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Old 20th March 2013, 02:33 PM   #1
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Default Hammond A0-29 project organ amp

I'm starting to revive a disgarded Hammond M3 I aquired 15 years ago... it had all but it's V1, V2 and V4 tubes removed and was stored in a churches back room. I bought all the missing tubes and tried to fire it off using the correct Hammond start/run switch procedures and it made horrific un-oiled tone generator bearing screech. I ordered two bottles of Hammond oil and finally got the tone generator's starting switch procedure to spin up quietly, and then remain spinning when only run switch was engaged (as it should) -finally! Shortly afer (6-7 minutes) after this success, I smelled electronic parts going up in smoke and saw a small trail coming up from under the power transformer end of the amp chassis. I killed power imediately, removed the four screws securing amp, gently flipped it over and saw a puddle of electrolyte goo under the c62 filter cap location. I also observed two 3" long ceramic 10 watt power resistors showing signs of excessive current with one cracked. They were mounted between two phenolic termal strips with wires going off in different directions. In the service maunual these are not pictured/drawn in the componet layout drawing on page 54, although they are located right beside three caps (c57,c31, and c5) mounted in the exact same fashion that do show up on in the underside of the amp figure on page 54 of the maunual? Further investication reveals the speaker connector plug has a second set of wires ("aftermod" I think) going to an additional 1/4 in female jack, to most likely hook up an extention speaker. I removed the plastic cover to inspect and the two connections were touching "dead shorted" together. So.....who knows what damage that short did to the output amp or power supply circuitry.

I'm a complete novice at tube amps, but have some solid state experience from long ago in the late 70's/early 80's. I'm wanting to get back into my long lost love. This appears to be maybe a common output amp "shorted speaker cable" type of failure. I obviously need to replace the one leaking chassis mounted filter cap, and its nearby smoked resistors. I'm not sure yet what resistors these actually are, but I guess I will reverse engineer what I observe, tracing their physically attached wires and connections, and figure out where they are in the schematic, and possibly identify their values/wattage. As a tube newby, socketed tube chassis, I assume one can remove the tubes and do fairly static component measurements of surrounding supporting components to insure there is no major damage to them a little easier than soldered in solid state components? Are there any other tips or tricks one of you with much more experience can share (like what should the output tranformer meaure on each side?) or can you point me in the direction of some decent resources or reference guides you recomend?
I have started to aquire a little gear - two tektronics scopes, and auto transformer, so I have some basic capabilities on my bench!
Thanks in advance from a tube newby!

Last edited by JeffMoJAD; 20th March 2013 at 02:51 PM. Reason: fixing horrible typos!!!!!!
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Old 20th March 2013, 03:32 PM   #2
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Usually the transformer doesn't blow up, either the output from a shorted speaker, or the power from a shorted electrolytic cap. Usually R98 the 50 ohm 10 W will burn up. A 47 ohm would be cheaper these days. The M3 schematic is on captain-foldback.com on Ontario, which is a good place to buy Hammond parts if you are in Canada or the UK. All the original e-caps should be replaced in all electronic chassis in this organ. This includes the electrolytic cathode capacitors that will burn up the 6C4's or 12AX7's if shorted. Some cathode caps are in cardboard wrappers with a plus on one end under the chassis. You can get M3 kits with a tall can cap from tonewheelgeneral.com in MO or bborgan.com in IL, but I like to put long life (>3000 hour service life) radial lead caps under the chassis and plug the holes with wire mesh. See posts 22 and 25 of this thread: Vintage Magnavox tube amp
A fuse in the blue wire before the power switch is also useful. 2 amp for just the organ, 3 or 4 amps for a tone cabinet or leslie speaker plugged in.
Don't work high voltage with the unit plugged in. Wear safety glasses, solder splashes. Don't touch any metal under the chassis until you measure it as under 25 VDC. More than that can stop your heart. Use one hand at all time until that measurement is complete- use a clip lead to chassis on the meter for the negative probe. Wear no jewelry, 3 VDC can burn your finger off at high current.
A whole pack of M3 fans hangs out on organforum.com. I have an H100 with more keys and pedals, and had to change 71 e-caps for decent sound. You only have under a dozen.
The M was used by Booker T & the MGs on some hits, and other british groups used them. It has a very traditional Hammond sound.
Have fun.
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Last edited by indianajo; 20th March 2013 at 03:41 PM.
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Old 20th March 2013, 06:42 PM   #3
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thanks for all the good info.... I will investigate further tonight and get the caps on order too! I also just borrowed a friends Sencore tube checker to make sure tubes are all still good tonight!

