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Testing Dynaco output transformers ... on Fri Nov 06, 2009 8:13 am
Bob Latino
Admin
Is that Dynaco A-470 output transformer on your ST-70 good or bad ? Did you buy a pair of A-470's off Ebay and your not sure if either or both are OK? All the original A-470 output transformers were very well made but understand that they sometimes do go bad. Those still in use have looked at 400+ volts on the primary side for 40+ years and many thousands of hours of playing time. It is possible that one of the primary side windings can burn out after being hammered with high voltage for that many years. You can check the transformer in or out of the amp. If you want to check it out inside the amp first make sure that the amp is off and then remove all the tubes. Take off the bottom cover and use an insulated screwdriver to short the 4 sections of the quad cap to the chassis to remove any residual high voltage storage. Temporarily unsolder the RED wire for this transformer from the quad cap and just let it hang in the air.
1. With your multimeter measure the resistance from ALL the primary wires (GREEN, BLUE, GREEN/WHITE, BLUE/WHITE and RED) to ALL the secondary wires (YELLOW, ORANGE, BROWN, BLACK). You should get no continuity or an infinite resistance from any of the primary wires to any of the secondary wires. If you get a low resistance from any combination of ONE primary and ONE secondary wire there is a short in the windings somewhere and that transformer is bad and must be replaced.
2. Measure the resistance from your primary RED wire hanging in the air to the other four PRIMARY wires. You should get readings of about 15 ohms to maybe 200 ohms. If you get an "open" on any measurement the transformer is bad and must be replaced.
3. Measure the resistance from the BLACK secondary wire to each of the other secondary terminals (BROWN, ORANGE and YELLOW). In all cases you should get a very low resistance maybe between .5 ohm and 2 ohms or so. If you get an open and measure nothing then one of the windings is bad. If it is just the 8 ohm tap you might still be able to use your 8 ohm speaker on the 4 ohm tap but IMHO you should replace the transformer.
If the transformer measures OK as outlined above it *should* be good BUT remember > You are only really testing the transformer for continuity and only applying 9 volts from the battery of your multimeter. When hit by 400+ volts on the primary side an issue could crop up that doesn't show up on the continuity test.
Bob
1. With your multimeter measure the resistance from ALL the primary wires (GREEN, BLUE, GREEN/WHITE, BLUE/WHITE and RED) to ALL the secondary wires (YELLOW, ORANGE, BROWN, BLACK). You should get no continuity or an infinite resistance from any of the primary wires to any of the secondary wires. If you get a low resistance from any combination of ONE primary and ONE secondary wire there is a short in the windings somewhere and that transformer is bad and must be replaced.
2. Measure the resistance from your primary RED wire hanging in the air to the other four PRIMARY wires. You should get readings of about 15 ohms to maybe 200 ohms. If you get an "open" on any measurement the transformer is bad and must be replaced.
3. Measure the resistance from the BLACK secondary wire to each of the other secondary terminals (BROWN, ORANGE and YELLOW). In all cases you should get a very low resistance maybe between .5 ohm and 2 ohms or so. If you get an open and measure nothing then one of the windings is bad. If it is just the 8 ohm tap you might still be able to use your 8 ohm speaker on the 4 ohm tap but IMHO you should replace the transformer.
If the transformer measures OK as outlined above it *should* be good BUT remember > You are only really testing the transformer for continuity and only applying 9 volts from the battery of your multimeter. When hit by 400+ volts on the primary side an issue could crop up that doesn't show up on the continuity test.
Bob
