Power Transformer Problems
These come in several flavors.
- Open windings, which causes the amp to simply not operate, no sound at all.
- Internal shorts to the transformer core, which is connected to the chassis. This causes the amplifier to blow fuses or the transformer to overheat, and may cause electrical shocks if the short is from the primary winding.
- Internal shorted turns, which will cause the transformer to overheat with no external load on it.
- Internal shorts from winding to winding, which cause one winding's voltage to "leak' into the other winding, possibly causing overheating or just mis-operation if not much current is involved.
- Unplug the amplifier.
- Open the case up and identify where the Power Filter Capacitors are. Connect a cliplead to the chassis, then clip the free end of the clip lead to the plate lead of any preamp tube to drain any charge from the filter capacitors. The plates of a 12AX7 are on pins 1 and 6.
- Identify the terminals of the power transformer.
- Using an ohmmeter, measure the resistance of the primary
(usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.
- Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
- If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
- If it is now less than 1M ohm, the transformer is failing, and should be replaced.
- If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.
- Measure the high voltage winding (usually red - red/yellow - red if it is centertapped, red - red or red - red/yellow if it is not centertapped) resistance from end to end. It should be less than 1K ohms. If it is over that, the winding is open, and the transformer should be replaced. The centertap, if present, should be tied to the chassis, perhaps through a standby switch.
- Measure the rectifier heater winding (usually ? - ?) resistance from end to end. It should be less than 10 ohms. If it is more than that, the transformer is bad. Measure the
resistance from either end of the winding to chassis. If it is less than 100K ohms, unsolder the leads and measure again. If this reading is less than 100K ohms, the transformer is bad.
- Measure the heater (filament) winding(s) (usually green - grern or green - green/yellow - green) as you did the primary, with the same conclusions EXCEPT that the heaters are usually tied to the chassis through the centertap (green/yellow) or through a hum balancing pot or fixed resistors of 100 - 500 ohms, so that having it conduct to the chassis is OK.
- If none of the previous tests indicate a bad transfomer, make sure all leads are soldered correctly back in their original positions if you unsoldered them.
The following procedures involve measuring hazardous voltages and performing operations inside the chassis with the AC power turned on. Do not attempt them if you
do not know how to do them safely. See the safety warning at the beginning of the debug page. Note that these procedures may cause the power filter capacitors to become charged and dangerous even with the AC power turned off.
Remove all tubes from the amplifier, leaving it in a position where you can probe the transformer terminals.
- Set your test meter to it's highest AC voltage scale (at least 750 VAC, preferably 1000VAC).
- Turn the power switch off.
- Plug the line cord in.
- Clip one meter lead to one side of the power transformer primary winding. Clip the second meter lead to the other side of the power transformer primary.
- Be certain that you have not placed the meter leads where they can cause a short.
- Check to be certain that you are not touching the amplifier except for the power switch.
- Turn the power switch on.
- Watch for the meter reading. It should read apporximately the same as your local AC power source (120VAC in the USA). If it does not, the wiring leading to the power transformer is bad, and you should check that wiring.
- Turn the AC power switch off.
- Disconnect the meter leads. Connect one meter lead to one side of the filament winding (usually green - green or green - green/yellow - green) and the other meter lead to the other end of the winding.
- Using the same cautions as above, turn the amplifier on, then off, noting the voltage reading. It should be the correct value for the amplifier, or slightly higher because of the
high current it normally carries. This is usually 6.3VAC nominal, and may be as high as 7.3VAC without unduely alarming you. If it is lower than 6.0V or above 8V, something is wrong with the transformer. Do this for each filament winding if there is more than one.
- As above for the heater (filament) winding(s), measure the voltage on the heater
winding for the rectifier tube. This voltage should be 5.0 to 5.8VAC.
- Using the same cautions as above for the primary, read the voltage on one half of the high voltage winding, from center tap to one end and then to the other end if it has a centertap, or from end to end if the high voltage is not centertapped. This voltage should be 250 to 500VAC, perhaps higher, depending on the designed B+ for your amplifier. If it
is not reasonably close to the proper AC voltage, the power transformer is bad.
- Turn the AC power switch off If it is not already off.
- Unplug the AC line cord.
- Using a clip lead, connect one end to the chassis. Connect the other end the the socket lug for the plate of a preamp tube - pin 1 or 6 in a 12AX7. Let the clip lead remain for two minutes, then remove it. This drains any DC voltage from the power filter capacitors that may have been placed there during the test. Be certain to remove the clip lead from the amplifier.
- If none of these tests show improper results, the power transformer may have an internal short.
- Unplug the amplifer. Remove all the tubes. Open the chassis, and connect one end of a clip lead to the chassis, then touch the free end of the clip lead to each terminal of the power filter capacitors as above to drain away any charge in them. Remove the clip lead.
- Using an ohmmeter, verify that the resistance to chassis from the power transformer primary windings is over 100K ohms. If it's lower than that, you need to find out why before proceeding.
- Make certain that no foreign objects or personal parts are contacting the amplifier.
- Turn on the AC power switch. Leave it on, periodically touching the outer metal shell of the power transformer to see if it gets hot. If it makes a loud hum, gets noticeably hot to the touch, or emits a burning odor, it is defective. Leave the transformer on for ten minutes, or until it gets hot, whichever comes first.