Owner`s manual

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Dynaco Stereo 70 Repair Guide

I. Introduction

A. Tools Required - Before you begin you should have a few basic tools on hand to help

you identify and repair the problem. Please obtain the items outlined in the following list:

• RCA shorting plugs (Fabricate using Radio Shack # 274-339)

• Digital Voltmeter (Inexpensive Model from radio Shack)

• Soldering Iron & Associated items (See the CAE Soldering Tutorial Tech Note @

http://www.curcioaudio.com/sup_3.htm

)

• Screw drivers, Pliers (including needle nose), wire cutters & stripper (See the CAE

Recommended Tools Tech Note @ http://www.curcioaudio.com/sup_3.htm

)

• Wire Jumpers with Alligator Clips (Radio Shack # 270-180)

• 100 ohm 2 Watt resistor (not a critical value, any value near 100 ohms will be fine)

• 20 fuses rated at 3 Amps, Slo-Blo (not larger nor smaller)

• Documentation (schematic diagrams and parts list) for defective component – Note:

You will find a full copy of the Dynaco Stereo 70 Owner’s manual at our website (

http://www.curcioaudio.com/dynadr_3.htm )

B. General Discussion

In this document, I hope to provide you with some methods and techniques that will

guide you to repair most any problem in your Stereo 70. However, it is absolutely critical

that you are certain that you have the skills to safely operate the tools recommended and

that you always remember that you will be working with lethal voltages. The electrolytic

capacitors will continue to store and present even after you have removed the power and

disconnected the line cord. Once again, always assume that lethal voltages are present

and work accordingly. That means never place yourself in a position to be the conductor

between the circuit and ground (let the jumper become the more direct current path).

The most effective technique for the diagnosis of problems with your components is the

Isolation & Elimination method. With this methodology, we will initially eliminate the

functional components until we isolate the defective component, section, subsection and

finally the part. For example, if you have hum in your system, it is important to determine

if the hum is being generated inside your Stereo 70 or is being injected into the Stereo 70

from your preamp or from poor shielding or thru the power line, among other paths.

Using the Isolation & Elimination methodology, we would first place RCA shorting

plugs at the input to your Stereo 70. If the hum is still present, it is safe to eliminate

everything but the power amplifier and associated components. Alternatively, if the hum

is eliminated, you can safely conclude that the problem exists upstream of the power

amplifier. In this case you would proceed to examine the components upstream.

Reconnect your preamp to the Stereo 70. Remove all input sources from the input of the

preamp. Select a line level input, such as the tuner input (with the preamp’s selector

Summary of content (16 pages)

