I have an amp I'm working on with some strange problems.
There is a section of the amp with a darlington transistor, TIP142, that has a diode in parallel with a resistor on the emitter. I've not seen this type of design and I'm trying to understand the purpose and operation of that section of the amp.
I have attached a snippet of the schema showing this arrangement. With the resistor in parallel, it's not really preventing current from flowing the opposite direction. On the forward direction, it seems like it would prevent any voltage larger than ~0.6v across the resistor as above that the diode would effectively short out the resistor.
Any help understanding the operation of this would be appreciated.
There is a section of the amp with a darlington transistor, TIP142, that has a diode in parallel with a resistor on the emitter. I've not seen this type of design and I'm trying to understand the purpose and operation of that section of the amp.
I have attached a snippet of the schema showing this arrangement. With the resistor in parallel, it's not really preventing current from flowing the opposite direction. On the forward direction, it seems like it would prevent any voltage larger than ~0.6v across the resistor as above that the diode would effectively short out the resistor.
Any help understanding the operation of this would be appreciated.
Quiescent current stabilisation. Darlingtons have been reported to have high thermal sensitivity (moreso than discrete pairs) and this is a traditional means of stabilising the current. The idea is that a higher than normal emitter resistor (4.7 ohms) can be used to set the current and as long as the voltage across it is less than 0.6V or so, as you say, it will not conduct much. But as 4.7 ohms is far too high a value for high currents the diode will conduct at higher outputs.
The downside is that this creates an additional non-linearity when the diode starts to conduct so would contribute a sort-of crossover distortion.
The downside is that this creates an additional non-linearity when the diode starts to conduct so would contribute a sort-of crossover distortion.
Ah, thanks for that. This is a guitar amp with a very weird problem.
When you turn it on, it presents about 5v DC to the speaker and the rails are down about 10v. If you open the circuit to the speaker, which can easily be done by inserting a 1/4" jack into the external speaker connection and then pull it back out, it works perfectly. The 1/4" jack isn't connected to anything, it just opens the connection to the speaker as it disconnects the internal speaker when you use an external cabinet. So plug it in, take it right back out, and the amp works until you turn it off/on again.
In tracking around voltages and such, I found about 5v across that resistor when it's not working and 0 when it is. Thinking there should never be 5v across that I was looking at that section. So I tried using a wire to quickly short between the bottom of that diode back to the left side of R31, which also does the trick. So I was thinking that darlington transistor or diode had a problem, but taking it out and checking it it tests out good. Replacing it with another doesn't change anything. The diode also checks good. Since I wasn't clear as to what the purpose of that diode was I thought I should at least learn that before I proceeded with other things. I don't have a diode like that so haven't replaced it yet, maybe it's doing something weird?
gabo
When you turn it on, it presents about 5v DC to the speaker and the rails are down about 10v. If you open the circuit to the speaker, which can easily be done by inserting a 1/4" jack into the external speaker connection and then pull it back out, it works perfectly. The 1/4" jack isn't connected to anything, it just opens the connection to the speaker as it disconnects the internal speaker when you use an external cabinet. So plug it in, take it right back out, and the amp works until you turn it off/on again.
In tracking around voltages and such, I found about 5v across that resistor when it's not working and 0 when it is. Thinking there should never be 5v across that I was looking at that section. So I tried using a wire to quickly short between the bottom of that diode back to the left side of R31, which also does the trick. So I was thinking that darlington transistor or diode had a problem, but taking it out and checking it it tests out good. Replacing it with another doesn't change anything. The diode also checks good. Since I wasn't clear as to what the purpose of that diode was I thought I should at least learn that before I proceeded with other things. I don't have a diode like that so haven't replaced it yet, maybe it's doing something weird?
gabo
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D12 can't be working as intended if you're seeing 5V across the combo of D12 and R37. Suggests that D12 may have faulty series resistance.
Are you able to publish more of the amp schematic? BTW, are you powering the amp with a regulator?
Are you able to publish more of the amp schematic? BTW, are you powering the amp with a regulator?
I pasted a link to the MR-752. It shows very low esr, so your part must be defective.
https://www.mouser.com/datasheet/2/308/MR750_D-1811774.pdf
https://www.mouser.com/datasheet/2/308/MR750_D-1811774.pdf
You might double check diode drop directly across the diode's leads to rule out defective trace/solder joint.
Yes, I am suspecting some board issues. It's quite old and even though the board looks ok I think there are some issues. It's just too flakey in many ways. I've poked around and inspected with my magnifiers and I just can't find anything concrete.
D5, doesn't exhibit this behavior. One thing about it that's very consistent, if you put that external speaker plug in/out, it works every time or at least it does for now.
