The Tektronix 556 is a dual-beam scope, the successor to the Tektronix 555. It uses letter-series and 1-series plug-ins. A rack-mount version, the R556, also exists. The 556, which is wider than the average 500-series scope, fits in the 205 cart. The 547 and 555 were the last 500-series scopes produced.
The 556 is a true dual-beam scope and none of the signal path is shared between the two beams. Electrically speaking, the power supply and the calibrator are the only parts that are common to both beams. The 556 has almost complete left-right symmetry, corresponding to upper and lower beams. Although the beams can be operated completely independently, the 556 provides many options for routing signals between the vertical plug-ins, triggers, sweeps, horizontal and vertical amplifiers. For example, a common use for a 556 is to view two simultaneous single-shot events, both sweeps being triggered by one of the traces. For this purpose, the 556 allows the output of one sweep generator to be routed to the horizontal amplifier of the other beam, so it drives both beams. Many other more sophisticated modes are also available.
Comparison with other scopes
The 556 is a one-piece unit whereas the 555 has the power supply in a separate box. Triggering and sweep circuitry is integrated into the 556 mainframe, as opposed to the removable timing units in the 555. The 556 takes two letter-series or 1-series vertical plug-ins. The 556's bandwidth is 50 MHz, whereas the 555's is 30.
The 555 and 556 both use separate HV transformers for the lower beam and upper beam voltages. In both models, the lower beam HV transformer produces the CRT anode voltage.
The 556 superficially resembles the 565, which is is also a dual-beam one-piece scope, but the 565 takes two 560-series vertical plug-ins and has much lower vertical and trigger bandwidth. Also, the 565 uses a single HV transformer to produce all CRT voltages, whereas the 556 uses two transformers.
The triggering in the 556 is done using tunnel diodes and the trigger circuit sits on its own small PC board just behind the trigger source selection switch.
The CRT circuit of the 556 uses two HV transformers. The lower beam transformer, T1301, produces:
- CRT anode voltage: +8150 V
- lower beam cathode voltage: -1850 V
- lower beam control grid (blanking) voltage: varies, about -2000 V
The upper beam transformer, T1351, produces:
- upper beam cathode voltage: -1850 V
- upper beam control grid (blanking) voltage: varies, about -2000 V
The CRT in model 556 is the T5560.