Owner`s manual

4
houses two identical channels each consisting of a 7199 signal tube and associated
resistors and signal capacitors. The 7199 contains two sections – the pentode
section (where the input signal from your preamp is initially amplified), and a
triode section where the signal is split into two equal, but out of phase signals (out
of phase means that one signal is the mirror image of the other). For push pull
amplifiers (like the Stereo 70) it is necessary to deliver two out of phase signals to
the output section. Failures in the driver section usually manifest themselves as
differences in channel level, distortion, buzzes, crackles, hissing, or other
disharmonious sounds. Most often the problem appears in only in one channel.
The signal from the driver board circuit is sent to each of the four output tubes
(individually) via a coupling capacitor (0.1uF). Note that the voltage on the driver
side of the coupling capacitor is at hundreds of volts (positive) and the voltage at
the output tube side of the coupling capacitor is at the output tube control grid’s
(pin 5) negative voltage. If the capacitor becomes only moderately leaky, current
will flow into the bias circuit of the output stage and as a result it will be
impossible to bias the output tube correctly. This is an uncommon failure however
it is worth mentioning since it is in concert with our discussion in the output
section.
Frankly, the driver stage is one of the two sections of the Stereo 70 that was not
designed with longevity in mind. The board material is phonelic (really pressed
paper) that absorbs moisture and does not hold up well to the typical heat present
inside the Stereo 70. The solder traces are very thin and the pads are small and
combined with the inferior board material the adhesion is marginal. This makes
the removal and replacement of components very difficult usually causing board
damage. Therefore we strongly recommend replacing the driver section
should
the diagnosis point towards the driver board. In fact, it is recommended that the
driver board be replaced irrespective of the analysis to dramatically improve
reliability and performance.
III. Troubleshooting
A. Isolation & Elimination – e.g. Make Sure the Problem is in Your Stereo 70
Unless you have a single component that simply won’t power up, it will be necessary to
isolate the component causing the problem. Accordingly, if your Stereo 70 fails to power
up or trips its fuse, proceed to Section III-B below. Regardless if the problem is hum,
hiss, or any other sonic defect, in the beginning your loudspeakers have been your only
instrument telling you something is not right. To be sure that the problem is indeed within
your Stereo 70, you will need to fabricate a pair of RCA shorting plugs. You will find in
the future that they will become a very valuable diagnostic tool.
RCA shorting plugs can be fabricated by connecting the inner (+) terminal to the outer (-)
terminal of a standard male RCA plug. Take the time now to create a pair. They will
help you isolate the problem and determine where the problem is originating. Given the