Tenma 72-3055 oscilloscope questions....

rumpfy said:

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.

Click to expand...

You mean open up the scope, and look at the inside part of VOLT/DIV rotor switches?

yes please.

Well I got the trace 1, Volt/DIV knob off, and the outer knob was stripped, and not making any contact with the rotor. It seems the trace #2 outer knob is also stripped, but it must be stuck at a setting that is close to optimal.
When I turn the outer rotor with my fingers, it is firm, goes click-click-click, and has stops in the right place. So I just need to get the outer knobs re-seated to the outer rotors.

I got the case open, and the it needs some serious dust-off before I take any pics.

Thanx KeepItSimpleStupid the penetrating oil was just the right trick to work the inner knob out.

Well done.
Just be careful with the 'dusting off'.
I would like to see some pics before you do too much; dust or no dust. I'm keen to see the basic structure.
With the knobs, you can see to that part. Are you saying the grub screw in the knob has stripped the thread in the knob boss? If so, it may be possible to tap the thread to a larger size presuming the thread in the knob is made into a metal boss and not plastic. Just think it through slowly.
Also, was the knobs stuck because of corrosion?
The construction of the switches is based on a clicker plate at the mounting point at the front end of the switch. Generally the boss for the switch shaft is brass. This needs a tiny bit of lubrication, but not much, because the switch doesnt get rotated a lot. Too much lube can travel back into the switch elements over time. Just a drop of the penetrating oil will do.
When you put the switch knobs on the shafts there will a means of locating the knob to give the correct calibration. This will either be flat on the shaft or a small hole into which the grub screw will locate.

Here are a bunch of inner pics

Got all that.
The knob shows the boss is still stuck on the shaft.
Been looking around the net and my guess is the cro is made in japan. You will have to solve the knobs etc yourself.
The construction is quite different to what I thought and is probably around 1980.
The cro looks like it has a 2kV EHT potential. I found diagram for the tube. The accelerating voltage is -2.2 kV at the cathode with the deflection plates at +80 volt.
The physical arrangement will be difficult to fault find; looks like 2 PCB's facing each other. 81221 seems to show the back of the attenuator. You need to confirm this. Try to trace the connection from the input plug through the AC/gnd/DC switch to the PCB or the attenuator.
I note in pic 81219 that the component numbering seems to be grouped into logical function blocks with a 3 digit code. In 81218 I can see 'R 29-' and 'P905' and 'P904R'. To the left I can see 'P403' and 'R433R'. The input stage might be say the 100 group, the output stage amplifier feeding to the crt pins might be group 400. and so on. There will be two identical vertical amplifiers and two attenuators and a particular resistor may be numbered R131L and R131R for the channel 1 and channel 2; Or it may be that it is R131 for channel 1 and R132 for channel 2.
The problem is to find out why the channel 1(?) dont go. I think the calibrator is fixed at 0.5 volt. You need to make a voltage divider to reduce this to 0.005 volt. Feed this signal into both channels and set the voltage divider switches to the 5 mV range. In this condition, the attenuator will be effectively switched out of circuit and there should (must) be a direct connection from the input terminal to the input point on each vertical amplifier. (there may be some small series resistance in the circuit but this will probably be less than 100 ohm.). For this test, set the DC/gnd/AC switch to DC. (In the AC position, a series capacitor is used to block any DC component from the input signal). Take your time with this and get back to me in a couple of days

Not sure how to load a picture.
For the 150BTB31A tube:

pin numbers

1 and 14 filament 6.3 VAC
2 first grid -2.2 kV
3 Cathode -2.0 kV
4 Focus ? -1.7 to -1.2 kV
5 Astigmatism +55 to +160 volt
6 +45 volt (may be a blanking function)
7 Y+ At+80 volt (Vertical deflection) tracing these leads back to the PCB will identify the circuit group for the vertical amp. (hopefully)
8 NC?
9 Y- At +80 volt (Vertical deflection) """"""""""
10 NC?
11 X- At +80 volt (Horizontal deflection) tracing these leads back will lead to the horizontal amplifier.
12 X+ At +80 volt (horizontal deflection)
13 NC?

NB: there is a coil wound around the front face of the tube. This is for 'trace rotation' to align the trace with the graticule. (dont fiddle with this; Dont think it needs adjustment by the look of your screen shots)

Well the outer knob screws had worn grooves into into the rotors, so I reset the knob onto the rotor, moved it forward about 1mm, reset the screws and tightened them down. As long as I am careful and don't turn with too much force, the knobs now work. It appears the previous user(s) must have turned the knobs with a lot of force and stripped the rotors. The inside of the knobs, the screw, and the rotor all look like brass.

Once I got the knobs reset, and aligned right, I was about to get both traces to work correctly.

I found an interesting video on utube.
The guy is 'bap8guy' and he may have a circuit diagram or better still, a manual.
Also have a few manuals for cro's that are 2 channel 20 mHz and use the 150BTB31 crt . All around the same vintage as the tenma. I have Philips PM 3200, BWD 820(aussie design), Hitachi V202. All these give a good general idea of whats what.
The hitachi comes from:
https://www.ebaman.com/index.php/remository/func-startdown/21653/