Tenma 72-3055 oscilloscope questions....

[Teufelwolf]

So I picked up a Tenma 72-3055 Dual Trace Analog Oscilloscope as an impulse buy (it was $50.00 shipped).
Ok, so it powers on, and it seems to do things when I move the time base and vertical on input #2. #1 seems to be have an issue with the horizontal, but I am not sure if it is the scope's insides, or if the knob is stripped, it has a very lose feel to it. Can not figure out how to get the knob off to check, it is one of those knobs with an outer ring, and inner knob that also pulls out. The out ring has a small hole, but I can't tell if is is a screw or some sort of hex nut.

It came with one probe, basically a BNC connector, cable to a probe. Now looking at pictures of other oscopes, I notice that most (all?) have a probes that have probe tip, and a ground wire. This one doesn't. The O-scope does have a ground connector. Do I need a probe with a ground clamp, or can I just clip the o-scope ground to the circuits ground?

Anyone have any experience with these, any tips pointers? I am just using it for very basic projects and experiments most on breadboards at this time.

 

[rumpfy]

sometimes a small Allen key is needed to get the knobs off.
Whateve, just take your time with it and all will become clear.
Does the CRT have around or a square screen?
Generally these dual trace CRO's have a single time base and a switching mode for swapping between trace 1 and trace 2.
Never heard of Tenma. Have you a manual?
For many measurements, just any old ground lead will do. But for high frequency or low level measurements, an attached ground clip will give a better trace.
Congratulations on the wisdom of buying a useful test instrument. Hope you get it sorted.

 

[Teufelwolf]

Square screen, no manual. It has a single time base, and two volts/div. settings.
Thanx for the probe info. I'll post a picture of the beast after the sun comes up...

 

[KeepItSimpleStupid]

1. probably a very small Hex Key. Possibly metric, English or very rarely Bristol. Could even be Torx.

MAKE SURE you have the correct wrench. Examine the "hole" carefully. I'll bet you will make out a HEX depression. You don;t want a rounded hex by using the wrong tool.

Some other hints. Since you said two knobs, there can be two such fastening screws, one for each knob. Their could also be two per knob, 90 degrees apart. If there is a single screw, their is usually a "flat" for the set screw to stop onto. One or both of the knobs could "pull off".

You may be able to see the shaft rotate on the other side.

Also inspect for separation points. Rarely, the "cap" can be removed revealing odd captive methods.

The inner knob may pull out to engage a switch.

2. Post a pic of your probe. Notice if there are any screwdriver holes at the probe or connector. The screw is only there if the probe attenuates. e.g. x10, x100. The screw adjusts the capacitance and "compensates" the probe, so square waves from the calibrator output are made square. i.e. sharp corners.

Inspect your probe for x1, x10 and gnd switchable. The ground leads on nice scope probes clip into place into a slot in the probe body.

Isolating your work with an isolation transformer is the preferred way to work.

Somewhere your work and your scope has to share a common ground. That may be the ground connection on the wall outlet or forced to be that. Usually high frequency signals require the close ground clip.

The probe could be something weird like a demodulating probe.





Loose knobs will usually rotate n*360 degrees or get "stuck" while rotating.

 

[RichTheDude]

As per Kiss said, the probe should have a place for a ground lead. Most of the time, they just clip onto the probe as in this picture (clearest pic I could find) http://i01.i.aliimg.com/wsphoto/v0/...ries-b-font-Neutral-packing-40M-Gray-font.jpg

Could be that your just missing the ground clip and lead.

 

[Teufelwolf]

Here are some pictures... The probe it came with looks pretty beat up, and appears to be missing the ground clip, so I just ordered a new probe.
Here are some pictures of the scope...

 

[RichTheDude]

You can test your scope and your probe out instantly. See the 0.5Vpp probe adjust clip/post? Put the tip of the probe on there (sometimes scopes have a GND post next to this). Typically you would use this with a non ferrous adjuster to "twiddle" the compensation cap (looks like it's next to the BNC on your "beat up probe" probe) such that edges are as fast as possible, and the flat parts of the square wave are as flat as possible.

Normally this square wave is 1kHz (check your scope specs), so you can verify your time base and input (Y) channels all work as expected.

 

[Teufelwolf]

Thanx again, by non ferrous adjuster - do you mean a non-metal screw driver to adjust the little screw inside compensation cap?

 

[RichTheDude]

Yes, but I should add I have used a normal terminal screwdriver LOTs of times. Thinking about it, I do not think it matters at all seeing as its a capacitor, I have some twiddlers somewhere which are brass molded in a bit of plastic. I think the non ferrous aspect would be more important for twiddling inductors in radios etc.

 

[KeepItSimpleStupid]

The setscrew looks like a hex driver, Now, the problem is: What size and is it metric or English?

e.g. https://www.harborfreight.com/36-piece-sae-metric-hex-key-set-94725.html

Place the screw vertical and put some penetrating oil on the screw/threads.

