Mike's Hobby Blog

Those of you who have followed my blog probably know how much I rely on my Tektronix 465 dual channel oscilloscope. I don’t own a modern scope or any logic analyzer at all. This scope is my main go to device, when more information is needed than what a simple DMM can provide.

Sunday, right at the end of my dummy load exercise, a serious issue with the faithful 465 suddenly appeared. At that point, the main trigger level control failed to function. I could no longer set the trigger “A” level, which pretty much makes this scope useless.

Before I go into what happens next, I’ll describe a little history of this unit. Prior to Sunday, my treasured 465 had a couple of other, less important problems.

The “B” trigger slope switch didn’t work properly, so the “B” trigger would only trigger on down slopes.

The horizontal position knobs didn’t work very well. The coarse knob sort of worked, but the fine knob was useless.

The “B” trigger level and slope control was damaged when the scope was shipped to me and the scope took a hit to the front corner. I disassembled the switch and straightened the bent components when I first got the scope and got it working. My repair was less than perfect, and after a bit of usage, the slope switch broke again. I don’t use that feature very much and didn’t bother to fix it again until a couple of months ago. I decided I wanted it to work right and I went in and fixed the slope switch again. This time, I think I bent the sheet metal just right and the switch worked just like new when I was done. If you are handy, these switches are surprisingly repairable, but you need to tweak things, just right. Even though I fixed the switch, itself, the slope function still didn’t work. It still only triggered on the down slope. There must have been some kind of problem in the circuit. Anyway, being a little intimidated by the apparently complexity of the electronics in this scope, I decided to leave good enough alone, and left the “B” trigger control in the broken state.

The other issue was that the coarse horizontal position control was always a bit jumpy. It worked well enough that I could position the start of the trace in the general area that I needed it, but it wasn’t a pleasure to use. The fine control was just about useless, since I first received the scope.

There is one other issue with scope that showed up in the last few weeks. I think it started with a rather large cup of coffee that I spilt on my work bench. I cleaned it up, but I didn’t realize until a week or so ago that some of the coffee had dripped off my bench onto the front of the scope which is kept stored on the floor right next to that bench. Well, when using the scope a week or so ago, I noticed that controls were incredibly sticky. The buttons were most noticeably sticky and I had to force them to move. It was very puzzling, until I remembered spilling the coffee. At that point it had all dried up and there was little I could do, except keep using the scope and hope the controls would loosen up over time. The alternative would be to disassemble the scope and clean the controls. Do to the complexity of construction of this device, this is something that I was hesitant to undertake.

That brings us up to last Sunday, when the trigger level control malfunctioned.

to be continued…

This simple circuit can be used to test the capacity of low voltage, high value capacitors.

Use the well known RC formula: Time = Resistance * Capacitance

Connect a DC voltmeter between TP1 and TP2. Start with the 10 volt power supply switched off and the capacitor discharged. The voltmeter should read close to zero. You can short positive and negative sides of the capacitor with a resistor to discharge it.

You start the timer and switch on the power supply. Stop the timer when the voltage reaches 6.3 volts.

Since time and resistance is known you can now solve for capacitance.

Capacitance = Time/Resistance.

So if you have a 470 uF capacitor, with a 10K series resistor in the circuit, it is supposed to take .000470 * 10000 = 4.7 seconds to charge to 6.3 volts. If, in your test, it actually takes 10 seconds to reach 6.3 volts, you can calculate the actual capacitor value. In this example, 10 seconds /10000 ohms = 1000 uF.

You can increase the value of the resistor to check lower valued capacitors, but it will hard to check real small capacitors with this circuit. You can use a 555 timer circuit to check real small capacitors, but thats a subject for another blog posting.

I used this 99 cent design from the web to build a basic ESR tester.

https://archive.today/301HS

After building and testing it on a solderless, plug in bread-board, I transferred it to a more permanent soldered PCB.

The idea of this design is that capacitors all have inherent resistance. This resistance can be determined by sending a fairly high frequency sine wave through the capacitor, as except for the internal resistance, a capacitor of more than a couple of uF will act like a short. The resistance can be calculated by putting the capacitor in parallel with a known resistance and using the formula for resistors in parallel.

1/R1 + 1/R2 = 1/Rtotal

Though a proper ESR tester uses a sine wave, the square wave which this circuit generates, is good enough for purposes of basically determining if a capacitor’s internal resistance is higher than expected.

I used a 10 ohm resistor in parallel with the capacitor. If the capacitors resistance is pretty high at 10 ohms, then the amplitude of the square wave, as seen on a scope at the input to both the 10 ohm resistor and the capacitor, will be halved. A low ESR of say 1 ohm will result in the amplitude of the wave decreasing by 90%. A higher resistance, such as 100 ohm will have little effect on the amplitude of the square wave. So, depending upon the specification of the capacitor, you can determine if behavior is as expected or not. Most capacitors will have a low ESR of say 1 ohm or less, which results in a dramatically reduced amplitude when attached in parallel with the 10 ohm resistor.

