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!