Lesle tube amp giving me fits.. Amp guys can you help?

[Rich Rice]

Wow. I got a nice old Leslie, untested. Fixed i up cosmetically, gave everything a thorough cleaning and polishing- fixed the seized up rotors and got them spinning nicely. Then I cleaned up the tubes, chassis, sockets, etc. Looks beautiful.. Upon inspection (prior to plugging it in) I spotted a resistor that was toast. It connected between the cap cans. I removed and replaced it, installed the amp into the cab, connected the speakers and relays, then applied voltage. Nothing. Checked the fuse, it was blown. Since I had not checked the fuse prior to powering it up, I replaced it- blew immediately. Not a flash, not a glimmer in the tubes, no smoke.. Nada. This one has a solid state rectifier, so my thoughts turned to that. I pulled the amp apart, looking for burned components, bad connections, loose wires, etc., all looked good except for some dull solder joints in the rectifier circuit. I reflowed all the iffy looking joints, plugged it in- poof went the fuse. So I pulled the amp again, and clipped the transformer wires that feed the diodes- trimmed the ends back so I could check the voltage coming off the transformer. Put in a new fuse, pulled all the tubes, applied power, zero volts. Checked the fuse, blown. Not sure if the transformer is toast or if it could be something else. My next step was going to be to replace the four diodes, but that doesn't appear to be the problem.. Any suggestions or help is greatly appreciated. This isn't my strongest suit, but I've been messing with this stuff for years. I think another, more experienced guy would have some idea of where to go from here. I'm running out of ideas, and don't want to do something stupid.. How frustrating..

http://www.captain-foldback.com/Lesl...matics/610.gif

Thanks for taking the time!

[Alex R]

Few electronic faults can be traced visually. We need a better basis than guesswork. One thing you could and probably should do is to replace all the electrolytic caps. Then try running it with no tubes in, briefly. Check voltages on tube pins and if they look ok (they will be higher than normal, much higher in the preamp), try installing tubes. One of them will prob pop the fuse again; replace it. NB this is only one approach, based on my assumption that you have no test gear.

If you can set up a light bulb limiter that would be a great step forward, so would a decent voltmeter.

That burned resistor likely signals a short upstream of it somewhere. A tube, a cap are the likely suspects.

[Leo_Gnardo]

I'm running out of ideas

Old amps - including Leslies - need hi voltage filter caps replaced. Most Leslies used a multisection cap, and (in the good ol' days) to make things easier didn't even solder leads onto the cap, just plugged it into a socket. Unfortunately the only multisection caps I know of that might fit are CE and have a reputation for going bad fast. You could remove the Leslie multisection cap and solder in individual caps - there's plenty of room inside - save yourself a lot of headache and frustration with those CE's.

Could also have a dead short in output tubes.

[AlG]

clipped the transformer wires that feed the diodes- trimmed the ends back so I could check the voltage coming off the transformer. Put in a new fuse, pulled all the tubes, applied power, zero volts. Checked the fuse, blown

Does that mean that you only have the transformer hooked up for your last test??? Or are you only disconnecting some of it?



If so chances are your transformer is shorted, which is common when the tubes lose their gate voltage.

[Rich Rice]

Thanks for the reply! I have some test equipment, and the resistor didn't appear to be burned at all, this amp was exposed to extremely high humidity, and sat unused for several years. I had planned on replacing the electrolytics- just didn't get there yet. I have a frequency generator, oscilloscope, digital VOM, AC voltmeter.. Just not a ton of troubleshooting experience..

If the electrolytics are toast, would that cause the fuse to blow immediately?

[Rich Rice]

Old amps - including Leslies - need hi voltage filter caps replaced. Most Leslies used a multisection cap, and (in the good ol' days) to make things easier didn't even solder leads onto the cap, just plugged it into a socket. Unfortunately the only multisection caps I know of that might fit are CE and have a reputation for going bad fast. You could remove the Leslie multisection cap and solder in individual caps - there's plenty of room inside - save yourself a lot of headache and frustration with those CE's.

Could also have a dead short in output tubes.

Thanks for the reply! I guess I'll go get some caps this morning.. I'm a CE dealer, but have heard the same thing bout their can caps on the Organ Forum.. This model has two of those can caps, but I can read the values on there.. Good ink, anyhow..

I just got nervous about the transformer, and am pulling high voltage now that it is disconnected from the whole unit. Big load off my mind..

[Rich Rice]

Does that mean that you only have the transformer hooked up for your last test??? Or are you only disconnecting some of it?

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If so chances are your transformer is shorted, which is common when the tubes lose their gate voltage.

I had only disconnected the two red transformer leads that feed the 4 diodes. I was worried about the transformer after that, so have since clipped the rest of the leads and am pulling high voltage from it now that it's completely out of the circuit.. that particular pair is giving me 266v.

[AlG]

Ah, good stuff. Rules out the transformer.

[Leo_Gnardo]

I just got nervous about the transformer, and am pulling high voltage now that it is disconnected from the whole unit. Big load off my mind..

So it looks like the transformer's OK, thank goodness for that, you don't want to have to buy a replacement.

In the future, assume old capacitors are bad until proven good. Too many stories of "I crossed my fingers for good luck & plugged the amp in, then smoke came out." That kind of "testing" may prove filter caps are bad, but it also kills transformers. FWIW withing the last few days I've had a streak of amps come in with 20, 25, and even 30 amp line fuses installed. Amps that simply needed a new bias filter cap, new output tubes, or new hi voltage filters.

[Rich Rice]

Ah, good stuff. Rules out the transformer.

That part of it, anyhow.. Yep- I was really worried..

I'm pulling 198 volts from the tap that feeds D3 & D4. Not sure what to make of the other windings or how to measure them- but I guess if the big ones are OK the rest are likely Ok, too.. I sure hope so.. lol




The red pair=266v
The blue pair=198v
Not sure what to make of the other wires on there. I've learned (the hard way) that if I don't know what I'm doing, don't do anything.. gets expensive really fast..

[Rich Rice]

Of course, just because I can find the cap values doesn't guarantee I'll get them in the right places, or the right direction.. LOL

[Rich Rice]

I think these are the ones I need to replace.. Not sure if the diodes are good or not.. also not sure if they are specific diodes- but I would hate to yank them if I don't have to..

[Rich Rice]

Argh.. Just went to my local electronics supplier, they couldn't hook me up with any caps that are even close. Bummer.. I guess that's what the internet is for, but I figured they would have something close.. Nope.

[soundguruman]

Argh.. Just went to my local electronics supplier, they couldn't hook me up with any caps that are even close. Bummer.. I guess that's what the internet is for, but I figured they would have something close.. Nope.

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[52 Bill]

The red pair=266v
The blue pair=198v
Not sure what to make of the other wires on there. I've learned (the hard way) that if I don't know what I'm doing, don't do anything.. gets expensive really fast..

The only other winding is the green wire for the filaments and bias supply. You should get about 12vac to ground on that lead.

Yikes, 460uF of capacitance on the doubler/215 volt supply! That's something you don't see very often.