PAGE 1
Dynaco Stereo 70 Repair Guide I. Introduction A. Tools Required - Before you begin you should have a few basic tools on hand to help you identify and repair the problem. Please obtain the items outlined in the following list: • RCA shorting plugs (Fabricate using Radio Shack # 274-339) • Digital Voltmeter (Inexpensive Model from radio Shack) • Soldering Iron & Associated items (See the CAE Soldering Tutorial Tech Note @ http://www.curcioaudio.com/sup_3.
PAGE 2
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.
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2) The Output Stage – There are two output stages, one for each channel. Each output stage consists of : a. the output transformer, b. output tubes (2 per channel), and c. cathode bias resistor (the original value was 15.6 ohms and had the appearance of a white ceramic tubular device connected to pins 1 & 8 of both output tubes). d.
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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).
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typically high input impedance of most tube circuits, simply leaving the inputs open will probably cause stray signals to mislead your investigation. FYI – the output should be connected as it would be normally (e.g. in a power amplifiers the speakers are connected). The steps are as follows: 1. Power Off 2. Starting at the power amplifier, remove the connections at the input and place the RCA shorting plugs into the inputs. 3. Apply power and observe.
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A. DEAD - Won’t Power Up or Trips Fuse 1. If your Stereo 70 is simply dead (no filaments illuminated) and the fuse does not trip - follow the steps in this section. If the fuse trips immediately, go to section A2 below, and if the fuse trips slowly (after the amp warms up) go to section A3 below. a. Insure that you actually have power at the plug (try a lamp or another component connected to the same receptacle). b. Is the power switch in the ON position ? c.
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Remove the cover and locate the power supply section. This is the area near the Power transformer, Quad Electrolytic Capacitor, rectifier tube, Filter Choke, and Bias Supply (selenium rectifier & two electrolytic capacitors located underneath the chassis). Follow the Quad Electrolytic Discharge Procedure as described below a. Remove the rectifier tube (or diodes if your amp has been modified) and the four output tubes. b. Replace the fuse, connect the amplifier to the AC mains and apply power.
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capacitor (only about 9 volts) and during this time the reading will not be stable. After a few seconds the reading will stabilize. Make note of this reading – it should be at least a few K Ohms. Any reading of less than 5K ohms is suspect and probably indicates a defective capacitor. To be sure you could remove the capacitor from the amplifier and repeat the test but our standard policy is to replace capacitors that measure in this range.
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composite of modern reliable capacitors. Both the voltage and capacitance ratings are raised giving higher reliability and performance. c. Return tow of the four output tube – one channel’s pair. Select DC Volts on your digital voltmeter and a voltage range of 2 volts and monitor the voltage across the 15.6 ohm bias set resistor (for the channel that now has the output tubes installed). Adjust the associate bias potentiometer fully counterclockwise.
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c. Remove power. Exchange two of the output tubes from one channel with other two output tubes from the other channel. Reapply power and adjust the bias according to the Dynaco Owner’s manual. • If the no sound condition has reversed channels, replace the output tubes. • If the no sound condition remains with the same channel, go to the next step d. Remove power. Exchange the small signal tubes (7199’s) from one channel to the other. Apply power and an input signal.
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transformer is defective. If the other channel reads incorrectly as well, follow the instructions in the next section (2b) below. 2. If your Stereo 70 has NO SOUND FROM EITHER CHANNEL and the filaments are illuminated – follow the steps in this section a. Apply power to your Stereo 70. Check to see if the filament is illuminated in the GZ-34 rectifier tube. • If it is illuminated proceed to the next step (b) below. • If it is not illuminated replace the rectifier tube.
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Remove the input connections from the power amplifier and insert the RCA shorting plugs. If the hum remains, the hum is obviously being created internally. Defective component possibilities in increasing order of probability include the power transformer (really unlikely), rectifiers (vacuum tube or diode), a tube with heater to cathode internal shorts (in other words, a defective tube), or open electrolytic capacitors (most likely).
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Hum can come from many sources both internal and external. In the previous section we focused our attention to hum that can originate internally to the amplifier. In this section we will focus our attention on external sources of hum. One of the main causes of hum are ground loops – caused when a circuit has more than one ground path for the power supply or signal currents to follow. The IR drop associated with each path will be different and therefore result in a voltage differential within the circuit.
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problem is further back (towards the source). Next, place the RCA shorting plugs into the phono input and select that input. Once again, power up and observe. The presence of hum now points to the phono stage inside of the preamp. Otherwise it is due to turntable, arm, cartridge grounding and / or shielding. If from the above series of tests you identify that the source of hum is internal to your preamp, the most likely causes are the tubes or power supply.
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G. CHANNEL IMPALANCE Left to right channel imbalance is very similar to hiss in its casual conditions. Once again switch the tubes from channel to channel to identify to isolate the defective area. If you determine that the tubes are the cause, replace the tubes. Other causes can be very elusive including problems inside local or global feedback loops. This can be very difficult to identify and we suggest contacting us here at CAE or a local qualified technician.
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QUAD Capacitor Replacement Module - A specially designed printed circuit board that contains a composite of high quality capacitors that provides both higher voltage (to increase reliability) and greater capacitance (to improve performance) compared to the original. Rectification provisions for either solid state (diodes) or the original vacuum tube rectifier. The board also includes a complete bias supply including rectifier, filter caps, and associated resistors.