Do NOT start amp WITH a "regulator" (dim bulb current limiter? Current limited power supply?) AND a speaker connected simultaneously.
Many amps "wake up stupid" (hey, just like me) for a couple seconds until all internal electrolytic capacitors charge to working voltage; the limiter/regulator prevents that and you have a bunch of uncompensated DC at the output, it can´t recover from that.
Start amp WITH regulator WITHOUT speaker, THEN reconnect it.
If amp works fine, plug it straight into wall outlet.
As of the D12/R37 trick, it´s an old school trick to increase stability, as was mentioned above.
Huge 4.7 ohm emitter ballast resistor "makes it stable" (instead of typical 0.33 ohm) BUT will, drop huge voltage and waste power, hence the diode bypass.
Fell out of favour long ago, that Crate amplifier must be at least 20 y.o. , if not more.
Many amps "wake up stupid" (hey, just like me) for a couple seconds until all internal electrolytic capacitors charge to working voltage; the limiter/regulator prevents that and you have a bunch of uncompensated DC at the output, it can´t recover from that.
Start amp WITH regulator WITHOUT speaker, THEN reconnect it.
If amp works fine, plug it straight into wall outlet.
As of the D12/R37 trick, it´s an old school trick to increase stability, as was mentioned above.
Huge 4.7 ohm emitter ballast resistor "makes it stable" (instead of typical 0.33 ohm) BUT will, drop huge voltage and waste power, hence the diode bypass.
Fell out of favour long ago, that Crate amplifier must be at least 20 y.o. , if not more.
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Thanks for that. Yes that amp really isn't worth fixing to be honest. It's not that good and it's very old, probably older than 20. Just something I checked and it sort of got to be one of those things that bugged me as to what was going on. So as much of a learning experience as anything else.
Oh, Crate amps are VERY good, only problem being that they are usually not expensive and specially "Jimi Hendrix didn´t use them". 🙄
And what´s the real problem anyway?
What are you fixing?
And what´s the real problem anyway?
What are you fixing?
That was in post 3, hard to see since it wasn't the first post, should have started with that I guess. haha.
This is a guitar amp with a very weird problem.
When you turn it on, it presents about 5v DC to the speaker and the rails are down about 10v. If you open the circuit to the speaker, which can easily be done by inserting a 1/4" jack into the external speaker connection and then pull it back out, it works perfectly. The 1/4" jack isn't connected to anything, it just opens the connection to the speaker as it disconnects the internal speaker when you use an external cabinet. So plug it in, take it right back out, and the amp works until you turn it off/on again.
Of course when I got it, the owner didn't know about the 1/4 jack thing. It just would turn on, make hum and not work. I found that 1/4" jack oddity just by deciding to try an external speaker, plugged in the jack without it being connected to the speaker, had some other issues, unplugged it and all of a sudden it was working. Until I turned it off and back on again.
Then you do not have a problem 😉
I already answered to that.
Amplifiers are not used plugged into a current limiter.
Any problem when used "normal"?
I already answered to that.
Amplifiers are not used plugged into a current limiter.
Any problem when used "normal"?
Of course there is, turn it on, makes a loud hum and doesn't work. Do you really think I would be looking at it if it worked? The guy who owned it has owned it for years and used it a lot, it stopped working and he asked me if I would look at it.
JMFahey explained my question/concern about regulation, but the power stage of your amp isn't regulated anyway.
I recommend fixing the D12/R37 problem first--- that may be all that's wrong. Provoke the amp into the failed state and probe differentially across D12 leads with your VM. If you see find excessive voltage, you'll know definitively the diode has failed. If no volts across D12, probing with meter should reveal volts between a diode lead and a presumed connection that is in fact open.
I recommend fixing the D12/R37 problem first--- that may be all that's wrong. Provoke the amp into the failed state and probe differentially across D12 leads with your VM. If you see find excessive voltage, you'll know definitively the diode has failed. If no volts across D12, probing with meter should reveal volts between a diode lead and a presumed connection that is in fact open.
WITH a regulator or WITHOUT a regulator?
If WITH a regulator, you do NOT have a problem.
SPECIALLY because:
Who knows?, I don´t know you from Adam.
I DO know you are baffled by something which has been explained above.
Again: WITH a regulator or WITHOUT a regulator?
Who cares?
"Everything" works perfectly until it dies.
Works perfectly means it works perfectly in my book so if it does now ..... where´s the problem?
Geez man, it doesn't work. With a regulator or without a regulator, It doesn't work. I don't know how else to explain it. I'm not baffled by anything that was explained, I am baffled about what is causing the odd behavior in this circuit, but that always happens until I figure it out.
It's ok man, all I really wanted to know was what was the purpose of the diode in parallel with the resistor on the emitter of the darlington transistor. That question was answered. Thanks.