I had someone Email me and ask me how to check out a used Dynaco A-470 transformer to see if it will operate properly when installed in an amp.
First of all you can never be totally sure that an output transformer will operate properly once the center tap of the primary is hit with 400+ volts but you can do some resistance (continuity) tests to check if it "should" operate as designed.
On the primary side of the transformer. Measure the resistance from the RED center tap to each of the other four wires (BLUE, BLUE/WHITE, GREEN, GREEN/WHITE) You should get continuity to all of the other four wires with a relatively low - less than 200 ohm resistance. Most measurements will be from about 15 ohms to maybe 150 ohms.
Next check the resistance on the secondary side of the transformer where the BLACK, BROWN (4 ohm tap), ORANGE (8 ohm tap) and YELLOW (16 ohm tap) wires are. Measure from the BLACK wire to each of the others. In all cases you should get a very low resistance - probably under 2 ohms in all cases with the lowest resistance to the BROWN wire and the highest to the YELLOW wire.
If the transformer checks out as above it should be good. Personally I would take off the bells and look inside for signs wear on the leads. If anything looks frayed I would slide some shrink wrap tubing over frayed area.
One other thing on Dynaco transformers especially the ones with the cloth leads. After years of heat the wire colors can fade and sometimes it is virtually imposible to tell the actual color. This is true especially on the BLUE and GREEN leads which may fade to a pale aqua color and look almost identical. If you take off the bells then many times the TRUE color will show under the bells.
Bob
First of all you can never be totally sure that an output transformer will operate properly once the center tap of the primary is hit with 400+ volts but you can do some resistance (continuity) tests to check if it "should" operate as designed.
On the primary side of the transformer. Measure the resistance from the RED center tap to each of the other four wires (BLUE, BLUE/WHITE, GREEN, GREEN/WHITE) You should get continuity to all of the other four wires with a relatively low - less than 200 ohm resistance. Most measurements will be from about 15 ohms to maybe 150 ohms.
Next check the resistance on the secondary side of the transformer where the BLACK, BROWN (4 ohm tap), ORANGE (8 ohm tap) and YELLOW (16 ohm tap) wires are. Measure from the BLACK wire to each of the others. In all cases you should get a very low resistance - probably under 2 ohms in all cases with the lowest resistance to the BROWN wire and the highest to the YELLOW wire.
If the transformer checks out as above it should be good. Personally I would take off the bells and look inside for signs wear on the leads. If anything looks frayed I would slide some shrink wrap tubing over frayed area.
One other thing on Dynaco transformers especially the ones with the cloth leads. After years of heat the wire colors can fade and sometimes it is virtually imposible to tell the actual color. This is true especially on the BLUE and GREEN leads which may fade to a pale aqua color and look almost identical. If you take off the bells then many times the TRUE color will show under the bells.
Bob
