"Lafayette" LSG-10 Signal Generator

[MattyMatt]

I'm now onto a new project.... Now that my scope is working properly.

I have a bit of electronic gagetry, mainly because I tend to snag interesting looking things, and I am an aspiring Ham radio operator... so I love learning about all kinds of stuff, and honestly, what better way to learn how something works, then to try to fix it .

Okay, so, as the title of this states, this is a "Lafayette" LSG-10 Signal generator. The "Lafayette" part is because it wasn't actually manufactured by Lafayette... as far as I can tell.

Stark also had this same model, as did Leader. Leader I believe is the true manufacturer, as this unit says Japan on the back.

At any rate, my first instinct is to plug it in, turn it on, and lets see what works and what doesn't. Well once I switched it on the power light lit up, that's a good sign.

So, I decided to start off with the basics, considering this is an RF generator I know I am not going to hear anything coming out of it, so I decided to use my newly repaired scope to see what I could find. Well to my surprise, it seems to be actually generating a wave... bonus! However, after digging around a bit, I have not been able to change the wave at ALL... now, maybe I'm just being silly but there should be something that I can change that will either change the amplitude or the frequency of what I am seeing, right?

Okay, so I tear into this little device, just to see what I am getting into, and i'm greeted with a wonderful surprise.... TUBES! Sorry, I really love tubes, or valves, or whatever you would like to call them... they are wonderful, and yes, I'm a bit of an audio geek as well.

Anyway, I was just wondering if I am completely off my rocker, but I was kind of hoping to see if I could manage to get some kind of function generator, if anything else to verify stuff on the newly repaired scope.

At any rate, I have attached some pictures of the Signal Generator, with a pic of what its output looks like, along with I believe to be the correct circuit diagram.


Let me know if I'm completely ass backwards, and trying to do something I can't, or if I'm in my right mind, and something is broken.

Thanks again!
Matty-

 

[Resqueline]

Tubes are cool, and I'm sure you can get the generator up & running. What frequency range does it have, and what frequency is it putting out now?
One thing that comes to mind is to check all the switches. They're most likely silver plated and could be severly oxidised.
If that's ok then measure the high-voltage & all tube voltages (anodes, grids, cathodes). Verify heater colors.
Some of those old paper/wax capacitors are prone to developing high leakage currents.

 

[daddles]

Looks like something from around the early 1950's. It's extra cool that you have the schematic for it and I'm with Resqueline in betting you'll be able to get it to work fine. Something well-made that you can maintain that has parts you can still get should work indefinitely. Thus, you might want to pick up some spare vacuum tubes, verify they work, and safely stash them somehow in the back of the instrument.

Once you get it working, use your DMM and scope to monitor waveforms and voltages at each point in the circuit and document them. Then troubleshooting will be easier later when needed.

One thing I do for old stuff is to take the thing apart and use an antiseize compound (any auto parts store will have it) on all the threads (including the set screws for the knobs). You only need a small amount. This virtually guarantees you'll be able to get things apart later. There aren't many things as aggravating as having a set screw's socket strip out because the set screw has rusted in place. You spend an enormous amount of time on something that should only be a few seconds of work.

 

[duke37]

There are two outputs, one audio frequency and is fixed and one radio frequency which is variable in frequency and can be modulated by the audio frequency.
I assume that you are looking at the audio output. you can check the RF output with a radio placed nearby and swinging the tuning till the radio and gen match.

 

[daddles]

It appears the audio jack will give you access to the internal audio oscillator (they were typically around a 400 Hz sine wave) and you can also use it to input an external modulating signal.

 

[Rustynutz]

Don't expect anything to cleaver out of this.
on the plus side looks nice and clean inside, and you should get hours of fun fixing it up?

 

[MattyMatt]

Sorry it took so long for me to get back to everyone, but here's some answers and questions and such:

What frequency range does it have, and what frequency is it putting out now?

110khz to 130mhz (I think... there are multiple labels towards the bottom of the scale, I think it has to do with harmonics, but the lower numbers start @ 120mhz to 260 mhz.

Speaking of Hz, I love when things were labeled in KiloCycles and MegaCycles

As far as what its putting out now, which output were you referring to?

One thing that comes to mind is to check all the switches. They're most likely silver plated and could be severly oxidised.

This i am going to be doing slowly, I got some stuff to clean with, but I will probably be taking a bit to get back to you on this one. some of the front terminals look as if they are pretty bad. Switch elements may be worse...

If that's ok then measure the high-voltage & all tube voltages (anodes, grids, cathodes). Verify heater colors.

The heaters are a nice orange color right now, not sure what the voltage should be on the tubes... and my tube tester apparently doesn't test anything more than the filament. my Line voltage is right about 115VAC.

Some of those old paper/wax capacitors are prone to developing high leakage currents.

Yikes! any suggestions on how to test and what to replace with?

One thing I do for old stuff is to take the thing apart and use an antiseize compound (any auto parts store will have it) on all the threads (including the set screws for the knobs). You only need a small amount. This virtually guarantees you'll be able to get things apart later.

That's a wonderful idea! I do that for all kinds of car stuff.... never really thought to bring it inside, I'll give that a shot

There are two outputs, one audio frequency and is fixed and one radio frequency which is variable in frequency and can be modulated by the audio frequency.
I assume that you are looking at the audio output. you can check the RF output with a radio placed nearby and swinging the tuning till the radio and gen match.

Yes, I was hooked up to the AF side (Audio frequency I presume) when the shots were taken. 10V/5ms /DIV was where my scope was at... so actually this may answer Res's question about 60 cycles.

Don't expect anything to cleaver out of this.
on the plus side looks nice and clean inside, and you should get hours of fun fixing it up?

Yes it was VERY clean... nice to see... and yeah I will have a BLAST hooking this up and getting it to work.

 

[(*steve*)]

Yikes! any suggestions on how to test and what to replace with?

There's lots of site's on the net that talk about this. I'd advise reading some of them because you may be interested in just making it work, or making it look "original".

Here are some sites: here, here, and here (the last mentions "stuffing").

Summary is:

* typically do not replace mica
* replace all paper (some have a plastic case!) -- replace with polyester
* replace electrolytic with electrolytic.
* be careful with point to point wiring (insulation and inductive effects)

 

[Resqueline]

As far as what its putting out now, which output were you referring to?

Uh, you tell me, what output you were referring to that had a fixed frequency & amplitude, AF/RF..?
The AF is both in & out and provides a fixed audio frequency with a variable amplitude in the INT position (as far as I can tell).

Yikes! any suggestions on how to test and what to replace with?

By measuring actual in-circuit operating tube voltages..

 

[Rustynutz]

As regards paper electrolytic caps
A mate of mine come up with a good idea, as the new caps are smaller you can put them inside the original paper covers probably with some packing and keep the original look wile maintaining a working machine.

just a thought for restoration projects.

 

[(*steve*)]

A mate of mine come up with a good idea

Along with many, many other people I'm afraid.

It's called "stuffing" and I mentioned it above