Owner`s manual

8
capacitor (only about 9 volts) and during this time the reading will not be stable.
After a few seconds the reading will stabilize. Make note of this reading – it
should be at least a few K Ohms. Any reading of less than 5K ohms is suspect and
probably indicates a defective capacitor. To be sure you could remove the
capacitor from the amplifier and repeat the test but our standard policy is to
replace capacitors that measure in this range. Unfortunately replacements are very
unreliable however CAE offers a small Quad Capacitor Replacement module
(P/N PC-S7U) specifically designed for the Stereo 70 that contains a composite of
modern reliable capacitors. Both the voltage and capacitance ratings are raised
giving higher reliability and performance.
3. If the fuse trips slowly (unit stays powered for a few seconds as the amplifier
warms up and then the fuse trips) the power transformer is probably (but not
assuredly) not the problem. In this case the fuse is responding to excessive current
after the vacuum tube rectifier reaches temperature (and begins to conduct). The
most common paths of excessive current in this case are into the output tubes
(when improperly biased or shorted) or into the infamous Quad Electrolytic
Capacitor). It is unlikely that the rectifier would fail in this manner but it is
possible so we will include that possibility in our tests. We will check the output
tube circuit first.
a. Remove all four of the output tubes.
Keep the rectifier tube in place. Replace
the fuse, power up again and observe.
• If the fuse does not trip
, measure the bias voltage at pin 5 of each of the four
output tubes (relative to chassis). You should be able to vary the voltage reading
from negative 39 VDC (approx) to negative 22 VDC (approx) as you rotate the
bias adjustment potentiometer associated with that channel. Note that one bias
adjustment potentiometer adjusts the bias on both tubes in the same channel
simultaneously – and be sure to check the pin 5 voltage on both of these tubes.
If you do obtain the correct reading, one or more of the output tubes may be
defective - check and replace any defective output tubes. If you cannot obtain a
correct reading, one or more of the Bias supply components (selenium rectifier,
2X electrolytic capacitors, or Bias Potentiometers) is defective. You may obtain
a complete bias supply replacement kit from CAE (P/N ST7-BIAS).
• If the fuse trips again
, replace the rectifier tube.
b. Replace the fuse, power up again and observe.
• If the fuse does not trip
keep the new rectifier tube and go to section 3c
below.
• If the fuse trips again
, the Quad Electrolytic Capacitor (the large tubular
aluminum can located above the chassis slightly left of center on the top of the
amplifier) is defective and must be replaced. Unfortunately replacements are
very unreliable. However CAE now offers a small Quad Capacitor Replacement
module (P/N PC-S7U) specifically designed for the Stereo 70 that contains a