I'm on the home straight restoring a Hammond L100 series all valve tonewheel organ. It is a wonderful instrument, and fully working now except for the reverb tank which has me stumped, and I could do with some help for ideas please. I've tried, wherever possible, to re-use original parts, so I'm trying to repair the original reverb tank, which shouldn't be too hard you'd think.........
The original reverb tank is a 1967 Gibbs 17.5" 2 spring, which is said to be the same as an Accutronics unit beginning 4FBxxx. So it has a high impedance input, and as expected the input coil measures about 200R dc, same as the output coil. In fact, the input and output transducers appear physically and electrically identical, is this correct in the 4FBxxx models ?
Anyways, touching either of the springs produces normal levels of output twang, so the output transducer seems to be working. Correct level of drive to the input transducer produces absolutely no output, however.
Because the input and output transducers seem identical, it's possible to swap the drive and output over. Under this condition, the input transducer is now the output tranducer, and again there is normal sounding spring twang. So I take this to mean that the coils and magnets are OK for both coils, as are the connections.
The tiny magnets are in place in the gaps, free to move.
Swapping in a Accutronics reverb tank from a fender twin reverb for a few seconds as a test (drive impedance is wrong so just a brief test), produces good reverb. Drive circuit produces loads of clean signal.
So I'm stumped..............ideas to try ?
The original reverb tank is a 1967 Gibbs 17.5" 2 spring, which is said to be the same as an Accutronics unit beginning 4FBxxx. So it has a high impedance input, and as expected the input coil measures about 200R dc, same as the output coil. In fact, the input and output transducers appear physically and electrically identical, is this correct in the 4FBxxx models ?
Anyways, touching either of the springs produces normal levels of output twang, so the output transducer seems to be working. Correct level of drive to the input transducer produces absolutely no output, however.
Because the input and output transducers seem identical, it's possible to swap the drive and output over. Under this condition, the input transducer is now the output tranducer, and again there is normal sounding spring twang. So I take this to mean that the coils and magnets are OK for both coils, as are the connections.
The tiny magnets are in place in the gaps, free to move.
Swapping in a Accutronics reverb tank from a fender twin reverb for a few seconds as a test (drive impedance is wrong so just a brief test), produces good reverb. Drive circuit produces loads of clean signal.
So I'm stumped..............ideas to try ?
Replace the reverb unit. The transmitter is almost certainly faulty. I use Accutronics Reverb Tank 4BB3C1D | eBay
From your description, the Fender tank worked OK?
From your description, the Fender tank worked OK?
It might be (relatively) easy to pick up another genuine vintage tank, would need to begin 4FBxxxx because the drive circuit is transformerless parafeed, so needs a high impedance input/transmitter. I figure it's OK to run a fender tank for a short time as a test, which is what I did to verify driver and receiver OK. Also checked the drive level using a 'scope, FWIW.
However, I think to myself 'what can possibly be wrong with the transmitter, when it works OK in reverse as a receiver, at least as far as spring twang is concerned ?' Coil, magnet must be OK to pass that test I think.......so why doesn't it just work ? It really is as simple as a tiny magnet attached to the spring suspended between poles of a U shaped core.........
Chances are if I can figure out why it doesn't work, I can fix the original.
From the description, it looks kind of impossible. Therefore it has to be some silly but non-obvious problem: a failed insulation grounding one of the terminals could be one (but there are certainly many other possibilities: you have to think outside of the box, and figure out what's really going on).
Thanks - in this case the transmitter winding definitely isn't open circuit, and it works as a receiver to spring twangs.......also this unit is a Gibbs, and isn't sealed which might hold a clue. It was, as might be expected, full of near 50 years of crud - which I carefully cleaned out leaving the ever so simple transducers and springs. I suppose that any vintage reverb tank might suffer similar blight, not knowing what the blight is exactly..........
Ever known a spring fail to transmit vibrations ?
You have two problems. One is the intellectual puzzle of why this unit doesn't work, even though the couple of test you came up with seems to say OK. The other is getting the system to work. The thing to do is take care of the second problem, and leave the first for your leisure.
You already know a substitute pan works. SO you at least have that option for repair.
I am not sure how you tested the drive. You got both ends to act a output, so it tends to mean the drive is not really getting to the transducer. It could be as simple as the male plug doesn't make good contact in the pan jack, while the same plug does make OK contact in some other jack. Connect it up, verify it doesn't work, then flip it over and connect your scope right to the inner wires of the pan.
You already know a substitute pan works. SO you at least have that option for repair.
I am not sure how you tested the drive. You got both ends to act a output, so it tends to mean the drive is not really getting to the transducer. It could be as simple as the male plug doesn't make good contact in the pan jack, while the same plug does make OK contact in some other jack. Connect it up, verify it doesn't work, then flip it over and connect your scope right to the inner wires of the pan.
Think you're right, Enzo it's curiosity that drives me to understand it. I could always fix it with a new standard fender tank and adapt the driver, but only if the original really is unsalvageable.
Some further tests today suggest that the springs might be so lossy that nothing/very little reaches the far end ! They were pretty corroded and loaded with crud before cleaning. So I'm going to replace the springs and see what happens.
Some further tests today suggest that the springs might be so lossy that nothing/very little reaches the far end ! They were pretty corroded and loaded with crud before cleaning. So I'm going to replace the springs and see what happens.
OK, so shortening the springs (by cutting and joining) by about 2" restored normal operation
I think what was happening in the old corroded springs is that there was no gap between the turns, so quite a few of them were touching or separated by crud/corrosion. Seems that the turns need to be physically separated for the reverb line to work.
I'll find new springs, but for now the original tank works well. Nice.
I'm amazed at the outcome, wouldn't have thought that cause possible. Thanks and hope this helps someone someday !
I think what was happening in the old corroded springs is that there was no gap between the turns, so quite a few of them were touching or separated by crud/corrosion. Seems that the turns need to be physically separated for the reverb line to work.
I'll find new springs, but for now the original tank works well. Nice.
I'm amazed at the outcome, wouldn't have thought that cause possible. Thanks and hope this helps someone someday !
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