Tektronix 175, Manual

Page: 43 / 124

Maintenance — Type 575
perform checks on the power supply operation. One cause
of improper regulation by a power supply is incorrect load ­
ing. To check power supply loading, shut off the power and
check the resistance of the power supply output bus to
ground. The — 150-volt bus should measure approximately
6 kilohms, the -4-100-volt bus approximately 90 ohms, and
the +300-volt bus approximately 17 kilohms.
If none of the preceding checks determine the cause of
the trouble, the improper regulation is probably due to a
change in value of one or more of the resistors or capacitors
composing the voltage divider networks. The resistance net ­
works in the grid circuits of V604, V624, and V644 are par ­
ticularly critical since they determine the output voltage of
their respective power supplies. Use resistance checks to
isolate the defective part or parts.
The following information may be used as a quick index
to troubleshooting the Low-Voltage Power Supply.
If the output voltage is high with excessive ripple, check:
1. For high line voltage.
2. The amplifier tubes (V604, V624, and V644).
3. For insufficient loading.
If the output voltage is high with normal ripple, check:
1. For proper resistance values in the dividers |R61 3 and
R61 7; R636 and R638; and R662, R664, and R666).
If the output voltage is low with excessive ripple, check:
1. For low line voltage.
2. The series regulator lube (V607, V627, or V657|.
3. For excessive loading.
4. Open or leoky filter capacitors.
5. Rectifiers |V602, SR620, or V642).
If the output voltage is low with normal ripple, check:
1. The resistance values in the dividers.
2. The capacitors shunting the dividers.
If the output voltage is normal with excessive ripple, check:
1. Filter capacitors at the output of the rectifiers and at
the output of the power supplies.
2. AC bypass capacitors in the grid circuits of the reg­
ulator amplifiers.
3. Regulator amplifier screen grid circuits.
Troubleshooting the CRT Circuit
If no high voltage is available from
either
the positive
or the negative high voltage power supplies, the trouble is
probably due either to a defective oscillator stage (V810)
or high voltage transformer (T8011. The oscillator con quickly
be checked by placing a neon bulb in the field of the high
voltage transformer, T801. If the bulb glows, the oscillator is
operating and the trouble is probably located in the sec­
ondary windings of T801. It is unlikely that both rectifier
tubes |V81 2 and V822) would simultaneously be defective but
the possibility should not be ignored.
Fig. 4-3. Chocking oporation of tho high voltage oscillator by tho
placement of a neon bulb in tho field of the high voltage trans­
former.
If the neon bulb does not glow in the transformer field,
the oscillator is not operating. In such a caso the oscillator
lube, V810 and the regulator tube, V804 should be checked
by substitution. If this docs not correct the trouble, check all
components of the oscillator circuit including the high voltage
transformer.
If unregulated voltoge is obtained from both high voltage
power supplies, (a lock of regulation is indicated if the dis ­
play changes size and becomes defocused as the intensity
setting is changed or as the line voltage is varied between
105 and 125 volts) the regulator tube, V804 and the oscillator
tube, V810 should be checked by replacement. The voltage
divider containing the INTENSITY and FOCUS controls can
also cause o lack of regulation if one or more of the re ­
sistors is defective. However, if the voltage divider is al
fault, the trouble will also result in a badly defocused and
distorted display or no display at all, thereby giving a more
direct indication of this type of trouble.
If both high voltage power supplies arc operating cor ­
rectly and the FOCUS and INTENSITY voltage divider is
normal, the trouble can only be the cothodc-roy tube or the
ASTIGMATISM and GEOM ADJ controls. The ASTIGMATISM
and GEOM ADJ controls can easily be checked by voltage
readings. If the entire circuit checks out properly and the
trouble still exists, replace the cathode-ray tube.
Troubleshooting the Horizontal Amplifier
Troubles occuring in the horizontal amplifier can generally
be classified as either amplifier unbalance or as abnormal
gain. These two troubles wil be discussed separately in the
following paragraphs.
Amplifier unbalance is indicated if one or more of the fol ­
lowing conditions exist: if the beam is deflected off the foce
of the crt, if the POSITION control does not have sufficient
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