th Feb 2017, 4:26 pm   #9
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Default Re: Signal Tracer

You can also still get the Radioshack version quite cheaply:

http://tech-shop.co.uk/product/radio...dio-amplifier/
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Old 28th Feb 2017, 6:59 pm   #10
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Default Re: Signal Tracer

Quote:
Originally Posted by mhennessy View Post
An easy approach would be this: http://www.velleman.co.uk/contents/en-uk/p261.html
...........
It wouldn't take long to build the Velleman kit (or equivalent) into a small plastic or wooden box.
It should fit into a tobacco tin for the genuine vintage "look", maybe smaller pots might be needed.

I'm not sure the phase shift oscillator would generate dc-light injection the same as a multivibrator, though.
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Old 28th Feb 2017, 7:44 pm   #11
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Default Re: Signal Tracer

Old Dell PC internal speakers have a dinky wee IC audio amp ready built - designed for 5V but they start to work at about 1.5V. I used to rob them from work machines because it was quicker than going into settings to shut all the silly Windows noises up.

Cheers,

Colin.
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Old 28th Feb 2017, 8:43 pm   #12
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Default Re: Signal Tracer

Another trick which I have used (I can't remember where I got it from, but I don't think it's original to me) is to start with a cheap pocket AM radio. When I did this such radios were on sale new for a couple of pounds...

Anyway, start by disconnecting the earphone socket and wiring the speaker directly to the radio. Now break the connection to the 'top' of the volume control and wire the now-disconnected socket : sleeve to the earthy end of the volume control, tip to the top end of the volume control, break contact to the signal from the detector stage that did go to the top of the volume control before you disconnected it.

The result is a working radio that can be used as an audio amplifier (signal tracer, etc) by plugging the signal into said socket. Doing so disconnects the radio part and feeds the signal into the audio amplifier.
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Old 27th Apr 2017, 9:31 pm   #13
Neil Purling
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Default Re: Signal Tracer

Tony's idea is a good one. I have a rather 'cosmetically challenged' Benkson 323.
I had to remove the earphone jack and wire the speaker direct. Be easy enough to replace the jack & drill the case for the wire & plug carrying the signal from the detector.
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Old 27th Apr 2017, 10:23 pm   #14
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Default Re: Signal Tracer

I built one back in 1977 and still use it. See here http://www.americanradiohistory.com/...PW-1977-07.pdf

Page 193. A very simple circuit using an FET, a 741 op-amp, a few resistors and capacitors on a small piece of veroboard. You can build it into any sort of small case but I bent up the aluminium and made the one described. I use mine with a pair of 'stethoscope' style headphones that came from a Grundig dictation machine. I added a diode RF probe and it's sensitive enough to pick up a signal from a ferrite aerial. It was invaluable when tracing audio switching faults on tape and cassette recorders.

I doubt it took more than a couple of hours to build and it was in use almost immediately. Never failed and still has all it's original components. I prefer an earpiece when signal tracing. There is no danger of acoustic feedback and you can hear weaker signals without straining....plus if you are in a workshop environment (I was), it doesn't annoy other engineers.
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Last edited by Sideband; 27th Apr 2017 at 10:35 pm.
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