Tektronix 211 – Battery Replacement, Internals, minor repair

Yeah, one of the “cute” oscilloscopes.

IMG_4018

In this scope, there are no tubes, no +500V DC power, no bumblebee capacitors! Not even a single significant electrolytic capacitor, so there’s no room for the typical drama. I’m a bit depressed about it.

As always, if you are new to this instrument, the starting point would be Tekwiki. It covers all the basics and essentials that you would need to know.

http://w140.com/tekwiki/wiki/211

This is a 500 kHz oscilloscope, at its best. At the highest sensitivity, it is at 100 kHz. Well, who cares, it is battery operated and floating. Case isolation during battery operation of up to 700V DC from common (shield) of probe.

Here is the video explaining what I did.

However, here are some details which are exclusive for the “Readers”

Now, there is not much to fix or understand, however, there are some interesting aspects around power supply. I been through similar aspects in several HP 34XX multimeters.

A few key differences compared to a typical scope –

  1. Probe – Wired directly to instrument, no BNC or other connectors.
  2. No “front” panel. We observe only the CRT on the front. In order to save space and keep it small, they moved the controls to the side and made use of the length of the instrument.

It’s a simple instrument – There are 3 boards and 3 ICs. Below you can find the mapping of the board to the functionality.

211-Board.png

The U65 is for trigger and sweep generation. The U25 is the 1st vertical amplifier (after input FET). The U106 is a dual amplifier chip to drive both H and V signals, which in turn drive the deflection plates via output transistors.
The power supply is a little more interesting. As in any battery + line operated instrument, all internal voltages are generated from a DC to DC converter or inverter + rectifier (or whatever you want to call it).

211-PS-1.png

The incoming AC is rectified and used to charge the instrument. It can also operate directly on AC.
There is no transformer, rather a capacitor divider. (Yes, a capacitor is used as a resistor because the input is AC and capacitive reactance is used to drop the voltage!)
Here is a quick summary of the different options that I mentioned above.

schemeit-project (6).png

The transformer is a piece we are all familiar with, the resistor option is common in old radios where the filament string acts like a divider too.
Why do they use the capacitive divider here? Maybe because there is no heat from resistors, no weight from transformers and it saves space. But what is the side effect?
The input needs to be a sine wave, as reactance Xc is based on the frequency (sine) which is basic ac-circuit theory.

X_{C}={\frac  {1}{\omega C}}={\frac  {1}{2\pi fC}}

 

If the input f is not pure sine, the capacitor will have different reactance to the harmonics in the input, changing the output voltage of the divider.

The total impedance (Not resistance, as this is a complex load) to the AC source

imp.png

is explained in all of the ac circuit theory books, so I will not repeat it here.

Here’s another question for you – why is the total impedance NOT Xc+R? (Yeah, like V/(R1+R2) rather the square root? You should find out and dig deep enough to arrive at why a sine wave is written as

sin

Perhaps not the math and the formulae, but “Why” behind it 🙂 If you already know the answer, please forgive me.

I find it funny that no one explains the “why” behind math, people just suck up to the equations 😉

One point to note is the power factor. Since this load is really small, it does not create a huge problem (maybe negligible), however if this is used for large loads, the PF will be an issue and will need to be corrected.  I have not done a detailed analysis of the input, but I do see a common mode choke in the input – for HF filtering –  it may also help with PF correction (?)

I digress a lot, time to change my drink, perhaps 😉

Based on the operating frequency and AC voltage, the capacitor value needs to be adjusted, i.e. C210 and C212 in this case. Caps.png

 

211-PS.png

It is very important to note that there is no filter capacitor after the bridge. The battery is the capacitor. So beware of two situations –

1. Do not remove the battery and operate the instrument, it may not work as input to the DC to DC converter is not filtered DC.

2. If the battery is bad or is loading the rectifier output the unit will not work, as the battery can sink the current from the line.

Moving on to the next section, it is a DC to DC switching converter. This section generates all the required voltages for the instrument, +5.6 V, -5.6V, +65V and -1000V.

211-Dc-DC.png

 

This is a very simple and straight forward schematic. Q231 blocks the switching converter from operating in case the battery voltage is too low, by cutting the Q235 off. Q242 and Q249 are the main switching trans. There are 4 Secondary windings on T250,  for +5.6 V, -5.6 V and +65 V.  Tap 6 is developed to 1 kV using a voltage multiplier. And the last one is used for CRT heater – elevated to 1000V but not rectified.

I had only one problem when I was working with my instrument – The battery was dead. Thankfully, it’s not a complex surgery, and it’s rather easy to get new 10 x AA Ni-Cd and put it in.

Here is a picture of the old one –

IMG_4006 2.jpeg

And the new (made with off-the-shelf Ni-Cds (1000 mAh))

211-newbat.jpg

As always, these things won’t leave me without creating a fuzz. The surgery was completed successfully, and the patient died. I wonder why?

Here’s what happened – The scope will not power on after replacing the battery. As always I suspected every possible complex scenario like a shorted cap, a broken transistor, a pregnant transformer, etc. But eventually, the problem turned out to be the power switch. Pin 3 and 4 of power switch connector P6 on the power supply board was open, no matter what position the switch was in.

I drank some alcohol to clean the power switch, and of course, it didn’t work. I tried heating my joints and PCB solder joint to the switch to fix potential cold joint. I did fix the issue to some extent with an intermittent connection and ended up with a burned nose tip for me (but hey, on the other hand, it did fix my cold :P)

At this point, I seriously started suspecting the alcohol and other spiritual substances I was drinking and smoking. They can create components and PCBs which never exist in the instrument but seem to be present in front of you and make you troubleshoot it for hours. These substances, however, reduce the signal and noise level from the wife, acting as an attenuator. More on this topic to be covered in detail in my upcoming book “Installing, configuring, managing, and troubleshooting Wife 1.0”

I then watched the news for some time and made myself sober with panic. Then, I took out the input board and switch. The problem was the trace connecting the switch terminal to the PTH for the cable coming from connector P6. This trace was broken internally.

pcb.jpg

This could be due to the mechanical stress presented by the switch, or maybe a bad PTH to start with? I wasn’t sure. Anyway, I soldered the cable directly to the switch terminal.

That brings me to an important point. P6 is the connector to the switch. The left two points control the DC to the scope. The right pair of pins control line AC to the line capacitor. Be careful. Line voltage is available on the power supply board as well as near the power switch on the input board.

connector-211.jpg

And that was it, its all up and running since it was a little cold in the night and there was only enough whiskey left for me, I put the hood over it.

Tek_211-wHood.jpeg

Now comes the important question – Where the hell do I keep it? I am literally sleeping on top of junk instruments due to a lack of space!

Ah, found the perfect spot!

IMG_4111 2.jpeg

Oops, there is some more space on top of it, let me look for another 200 series scope. (Don’t tell the wife!)

Note: The CA plug-in in the slot was donated to the wife as a trivet.

 

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