LTD 400 troubleshoot

Jameson Koweek · Post by **Jameson Koweek** » 19 May 2024 5:10 pm

Just picked up an LTD 400 today, at a discount on account of the fact that it is not working. The light powers on, and there’s a faint noise of life through the speaker. The speaker wiring is all good, I ran the speaker through my milkman the amp 100 and it sounds great. I tried unplugging the reverb wires from the chassis but that didn’t help.

Any advice on where to start troubleshooting it?

Thank you.

Bill A. Moore · Post by **Bill A. Moore** » 19 May 2024 5:30 pm

Aggravate the effects loop jacks for a start could also jumper with a patch cord.

Dennis Detweiler · Post by **Dennis Detweiler** » 19 May 2024 8:29 pm

Sounds like Molex connectors. Sometimes if you pound on the top of the amp it will temporarily connect. You can pull each connector from the boards and reconnect and may get it to work for a while. I recently replaced all of the Molex pins and soldered the wires to the pins. My old 1974 works like new again. Dead quiet and sounds great.
Sometimes the 6 power transistors will drop power intermittently for the same reason. They plug into their sockets. However, the Molex connectors are usually the culprit.

Jameson Koweek · Post by **Jameson Koweek** » 19 May 2024 9:58 pm

Excellent suggestions, however, unfortunately, this amp has neither molex connectors nor an effects loop. I don’t know if later models included these features, but I have two 1/4” inputs on the front, a 1/4” output jack for the speaker (a JBL manufactured for Peavey) and 1/4” jacks labeled “switch” and “booster”

I did notice, since posting, there appears to be some white residue coming from under the caps to a few of the transistors on the back.

Dennis Detweiler · Post by **Dennis Detweiler** » 20 May 2024 6:06 am

No Molex connectors connecting the circuit boards inside? Is it hardwired? Description sounds like the 70's LTD with the smaller cabinet version of the Session 400? Post the serial number which should be on the back panel board. Or a photo. You'll have to pull the chassis out to see the Molex connectors wiring harnesses.
The white residue is the heat sink paste used during assembly. It helps dissipate heat to the chassis panel.

Jameson Koweek · Post by **Jameson Koweek** » 20 May 2024 7:09 am

My mistake Dennis, I realized after I had posted this they might be internal. I have a LA 400 where the speaker out is molex connected. I will look in to this today.

Robert Parent · Post by **Robert Parent** » 20 May 2024 7:12 am

I had one that the component leads on the circuit board would break. My guess is that the leads were bent to sharp and after vibration would break. It happened many times over several years.

Robert

Jameson Koweek · Post by **Jameson Koweek** » 20 May 2024 9:37 am

Serial number 5A-113749

According to the Peavey serial number lookup, it was made in 1975. I tried disconnecting and reconnecting the internal connectors to no avail. Also, no visible broken component leads. This one might have to go to a real amp tech. Either way, the speaker alone seems to be worth more than I paid for the whole thing so I’m happy to have it.

Jameson Koweek · Post by **Jameson Koweek** » 20 May 2024 9:57 am

Do the power transistors in the back eventually fail? It seems that three of the six are RCA (which I think are original) and three are Motorola 15024s.

I should mention that the previous owner said the amp was working fine, and then was put in storage, and came out non working

Dennis Detweiler · Post by **Dennis Detweiler** » 20 May 2024 10:10 am

Yes, I would just let a tech work on it. If you're not familiar with probing loose connections with a proper probe, you could receive a possible deadly shock. If the filter caps are original, it would be good to replace them. If it worked when put in storage, it's probably a simple fix that shouldn't be costly for a tech. If the tech doesn't have a schematic, someone on the forum including me can send one.

Bill A. Moore · Post by **Bill A. Moore** » 20 May 2024 5:42 pm

About as deadly as a toaster or fan! There is wall voltage on the input to the fuse.

Jameson Koweek · Post by **Jameson Koweek** » 20 May 2024 6:11 pm

Wall voltage to the fuse plugged in and turned on, or unplugged?

Dennis Detweiler · Post by **Dennis Detweiler** » 21 May 2024 7:27 am

I'm talking about probing around inside on the circuit board improperly (capacitors) while the amp is turned on. Many years ago a local musician was probing on his circuit board with a screwdriver and was killed from contact with a filter capacitor. When turned off, they still hold a charge if it's a healthy capacitor. Electronic rat and mouse traps use capacitors to instantly kill them.

Bill A. Moore · Post by **Bill A. Moore** » 21 May 2024 8:13 am

Dennis, I would like to see the documentation from someone killed discharging a capacitor. I have searched and found no evidence of death from high voltage/low amperage shocks like those found in tube amps. I have worked around ignition voltages most of my life, (up to 100KV), and of course got bit a few times. The reaction to the shock may cause you to hurt yourself!
SS amps of course use much lower voltage, but still wall voltage has the capacity to kill if it crosses your heart! I built an amp for a good friend a few years ago, who has built several before he had a pacemaker installed. The low amperage shock can kill in his case!

Jameson Koweek · Post by **Jameson Koweek** » 21 May 2024 8:45 am

I should clarify, while I may be heathen enough to have been working on this in just my underwear, I am smart enough to at least have put on some rubber soled sandals, turn off the amp and unplug it, and certainly not dumb enough to go rooting around with a metal implement.

I cleaned and refitted all the molex connections, and visually inspected the legs of all the visible components, and found no easy fixes. It’s off to my amp tech later this week.

Dennis Detweiler · Post by **Dennis Detweiler** » 21 May 2024 8:58 am

Bill, I never knew this musician. He was a good friend of a local steel player. Sadly both are gone now. Perhaps he had a pacemaker or some kind of heart condition? I do know that you can kill rats or mice with a battery charged capacitor when the animal connects with a positive and negative mounted plate (front feet on one plate and back feet on the other) on the floor of the trap. I'm not an electronics tech, but have replaced caps on my amps after I've discharged them. Also, have probed (pushed) components with an insulated probe and have found broken leads and bad solder joints. Just the simple things. Beyond that, I leave it to the professionals.

Bill A. Moore · Post by **Bill A. Moore** » 21 May 2024 1:12 pm

Dennis, ohms law applies, in the case of rats, sure, easy to kill as there's not much resistance to the flow. Humans need 100 to 200 milliamps across their heart to be fatal! Most of us have Megohm values between left and right hand, and we may receive a shock, but not fatal.
Wall voltage can kill! Maybe only 120V, but most outlets are 15A fused!