I will definately bleed the caps down and work with only one hand in or on the chassis too!!!!!!
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Old 20th March 2013, 06:57 PM   #4
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I also have a M-102, both a model 145 and Model 147 leslies, a Leslie combo preamp, Rhodes 73 stage piano, Fender 5 string Jazz bass, 6 piece 1972 Ludwig, 9 piece Tama double bass, and 11 Slingerland double bass drum kits, as well as various other instruments left behind by my kids and percussion accessories I've collected. I additionally own a Kustom 200 lead head from the 70's, a '66 Blackface Fender Bassman, a Peavey TNT50, CS-400, CS-800, CS-1000, a vintage ten band Peavey 4 rack space EQ. I also have several other additional Mackie and QSC power amps, crossovers and EQ's in multiple racks too!
I take good care of it and it takes good care of me!!!!
thanks again for your help and I will fill you in on where I get with the rising of the M3. I still have to add Leslie hookups on both my hammonds and found the schematic for the old "wurlitzer model" adapter hookup and already got two Amphenol 6 pin connectors. I'm still trying to find where I put my salvaged half moon Lelsie swithces off an old Hammond T-112 solid state combo (new ones look to be quite hight!) that burned up.
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Old 20th March 2013, 07:59 PM   #5
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Well, you're equipped. Pity you live in FL, I've been looking for a partner to play two organ two organist pieces, or two piano two pianist pieces. Two H100's one trailer, one inverter, it can be as loud as the 4th of July. I've been working up Washington Post March and They Call Me Da Breeze on the organ, who needs Lang Lang? (National 4th of July Celebration on the Mall 2 years ago).
I buy my caps mostly from newark.com in NC, they have the hours life in the selector table. Also, UPS surface, from Gafney NC, if I get the order in by 10 AM sometimes it comes by 1600 the next day. I'm pretty sure you could get 1-2 day service in FL, too. I get the $6 business rate since my garage used to be a body shop. Now it is an organ and transmission shop (amateur).
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Old 21st March 2013, 01:26 AM   #6
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If i were you I would remove the rectifier tube from the main amplifier and see if the transformer is making AC voltage on the pins of the rectifier. Also make sure that the tube heaters are lighting up. The resistors will be easy to source from mauser. Most likely what happened is your can cap shorted out and caused the resistors to burn up in the power supply. If your transformer is making power still, there is hope. I made new caps for my hammond amplifiers using perf board and electrolytic caps. I cut out the can caps and made a spot to bolt down the perf board. Re wire the supply with new resistors, and you should be up and running.
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Old 21st March 2013, 11:39 AM   #7
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Yes, I should have emphasized that all those big resistors before and on top of the can cap should be measured before you make your order. Any of them could be burned out. Be sure the cap is discharged with a resistor to chassis before making the measurement, or remove them. I use 2-3 watt metal film resistors from Vishay or Multicomp (newark house brand) instead of carbon comp 1/2 watt. They have come in from Thailand or India. Buying Xicor resistors from Mouser pretty much guarentees that your resistors are made in a country that recently found 14000 dead pigs in the city water resevoir. I install radial lead caps on solder terminal strips from tubesandmore.com, triodeelectronic.com,or electronicsurplus.com. All also have attractive prices on non-ROHS 600 volt rated 24-18 ga wire. Surplus has some cheap diodes and transistors (and some rediculous ones).
Also, Mouser has missed same day pack out on a 1515 Wednesday order twice, shipped it 2nd day air twice, which guarenteed it came Monday afternoon. Surface UPS is not managed so carefully to ensure that it doesn't come too soon. And Newark has shipped same day on every order that was warehouse complete that I have got in before 1600. (There was one order where somebody else bought the sale parts before I did, one should specify fill or kill on the text box.) Mouser is 1000 miles from here, always takes 3 days or more surface. Pensacola is the same distance. UPS charges $2-3 more for 1000 miles surface than 600, may as well ship USPS from Mouser and possibly get free Saturday delivery. The Post Office sent a special truck out one time at 1630 on Saturday afternoon with a Priority Mail box from Mouser. Mouser has transducers and insulators you can't get from Newark.
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Last edited by indianajo; 21st March 2013 at 11:47 AM.
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