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Old 16th April 2007, 03:26 PM   #1
rudy129 is offline rudy129  United States
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Default Dynaco Stereo 70 Output Transformer

One of the channels on my st70 only plays with the 16 ohm taps being used. The 4 & 8 ohm taps play extremely low. All the voltages are what they should be at the output tubes. Has anyone ever heard of a problem like this? Maybe 4 & 8 shorted?
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Old 17th April 2007, 03:34 PM   #2
BudP is offline BudP  United States
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Rudy,

What are the voltages at the 4.8 & 16 ohm taps on the "bad" transformer Vs what they are on the good one? To do this properly you should have a dummy resistor for each tap but that is not a likely thing to have laying around.

So, keep the 16 ohm tap on both transformers loaded with a 16 ohm speaker and measure relative voltages of all three impedance taps. It is pretty unusual, when a winding goes bad, not to have a power loss on all three, as they are usually series connected internally, each tap using the preceding winding for part of it's turns.

It is possible that Dyanco wound the 16 ohm winding as a completely separate winding, they did every other kind of strange thing to these outputs. If your measurements show a voltage drop on 4 & 8 ohm taps you will need to replace the output. Your choices there can be interesting. It is unlikely that even the Dynaclone transformers will be a perfectly matched audible replacement So you may need to budget for two new transformers. Tell us what you find.

Bud
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Old 18th April 2007, 11:25 PM   #3
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Hey Bud,
The St.70's output, the A470 is a single secondary design, Common-4-8-16
cheers,
Douglas
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Old 22nd April 2007, 09:39 PM   #4
rudy129 is offline rudy129  United States
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This is weird. I took an output transformer from another st70 and replaced the one with the low 4 & 8 ohm taps and this one does the same thing. Obviously the transformers are good so what could be causing only the 4 & 8 ohm taps not to play on one channel?
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Old 22nd April 2007, 10:07 PM   #5
SY is offline SY  United States
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Well, then there's either a miswiring or unintentional short at the output terminal strip. You can confirm by connecting the speaker and the feedback loop directly to the transformer wires pulled away from the terminal strip.
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Old 22nd April 2007, 10:42 PM   #6
rudy129 is offline rudy129  United States
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Thanks, found it!! Positive was touching chassis.
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