The AO-28 tube pre-amp is found in the Hammond B-3 as well as several other models. C-3, A-100, RT-3, D-100 etc.
Designed in the fifties, this pre-amp remained in production until 1975. And, it remains fully functional and serviceable today. This discussion is designed to help owners do some basic troubleshooting and, we’ll get into some depth of circuit operation as well as common problems usually found here.
First, let’s look at the preamp and determine where things are located.
- T-7 Power Transformer.
- V-8 6X4 Rectifier Tube. This is the one tube most prone to failure.
- V-3 12BH7A Output tube. Also serves as the vibrato drive tube.
- V-4 12AX7A Preamp tube.
- V-1 6AU6A Input tube. This is the non-vibrato input preamp.
- V-2 6AU6A Input tube. This is the vibrato input preamp.
- V-6 6C4 Percussion tube. This is the percussion keyer tube. No audio goes thru it.
- V-5 6C4 Percussion tube. This is the percussion preamp tube.
- V-7 12AU7A Percussion tube. This is the voltage controlled amp for creating the percussion decay.
- T-4, T-5, T-6 are percussion transformers located under the chassis.
- T-3 Output transformer.
Input StagesIf the vibrato switches are set to vibrato off, the signal from the matching transformer goes into Point “A”, a solder terminal located on the preamp. If you had a problem with the non-vibrato sound, it would likely be in this area. Basically the signal is routed to the input grid (pin 1) of the 6AU6A tube for amplification. The voltages shown are usually the first thing to check. If the tube is good, then the two most likely sources of problems will be R6 (plate load resistor) or R7 (screen grid resistor). If you check the plate voltage for example and it reads low, then the plate load resistor R6 is probable going bad. (opening up) If however the voltage on the plate reads high, then the screen grid resistor R7 is going bad. (opening up).
Since the vibrato (V1) and non-vibrato (V2) tubes are the same (6AU6A), you can swap them to determine if it’s a tube problem.
Also check pin 7 for the 1.7V from V2.
- Plate load resistor R6 opens up and the plate voltage goes down.
- Screen grid resistor R7 opens up and the tube stops conducting.
- The tube itself goes bad.
- The capacitor C-5 can cause a “thump” when switching the vibrato tabs.
Again the plate and screen voltages should be checked on V2 like above. Additionally, the voltages on V3 should be checked. Any loss of amplification here will cause the vibrato to either be weak or not work.
- Plate load resistor R19 opens up and the plate voltage goes down.
- Screen grid resistor R20 opens up and the tube stops conducting.
- The capacitor C-11 can cause a “thump” when switching the vibrato tabs.
- The tube itself goes bad.
- V3 fails.
Volume Control SectionV4 is a dual triode tube. The first stage V4a accepts signals from the non-vibrato preamp, the vibrato scanner, and the percussion circuit. These signals are again amplified to a sufficient level to drive the expression control. The main service concern here would be the plate voltage. If the plate voltage is low, then the plate load resistor R29 is likely going bad (opening up). If it is high, and since there is no screen grid, the tube itself is likely going bad.
In the expression control circuit, you will see there are two plates shown after V4(a), and a third movable plate shown going into the next stage of V4b. The higher plate is connected directly to the output of V4a, while the lower plate has some RC (resistive/capacitive) circuits shown. If the movable plate is closer to the higher stationary plate, the volume is louder. As it moves to the lower stationary plate, it gets softer.
Because of the RC network, the signal also gets a tone change. In other words, the high frequencies are attenuated faster than the lower frequencies. This means at lower volumes, the bass will actually remain more present than the highs. Since we hear different frequencies differently at different volumes, (see fletcher-munson curves) this acts like a loudness curve in a home stereo.
The second stage of V4(b) picks up the signal from the expression control, amplifies it and sends it to the output stage through the tone control. Again the plate voltage is the primary check. Determine if the tube or plate load resistor is bad. A failure in either stage of V4 will usually kill all sound in the organ.
The tone control is a passive circuit and in reality, it only attenuates the highs. It can be a source of problems if the potentiometer R40 is scratchy. If you have the preamp turned up to expose the underside, always clean this pot with some good quality cleaner such as De-oxit.
Notice C22 which is a feedback capacitor in the second stage of V4(b). This cap takes some of the output from V4(b) and sends it to its input. Since the output is 180 degrees out of phase with the input, it reduces the gain of that stage. It is set at the factory and in general, you set it by turning the screw located in the expression box clockwise all the way in. Then turn it counter clockwise 1 1/2 turns. This should not be played with as a volume control. Even though it does affect the volume, it also affects the frequency response and should be set close to the factory setting.
Also notice the reference to the volume tablet and the E and F terminals. This is where the volume soft/loud tab connects. By shorting out C24, the volume is raised. I mention this because there is a typical problem where when switching the soft/loud tab, you hear a thump or pop. This is due to C23 going bad. (leaking DC)
- Plate load resistor R29 opens up and the plate voltage goes down.
