Dynaco Stereo 70 Output Transformer

rudy129 · 16th April 2007, 03:26 PM

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?

BudP · 17th April 2007, 03:34 PM

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

Bandersnatch · 18th April 2007, 11:25 PM

Hey Bud,
The St.70's output, the A470 is a single secondary design, Common-4-8-16
cheers,
Douglas

rudy129 · 22nd April 2007, 09:39 PM

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?

SY · 22nd April 2007, 10:07 PM

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.

rudy129 · 22nd April 2007, 10:42 PM

Thanks, found it!! Positive was touching chassis.

16th April 2007, 03:26 PM	#1
rudy129 diyAudio Member Join Date: Aug 2004 Location: New York	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?

18th April 2007, 11:25 PM	#3
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	Hey Bud, The St.70's output, the A470 is a single secondary design, Common-4-8-16 cheers, Douglas __________________ the Tnuctipun will return

22nd April 2007, 10:07 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

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17th April 2007, 03:34 PM	#2
BudP diyAudio Member Join Date: Feb 2007 Location: upper left crust, united snakes	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

22nd April 2007, 09:39 PM	#4
rudy129 diyAudio Member Join Date: Aug 2004 Location: New York	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?

22nd April 2007, 10:42 PM	#6
rudy129 diyAudio Member Join Date: Aug 2004 Location: New York	Thanks, found it!! Positive was touching chassis.

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