Owner`s manual

switch) and place the RCA plug into the selected input. Apply power to both the amp and

preamp. If the hum is present, the problem is near the preamp line stage. If the hum is not

present, place the input shorting plugs into the preamp’s Phono Inputs and select the

phono input with the preamp’s selector switch. Observe again - hum or no hum; phono is

implicated or not. Once you have identified the problem component it is time to dig into

that component and begin the I&E process again to zero in on the defective part or parts.

Using the isolation & elimination technique, it is possible for you to identify and repair

many problems within your entire system. However it is very important that you do not

make assumptions and skip the top tier isolation steps. I have been doing this for many

years and still continue to cause myself hours of wasted time by thinking that I already

“know” which component is at fault.

A huge benefit of working on two channel components is the ability to compare the

operation and measurements of the “good” channel to the defective channel. The

differences can be very informative in locating the defective part in the non-working

channel. During the course of this discussion you will be asked to switch the tubes from

channel to channel as an isolation technique. In general, if the problem moves with the

part switch, the part is defective. However if the problem remains in the same channel,

the part is probably OK and the fault lies in the circuitry associated with that channel.

This also applies to voltage measurements - you can compare the readings from channel

to channel to help you understand what is expected vs. what is defective.

For you to repair and maintain the performance of your Stereo 70, it is very helpful for

you to read and understand the three principal sections of your Stereo 70 and how they

interact. The next section discusses each section briefly.

II. Stereo 70 Overview :

Tube power amplifiers are relatively simple devices and consist three main sections

1) The Power Supply – The power supply consists of

a. The line cord,

b. Fuse & Fuse Holder

c. Power switch,

d. Power transformer,

e. Rectifier (tube is stock, modified may be diodes) & Tube Socket

f. Choke,

g. Filter capacitor (the Quad electrolytic “can”), and

h. Bias supply. The Bias supply consists of the rectifier diode (in the original

it was a selenium “stack”), filter capacitors (2), fixed resistors (2), and bias

pots (2, one for each channel).

Because the power supply is common to both channels, symptoms that effect both

channels (dead amp, fuse tripping, hum in both channels, weak power in both

channels, etc.) is usually the result of a problem in the power supply.