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Does anyone have a Lafayette LR-3500 that they could check something for me?

EastPoint

EastPoint

Recycler
Subscriber
Hi folks. I have a Lafayette LR-3500 here with a right channel that's acting up. I want to check and make sure the outputs are in the correct locations, since this was worked on in the past. If anyone has an LR-3500, would you mind popping the top and seeing which output transistors are on top for each channel? Are they the same one, or different? Thank you!
 
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What does acting up mean?

The service manual documents the location of all the transistors in the receiver.

From memory, looking down from the top, Left Channel top transistor Q1 is 2SC1619; bottom transistor is Q3, 2SA808.
Right channel top transistor is Q2, 2SC1619; bottom transistor is Q4, 2SA808.

TO BE SURE: All 4 transistors have the outer case as the collector. The "SC's" are NPN transistors and the "SA's" are PNP transistors. The NPN transistor (SC) outer case will have a positive voltage (roughly +38 volts). The PNP transistor (SA) will have a negative voltage (roughly -38 volts).

If you're going to poke around the innards of the receiver, get the service manual.
 
Nevermind, I got the manual and figured that part out. The right channel has no voltage on the bottom output--it has the 35v on the case but nothing on the emitter or collector. I checked basically everything on that channel and I am stumped.
 
On the NPN transistor, if the collector case has roughly +35 volts on it, the base should have about +0.58 volts and the emitter should have 0.01 volts. You should also check the driver transistor Q10, 2SC1124 voltages. Also the emitter resistors on the output transistors (should be 0.47 ohms) and the driver emitters (should be 150 ohms)
 
On the right channel, the NPN output has .013v on the emitter. The base reads .22v but I’m not sure there’s anything there—I think the meter is just drifting because there’s no connection.

The emitters are .47 ohm 3W and are good, and the 150-ohm resistor is good too.

Q10 has 0v emitter, .558v base, 34v collector—measuring left to right and it’s an EBC transistor.
 
EastPoint said:
On the right channel, the NPN output has .013v on the emitter. The base reads .22v but I’m not sure there’s anything there—I think the meter is just drifting because there’s no connection.

The emitters are .47 ohm 3W and are good, and the 150-ohm resistor is good too.

Q10 has 0v emitter, .558v base, 34v collector—measuring left to right and it’s an EBC transistor.
Click to expand...

Q10 base should have about 1 volt, emitter should have +0.58 volts, and collector around 28 volts.

Does the relay click in when you turn on the receiver?
I'll ask again: What does acting up mean?
 
So, when I got this the left channel would work, but the right channel would make it go into protection. The right channel was weaker as well. So if I had the balance all the way to the left, no issues. If I put the balance to the right, ANY volume over a whisper and it kicks into protection, kicks out, kicks in again, etc.

I rebuilt the preamp section with new transistors (other than the two FETs) and new caps. No change.

I replaced the relay (a previous owner had pulled the cover off and tried to jam the relay closed). No change.

I replaced all the electrolytic caps on the amp. No change.

I replaced all of the small signal transistors on the right channel. No change.

I swapped ALL the larger transistors left-to-right. No change.

I replaced both outputs on the right channel with MJ21193G/MJ21194G outputs. No change.

I fiddled with bias, no change.

However, at some point today it lost the volume on the right channel as well. It plays in a whisper on the right side with the volume ALL the way up. If you inject a signal directly into the right channel, then it starts going into protection again like before.

The two emitter resistors on the right channel appear to be non-original, which is bizarre. Everything else looked original. None of the outputs had any sort of heat-sink compound anywhere. Bizarre.
 
EastPoint said:
So, when I got this the left channel would work, but the right channel would make it go into protection. The right channel was weaker as well. So if I had the balance all the way to the left, no issues. If I put the balance to the right, ANY volume over a whisper and it kicks into protection, kicks out, kicks in again, etc.

I rebuilt the preamp section with new transistors (other than the two FETs) and new caps. No change.

I replaced the relay (a previous owner had pulled the cover off and tried to jam the relay closed). No change.

I replaced all the electrolytic caps on the amp. No change.

I replaced all of the small signal transistors on the right channel. No change.

I swapped ALL the larger transistors left-to-right. No change.

I replaced both outputs on the right channel with MJ21193G/MJ21194G outputs. No change.

I fiddled with bias, no change.

However, at some point today it lost the volume on the right channel as well. It plays in a whisper on the right side with the volume ALL the way up. If you inject a signal directly into the right channel, then it starts going into protection again like before.

The two emitter resistors on the right channel appear to be non-original, which is bizarre. Everything else looked original. None of the outputs had any sort of heat-sink compound anywhere. Bizarre.
Click to expand...
Your post is confusing.
You start out by saying: "when I got this the left channel would work, but the right channel would make it go into protection. The right channel was weaker as well."
Then you go on to say you replaced this and that and there was no change
Then in the second to the last paragraph, you say: "at some point today it lost the volume on the right channel as well. It plays in a whisper on the right side with the volume ALL the way up."
SO, if you lost volume today in the right channel, when did the right channel start to work?

You have several things working against you. Ad hoc part replacements without knowing what is causing the initial problem, is never a good idea. The source of the problem needs to be found. Just replacing parts can add other or similar problem issues to what you had initially which makes a solution even harder to find.

The original output transistors can still be bought online. You say you replaced them with MJ21193G and MJ21194G. Did you compare the specs between these transistors and the original ones? Biasing and resistor values around the driver and output stages are critical which can lead to non-functioning stages and screwy bias levels. Resting current for the output stages should be 50 ma.
 
The MJ21193/94 are mentioned on AK as good subs for the originals.

The right channel initially worked but was kind of weak and would go into protection. It is now much weaker and doesn’t go into protection because it doesn’t have enough volume to kick it into protect.
 
Just picking up a LR-3500. Anyone here know where I can get a copy of the service manual? Haven't been able to find one online
 
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