0.050 is a popular size for knobs, but yours could be metric. I just got a bunch of turns counting dials from China with a bunch of 0.050 wrenches for each one of them.

 

[rumpfy]

The CRO looks like about a 1965-1975 vintage.
Is it valve or transistor or a mixture.
The allen key could be 1/16 th inch if its US made, or 1.5 mm if Japanese.
I get a feeling the CRO is made to compete with a Tektronix of the early 70's vintage. I note the timebase settings for 'H' and 'V'. These are typical timebase selections for working on analog TV sets.
I would guess the calibration signal is derived from mains frequency; 60 Hz
Just keep asking questions and you'll get it sorted.

 

[RichTheDude]

I would say the scope is 80s/90s vintage personally, I think TENMA is a relatively recent brand. A slightly spruced up version of this scope is still available at Farnell/Element 14 https://uk.farnell.com/tenma/72-6800/oscilloscope-2-ch-20mhz/dp/4911842 . I started out with a similar scope, but it was made by Hameg.

In the UK you can buy a set of metric and imperial allen keys for a £1, they are ideal for jobs like this.

 

[KeepItSimpleStupid]

Here is where the Tenma stuff is/was sold. **broken link removed** The blurb says Tenma started in the early 80's.

 

[Teufelwolf]

Well even if it is only half working, it was a good deal. Locally even 1960's 70's vintage oscopes are selling for $100+ w/o probes. I got this for $25.00 + 25 shipping.

 

[vtech]

TENMA has been a private label name for MCM electronics(part of Newark/Farnell/Element14) for some time now. Many of their products are very similar--if not copycats of brand names. like Tektronix or leader etc. Not top of the line but over the years I have used several of their products with no problem. If you need to know anything about TENMA, your best bet is to contact MCM out of Dayton, Ohio.

 

[Teufelwolf]

Well I got a pair of new probes ($12 from China), and got around to working with the scope again. It seems that input one voltage knob is pretty messed up. I got that hex screw out, even though it was pretty badly stripped, but I still can not get the knob off to see what the issue is. The inner knob just will not budge. Not sure if it should pull off or screw off, tried both with limited forced and it doesn't budge.
Input two seems to work OK. The 1st pic is input two hooked to the calibration plug. I get the square wave pattern that a few sites mention for calibration.
.

The 2nd pic is the input One, seems the voltage divider is set too low, and the outer knob is not making contact with the rotor.

Still pleased with it, as I'm sure a single trace will suit my needs for now.

 

[KeepItSimpleStupid]

OK, I think you should do the following. Turn the instrument so the screen faces up. Place some small amount of penetrating oil e.g. WD-40) between the bushing and shaft.

Can I assume the inner know has no set screw in it? Take a pic of the inner knob.

 

[Teufelwolf]

I'll try that after I get back from my weekend excursion.
Yes the inner knob has the set screw removed. Neither of the outer knobs has a stop, yet from the dial marking, it looks like there should be a stop - if you look at the photo of the front, you can see the outer Ch1 Volts/Div knob pointer is at the 6 o'clock position. Don't know if that is normal, or if a previous user(s) broke the stops.

 

[KeepItSimpleStupid]

On most scopes I've seen stops, but there is no need for them.

You then don't have th luxury to turn a knob against a stop. Time to look if you can hold onto the shaft on the inside. This https://www.irwin.com/tools/locking-tools/the-original-long-nose-locking-pliers-with-wire-cutter series of vise grips I find helpful.

 

[rumpfy]

1. Getting the screw out and the knob off:
not sure what has happened, but if the screw is out, it is possible that there is some shaft material jamming the knob on to the shaft. The grub screw can make marks on the shaft which will jam the knob boss. Shaft is probably steel and the knob boss is probably brass, so there may be some corrosion binding the knob boss to the shaft as well. Kiss's posts are relevant here.

2. Attenuators.
You need to get the covers off and have a look at the switches and post some pics.
The CRO is DC coupled and the variable attenuator is most likely a variable resistor in the cathode (emitter circuit of the input stage). The voltage divider is a 3 stage affair with a basic sensitivity of 0.005 V per cm. There will be a 1:2;5 divider and there will be a 1:10 stage and either another 1:10 stage or a 1:100 stage to give all the required attenuation for the input voltage range of 0.005 volt to 5 volt.
The nice thing about a dual trace CRO is that you can use the good channel to fault find the dud channel. If you apply the same input signal to both channels and then check the signal level throught the attenuator, you will find the problem area. I note the CRO is a 20Mhz design and the attenuator may have some fancy attenuator block assemblies. One of these may be at fault. For a 10 Mhz design, the attenuator can be made of discrete components, but at 20 mhz, the attenuator may have to be of a hybrid construction to get a smooth bandwidth.
I note the trace for the dud channel appears to have a large spike. This is consistent with an open circuit resistor in one of the attenuator stages. You might like to check each channel at the 5 mVolt gain setting. At this setting, the attenuators will be effectively out of circuit. Your problem may of course be due to the gain switch being deranged; so this should be sorted first.