Built myself a retro signal tracer for tube radios
 

Here is the homebrew signal tracer project I made. I tried to keep it retro in styling and electronically. It is 100% "hollow state" --there are nothing but tubes in the circuits right down to the 6AL5 small signal rectifier to drive the meter.

The speaker is a 5 watt, 4" unit. Fidelity is good, as it must be to find sources of distortion or excessive hum in an amplifier or radio. There is an output for an external meter or scope if specific (relative) signal voltages are needed or if the audio waveform or RF modulation envelope (demodulated) is to be observed visually. I also put in a noise test circuit with a momentary push button that puts out B+, through a high value resistor, to the audio input probe. Any roughness in this current flow caused by a cold solder joint or suspected "noisy" component is amplified by the audio amplifier. That test works similar to the old Eico 147 signal tracers.

I prefer a meter to an eye tube, so this meter was salvaged from the junk box. It is an old HP 400 series meter. The RF input probe uses a subminiature 5603WB triode as a grid leak detector to demodulate AM radio signals and amplify the corresponding audio signal, which is then sent to the audio-taper gain control potentiometer in the grid circuit of the 1st 12AX7 dual triode amplifier. The other half of this tube gives another stage of gain and feeds the grid of a 50C5 power pentode to drive the speaker though an audio output transformer. The secondary of the audio output transformer also feeds a signal to a 6AL5 dual diode tube to rectify the signal to the meter. The meter gives a relative signal level, and is responsive enough to bounce along with audio of a radio station or give a steady reading from a tone injected into the device being tested. The speaker can be muted by a single pole double throw switch that cuts out the speaker and replaces it with a 5 watt, 10 ohm load resistor. The power supply is a 35W4 half wave rectifier tube filtered by an RC pi filter.

The unit is sensitive enough to give a clear signal right from the antenna lug on an AM radio that is tuned into any reasonably local radio station, or can be turned down enough to get a signal right off the plate of a typical radio's output tube. It should be handy for audio amplifiers and phonos as well as radio work.

The faceplate and chassis are made from 6061 sheet aluminum. The cabinet is 1/4" red oak with poplar framework. The probe housing is made from 3/4" brass tubing, insulated with heatshrink.
I am still waiting for a 4 pin jack to arrive for the final hookup to plug in the active RF probe, and also have a black hinged handle ordered for the top of the cabinet.

(Note: I do not claim that this tracer meets UL or any other safety standards, but it is only for my use, and I know where and how I'm using it.)

http://i553.photobucket.com/albums/j...ps7c075aaf.jpg

http://i553.photobucket.com/albums/j...psaac37b79.jpg

http://i553.photobucket.com/albums/j...ps2ae50548.jpg

http://i553.photobucket.com/albums/j...ps820620ca.jpg

http://i553.photobucket.com/albums/j...ps62933cf0.jpg

http://i553.photobucket.com/albums/j...ps37cd7961.jpg

http://i553.photobucket.com/albums/j...psf01810b0.jpg

http://i553.photobucket.com/albums/j...psc68e56e2.jpg

http://i553.photobucket.com/albums/j...ps6549e149.jpg

http://i553.photobucket.com/albums/j...pse19cc39d.jpg

http://i553.photobucket.com/albums/j...ps0675b3b2.jpg

Very nice!

Thanks for the pictures.

Thanks. It was fun. I need new pics of the proper jack installed for the RF probe and the handle on top where I can wind up all the cords.

Ya done GOOD ! (grin)

Outstanding job!

I'm convinced that solid state electronics and overhead valves for automobile engines are passing fads. :D

LOL. Thanks guys.
I used it to test a phono pickup and a weak IF stage in a radio so far. It works great. I have a factory built Eico 147 Tracer that has been restored and works well, but I kind of prefer this one.

A great deal of craftsmanship is evident. I would use a microphone connector from a scrap CB rig, for the probe connector. They usually have several pins for the PTT function. :thmbsp:

That's exactly what I ended up using! :thmbsp:

What did you use for punching the holes in the aluminum chassis. They look a lot cleaner than a drilled hole.
BTW, I never saw a 6AL5 of that style. It must be foreign make.
I would've liked to have seen the inside of the probe.
I have a fairly new Hickok, signal tracer, set tester. There's a tube in the probe, as well. :yes:

The is nicest bit of home-brewing I have seen in years. :thmbsp::thmbsp::thmbsp:

VERY NICE! I want one!

With a cabinet like that, It belongs in the living room, instead of the work shop. :thmbsp:

Thanks guys.
dieseljeep: i used a step drill bit. I will take pictures of the probe insides and the finished unit with the mic jack...I forgot.

The 6al5 is in fact a european four numbered equivalent drop in for an American 6al5. I can't remember the number. The insides are the same but the bulb is taller.

