Heathkit T-3

[Throstur]

I bought a Heatkit T-3 signal tracer on ebay and it seems to be in great shape and works fine as far as I can tell. I have never tried or even seen one before! It did not come with a manual. I understand how to use it on a radio with the RF probe.

Can you help to understand how I should use it to trace a problem in a tube amp?. Should I inject a signal into the input of a faulty amp, with the amp turned off? Then trace the signal through the circuit with the audio probe by touching the components? Also, do I need a signal generator to do this, or can I use something else, or even make a simple signal tracer? All the best, T

[jjasilli]

You must have the amp under test ON.  Signal can't go through a tube unless the tube is ON (and bias is not cold enough to shut down current flow inside the tube). 

You 1st need to generate signal into the amp under test.  You can use the output of a cassette player, FM tuner, even an electronic guitar tuner.  jhadhar65 has posted a download to turn your computer into a signal generator:  http://www.monsterfactoryamps.com/downloads.htm

To use your signal tracer, connect its +input lead to a signal point in the amp under test, and the (-)lead to the chassis of the amp under test.  Connect to signal at a point in the amp under test where there won't be hi voltage DC. E.g., after a coupling cap is a good place. 

Go to Doug's Library Section and check-out Listening Amps.  That's what Doug calls a signal tracer.

For manuals, google "heathkit schematics"  or "heathkit manuals".  There are sites from which you may be able to download free schematics or manuals for your heathkit model.  If nothing free is available checkout Mike's Manuals on line.  Also keep an eye on eBay.  Various heathkit manuals come up for sale there.

Some people like to use signal tracers as mini guitar amps!

[Throstur]

Thanks a million jjasilli. This answer is  as good as a manual for me. I will however keep my eye´s on fleabay for a manual. All the best, T

[PRR]

Big file:
http://pdf.textfiles.com/manuals/SCANNERS-F-R/Heathkit-T-4-MANUAL.pdf

That's for a T4; I bet your T3 is super-similar.

It's just a dinky gitar amp, with the input well protected against large voltages. Feed signal to sick amp, verify that signal gets past the jack, then follow along noting that signal gets bigger at each stage (you have to turn-down the T3 to keep a reasonable volume).

[tubesornothing]

Man I'd get one of those just for the cool factor of those electronic eyes...

[Throstur]

Yes I find i a very cool, and I hope it´s useful to. This one is in excellent condition. I only paid 43 USD for it and that is not much IMO.

Thanks PRR for the file, very intesting. T-3 is larger than T-4 because it has a Watt meter circuit.

[PRR]

> I bet your T3 is super-similar.

T3 is actually quite different from T4. Here's T3:
http://www.tech-systems-labs.com/schematic/T-3SCH.gif

T4 is practically a small 12AX6 Champ. Smaller output bottle. And for guitar-only, we know we'll never overload the first stage, so the volume pot is between 1st and 2nd stages; in signal-tracing, we may get ANY size signal, so the volume pot is put at the very front. But a T4 would be a groovy mini-Champ.

T3 is more like the pentode-preamp Champs, plus an extra pentode used only for "RF" signals. I bet the 12C8 is supposed to overload on almost anything. If the "anything" is an AM-modulated wave, the modulation (audio) comes out of the overload.

Feeding guitar right into T3's "Audio" input makes sense. KAY used to sell sleezier "amplifiers".

Three-position switch: MUST be in normal position for most work. The middle position injects 200V DC into the amplifier being tested! The idea is that bad contacts and arcing caps will not like this treatment, and go from intermittent to dead. Yeah, when you repair $9.95 radios a dozen a day, what the heck, zap it. I would not want that on most guitar-amp work, not without careful thought about what could or should happen.

The 3rd position of the switch mutes the audio and wires the eye to a front-panel "wall outlet". Plug-in a dubious appliance, wiggle the watts pot until the eye blinks, you know how much power it draws from the wall. This is VERY rough. And in most cases, not real useful. If it pulls much less than nameplate power, you will already know cuz it won't work right. If it pulls much more than nameplate power, it probably burned-up before you got to the tester. And it will be inaccurate on high-efficiency (solid-state rectifier) power supplies. Still, you may as well plug in a 100W incandescent lamp and see it work.

If you have a power amp (SE or push-pull) and you do not know if the OT is good, disconnect primary and jumper to the P B+ P terminals, keep secondary to a good speaker, play guitar through the T3 with T3 speaker muted. You should get the full 4 Watt goodness of the little 12A6 bottle. Well, maybe 2W due to impedance mismatch, and maybe weak bass because the T3's OT is probably skimpy, but clearly "works", not "dead" or "only sizzle".

Speakers can connect to VC and C, and should play like several Watts.

Feeding gitar into "RF Input" will "work", and probably be VERY annoying loud fuzz. Note just 14V on plate!

If you are tracing for very small hiss, yank 12C8. Even when the RF input switch is short, this stage adds hiss.