Did I blow up my Leslie 760?

[The_Doctor]

So I just bought a Leslie 760 that was in a climate controlled storage unit for at least 7 years. I do not know anything about the history of the 760 before that. My B3 has a Trek II SSP-3A preamp & the 9 pin connector I was using with a Leslie 710 (with the LCO-1A crossover). I did not like the sound of the 710 and the 760 looked like a great choice, being 9 pin and all. I got the 760 home and hooked it up, it worked great for about a minute until I "pulled out all the stops" and gave it full volume. All the bass disappeared and now no sound comes from the bass speaker except a popping noise. I thought I blew the speaker so I hooked the wires up to a regular external speaker but the same noise comes through and nothing else. When you first turn on the organ there is a whistle noise that goes up in pitch and the way down in pitch until it settles on the popping sound. Is anyone familiar with this issue? I have attached a video of me turning on the organ with the 760 attached making the whistle sound and popping. The treble speaker seems to still be working ok. What have I done? :'(

[tpappano]

First thing to check is that the various connectors are making good contact and are snug. On the back side of that black aluminum plate are the actual amplifier and crossover circuit boards. There are two 2-pin connectors that go to the speakers, and several single-pin connectors that carry the organ signal to the crossover board and then to the bass and treble amp modules. Make sure these are all snug, and maybe un-plug and re-plug them a couple times to clear any corrosion that may be present. Also, turn the level adjustment controls back and forth a little, they can get grungy inside and make poor contact. Coming from each amp module is a cable with a 5-pin connector that plugs onto the power supply board. Same deal here, poor contact can cause problems. Vibration and ordinary wear and tear can cause intermittent failure of the solder connections under the power supply board, too. If it still won't work we can dive deeper. Having two separate amp modules makes it possible to do some fault finding by swapping various connectors. I find that when I "crank up the volume" vibration from the bass (and even treble) can really shake up bad connections in cables, tube sockets, you name it 8)

[Jyvoipabo]

Hello,

I think I hear an engine noise.
You should unplug them, this would facilitate the diagnosis.

Amplifiers are likely to send DC voltage to your speakers and cause them to smoke.

By disconnecting and reconnecting the speakers, if you hear a loud honk, there is a problem.

Or, you have to check ~0 Volts between the speaker connections.

JP

[Stefank]

OK, so if I understand well:
- it was functioning correctly for a while.
- than you "pulled out all the stops": to avoid misunderstanding on slang: that I guess means: "turned up the volume big time"
- than bass stopped but treble kept going

Than.... same sound with different speaker connected to organ ?????
- If the latter is true the Leslie should still be there but the organ got broke.

Anyway:
- a speaker often does not completely disconnect, especially the old speakers are more likely to get mechanically damaged by popping to the "end of movement". After that they will still function but just give squircking sound.
- more likely to have the amplifier or speakertransformer "somewhat dead"
- Amplifiers and transformers "like to die" from heat, but if you were just playing for few minutes than they were not yet at burn-level.
- a plug (as suggested) may be the issue, but why would that break when turning up the volume? Still,... apart from it "not being so logical to happen with volume increase", it IS logical for something that has not been used for long time.

Like tpappano says: just check all connections and plugs. also unseat all tubes and reseat them.
What I would do (but this is NOT a recommendation for reason of safety): put a knife between some keys to let it sound (yep, become Emmerson), than wiggle all connections and tubes to see whether the sound comes back. When doing this: 1 hand only, other hand on the back. 1 hand on the back is the golden trick I learned many years ago when servicing high voltage devices. That way you may touch a live wire, but current will never flow through your hart from one hand to the other. Stick the fingers under your belt to keep yourself from accidentally forgetting this (as said, tis is NOT a recommendation, it's just what I often do).

When I got my L122 few weeks ago I also had to reseat all tubes and connectors to have it sound "crackfree". Now no problems.

[David Anderson]

I agree with tpappano's advice. There are a lot of plug-in connections inside 760s, and they do sometimes corrode. The pin/socket connectors on the circuit boards can suffer from cracked solder joints. Realistically, that's also the only repair strategy here that doesn't involve more complicated electronic diagnosis using meters and schematics.

[The_Doctor]

Thank you everyone for the advice - when I get home from work tonight I'll break out the isopropyl & wire brush and start on the connectors. I will also try tpappano's idea of swapping the connections for the treble and bass and seeing if that changes problem, then I will know for certain which board is bad. I wasn't sure if anyone had run into this exact problem before, it would be easy to remember that whistle noise if you had ever heard it. Thanks again, I'll post back on here with my progress!

Edit:
Also to clear up with what Stefank suggested - there aren't any tubes left in my B3, the Trek preamp is solid state (just like the 760). My B3 is connected to a PR40 (main echo ensemble) and it still plays beautifully through it, so it is definitely something on the Leslie that went wrong.