I was thinking about a EAA91, but you said four numbered.
BTW, I'll keep the step bit idea in mind. :thmbsp:

A sharp step bit does a great job on sheet metal if you keep it from walking around. I drill a 1/4" pilot hole with a regular bit, then clamp the work piece to a chunk of 2x4. Then I drill down through the metal and into the wood underneath. It helps keep the bit from wallowing around or snagging. A little de-burring, if needed, with sandpaper and I get a neat hole for tube sockets or whatever. I have been meaning to buy a set of Greenlee punches, but they are expensive...well so is a step bit, but I already have that.

Here are some final pictures:

http://i553.photobucket.com/albums/j...psf4f3335e.jpg

http://i553.photobucket.com/albums/j...psb6adbac4.jpg

http://i553.photobucket.com/albums/j...ps2b7dcbef.jpg

Why a hot chassis?
 

I keep looking at this thread, and can't keep my mouth shut any longer

1. Beautiful job on the cabinet

2. Awesome metal work

3. Why did you use a hot chassis? This thing could kill someone, and an isolation transformer capable of powering it is cheap, i.e. $20 at digikey or mouser. It would also prevent ground loop/hum issues during use, and make it a more useful test instrument.

It's kind of hard to tell if the chassis is the B-. It looks like the negative on the electrolytic is connected to a B- line of the terminal strip.
It's easy enough to use an isolated B- line, bypassed to the chassis by a .1 mfd cap to the chassis. Maybe, that's what was done.
As a retired electrician, I can't stress enough that your work bench has a GFCI receptacle.
I make portable GFCI's, by using a plastic single device box, a three wire power cord and a GFCI receptacle.
Even if your place, doesn't have grounding receptacles, using an adaptor will give you the same protection, using the portable GFCI. :thmbsp:

If you click on the images it goes to his photos, and this one is also there

http://i553.photobucket.com/albums/j...ps28dd0903.png

It confirms what I thought - hot chassis.

Merry Christmas all!

Double post

Thank you for the kind comments.

As I wrote in my very first post:
This is an experimental unit for my own use. The chassis is connected to the ground prong of the cord and is fused. It is still, in my opinion, safer than the hot chassis vintage radios that it is promarily intended for. An isolation transformer is a must.

This entire device is posted for, as they say, "entertainment purposes only."

cheers
Rob

The wide blade is the "neutral", not the "ground"... There's a variety of scenarios which can result in the neutral becomin live, so a polarized cord with the neutral conductor bonded directly to the chassis isn't really enough to say the chassis is grounded.

In the context of using it only at home and only with a known safe outlet I agree risk is minimal, I just had to bring it up :) plus you never know, some day this may pass from your hands into those of some wide eyed kid who has never heard of a live chassis.

I understand, but the wide blade doesn't go to the chassis. Can you see the legend on the left there where the chassis ground symbol is different than the B- ? The center prong on the plug...the widest and longest center one in the picture...does not represent the prong in the grounded circuit conductor (often technically incorrectly referred to as "neutral" in a 120V single phase circuit). The prong to the chassis is the system ground. The metal chassis is at the same potential as the earthed grounding conductor of the building wiring.

Scroll down and see the right hand column: http://www.electronic-symbols.com/el...rs-symbols.htm

Yes, on the schematic, but the photos in post #1 show very clearly a two prong ungrounded cord, with one of the conductors attached to the chassis, and not a three prong grounded power cord. Also, the schematic also shows signal ground to the probe connected directly to the neutral wide blade, rather than to the third prong. If there was a three prong cord, and the safety grounded third prong was also connected to the probe, of course this would be an entirely different matter, I am only making a comment on what I see.

Ah yes, the three pronged cord was not installed yet. The schematic reflects the construction plan, the photos are only of the construction in progress. It had a two prong cord for testing and adjustment. I did not have a good computer type cord on hand during construction. There are also part missing in the photos. I thought it obvious that the schematic would be referred to, and not the under constuction photos--missing parts and all--as to how I was building my tracer. The signal ground is through a capacitor to allow the chassis to act as a shield against feedback or external noise. The chassis itself is in fact hard wired to the earth prong and the power conductor is fused. The fuse should blow if the chassis is cross connected to any "hot" lead.

I don't understand why you posted the schematic above, and stated that it (the schematic) confirms it to be a hot chassis, but now are referring to the under constuction photos to state that it is a hot chassis?

This is where I get lost in your comments:

If you look at the GND alligator clip going out to the probe, and the shield for the audio in and the scope out, all those on the schematic connect back to the signal ground/ AC line neutral and not The chassis ground. Since you didn't actually use a grounded power cord and went to the trouble of using a heyco strain relief clip it looked permanent to me. If the GND clip was capacitor isolated like in the vintage designs this would obviously not be a point for discussion.

I never intended to sound argumentive and I apologize if this has gone in that sort of a direction. The only reason I even brought it up, is I once decided to use a polarized cord and not bother with isolating the chassis on a phono amp to get rid of hum, and ended up giving myself a huge shock with a miswired extension cord.

I didn't take it that way, and I'm sorry if I sounded that way.

I will delete reference to the schematic so that there will be no more confusion.