IN CASE OF DIFFICULTY
The design of the Mark VI is inherently trouble free.
However, a mistake in assembly or a defective component
can affect its performance or make it inoperative. The wiring
should be checked step-by-step to make certain that all the
connections have been made in accordance with these in-
structions. Since many times the builder will tend to make
the same error in checking his work as when building the
amplifier, the best procedure is to have someone familiar
with the product check out your work against the
instructions.
The tubes should begin to glow after a few seconds of
warmup (note, however, no signal will pass through the
amplifier for 30 seconds, because of the time delay relay for
the 7199 tube). If the tubes do not glow, there is probably no
AC
voltage getting into the amplifier, which may indicate a
blown fuse.
FUSE FAILURE
If the power fuse fails without apparent cause, always
replace it with a 5 ampere
AGC
/3
AG
type, if the amplifier is
wired for 120 volts or 100 volts line; use a 2.5 ampere
AGC
/3
AG
fuse if the Mark VI is wired for 220 volts or 240
volts line. Never use other than these sizes.
If the power fuse fails within about 30 seconds of turning
the amplifier on, disconnect (unsolder) the two red
B
+ leads
from the power transformer (terminal strip T-3 lug #3 and
lug #4), tape the lead ends to protect them, and install
another fuse. If the fuse holds, the four
B
+ rectifier diodes,
part #544323, should be checked. A properly operating
diode has a very high resistance when the probes of a meter
are connected in one direction, and a much lower resistance
when the probes of the meter are reversed. If one or more
diode is either open or shorted, replacement of all four is
suggested.
If the power fuse fails at turn on with the red transformer
leads disconnected, there is either gross miswiring or a de-
fective power transformer.
If the
B
+ diodes are not at fault, the 50 mfd, the 100 mfd
or a section of the dual-section 50 mfd capacitors may be
shorted or miswired. Generally shorts in this section will
show up by measuring the resistance from the "+" capacitor
terminal to ground (chassis). The amplifier must be un-
plugged from the
AC
line. The resistance should exceed
100,000 ohms.
TUBE FAILURE
If one or all of the 8417 output tubes glows a bright red in
its plate section after warmup, it is possible that a tube is
defective. There also may be a fault in the bias supply which
supplies negative voltage to the output tubes, measurable
at pin #5 of each 8417 (use a
TVM
or
VTVM
). The actual
value of the negative voltage will depend on the setting of
the bias potentiometer, but with the Biaset reading set to
"0", the negative voltage at the output tubes should be within
20% of minus (—) 26 volts. If the four tubes have different
negative voltages at pin #5, there is a defective component
or miswiring. A small difference indicates either a runaway
output tube or a leaking (defective) 0.5 mfd (0.47 mfd)
coupling capacitor on the
PC
-44 printed circuit board. If one
tube has no negative voltage, there almost has to be an open
connection from the bias supply to this tube.
If there is a single glowing tube and no
TVM
or
VTVM
is
available, it is easy to find the fault by interchanging tubes.
If any tube in the same
socket
exhibits the glow, the fault is
in the circuitry associated with that socket. If the same
tube
glows in any socket, it is more than likely a bad tube (in this
case,
all
tubes are suspect).
When it is necessary to change the 8417 output tubes, use
only a matched set of
four.
Even though only one or two in
a set may be bad, we recommend that all four be replaced at
the same time. Matched sets may be obtained directly from
Dynaco.
There are no substitute tube types for the 8417, 7199, or
6N030T used in this amplifier.
NO SIGNAL
If there is no signal coming through the amplifier, remove
the input cable to see whether insertion of the plug makes a
"thump" through the loudspeaker. If so, then there is a
defective audio cable, preamplifier or other associated equip-
ment, but not in the amplifier. However, if there is no
"thump" when inserting the plug, then the amplifier or the
speaker
fuse is at fault. Try a new fuse first.
A defective 6N030T delay relay tube will also prevent
signal from passing through the amplifier, because it controls
B+ voltage to the 7199 driver-inverter. Therefore, if the
power supply is putting out
B
+, and the output tube voltages
are normal, yet the 7199 has no
B
+ voltage, check the delay
relay for proper operation.
If a meter is available, voltage measurements should be
taken and compared with the voltage chart included in these
instructions. With an incoming line voltage of 120 volts
AC
,
departures of more than
±
10% from the values shown indi-
cate probable malfunctioning of some of the circuit
components.
HUM AND NOISE
If excessive hum or noise appear, it is necessary to deter-
mine whether they occur within the amplifier, for sometimes
noise in a preamplifier will be blamed on the amplifier. The
simple test is to turn down the front panel level control at
the input of the amplifier to see if the same noise is present.
If the noise is gone with the level control fully counterclock-
wise, it is in the preceding equipment which should then be
checked. If the noise remains, it is probably in the amplifier.
With the amplifier off, remove the 7199 and then turn the
amplifier on. With this tube removed, the feedback network
is cut so that the noise
should
be higher than with the tube
in place. If the noise remains the same or drops with this
tube out, the problem lies in the tube or circuitry surround-
ing this tube on the printed circuit board. First try a replace-
ment 7199.
If under the above test the noise does increase, more than
likely the 7199 and its associated circuitry are all right. The
problem then lies either in the circuitry of the output stage,
which would include the output tubes themselves, or in the
power or bias supplies. Excessive hum, for instance, can be
caused by the failure of a
B
+ or bias diode, or by a defective
B
+ or bias capacitor.
Sizzling sounds through the loudspeaker may come from
poor contacts between circuit elements and ground connec-
tions. Make certain that the single chassis ground connec-
tion in the Mark VI is securely soldered (capacitor C-l lug
"A", as indicated in the pictorial diagram).
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