I've bought old, broken and hence cheap, Tesla counter BM520. It's classical
piece of equipment build with TTL 74xx family chips and with nixie tubes for
The good thing about this kind of vintage equipment is that it was made to be
repaired and that there are schematics available, you can get even scans of the
original manuals easily from the internet.
This particular unit, apart from being very dusty, had a few broken transistors
and trimming pots in the power unit, which was fairly easy to spot and fix.
If you are wondering about the specs the maximum frequency is 12.5Mhz although
it seems to be working fine up to 15Mhz for me.
The timebase is driven by a 10Mhz crystal oscillator that is termostated to a
constant temperature and should be correct for six decimal places that are
shown on the display. I haven't verified the precision yet and I would expect
that the frequency is slightly off after all these years.
As I examined the boards I've been surprised that some of the digitron driver
chips 74141 were made by Texas Instruments. Originaly I've thought that this
particular unit has been repaired once already. But later on I've been told
that sometimes Tesla was too slow to produce chips and had to buy them from
capitalistic countries instead.
Here we are looking at the power source unit next to the 10Mhz oscillator, the
power transistors are actually mounted on the back of the unit in the classical
TO-3 packages. The board produces two voltage levels, that is +5V for TTL chips
and +12V for amplifiers.
And by the way the connectors on the right are printer interface, the data
shown on the display are available on these two connectors in TTL levels, it
would be quite easy to add USB interface with atmega to capture the measured
There were three faulty transistors in small metal can packages, which is
common mode of failure for these. The two transistors driving the power
transistors were simply dead. While the last transistor disconnected from the
board because one of the leads was completely rusted off. Such transistor can
be easily lifted off the board without much force. There are two reasons why
this happens to old tesla transistors, first of all when there was shortage of
copper transistor leads were made from iron with gold plating on the top. The
iron tends to rust off and eventually you may end up with hollow leads that are
composed mainly of rust and thin gold layer on the top. Secondly the small
metal cans are sealed on the bottom with a glass that contains additives to
lower the melting point, unfortunately some of the additives are also corrosive
which only speeds up process.
As I said earlier three trimming pots needed to be replaced, the wipers were
not making good contact anymore. This usually happens when they are close to
heat source which causes thermal cycling, which may have been the case here,
since the board is stacked between the main transformer and the power
Here is my hand-drawn schematic of the PSU with some annotations. The schematic
in the original manual follows how the components are placed on the board,
while this one is divided into a functional blocks.
Photo of the swapable boards before cleaning, the two boards on the bottom
separated by a metal shield are wideband amplifiers, the boards on the right
contain the logic circuits implementing the different counter modes in 74xx