- Plate load resistor R36 opens up and the plate voltage goes down.
- The tube itself goes bad. (12AX7a)
- C23 leaking causing a pop when switching the volume tab.
Output SectionV3 is the final amplifier of the preamp. This 12BH7A is a dual triode where one stage is for preamp output and the other is used after the V2 vibrato preamp to drive the vibrato delay line. The signal is taken from the tone control, goes into pin 7 input grid, and outputs on the plate to drive the output transformer. Obvious check points are pin 6 plate and pin 8 cathode. A shorted C26 could cause the cathode voltage to be low. An open R42 could cause it to go way up and result in little or no amplification. Output transformers rarely fail so a loss of plate voltage is unlikely. The transformer converts the single ended signal into a balanced signal. It’s output is the famous “GG” terminals. A quick check of the overall gain of the preamp is to turn the expression pedal all the way up. Pull out the four lower drawbars on either manual. Play a “C” chord in the middle of the manual. You should read about 3 to 4 Vac across the GG terminals.
- V3 12BH7a tube fails.
- Dirty Tone control causing static or no sound.
- C26 shorts or opens. Check the 12Vdc. An open cap symptom will be less bass.
- Output transformer can open up or get noisy.
PercussionThe percussion channel input terminal H gets its signal from the percussion switches. Either the second or third harmonic. The signal enters T4 transformer and V5 (6C4), the first stage of amplification. Not many problems in this circuit other than the occasional tube failure. T5 takes the single ended signal and changes it to a balanced signal. It drives the balanced amplifier V7 (12AU7A). Notice the third winding on T5. This carries the selected percussion harmonic that was borrowed from its drawbar back and replaces the borrowed harmonic through R50 and terminal J. By changing the voltage on the input grids, the gain of V7 can be varied. This is done through the center tap of T5′s secondary winding controlled by V6 (6C4). The percussion cutoff control sets the bias of V7 which adjusts the decay speed of the percussion.
From the Service Manual:
When a key is depressed the signal first plays loudly through the control tube, transformer T6, a high pass filter, and terminal D to the grid of V4. Immediately condenser C31 in the control tube grid circuit begins to discharge, causing the signal to fade away. Terminal K (Approx 25Vdc) is connected to the 1′ drawbar wire which is connected to the manual busbar. When an upper manual key is pressed, terminal K is grounded through the tone generator filters. This virtually grounds the plate of V6 (connected as a diode), stops conduction, and isolates cathode and control tube grid circuit. The grid then drifts from approx +25Vdc to about +15Vdc, at a rate determined by the time required for C31 to discharge through R57 and R58. At the completion of this sequence the percussion signal is blocked. No further percussion effects occur until all keys of the upper manual are released and the control grids can again rise to +25Vdc. The rate of this rise is fixed by the time required to charge C31 to +25Vdc through R55 and R56.
With all three percussion tubes removed, the organ will play normally, just no percussion. The transformers give little to no trouble but can fail. In some cases, cause static that can only be removed by replacing the offending transformer. In an emergency, you can pull the percussion tubes to get rid of the static and operate the organ with no percussion. These transformers are NLA but can be acquired from used instruments including the smaller and much cheaper M-3 series organs.
To set the percussion cutoff control, depress a key with no drawbars out and the percussion on. The sound should fade out completely. Adjust the control until the sound can be heard. Then turn it the other way until if fades out completely. You can further adjust it for the amount of decay you prefer.
More for information on a quick fix to dead percussion.
- Any of the three tubes fail.
- A short on the K terminal keeping the percussion cut off.
- Static in transformers or transformer failure.
VibratoAs far as the preamp goes, the vibrato signal is sent from V3 through Terminal C to the delay line. The delayed signal is scanned by the scanner and returned to the preamp on terminal D.
Learn more about the delay line and scanner.
- Check that the tubes are lighting up and the Generator is running.
- Check the left most rectifier V8. Try replacing it.
- Set up the organ with first 4 drawbars on either manual, full expression, hold down a C chord in the middle of the manual while checking for AC voltage on the GG terminals. If you have voltage, you may have a problem in the speaker amplifier.
- Make sure the expression linkage is working and properly adjusted.
Weak or no sound on either the Vibrato or non-vibrato setting:
- Swap V1 and V2 and see if the problem moves.
- Lift the chassis and check the voltages on the questionable channel.(V1=non-vib, V2=vib)
General static in sound or even when not playing:
- Slightly move each tube and listen for noise. Pull tubes, clean pins and replace.
- Try tapping each tube and listen for noise. Any microphonic tubes should be replaced.
This information is for educational purposes only and no claims are made that this information will lead to any successful repair. Benton Electronics assumes no responsibility to its use.
|Hammond Percussion Quick Fix||Working with the Worship Team|
|Hammond Percussion Quick Fix|
|Working with the Worship Team|