DYNACO Stereo 70 Manuale di restauro

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Dynaco Stereo 70 Power Amplifier

Restoration Manual

Detailed Procedure & Guide

Page 1

R0 4-03

I. Introduction

This document provides a step by step instruction for rebuilding and restoring the Dynaco

Stereo-70 power amplifier. It discusses most of the typical and a few atypical problems

that may be encountered with the restoration of the vintage Dynaco Stereo-70 power

amplifier and provides the solution needed to resolve these problems. Most of these

problems are age related and therefore any unit that has not been in service for over six

months should be subjected to the Post Storage Start Up Procedure (Addendum 2). If

your goal is to repair a problem with your Stereo-70 (such as hum, noise, fuse tripping,

etc.) you should review our

available at our

Stereo 70 Troubleshooting Repair Guide

website.

The most problematic components in the Dynaco power amplifiers are (in order of

severity) the Quad electrolytic capacitor, selenium rectifier, output tube sockets, output

tubes, and driver pc board. Of those, it is mandatory to replace the first three since if not

defective they are certain to be soon. In particular, the original Quad electrolytic

capacitor, encountering 21

century mains voltages, is operating at or near its design

limits and accordingly is simply unreliable. This is the main reason that we do not

recommend replacing it with another having the same ratings but rather one of the

aftermarket modules that extend the operating voltage and also moderately increase the

energy storage capacity. CAE offers two capacitor modules that meet these objectives

and additionally include a complete new bias circuit (that also replaces the selenium

rectifier). This is essentially a completely new power supply with higher ratings and

energy capacity.

The second most problematic component is the output tube sockets which corrode and

lose conductivity over time. These failures can be very costly since contact failures will

zero bias the output tubes causing the tube to conduct maximum current (you may have

seen the glowing red elements) dramatically shortening the life of the tube (if not

destroying it altogether). Unfortunately the degree of cleaning necessary to remove the

oxidation also removes the nickel plating (preferable to gold) thereby rendering the

cleaning process short lived. The solution is inexpensive - replace the sockets completely

using ceramic based sockets. This is a very worthwhile investment easily paying for itself

considering the cost of premium replacement tubes.

Finally the original Dynaco pc driver board is subject to failure not due to limitations of

the components (although the tube sockets are questionable) but rather due to the

substrate material of the board itself. The majority of the original boards were made from

an inexpensive and common consumer electronics composite phenolic base material. It is

prone to moisture absorption and retention and with the varying temperatures

encountered inside the Stereo-70, has expanded and contracted to the extent that the pc

foil either has or is about to delaminate. This material also becomes coated with grime

that is impossible to remove without harming the board. The symptoms of a defective

drive module are noises, popping and crackling of your amplifier. We realize that many