The amp is a Directed 1100D5 5 channel amplifier that I had for a few years. It has received several repairs over this time, mostly output transistor replacement. When I got it there was distortion on two of the four Class AB channels. I have replaced the TIP35/36 combinations in all of the channels on a few occasions trying to sort out this problem. Most of the time, inserting new outputs will cause the problem to go away, but it will usually come back once the amp is reinstalled. If I use only the RR, FR, and sub channels, there is no distortion or problem. I seem to recall the RL channel giving some trouble a while ago, but not right now. I need some assistance in tracking down the source of the distortion.
READINGS IN AB CHANNELS
The only odd thing I noticed is with R255 and R265. I cannot confirm my readings since the amp is now playing clearly on all channels until the problem reoccurs. What I did notice is that on both sides of R255 and R265 I have different readings on FL, FR, RL and RR channels. R*55 and R*65 are directly connected on one end. When FL channel was distorting I am sure that R255 and R265 had identical readings on all four ends, i believe it was 2.1V. The distortion is now gone and the voltage readings are now different on the unconnected ends of R255 and R265.
Channel
FL R255 0.9 2.0 R265 0.9-0.3
FR R155 0.3-0.3 R165 0.3-0.3
RL R455 2.1-2.1 R465 2.1-2.1
RR R355 0.5-1.4 R365 0.5--0.0
I am posting a pic of the circuit to assist anyone who can help me. Thanks.
READINGS IN AB CHANNELS
The only odd thing I noticed is with R255 and R265. I cannot confirm my readings since the amp is now playing clearly on all channels until the problem reoccurs. What I did notice is that on both sides of R255 and R265 I have different readings on FL, FR, RL and RR channels. R*55 and R*65 are directly connected on one end. When FL channel was distorting I am sure that R255 and R265 had identical readings on all four ends, i believe it was 2.1V. The distortion is now gone and the voltage readings are now different on the unconnected ends of R255 and R265.
Channel
FL R255 0.9 2.0 R265 0.9-0.3
FR R155 0.3-0.3 R165 0.3-0.3
RL R455 2.1-2.1 R465 2.1-2.1
RR R355 0.5-1.4 R365 0.5--0.0
I am posting a pic of the circuit to assist anyone who can help me. Thanks.
I tried that. Pushing on top of different capacitors and transistors within the circuit and also pressing on different areas of the board. I can't really say what makesbit come and go. If I go reinstall it tomorrow it will probably play fine on all channels for maybe half an hour to an hour, and then start to distort on the FL channel. I am uncertain about the RL channel since the FL is most audible.
I think I may be losing the negative rail as you suggest. When I was probing the outputs with the scope I recall something odd regarding the negative transistor for the FL channel. I remember getting no change on the display when first probed, but the distortion went away, and all outputs currently show + and - rail voltage. I will have to reinstall the amplifier and cause the problem to reoccur in order for me to track it down.
Can you identify what component(s) I can pro actively change that may be causing this intermittent issue? Very tired of having to keep disassembling this amplifier for this issue...
As mentioned, the only two things I have noticed is that both sides of R255 and R265 had the exact same voltages when distorted, but those readings are different when not distorted. It also seemed like the negative transistor had no output when distorted. I may be off-base here but that is all I have noted to this point. Thanks.
Can you identify what component(s) I can pro actively change that may be causing this intermittent issue? Very tired of having to keep disassembling this amplifier for this issue...
As mentioned, the only two things I have noticed is that both sides of R255 and R265 had the exact same voltages when distorted, but those readings are different when not distorted. It also seemed like the negative transistor had no output when distorted. I may be off-base here but that is all I have noted to this point. Thanks.
Ok, resurrecting this thread because the amplifier was never really fixed. I was using two channels of the midrange section and the sub channel for the while but have it back on the bench for another attempt.
There are two channels which give trouble with distortion, left front and left rear. They play mostly clean at first and seem to get distorted as the amp warms up. I was bench testing and was probing with the voltmeter and discovered that when I touched the capacitor located at C260 and C460 the distortion cleared up in each channel. I am still bench testing and the distortion has not returned since I resoldered these capacitors. Is this treating the symptom and not the cause? Does this indicate something else I should check?
Regarding the rail voltage, I am not losing rail voltage at any point during my testing whether distortion is there or not. However, the rail voltage is not even between positive and negative. Positive rail is 32v and negative rail is -29.8v. Is that something that can, or needs to be repaired?
There are two channels which give trouble with distortion, left front and left rear. They play mostly clean at first and seem to get distorted as the amp warms up. I was bench testing and was probing with the voltmeter and discovered that when I touched the capacitor located at C260 and C460 the distortion cleared up in each channel. I am still bench testing and the distortion has not returned since I resoldered these capacitors. Is this treating the symptom and not the cause? Does this indicate something else I should check?
Regarding the rail voltage, I am not losing rail voltage at any point during my testing whether distortion is there or not. However, the rail voltage is not even between positive and negative. Positive rail is 32v and negative rail is -29.8v. Is that something that can, or needs to be repaired?
The problem right now is only in the left rear channel. It is intermittent but when the channel is distorted, all three legs of the bias transistor will have the same voltage, usually around +2.2v. When the channel is playing clearly, there is usually around +1v on the center leg, +0.2v and +0.8v on the outer legs. This is almost the same bias transistor readings as the other three channels that play clean audio. The left front channel used to exhibit the same issues but has since stopped after some desoldering done within the channel. The left rear channel may sometimes play clean after some resoldering within the channel but so far the issue continues to return in that channel. The bias transistor measures the same as all the others when checked.
What can I check or change to try to solve this issue?
What can I check or change to try to solve this issue?
The distortion issue in this amplifier was resolved by desoldering and resoldering most of the channel connections close to the outputs.
The problem I have with this amp now is an intermittent loud, scratchy, static type noise in the left front channel only. The music playing remains clear and undisturbed during this occurrence. It occurs completely at random and I have no idea how to get rid of it.
I have resoldered all of the connections in that channel, I have cleaned out all switches and potentiometers, I have tried pushing on the board and vertical cards in the midrange preamp section, and have a manipulated all switches and pots in all positions in an attempt to cause the problem to occur.
With the random occurrence of this noise it seems like tracking it down will be tricky. Is there any common component that I can try changing to solve this intermittent noise?
The problem I have with this amp now is an intermittent loud, scratchy, static type noise in the left front channel only. The music playing remains clear and undisturbed during this occurrence. It occurs completely at random and I have no idea how to get rid of it.
I have resoldered all of the connections in that channel, I have cleaned out all switches and potentiometers, I have tried pushing on the board and vertical cards in the midrange preamp section, and have a manipulated all switches and pots in all positions in an attempt to cause the problem to occur.
With the random occurrence of this noise it seems like tracking it down will be tricky. Is there any common component that I can try changing to solve this intermittent noise?
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