DYNACO Stereo 70 Restoration 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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