COSSOR 339A

Front view [21K]

World War II
Dual Trace 4½" CRT with ~1.2Kv EHT
Resolves 2MHz
7 Valves
Size 13½" by 8½" by 19¾"
Weight : 40Lbs

NOTES

The roots of this model are in a pre-war design, the "3339". Many early scopes still used gas filled valves for the timebase oscillators (just like early televisions) but gas ionisation time led to such timebases being restricted to a maximum of perhps 10-15KHz. However the 3339 used the "Cossor Hard Valve Timebase" (which was, in reality, a circuit patented by Puckle in 1933) and enabled timebases to achieve 1MHz rates.. The 3339 also contained two single-valve 'Y' amplifiers each based around a Cossor 42SPT ; this valve developed as a video amplifier for TV sets and hence needed high gain, plate voltage and bandwidth. However, during the War the military wanted to standardise valve usage, leading to the 42SPT's being replaced by 807's (RF power valves!). The 807's had much lower gains and led to the 3339 model number being changed to the 339.

The 339's manual reads more like a sales brochure, enthusiastically spending pages on latest developmens such as a dual beam CRT (signgle beam was available as an extra option!) and includes technical information on how the electron gun overcomes trapezium distortion. The CRT is one of the early hard-vacuum CRT's, earlier CRT's often using residual gas for focussing and had the side effect of further limiting bandwidths.

The 339's CRT has a single electron gun and utilises a splitter plate to produce two seperate beams. The plate is placed between the Y1 and Y2 plates and acts as a shield between the two plates. Thus the two beam halves can be independantly deflected by Y1 and Y2. This method has a side effect in that a +ve voltage on Y1 causes a deflection in the oposite direction to a +ve voltage on Y2 i.e. one channel appears inverted.

The scope isn't particularly accurate. For example, the timebase switch switches various capacitors into the circuit to change the speed. Except there is no cap on the fastest switch setting, it instead relies on stray circuit capacitance.

Most 'scope amplifiers are preceeded by an attenuator. However, the attenuator for this set is placed between the amplifier and the C.R.T. !

And the weirdest thing of all ? As well as the electrostically deflected C.R.T., there are also some scanning coils ! These are actually intended to enable the 'scope to be used to display current readings.

After the war the 339 remained in production until 1947. However the manual, which was also available seperately continued to be available for another decade due to its extensive information on how to use an oscilloscope ; the manual was cheaply priced and hence of great use to students and people new to the subject.


Fame ! An appearance in "Television is Here Again" from 1946. Check out the soldering iron (chap on the left) ! Link to larger view [50K]	Advert from Wireless World magazone August 1946 [96K]

SERVICE DATA

Came with a photocopy of the original manual, which you can download from the service data page. As well as including the main service data there is extensive information how to use a 'scope for many diverse applications.

CURRENT STATE

Not as bad as the photo may suggest, but electrically in need of some restoration. One electrolytic has been replaced (1966), but the others are original and carry the date "May 1944".

There are two slots in front of the C.R.T. which are intended to locate a graticule, though the graticule itself is missing.

WHERE FOUND

September 2002 B.V.W.S. bash at Easton-in-Gordano for £3.

Chassis timebase and PSU [13K] Chassis underside [15K] Amplifiers and EHT rectifier [15K]

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Last updated
24th December 2005