Heathkit general purpose oscilloscope model 10-21

[punkykatt]

Let me see if I have this right. I will be using the vertical input only. Black banana jack lead to chassis ground, red banana jack lead to control grids and OPT secondaries to check sine waves.  What are the horizontal inputs used for?   :iamwithstupid:

[tubesornothing]

Its probably so you can do x-y plots.  In this case rather than use the scopes built-in time base as the X axis, you supply the X axis.  We dont use this much in guitar amp fixing, but it can be fun.

Here is a thread that discusses it:  http://www.el34world.com/Forum/index.php?topic=6948.0

And tuning a guitar: http://www.ymec.com/hp/signal2/tuning3.htm

[PRR]

> enough of my fancy probe stuff!

I have precision tools, but I also use a stone hammer when appropriate.

I'm just saying: this 'scope, guitar amps, punkykatt's level of experience.... clips on zip-cord will get the learning started.

Yeah, if the red lead lays on the PT while conected to first plate/second grid, he'll see a little hum (a wiggle that syncs solid on the "Line" sweep setting). Moving the red lead around will change it. That's good learning. Don't get that from books.

Eventually he learns why shielded probes are sometimes useful. Down the road he may want to hunt hum, and may need shielded leads to reduce stray lead hum. But unless his bench is very hummy (under the fusebox or next to the electric stove), he can dance around this a long time. And you can often find hum in an -amplifier- by watching the final output and killing stages in sequence. On a Champ, watching the speaker, if hum goes down when Volume is turned down, it's probably in the first stage (the only stage before the volume control). Or if hum remains, its probably the driver. Short 6V6 grid to ground. If it still hums, it aint the driver, the 6V6 is hummy (probably excess B+ ripple).

[PRR]

> What are the horizontal inputs used for?

What ToN said. Mostly a frill for fancy tricks. Not needed for Basic Waveform Display. But it didn't cost a lot to bring the H-sweep out to front jacks, and it -can- be useful.

Feed the same input to both V and H. Set H to "Ext". You get a diagonal line. If both inputs are the same and both V and H gains are the same, it is 45 degrees.

Now feed the H with the input of an amplifier, and the V with its output. The output is bigger. The slant line gets very tall, off the top/bottom of the display. Turn down the V gain. Now as you increase the signal, both input and output get bigger, up to a point; then the V signal (amplifier output) overloads. The slant-line becomes an S or Z shape. Add in phase shifts (ovals), bias shifts, sticking, it can get very messy. I'm not sure how much use this is.

A modification of this sold a lot of 'scopes. A radio speech transmitter has an audio signal modulated onto a radio signal. Compare the two X-Y, you can see if you are modulating too much or too little (distorted or faint). That's one of the first practical uses for 'scopes, and stayed common from AM all the way through SSB (not much use on FM or digital radio transmitters).

Feed a stereo signal Left to V and Right to H. If the stereo is "mono" you get a diagnonal line. If a signal is all in one channel or the other you get a vertical or horizontal line. For "pan-pot stereo mix" with no reverb, you get a splay of diagonal lines, angle proportional to that instrument's pan-pot setting. On my 2-mike live orchestral recordings I got a very diffuse cloud, each instrument and its reflections on a different slant.

[quayhog]

PunkyKatt, Manual leaving CA in todays US Mail.  I hope I put enough stamps on the envelope.   It should answer a lot of your questions.   

I've never found an OScope to be particularly useful in building or repair of guitar amps.  A  DMM and a good analog meter is all I really need.  I also have the matching signal generator to the IO-21 scope.

[punkykatt]

Thank you very much quayhog. Please let me know what all this trouble set you back and I mail you a check.  I have had great results finding amp faults with Doug`s listening device. I got the Heathkit for nothing, so Im fixing it up to add to my shop tools.  Yeah, signal generator, I have to get one of those now? Do you want to sell yours?  Thanks again.  Punky

[punkykatt]

Thanks again quayhog for the manual and schematic.  I did the test and adjustment procedures and everything seems to be operating correctly. By the way, the schematic does call for a 0.5A line fuse just as the original fuse was.  Next step.  Can I use my Marshall PowerBrake PB100 set on the dummy load for a load resistor? Also can I use my vintage Korg tuner WT-10A http://www.i2.i-2000.com/~kbrunner/tuners/wt-10a.htmas a signal generator? It has an output to go to an amp. (I tried it out on one of my amps and it puts out quite abit of (loud) signal compaired to a guitar signal even on the lower setting.   Thanks in advance for any help.  Punky

[tubesornothing]

With an ohmmeter, measure the power brake when set to dummy load. If it matches your speaker impedance, give 'er a go!

With the Korg tuner, can you measure the signal strength, ideally around 200mV work best (simulate humbuckers) or 100mV (for single coils).  Use your meter on AC or your scope.

[punkykatt]

Thanks ToN,  Powerbrake on dummy load, Im reading 1.2 ohms on the input jack. It dosent make any difference if the selector switch is on 8 or 16 ohm. This must be a reactive load, will that still work?
The Korg tuner puts out 315mV in Lo setting and 520mV in the Hi setting.
Is that too hot for the scope?  Thanks.

[tubesornothing]

Hmmm, I am not so sure about the power brake.  I'll have to let someone with experience with those comment.

The 315mV would not be too hot for the scope, but might swamp your preamp.  Depends on what circuit you are testing and what the intended guitar is.  E.g.  if you wanted to do distortion testing intended for a strat 315mV would be too much. However, if it was for active pickup, 315mv might be just fine.

You can always use a voltage divider to bring it down. 

[tubesornothing]

Also 315mV might cause some effect that you would not normally see.  E.g.  315mV input might cause the signal to get quite big and then cause some leading edge oscillation in a later stage.  This might never be seen if you play with single coil pickups.

The more I think of it, just use a pot as a voltage divider on the tone generator at the plug in.

[punkykatt]

Thanks ToN, I will install a pot in a small box with input and output jacks to drop the signal.  In the manual they show connecting the scope up to an amp with a resistor(same as speaker load) across the speaker leads and dont show a speaker. Im wondering if they do that just to mute the speaker?
Punky

[tubesornothing]

All I have for my dummy load is a 50W 8ohm resistor.  I think Aiken has a schematic on his web site on how to create a reactive dummy load.

[punkykatt]

Here is a schematic of a Marshall PB100 . Will this work in the dummy load(load box) setting  as a 8 or 16 ohm load for oscilloscope testing? Is the load resistor only used to keep a load on the amp when the speaker is disconnected?

[tubesornothing]

I'll have to let PRR or one of the other guys comment.  That big multitap transformer confuses me a bit when it comes to the dummy load.

[PRR]

If that load-box is set to a non-zero setting, and driven in the audio band (whatever Weber thinks that is) rather than DC, it "is" 8 or 16 ohms for your purposes.

Turn it to -30dB, add a little speaker (perhaps with 50 ohms in series) so you can hear what you are seeing.

[punkykatt]

Thanks PRR, I just wanted to be sure the Marshall Power brake would not interfear with the accuracy of the scope reading.