[David Anderson]

Other than a few very specific issues, it's not realistic to expect people to be able to listen to a problem and know exactly what the cause is. There are too many different causes that can present the same symptoms. It's like showing us a video of your car running rough and expecting us to know exactly why your car is running rough. There have been plenty of times when I opened something up thinking I knew what the problem was only to find it was something else I hadn't seen before.

One thing I would NOT do is swap the speaker wires for the horn onto the bass board. If, for some reason, you have a lot of DC on the bass output, it will fry your horn driver very quickly. Even a loud pop might damage it. These early SS amps have no speaker protection circuits on the transistor output stages. The horn driver is a lot more fragile than the woofer.

Edit: Current finding its way through a bad connection can make whistling noises.

[Jyvoipabo]

Good evening,

Forgive me, but 'the machine gun sound' at the end reminds me something.
I already had this problem. It came from the pedal which was poorly powered.

To find out, set the master volume of the leslie to zero and gradually increase it. If the machine gun noise increases with this setting, then it may be a B + problem (~ 30V).

JP

[David Anderson]

I don't think he's using a Leslie Combo Preamp Pedal. It sounds to me like he's powering the Leslie directly from a 9-pin hookup on the organ, in which case, the B+ from the Leslie isn't used.

[The_Doctor]

So tonight I cleaned the pins and connectors on the treble amp, bass amp, and power supply with alcohol and wiped around the legs of some stuff on the boards. I let it dry thoroughly and reconnected everything & the problem persists. I shut it down and I connected the bass speaker connector to the treble amp board and there was no more whistle & popping - so the problem is somewhere on the bass amplifier section (or it's output transistors) I'm assuming.

[tpappano]

Did you try turning the level pot back and forth a little? If that also didn't help I would do this next: move the bass speaker connector to the treble amp module, then also swap the signal input wires (P4 and P5, I think). Basically moving the bass audio and speaker to the treble amp. Leave the horn cable disconnected for this test. If you get normal bass sounds, then this at least proves the bass signal is coming out of the crossover module, and the bass amp module may indeed have a problem. (if you don't, it may be the crossover) Then, it's time to get out the voltmeter and see what is going on with the bass module and output transistors. The schematic shows what the various readings should be.

[David Anderson]

I would next unplug the 5-pin power harness to the Treble board from the Power Supply and plug the 5-pin power connector for the Bass board where the Treble board was previously connected. By doing this, you are testing the connector solder joints on the Power Supply circuit board. Both 5-pin connectors to the Bass/Treble boards on the Power Supply are identical, so it doesn't really matter, functionally, which 5-pin plug goes to which connector on the board. You can operate it safely with the Treble harness unplugged.

It is standard in refurbishments of SS Leslies to examine all solder joints on the Power Supply circuit board and resolder any that look suspicious.

If it stays the same, I would follow some of the Volume Control advice above. If the popping decreases with the Bass volume control, it is happening upstream of the volume control on the Crossover Board. If the Bass volume control doesn't affect the noise, it is happening on the Bass board, at which point, you might want to look at solder joints on the Bass board.

[tpappano]

Excellent point!

[The_Doctor]

Those are both excellent ideas! I didn't want to swap around any other wires or connectors earlier for fear of frying something, but I will try these suggestions out tomorrow morning.

[The_Doctor]

I would next unplug the 5-pin power harness to the Treble board from the Power Supply and plug the 5-pin power connector for the Bass board where the Treble board was previously connected. By doing this, you are testing the connector solder joints on the Power Supply circuit board. Both 5-pin connectors to the Bass/Treble boards on the Power Supply are identical, so it doesn't really matter, functionally, which 5-pin plug goes to which connector on the board. You can operate it safely with the Treble harness unplugged.

This didn't change anything, so hopefully the power supply is working properly.

Did you try turning the level pot back and forth a little?

At max bass volume the popping settles to intermittent, but when turning the pot the popping increases an decreases it's rate:

If that also didn't help I would do this next: move the bass speaker connector to the treble amp module, then also swap the signal input wires (P4 and P5, I think). Basically moving the bass audio and speaker to the treble amp. Leave the horn cable disconnected for this test. If you get normal bass sounds, then this at least proves the bass signal is coming out of the crossover module, and the bass amp module may indeed have a problem. (if you don't, it may be the crossover) Then, it's time to get out the voltmeter and see what is going on with the bass module and output transistors. The schematic shows what the various readings should be.

Moving both the connector and signal wires did indeed produce a clean bass signal! So I guess my next step is checking some voltages on the bass board and it's output transistors. I have downloaded a pdf of the service manual & I have a decent multimeter.

[tpappano]

Very good, sounds like you've clearly identified the bass module and/or output transistors as the culprit! Voltage checks should reveal something, there are only four transistors. If not one of those, it may just be a bad solder joint on the circuit board. The output transistors, at least, are as common as dirt 8)