2ap1 Oscilloscope

Oscilloscope Schematic 2ap1
Fig 2—Schematic diagram of the CRT, high-voltage and heater supply circuits for the tuning indicator.

C1-C3—33-ftF, 350-V electrolytic. C4—50-|tF, 500-V electrolytic. CS, C6—22-aF, 350-V electrolytic. C7—1 S-pF, 500-V electrolytic. D1-D3—1N4007. R1, R2—10 U, 1W. R3-R5—1 MO, VsW. R6-15 kO, 2W. R7—22 kO, 2W.

R9—-47 kfi potentiometer.

R11—1100-kfi potentiomter.

R15, R20—220-kfl potentiometer.

input to the scope if you're using a transceiver that has a high audio output impedance (such as the Kenwood TS-440 transceiver's fsk out circuit).

U1 drives two active filters (U2A and U2B), which are tuned to the mark and space signals of the mode being received. Table 1 provides a list of the modes, filter frequencies and controls used to adjust the filters.) For RTTY, I elected to use the high-frequency tones because the majority of HF transceivers use this combination. By using a switch at S1 that has more than the three positions shown, you can

Table 1

Modes, Tones and Controls add other tone selections. For instance, you might want to add the low RTTY (1275/1445 Hz) and packet-radio (1200/2200 Hz) tones and possibly CW (750-800 Hz in most transceivers).

The outputs of these filters are coupled to the horizontal and vertical amplifiers that feed the CRT shown in Fig 2. The horizontal and vertical amplifiers are each one-half of a 12AX7A (ECC83) dual triode. These tube sections furnish about 160 V P-P to their respective pins on the CRT. Admittedly, the tube is being pushed over its rated limits, but continues to operate without trouble.

The use of a tube for the horizontal and vertical amplifiers was dictated by the fact that the plate and heater voltages were already available in the old scope. The difference in the values of the plate resistors of the two amplifier stages is related to the different amplifier gains required to properly drive the CRT's horizontal and vertical deflection plates. (The horizontal deflection circuit requires more drive than the vertical.)

Although the CRT of my oscilloscope is a DG-7-32, the basic circuit of Fig 2 is similar in most old scopes. With some circuit modifications—and paying proper attention to the pin numbers—you can use other CRTs (such as a 2AP1), which are usually easy to obtain surplus or at flea markets.1


'A basic circuit for a modulation-monitoring oscilloscope appears in The 1988 ARRL Handbook. Specifications and pin-outs for CRTs can be found in older editions of the Handbook (those prior to 1979). —Ed.


Packet Radio SSTV

Frequency (Hz) and Control 2125 R1 1650 R2 1500 R3

Frequency (Hz) and Control 2295 R4 1850 R5

Shift (Hz)

+2 -1

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