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Dynaco Stereo 70 Power Amplifier

Restoration Manual

Addendums

Page 3

R0 4-03

Addendum 2 Dynaco Stereo-70 Post Storage Startup Procedure

& Reforming the Quad Electrolytic Capacitor

Dynaco vacuum tube amplifiers incorporate components that may undergo

electrochemical changes if left in a de-energized state over a long period (approximately

6 months). To preserve the operation of the components and operator safety is it

generally recommended that the reapplication of power be conducted in a controlled

manner. It is the purpose of this document to describe a suitable procedure for the typical

owner without access to laboratory test equipment.

The following components and test equipment will be required:

1. Digital DVM (inexpensive model from Radio Shack or equivalent)

2. 100K ohm / 3 to 5 Watt Resistor

3. 3" length of 1/8" heat shrink tubing

Procedure:

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. Remove the rectifier tube (GZ-34).

3. Connect the amplifier to 120VAC mains.

4. Switch on the amplifier (for approximately 1 minute). Examine the four output tubes

and two small driver tubes to confirm the illumination of the filaments.

5. Locate the 7 pin terminal strip on the underside of the amplifier to the left of the power

transformer. Using your digital voltmeter (set to DC volts) Measure the DC voltage on

lug # 4 (counting from the left edge of the amplifier) and confirm a reading of

approximately negative 65 VDC (+/- 10%) relative to the chassis. Failure to achieve this

voltage suggests a defective selenium rectifier or other bias supply problems.

6. Switch off the amplifier and disconnect from the AC mains.

7. Remove the remainder of the tubes (4X output, 2X 7199 driver) and label to identify

for re-positioning later. Install the Rectifier tube (GZ-34).

8. De-solder the red wire connected to lug 2 (See figure 1) of the Dynaco Quad

electrolytic capacitor (the other end is connected to V1/pin8).

9. Slide the 3" length of the 1/8" heat shrink tubing down the free length of the red wire

removed during the previous step. Tack solder one end of the 100K ohm / 3Watt resistor

to the free end of the red wire removed in the previous step. Slide the heat shrink tubing

upwards to completely cover the exposed any wire including the resistor lead. Connect

the remaining free end of the 100K / 3Watt resistor to lug #2 of the Dynaco quad

electrolytic capacitor.

10. Set your voltmeter to DC volts and set the range of your voltmeter to be capable of

reading 500VDC. Securely connect the Black Test lead of your voltmeter to V1/pin8.

Securely connect the remaining Red Test lead to Lug # 2 of the Dynaco Quad electrolytic