DYNACO Stereo 70 User manual

Dynaco Stereo 70 Power Amplifier
Restoration Manual
Addendums
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Addendum 1 Testing the Dynaco Stereo-70 Transformers
A. Power Transformer
1. Remove all tubes (4X output tubes, 2X driver tubes, 1x GZ-34 rectifier tube)
2. Disconnect the Red/Black Stripe wire connected to the Selenium rectifier – cover this
end with tape or 1/8” shrink tubing to prevent contact with other items
3. Install a 0.1A fuse
4. Connect the amplifier to 120VAC mains.
5. Switch on the amplifier (for approximately 1 minute)
6. Switch off the amplifier
7. Remove and examine the fuse
8. If the fuse is not tripped, the power transformer has passed the short test
9. Install a 1.0 amp fuse
10. Switch on the amplifier
11. Measure the AC voltage from chassis to pin 4 of the GZ-34 rectifier tube (should be
approx. 330VAC)
12. Measure the AC voltage from chassis to pin 6 of the GZ-34 rectifier tube (should be
approx. 330VAC)
13. Measure the AC voltage from chassis to the Red/ Black Stripe lead of the power
transformer (should be approx 60 VAC)
14. Measure the AC voltage ACROSS pins 2 & 8 of the GZ-34 – should be 5 VAC
15. Measure the AC voltage ACROSS pins 2 & 7 of the right channel output tube V7 -
should be 6.3 VAC
16. Measure the AC voltage ACROSS pins 2 & 7 of the left channel output tube V2 -
should be 6.3 VAC
17. If all of the above voltages are within 10% the power transformer is functional.
18. Switch off the amplifier and return the original fuse.
B. Output Transformers – This procedure checks both the left and right output
transformers simultaneously. A defect in either transformer will result in a test fail result.
It is beyond the scope of this document to delineate further details concerning any
failures without instrumentation specified in this document. However, if one of the output
transformers (in the pair) is defective it is questionable as to if further investment into this
particular amplifier is warranted and therefore we consider this test useful for go / no-go
purposes.
1. Remove all tubes (4X output tubes, 2X driver tubes, 1x GZ-34 rectifier tube).
2. The Stereo-70 must be completely disconnected from the AC mains and all inputs and
outputs must be disconnected.
2. Using a alligator jumper, temporary connect V3/pin3 to V6/pin3.
3. Using another alligator jumper, temporary connect V2/pin3 to V7/pin3.

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Restoration Manual
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4. Obtain another “test” receiver (preferably tube but not mandatory) and connect the
output of one channel to the Dynaco Stereo-70 Right Channel Speaker Terminals 8 Ohm
and Ground.
5. Obtain a 8 Ohm “Test” Loudspeaker and connect it to the Dynaco Stereo-70 Left
Channel Speaker Terminals 8 Ohm and Ground.
6. Have the other channel of the service amplifier directly connected to another
loudspeaker (control speaker) identical to the test loudspeaker. Set the test receiver to
monophonic operation.
7. Starting at very low levels (with the volume control of the test receiver fully
attenuated) bring up the level until you have a soft level available from both the test and
control loudspeaker. Note – it is not necessary to operate the test receiver at high or even
moderate volumes – it is only necessary to raise the level sufficiently to be able to discern
equal loudness from both the test and control loudspeaker.
8. With the balance control of the test receiver centered, the sound from both speakers
should be nearly the same. If there is a noticeable difference one of your output
transformers is defective.

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Restoration Manual
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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

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Restoration Manual
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capacitor. During the tests that follow it is important to remember that your DVM will be
floating at nearly 500 VDC relative the amplifier chassis. Most DVM’s are suitably
insulated however you should be advised nonetheless and take the appropriate
precautions.
11. Connect the amplifier to 120VAC mains.
12. Switch on the amplifier. Examine the Digital Voltmeter reading. It will begin at a
very high voltage (nearly 550 VDC) and begin to drop over time. This voltage represents
the normal charging of the quad electrolytic capacitor and actually is re-forming the
capacitor simultaneously via the 100K ohm resistor. Observe the reading over a 15 to 30
minute period. If after approximately 1 hour the voltage reading of the DVM fails to drop
to 10 volts or less, the condition of the Quad electrolytic capacitor is suspect and should
be replaced.
13. Switch off the amplifier, disconnect it from the AC mains, and wait at least one hour.
14. Slide back the heat shrink tubing and de-solder the 100K / 3Watt resistor from both
the red wire and lug #2 of the quad electrolytic capacitor.
15. Re-connect the red-wire from the previous step to lug #2 of the Quad electrolytic
capacitor.
16. Install the output tubes – the two small driver tubes will be installed later. Rotate the
bias adjustment potentiometers (located near the center of the chassis) completely
counterclockwise.
17. Connect the amplifier to the AC mains and apply power. Re-adjust the bias per the
Dynaco Owners manual.
18. Switch off the amplifier and re-install the two remaining driver tubes (7199).
19. Replace the covers and return your amplifier to service.

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Restoration Manual
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