In the following procedures you will be close to dangerous high voltages. Be careful.

●   The first of the procedures has you verify the voltages at the connector. The second requires you to remove the bottom cover for further testing. The second parts is only required if indicated by the results of the first.

●   The AC4/AC3 can be divided up into AC and DC sections as pictured below.

●   The supply also has 4 separate sections that supply different voltages.

●   The High Voltage (HV) section supplies approximately  +650 volts used by the tube final amplifier section.

●   The Low Voltage (LV) supplies approximately +250 volts to most other sections of the radio.

●   The Bias supply, supplies the needed negative (-) bias voltage to the amplifier sections.

●   Last is the filament supply. This supplies the needed AC voltage to all of the tubes in the radio.

●   The supply also routes the transmitter ALC and amplifier relay keying voltage to plugs on the rear of the AC4/AC3 for use by a remote amplifiers.

●   Unplug the supply from the outlet.

●   In order to get the AC4/AC3 powered up, we need to simulate the power on/off switch inside of the radio. This requires us to short pins #1 & 2 together with a jumper.  Use the figure below to following the circuit. Pin #1 goes to the black primary winding of the transformer. Pin two goes to the Fuse. By shorting pins #1 & 2 you complete the circuit and enable AC voltage to be routed to the primary of the transformer.

●   Plug the positive (+) lead of your DC volt meter into pin #10 on the connector. This is the High Voltage +650 Volts DC.  Make sure you set the meter up to read >650 Volts DC.

●   Hook the negative (-) test of your DC volt meter to the supply chassis.

●   When performing the following step, make sure that if you do not see a voltage within several seconds, you unplug the supply immediately.

●   In each of the following steps, you can choose to unplug the supply after each step. This will prevent damage to components while you are checking other sections. If you have 3 volt meters that can be hooked up at the same time, you can make sure all 3 voltages are present to make sure damage does not occur. 

●   In each of the following steps, you can choose to unplug the supply after each step. This will prevent damage to components while you are checking other sections. If you have 3 volt meters that can be hooked up at the same time, you can make sure all 3 voltages are present to make sure damage does not occur. 

●   Plug the supply into the AC outlet. Verify there is DC voltage present. It should be in the mid +650 volt DC range. If it looks ok, turn your meter to AC and verify you have very minimal AC voltage present. This AC reading should be less than 2 volts.

●   Move your positive lead to pin #11 and verify you have +250 volts, with less than 1 volt AC.

●   Reverse your positive and negative leads by moving your (+) lead to the chassis ground.

●   Move your negative (-) lead to pin #9 and verify you have negative  -20 to -60 volts present with less than a half volt AC present This is your bias voltage and can be adjusted by turning potentiometer R6 located on the front of the AC4.

●   Turn your meter to AC and put either lead in pin #6 and pin #4. You should read around 12-14 volts AC. This is the filament supply voltage.

●   If a problem is noted in any of the above steps. Unplug the unit from the AC outlet.

●   Remove the 10 screws securing the bottom cover.

●   Use the following pictures to help locate components.

●   Note any burnt components and replace.

●   If one of your supplies had a low voltage, the problem could be a bad diode. It’s easier to just replace them all than trying to isolate which one is bad.  Radio shack sells a good replacement, it’s their catalog number 276-1114.  It’s a 2.5 amp 1000 volt rectifier diode. A 1N4007 in addition to many more work just fine.

●   Resistor R3 is often the culprit in the 250 volt section.  It is susceptible to over heating, which causes it to open up resulting in no, or very little voltage output.

●   The most troublesome problem is the filter capacitors failing. They dry out or start leaking and can cause damage when they fail. Inspect around the base of each of the 5 filter capacitors for leakage. If any is noted, replace.

●   Unplug the AC4 and let it sit for several minutes before proceeding. Put your meter in the resistance mode. 

●   Put the negative lead on the chassis ground and the positive lead on each post of each capacitor. The meter should show a high resistance. The meter movement should start showing the resistance moving down scale (getting smaller) as the capacitor uses the internal meter’s battery to charge it up.  This indicates the filter capacitor is not shorted, or opened, and might be ok.

●   If when checking, the meter does not indicate the discharging of the capacitor in the above step, replace it.

●   If the meter shows a very low resistance, the capacitor is shorted and needs to be replaced.

●   If the meter shows a high resistance, and does not discharge, it indicates the capacitor is open.

●   Replacement capacitors for theAC4/AC3 are no longer available.  The easiest way to replace defective sections is to clip the post off of the effected defective capacitor and then solder a replacement to that removed post. 

●   Wrap this solder point in electrical tape to prevent it from later shorting to ground.

●   An alternative to this method is outlined in Technical Information Exchange #10, written by Daniel, W6SPC. It covers replacing defective R4 filter caps, but it can be easily applied to the AC4/AC3 unit. The general idea is shown here.

1.  The AC4 and AC3 are almost identical and can be interchanged for use on any Drake radio requiring one or the other. 

2.  The most notable difference is the AC3 is open frame, where the AC4 has an RFI protective cover.