The BM365U RC generator was manufactured by Czechoslovak Tesla national enterprise based in Brno in the 1960s and 1970s. Some pieces were labeled "RC Oscillator" on the front panel.
The BM365U is a slightly different version of the BM365. Unlike the BM365, the BM365U does not have a 120V/220V 50Hz mains voltage selector, it only works on 220V 50Hz.
For some reason, the BM365U is in a deeper cabinet in which the rear 40% of the depth is empty.
In contrast, the BM365 has a shallow cabinet that matches the dimensions of the chassis.
Maybe this solved an overheating problem - the heat couldn't spread in the shallow BM365 cabinet.
Otherwise, the BM365 and BM365U are probably identical. It is a portable low frequency voltage source of a sinusoidal voltage from 25 Hz to 30 kHz (the manual says up to 32 kHz).
Thanks to its small dimensions and low weight, it is suitable for repair practice at the customer's premises as well as for laboratory measurements and testing,
e.g. as a modulation signal source.
The output voltage can be adjusted from zero to 10V continuously with a potentiometer and in five ranges with a decimal divider (1:1, 1:10, 1:100, 1:1000, 1:10 000).
The second of the outputs passes only through this potentiometer, but not through the divider. This potentiometer has the mains switch built in.
On the front panel there is a light indicating power on (6V 50mA telephone bulb).
The specified power consumption of the device is approx. 25VA, weight approx. 5.6kg and dimensions WHD 260 x 210 x 155 mm (BM365), 260 x 210 x 220 mm (BM365U).
Frequency range: 25 Hz - 30 kHz in four ranges:
I. 0.025 kHz - 0.15 kHz
II. 0.15 kHz - 0.9 kHz
III. 0.9 kHz - 5.4 kHz
IV. 5.4 kHz - 30 kHz
The device is equipped with tubes:
2x 6F32 steep hf pentode in BM365, in BM365U replaced by soviet 6J1P (6Ж1П) or 6J1P-EV (6Ж1П-ЕВ)
1x 6L31 power beam tetrode
1x 6Z31 two-way rectifier
RC generator (RC oscillator) Tesla BM365 - schematic (this one still has the 120V/220V selector, component values are from old R10 series).
RC generator (RC oscillator) Tesla BM365U - schematic (almost like BM365,
but now without the 120V/220V selector, component values from modern E24 series and R24 resistor replaced by a trimpot).
RC generator Tesla BM365U - front panel (version with labels in English, there were also versions in Czech and Russian).
Tesla BM365U RC Generator similar to BM365.
RC generator Tesla BM365U - scale with 4 color-differentiated ranges.
Tesla BM365U RC generator - the cabinet is deeper than the BM365.
Tesla BM365U RC generator diagonally from behind.
RC generator Tesla BM365U from behind, for 220V 50Hz mains (no 120V option).
Tesla BM365U RC generator - mains fuse, mains input, label, sealed screw and an adjustment potentiometer.
Unlike in a BM365, there is no 120V / 220V mains voltage selector.
RC generator Tesla BM365U - nameplate.
Tesla BM365U bottom.
Tesla BM365U RC Oscillator chassis from above.
You can see how much empty space there is in the cabinet :).
The chassis was artificially extended with a frame on which there is a fuse holder and a mains connector and long pillars for screws (behind the right one is the long shaft of R13 potentiometer).
Tesla BM365U chassis from behind. On the left under 6Z31 you can see the hole for the omitted 120/220V selector,
which originally in the BM365 was just behind the back wall of the shallow cabinet, now sunken deep inside.
Tesla BM365U chassis from the side of tubes 2x 6J1P and 6L31, behind them C12 (100uF).
Tesla BM365U chassis from the power supply side with a transformer and a 6Z31 two-way rectifier tube.
Tesla BM365U - 2x 6J1P and 6L31 tubes with removed covers.
Tesla BM365U - 2x 6J1P and 6L31 tubes with the covers removed, top view.
Tube types in my BM365U: 6J1P-EV (2x), 6L31 and 6Z31.
Tesla BM365U chassis from below, rear view.
Tesla BM365U chassis from below.
Tesla BM365U chassis from below, front view.
Detail of the Tesla BM365U chassis from below.
Regulating lamps stabilizing the amplitude (4x in series 60V 50mA telephone lamp made by Polam).
No, they don't really glow when in use. There's only few volts on them.