Here is the tester connected to a capacitor.

There are a few connections required to operate this tester, but they are easy to make. I connected a bench supply on the right side and adjusted to 9 volts. The capacitor is connected with alligator clips on each side of the 10 ohm resistor, making sure the polarity is correct. The oscilliscope trigger input is connected to the 555 trigger. The oscilloscope channel A is connected to the high side of the capacitor and 10 ohm resistor, which is used to read the result. This circuit could also be used to test a capacitor without removing it from the circuit.

A few notes about my build – the schematic for the 555 timer version of this tester is missing 555 pin 1 connection to ground. Though the design is adjustable, I set up my tester to run at 100 kHz. Higher frequencies would be needed for capacitors below a few uFarad in value. I didn’t have a 10 ohm resistor handy, so I connected 2 22 ohm resistors in parallel to get an effective 11 ohm resistor. This value could be adjusted higher or lower to get better readings on higher or lower value ESR capacitors. The size of the square wave you are measuring is only a few 10s of millivolts so you will need to set the vertical scale on your scope appropriately. I used 10X probes, but 1X probes would result in better resolution. My cost was actually zero, as I had all the parts to build this in scrap bins.

I just purchased a broken Fluke 1953A Counter/Timer off of eBay.

This one I bought as broken for $32 plus $12 shipping. The Kenwood TS-530S I mentioned in an earlier post was supposed to be completely working, so this time I figured that I would just buy a broken unit and see if I could repair it. Before purchasing, I did look at the schematics of the unit, to get a good idea of the repairability of the design. I also did some google searches to see if I could come up with common modes of failure. There are a lot of counter timers on ebay. The reason that I settled on this Fluke was that this one had a oven for frequency control and the price was right. The oven maintains the frequency source at a constant temperature, so that fluctuations in environmental conditions don’t affect the accuracy of the unit. I found one site that listed the original list price of these units at $2295, I figured my $44 purchase was quite a deal. You can buy Chinese made frequency counters for around $100, but they wouldn’t have two channels and oven based frequency source and I doubt that they would have the long term stability of a relatively ancient Fluke.

When I received the unit, I decided to open it and take a look at the internals, before powering on. Inspection of the internals didn’t reveal any obvious damage, so I decided to power it up. Turns out it powered right on, but the self test didn’t work right. The self test uses the internal frequency source to drive the display. This wasn’t working right, as the displayed value wasn’t a consistent power of 10. After a bit of poking around and a power cycle, the unit didn’t power on at all. Now I figured that I had two problems to deal with, the incorrectly operating self test and the intermittent lack of power.

I found a note on a forum about a person that replaced a power supply cap to get his Fluke 1953A eBay find working, so I suspected that I might have a bad cap. I decided I’d pull all the power supply filter caps and test them out of circuit. I tested capacitance and found that all the capacitors seemed to have better than rated value for capacitance. Next, I built a simple ESR tester and ran that test on them. The 1000uF cap did seem to have a somewhat unusual ESR behavior. After spending a few hours trying some more iterations of the same tests, I finally came to the conclusion that the other caps were almost certainly good. I decided to replace the possibly bad one and found an equivalent replacement at a nearby Radio Shack. I installed all of them and proceeded to try to power on again.

Once again no display. I started probing with an oscilloscope. What I found, is that I didn’t have any power to the capacitors at all. Probing upstream showed that no power could be found past the on/off switch. After unplugging the unit, I unscrewed the nut holding the switch to the back panel and tested the switch with an ohm meter and found it that was faulty. Luckily I had an equivalent switch in my scrap box.

The switch on the left is the original. Note the damage on the top side and the long toggle. I’m guessing that this switch took a hit on the toggle which destroyed the switch.

I put the new switch in and powered on. This time the unit came to life, and even the self test worked. I’m not sure if the self test was fixed with the power switch swap or the capacitor change, but as long as it continues to work, I’m not going to pursue it further.

Testing accuracy against my Marconi 2218A reveals that after the oven warms up, the accuracy of the two units is within 10 Hertz thoughout their range. Take a look at this side by side shot of the two units.

Not bad, for a unit that was last calibrated in 2002. At some point I’ll work on getting the Marconi calibrated by beating it against WWV, after which I’ll be able to calibrate the Fluke against the Marconi.

Update – I just realized the Marconi has a limit of resolution of 10 Hertz, so these two units are, in effect, in perfect calibration with each other. I guess buying quality equipment does make a difference!