Output Transformer
If the output transformer shorts between windings it can lead to no output, weak output or funky sounding output. You can test the transformer by measuring the resistance between its leads.
Drain the filter caps and remove the rectifier and power tubes before making the following resistance measurements:
For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.
A
shorted primary winding will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.
The
secondary windings will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.
If a transformer
winding is open (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).
A
short between the primary and secondary windings will typically show less than 10 ohms resistance between the primary and secondary leads.
Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a
short to the transformer's iron core.
For comparison my healthy Hammond 125C push-pull output transformer measured: red center tap to brown 150 ohms, red center tap to blue 116 ohms, brown to blue 268 ohms. All secondaries measured .2 to .6 ohms between them. A Hammond 125GSE single-ended output transformer measured 53 ohms between the two primary wires. All of the secondaries measured .2 to .3 ohms. Both transformers measured an open circuit (no connection) between their primary and secondary wires and between all wires to the transformer outer metal shell.
If any of the above faults are present your best bet is to just replace the output transformer. For valuable vintage transformers it is possible to have them rewound.
From rob robs page.
