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Lafayette LR-5555A DC at speaker output

S

samuelcosmo

New Member
Hey all! My name is Sam, I'm from NYC and I have a computer repair business. Once in a while a friend will ask me to fix something cool and old, and I usually wind up on Audio Karma for some tips! So, first of all, thanks to everyone who contributes to this site!

I have a Lafayette LR-5555A that doesn't "click". Already checked the relay, it is good, as well as the output transistors (Sanken S80W). When I jump the relay, the right channel speaker output shows ~40V, and there's also 40V at every pin on that channel's output transistor. The left channel works fine.

I have a schematic (can upload if needed), but I know I'm missing something.... Any help appreciated. Thanks.
 
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samuelcosmo said:
Hey all! My name is Sam, I'm from NYC and I have a computer repair business. Once in a while a friend will ask me to fix something cool and old, and I usually wind up on Audio Karma for some tips! So, first of all, thanks to everyone who contributes to this site!

I have a Lafayette LR-5555A that doesn't "click". Already checked the relay, it is good, as well as the output transistors (Sanken S80W). When I jump the relay, the right channel speaker output shows ~40V, and there's also 40V at every pin on that channel's output transistor. The left channel works fine.

I have a schematic (can upload if needed), but I know I'm missing something.... Any help appreciated. Thanks.
Click to expand...
Your first clue: "When I jump the relay, the right channel speaker output shows ~40V, and there's also 40V at every pin on that channel's output transistor."
There is no "output transistor". There is a amplifier module (S80W) which consists of a number semiconductor devices. On the S80W, DC voltages on pin 2 should be roughly +42 volts and pin 7 should be roughly -42 volts. Voltage readings depend on your meter and line voltage. All the other pins should be roughly DC 1.2 volts, -1.2 volts, and 0 volts. The output DC voltage from the S80W (from pin 5 through the relay contacts to the speaker terminal and voltage measured to ground) should 0 volts. Your S80W in all probability is bad :(
 
Thank you for your reply. That's what I thought, but I unsoldered and swapped the two S80Ws, and its the same thing, on the same side! :wtf:
 
samuelcosmo said:
Thank you for your reply. That's what I thought, but I unsoldered and swapped the two S80Ws, and its the same thing, on the same side! :wtf:
Click to expand...
Substitution should never be the first course of action because you don't know if the component failed due to its own set of problems or something else in the adjacent circuitry went bad and caused the S80W to fail. If I were trouble shooting the problem, I would remove the suspect S80W. Then I would measure the DC voltages on each solder point (to ground) where the S80W pins are connected to see which have an usually high DC voltage. Then I would check the components connected to those pins in question to see if a component there was bad. With the soldering and unsoldering going on, make sure you don't bridge any adjacent paths or pads. Doing a quick look at the schematic, if capacitor C9/C10(left/right ch) shorted for whatever reason, approximately 30 volts DC would appear on the output pin of the S80W (not a good thing). I would also check Q1, Q2, Q3 or Q4 (depending on which channel you're working on) to make sure they are good.
 
You were right, it was the S80W's.... removed, checked voltages, replaced, bingo.

The thing sounds pretty good, but I'm trying to track down an annoying crackle. May start a thread for a little help with that. Thanks so much!
 
samuelcosmo said:
You were right, it was the S80W's.... removed, checked voltages, replaced, bingo.
Click to expand...
Happy Person.gif

The thing sounds pretty good, but I'm trying to track down an annoying crackle. May start a thread for a little help with that. Thanks so much!
Click to expand...
"Crackles" can develop in leaky capacitors, resistors breaking down as current is passed through them, and even transistors. An oscilloscope is very useful in tracking down crackles. Shot gun part replacements is not the answer to finding the problem.
 
I happen to have an oscilloscope..... I've only ever used it for biasing old fender amps. Where would you start? The crackles are the same regardless of volume/balance, however, if I flip the "hi pass" switch, they change in tone.

ugh!
 
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