Speco Portable Signal Tracer

[dieseljeep]

Originally Posted by dewdude

I really didn't want to go digging through that dusty, snake infested, spider infested, cobwebbed mess of a shed looking for the book. Still, after I looked more at the thing to see how the filaments are connected; I was confused. I googled for a while; could find nothing about this tracer.

So, back up the shed I went; and on the floor under a recently placed bin; I found the book; which was lucky because I expected to be up there a while and get bitten by something.

It's a Speco Model STAB. It actually used two batteries. "2-#2 flash light cells"; and any number of 67.5V batteries. There is also apparently a 1S5 tube in the probe; so I'll have to pull that apart.

I think I might actually have a transformer that puts out in the neighborhood of 60V; so I may be able convert it to AC.

schematic: https://flic.kr/p/nHDKWV
tech details: https://flic.kr/p/nFB9ih

The B+ source is relatively easy to build, the 1.5 volt, high current supply is a little more difficult. The schematic shows that the filaments are wired in parallel.
BTW, the 3S4 is only rated for around 67.5 volts, plate and screen, so I would try to stay around those parameters.

[dewdude]

Originally Posted by dieseljeep

The B+ source is relatively easy to build, the 1.5 volt, high current supply is a little more difficult. The schematic shows that the filaments are wired in parallel.
BTW, the 3S4 is only rated for around 67.5 volts, plate and screen, so I would try to stay around those parameters.

I was planning on building this:

http://www.antiqueradio.org/art/Powe...ySchematic.jpg


My filament draw is only .2 amps; and quite frankly; I'd those off batteries if I had to. But that has a 300ma 1.5V filament supply and 20ma b+ supply; the b+ draw is about 9ma. I think that transformer is only around $11; and the parts are small enough I could build where the batteries would reside.

[dieseljeep]

Originally Posted by dewdude

I was planning on building this:

http://www.antiqueradio.org/art/Powe...ySchematic.jpg


My filament draw is only .2 amps; and quite frankly; I'd those off batteries if I had to. But that has a 300ma 1.5V filament supply and 20ma b+ supply; the b+ draw is about 9ma. I think that transformer is only around $11; and the parts are small enough I could build where the batteries would reside.

That's an interesting looking power supply. I already have some of the parts for it.
I'll use a power transformer from a scrap VTVM, that wasn't worth repairing.
I can always use an extra power supply for a battery tube radio.

[dewdude]

Originally Posted by dieseljeep

That's an interesting looking power supply. I already have some of the parts for it.
I'll use a power transformer from a scrap VTVM, that wasn't worth repairing.
I can always use an extra power supply for a battery tube radio.

That power supply looked much safer than some of the transformerless ones I saw. There was another one on that page that basically rectified line voltage; which would have required all sorts of isolation.

The transformer I think is about $11; which doesn't seem too bad. We had a very similar supply at work that came as a kit (I had to assemble). It was different than this circuit; provided more voltages, but fundamentally the same.

It worked rather well. Never had a ground problem; so I'm pretty confident this design will work well.

Fair enough that's a pretty good argument for a power supply!

Well, this is what I told my boss. If he wanted me to get amp's fixed and off the bench in under an hour (because god forbid he pay me more than $12 to fix an amp); I don't have time to remember to turn the signal tracer on when I have to worry about high b+ and other issues. I can work safely and efficiently; but I have to do things like leave test equipment on during the duration of the day.