DYNACO Stereo 70 Manuale di restauro - Pagina 5
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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 / cool
cycles 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. Restoration Process
A. Preparation
1) You will first remove the top and bottom covers by removing the 4 (6 on some
units) located on the right and left sides of the amplifier. Both covers should be
free to be released.
2) Next, remove the tubes (4X output tubes, 2X driver tubes, 1X GZ-34 rectifier
tube) and label each to their respective positions if you are planning on retaining
the original tubes.
3) Using a 2" paintbrush and a vacuum cleaner hose gently dislodge and vacuum all
dust and dirt from the topside and underside of the amplifier. Be careful to get
into the small crevices under the transformers.
4) Before we proceed, it is important to confirm the operation of the power and
output transformers since you may want to reconsider the value of the project
should and of the transformers be defective. If you amplifier has been in service
before you began, it is fair to assume that the transformers are functional and
therefore you may omit the transformer tests and move to the next phase. The
transformer tests are described under Addendum 1: Testing the Stereo-70
Transformers.
B. Disassembly
During the disassembly, there are some components that will be retained and reused
while others will be discarded. Obtain a small box to hold the hardware and parts that
will be reused. When identified as retained, carefully de-solder all leads and wire
remnants, thoroughly clean the leads of all remaining solder and store them in a safe
Dynaco Stereo 70 Power Amplifier
Restoration Manual
Detailed Procedure & Guide
R0 4-03
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