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Time to test my new (eeh... old) scope.

per

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I finally got probes for my scope, and I then decided to test it on something computerized. I soon realized that my XT was the best target.

I didn't want to constantly hold the probe while tuning the scope, so I decided I wanted to meassure the output to the PC speaker. I attached the probe, as you see in image 166, and started the machine. When DOS came up, I entered Debug, and turned on the speaker output by typing "O 61 4B". The scope obivously started to display something else than the usual flat line. When I ajusted the x-sweep frequency of the scope, it looked about similar to image 163.

Then I started changing the values of the counter, as image 165 shows, and the frequency changed as expected. I tried to load values like 0001h, but I didn't got any clear ressult then, even my scope is able to read such high frequences. The value of 0002h gave ressults, as you see in picture 158, and note how round edges the waveform has! I also tried frequencies of round values, like 20Hz. See image 164 for this. Here you also note that the wave is not really square.

So why isn't the wave squared? There may be several reasons for this. First of all, it can be a fault in the scope, but it can also be something with how the IBM PC/XT handles the speaker output. After looking at the schematics, you can see that the signal is first run though a TTL logic AND gate, then through a 75477 IC, this signal is filtered by capacitor C53 (.01), and finally run through resistor R6 (33Ohm) before reaching the speaker connector. Maybe the capacitor is the cause of the strange waveforms, or maybe the 75477 doesn't have a clear squarewave (especially on high frequencies). I don't know.
 

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Chuck(G)

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The RC combination on the output of the 75477 certainly has something to do with the waveforms you're getting. Try sampling pin 7 of the 75477 instead.
 

Druid6900

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Somewhere on the body of the probe, there should be a hole with a screwhead in it. This is the probe calibration screw and trims a capacitor built into the probe (which will need to be done often, especially with an older scope.
Get a good trace of a square wave (there may be a calibration point on the scope itself for this) and adjust the screwhead with a non-ferrous tool onto the wave squares up.
If your probes don't have this feature, nevermind :)
 

dave_m

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probe compensation

probe compensation

What make and model scope probe are you using? Post a picture.

Most probes need to be compensated. There is a trim capacitor either in the handle or near the BNC connector. Some are hard to find.

The usual routine is to hook the scope probe to the calibrator signal on the scope. It is usally a 1 KHz square wave of 1 volt peak to peak. With a small screwdriver you tweak the trim capacitor until you get the best square wave. Normally you do this before using the probe on signals of unknown shape.
 

Chuck(G)

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What make and model scope probe are you using? Post a picture.

Most probes need to be compensated. There is a trim capacitor either in the handle or near the BNC connector. Some are hard to find. .

Look at the second photo he links to.

Heck, Dave, a BNC connector would be hard to find on this scope. Try SO-239 (UHF) connector. No calibration source that I can make out.
 

dave_m

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Look at the second photo he links to.

Heck, Dave, a BNC connector would be hard to find on this scope. Try SO-239 (UHF) connector. No calibration source that I can make out.

Wow Chuck, is that what they called a "Type N" connector? If so, I haven't seen that on a scope since 1966. The cable on that probe looks awful long too. I just noticed the huge hook on the end of that probe on the first photo. What was that probe used for, gaffing sharks? :)
 

Chuck(G)

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Wow Chuck, is that what they called a "Type N" connector? If so, I haven't seen that on a scope since 1966. The cable on that probe looks awful long too. I just noticed the huge hook on the end of that probe on the first photo. What was that probe used for, gaffing sharks? :)

Dunno, Dave. Just noticed that this wasn't your usual Tek 465. My guess is that it's intended more for the radio and TV service trade.
 

Dwight Elvey

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Hi
I suspect that it is a 1X probe and has no compensation.
Even so, notice that the picture does show a flat signal
on the top but a rising signal on the negative half. That leads
me to think it is something in the PC and not the scope.
The highest frequency picture is just that the scope is
running out of steam.
Dwight
 

per

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Hi
I suspect that it is a 1X probe and has no compensation.
Even so, notice that the picture does show a flat signal
on the top but a rising signal on the negative half. That leads
me to think it is something in the PC and not the scope.
The highest frequency picture is just that the scope is
running out of steam.
Dwight

No, I actually have to run the scope at x10 to get correct signals from the scope.

The probe actually got a calibration screw, and I did calibrate it before starting (I got the manual in PDF format, so I know how to do it). The kind of probes I'm using is the ones provided by Telequipment when the scope was new. They don't either got a BNC or Type N connector, but a plain old UHF Connector (Don't know if it is PL-259 or SO-239). The scope was originally meant to be used with TVs, and it got special trigger functions that triggers syncronized with the TV sync signals.

The scope is made in UK sometimes durning the 60's.

While I'm on it,
When I was testing yesterday, I tried to meassure the output from one of the RAM chips. Without notice, I managed to get the probe between two of the pins, and the monitor went black. I turned the computer quicly off. I turned the machine on again, and luckily, it still worked fine except for one thing: The first bytes of the EEPROM on the XTIDE card I'm testing was replaced with the same two repeating bytes, and hence I can't use the card untill I have reflashed it.
 
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dave_m

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Per,
Thanks for the info on the UHF connectors. It was very interesting. When you get a chance, can you post the probe manual?

You have a real piece of history there with that scope and its proper probe.
-Dave
 

Chuck(G)

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PL-239 = Plug
SO-239 = Socket

Fairly ancient RF connectors (WWII gear has them) and real buggers to install correctly, particularly on thin (e.g. RG-58 ) coax.

So you have the socket on your scope and the plug on your probe. N-type connectors would be better, but at the limit of your scope, it hardly makes a difference.

FWIW, N-type connectors are probably just as old as UHF, but have better better transmission and matching characteristics.

I'm curious why a fair number of off-brand oscilloscopes use the Tek knob scheme: dark grey outer knob with red inner knob. Anyone know? I just saw a Telequipment scope for sale with the same scheme.
 
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per

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Per,
Thanks for the info on the UHF connectors. It was very interesting. When you get a chance, can you post the probe manual?

You have a real piece of history there with that scope and its proper probe.
-Dave

I got the manual here:
http://www.thevalvepage.com/servdata/testeq/telequip.htm

Make sure to get the one for the D52. That's the model I have. It's using so-called split-beam, what means that there is a charged plate inside the CTR that splits the beam of electrons into two beams, hence two synced traces on the display (one for each input).
 
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