I have the amp in the title that had one channel fail. I replaced the output transistors and it seemed to work fine when I tested it. Original parts weren't available, so I had to substitute, and the suggested substitutions were very close in all specs to the originals.
I then stored the amp for later use. Today I hooked it up and it works, but it appears I'm getting DC voltage on that one output when I turn up the gain past a certain level... I say appears because I haven't tested yet, but once I pass a certain level on the gain knob, the woofer on that channel pulls in hard and stays there.
Below that gain setting level, that channel works no problem, just with less than the desired output.
Could that be a sign that the bias needs to be adjusted?
Any other thoughts?
Thanks!
I then stored the amp for later use. Today I hooked it up and it works, but it appears I'm getting DC voltage on that one output when I turn up the gain past a certain level... I say appears because I haven't tested yet, but once I pass a certain level on the gain knob, the woofer on that channel pulls in hard and stays there.
Below that gain setting level, that channel works no problem, just with less than the desired output.
Could that be a sign that the bias needs to be adjusted?
Any other thoughts?
Thanks!
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That's not likely a bias problem (although you can't completely rule anything out at this point).
If you leave the gain at the minimum level and increase the input from the signal source, does the amp still produce DC at the speaker terminals?
Does it produce DC if there is no load but the gain is increased as you did before?
What were the original outputs and the replacements you used?
If you leave the gain at the minimum level and increase the input from the signal source, does the amp still produce DC at the speaker terminals?
Does it produce DC if there is no load but the gain is increased as you did before?
What were the original outputs and the replacements you used?
No. And It generates the dc even without any input attached.
I'll check on the other questions.
On the gain pot for that channel, do you have DC on two of the pins and essentially no DC on the other 4 pins?
Old part: K D998
Replaced with TIP35C
Old part: K B778
Replaced with TIP36C
Also, I found I was wrong. It's not the channel I repaired that's having that problem. It's the adjacent channel. Ch3 was the one I repaired, Ch4 has the problem.
Also, you seem to be on the right track Perry, the DC appears to go away when I set the gain all the way up. Also, when it's got the DC, the output starts to oscillate when I touch the gain pot's metal post. If the gain is at a level where the DC doesn't occur, the oscillation doesn't occur either. I also noticed that it's not DC. It's a slow oscillation, which speeds up when my hand approaches it or if I adjust the pot.
Question: If I ever plan to bridge this amp, should I also replace the output transistors on ch4 with those I used on ch3?
Thx!
Replaced with TIP35C
Old part: K B778
Replaced with TIP36C
Also, I found I was wrong. It's not the channel I repaired that's having that problem. It's the adjacent channel. Ch3 was the one I repaired, Ch4 has the problem.
Also, you seem to be on the right track Perry, the DC appears to go away when I set the gain all the way up. Also, when it's got the DC, the output starts to oscillate when I touch the gain pot's metal post. If the gain is at a level where the DC doesn't occur, the oscillation doesn't occur either. I also noticed that it's not DC. It's a slow oscillation, which speeds up when my hand approaches it or if I adjust the pot.
Question: If I ever plan to bridge this amp, should I also replace the output transistors on ch4 with those I used on ch3?
Thx!
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The pot could be broken for that channel. Does it appear to have any damage?
Do you have any DC on the terminals of the pot? Compare to the other 2 channels.
You have to be careful when replacing those outputs with the TIP35C and TIP36C. They have different bias requirements. After you get it repaired, you need to drive the amp hard enough to get it to go into thermal protection. As it's heating up, remove the audio signal and check the idle current to make sure it hasn't drifted up. Do this every 3-4 minutes until it thermals. If, at some point, you see the idle current increasing, reduce the biasing for that channel.
If there are no biasing problems, the TIPs would be more reliable than the original transistors (assuming that the rail voltage isn't more than ±50v).
Do you have any DC on the terminals of the pot? Compare to the other 2 channels.
You have to be careful when replacing those outputs with the TIP35C and TIP36C. They have different bias requirements. After you get it repaired, you need to drive the amp hard enough to get it to go into thermal protection. As it's heating up, remove the audio signal and check the idle current to make sure it hasn't drifted up. Do this every 3-4 minutes until it thermals. If, at some point, you see the idle current increasing, reduce the biasing for that channel.
If there are no biasing problems, the TIPs would be more reliable than the original transistors (assuming that the rail voltage isn't more than ±50v).
I got the TIP35C and TIP36C, which handles 100V...
No visible damage on the pot.
On the pots I've got +0.39V on all pins of both pots.
I've driven this amp very hard in the past and it's never gone into thermal overload... How do you recommend I do so?
No visible damage on the pot.
On the pots I've got +0.39V on all pins of both pots.
I've driven this amp very hard in the past and it's never gone into thermal overload... How do you recommend I do so?
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What is the amp rating of the power supply you have?
Measure the resistance from the shield of the RCA for the oscillating channel to the RCA shields of the other channels and to the non-bridging speaker wires. Do you read ~0 ohms for each?
Do this with no power applied and no RCAs plugged into the amp.
Measure the resistance from the shield of the RCA for the oscillating channel to the RCA shields of the other channels and to the non-bridging speaker wires. Do you read ~0 ohms for each?
Do this with no power applied and no RCAs plugged into the amp.
I've got 2 rails on the power supply. One is 1.5A, the other is 12A. I've got a spare 12V car battery too...
Resistance from shield to other shields: 0ohms
Resistance from shield to non-bridging wires: 0ohms
Resistance from shield to other shields: 0ohms
Resistance from shield to non-bridging wires: 0ohms
Many times, oscillation is caused by a broken ground but those are what I'd expect to see.
Since you don't have a power supply (with an amp meter) capable of getting the amp to thermal, you'd have to do this in the vehicle and instead of monitoring the idle current, you'd monitor the DC voltage across the emitter resistors. Set the voltage across them (cold) to 0.001v DC (no signal, no load). Then, in the vehicle, check the voltage across the emitter resistors to confirm that it's not increasing. You'll have to set the volume to 0 when measuring the voltage.
Since you don't have a power supply (with an amp meter) capable of getting the amp to thermal, you'd have to do this in the vehicle and instead of monitoring the idle current, you'd monitor the DC voltage across the emitter resistors. Set the voltage across them (cold) to 0.001v DC (no signal, no load). Then, in the vehicle, check the voltage across the emitter resistors to confirm that it's not increasing. You'll have to set the volume to 0 when measuring the voltage.
So you want me testing all channels? Or do I just test the channel giving me trouble ch4? Or the channel I fixed ch3?
The oscillation is only on ch4...
The oscillation is only on ch4...
You probably only need to test the channel with the 35s and 36s but you could use the other channels as a reference.
You'll have to find the cause of the oscillation before you can do anything else.
With your multimeter across the defective channel's speaker terminals and the gains ~1/2, can you see the voltage going up and down on the meter?
You'll have to find the cause of the oscillation before you can do anything else.
With your multimeter across the defective channel's speaker terminals and the gains ~1/2, can you see the voltage going up and down on the meter?
Have I confused you? The ch3 has the 35s and 36s but isn't the one that oscillates at a specific gain setting. I thought it was at first, but found it isn't...
The channel that oscillates is ch4 which I haven't worked on.
The channel that oscillates is ch4 which I haven't worked on.
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