30 years old guitar player here also novice electrician. I like enjoy fixing stuff but this time I need help. My amp is not in a good condition I read 33V dc on speakers.. Ouchh..
First it was only one resistor that burnt by plugging the amp, after changing it then others smoked, then I noticed the wire going to power transistor is deattached. I replaced old tip29/30c's with modern 40/41c's, 204/207 pairs with also 547/x pairs, 2n3373 changed with mj15003 but I am still hearing the buzz and It's still 33V dc at speakers... What is wrong bias? I am just tired. I double check transistors BCE, EBC all right, I don't know what to do anymore... Feeling overwhelmed.
Amp is HH vs musian 100
At first I lost that 10 R at the 48 volt supply then bias one, then 1k feedback resistor... I am open to suggestions..
First it was only one resistor that burnt by plugging the amp, after changing it then others smoked, then I noticed the wire going to power transistor is deattached. I replaced old tip29/30c's with modern 40/41c's, 204/207 pairs with also 547/x pairs, 2n3373 changed with mj15003 but I am still hearing the buzz and It's still 33V dc at speakers... What is wrong bias? I am just tired. I double check transistors BCE, EBC all right, I don't know what to do anymore... Feeling overwhelmed.
Amp is HH vs musian 100
At first I lost that 10 R at the 48 volt supply then bias one, then 1k feedback resistor... I am open to suggestions..
Attachments
You probably should have asked for help before digging a deeper hole.
To help it looks like we will probably need some more information.
Still hearing what buzz?
What resistors burnt?
Which wire to which transistor was detached?
Why did you replace the old transistors? Did you measure them and find a short or an open?
Did you look around for any electrolytic capacitors that appear to be bulging and or leaking?
Any signs of corrosion, localized heating of the PCB, cracked solder joints, etc.?
Where is the switch or relay contact shown in series with L1 on the schematic?
Do you have a multimeter with a diode check function?
Do you have access to an oscilloscope?
To help it looks like we will probably need some more information.
Still hearing what buzz?
What resistors burnt?
Which wire to which transistor was detached?
Why did you replace the old transistors? Did you measure them and find a short or an open?
Did you look around for any electrolytic capacitors that appear to be bulging and or leaking?
Any signs of corrosion, localized heating of the PCB, cracked solder joints, etc.?
Where is the switch or relay contact shown in series with L1 on the schematic?
Do you have a multimeter with a diode check function?
Do you have access to an oscilloscope?
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@Markw4 Thank you for the reply !
I should have... Too much confidence bad for me...
Still hearing what buzz?
Hum maybe? When I plug the cab I am hearing bassy square wave-ish noise, it also used to be like that before I change the transistors, but I could hear tiny signal from the guitar getting amplified with that noise. Now input signal is not getting amplified at all, I only hear deep hum, which I think the sound of that 33 DC volt I measured at speakers output with speaker attached.
What resistors burnt?
First R16, then R25, R15, R32, R11.
Which wire to which transistor was detached?
Q6's collector, R16 was the first one to burn, and was the hottest, melted some of pcb plastic.
Why did you replace the old transistors? Did you measure them and find a short or an open?
I thought, I could solve the problem if I change them. Then looked for 'superior' parts... After I replace them, I checked them and two of them where short, D381 K78C and BC 207B.
Now I am comparing the datasheets between D381 and tip41c, they kinda look different, however AI told me that I could replace them, I might be tricked.
Did you look around for any electrolytic capacitors that appear to be bulging and or leaking?
There wasn't any, but replaced em all anyways, all the volts, values and polarities are double checked.
Any signs of corrosion, localized heating of the PCB, cracked solder joints, etc.?
None expect the ones that burned, and I replace them with same values, but there is one burnt resistor that reads around 10-11k ohm, but there isn't any 10k value in schematic, maybe feedback resistor R11 is that one? Just speculations.
Where is the switch or relay contact shown in series with L1 on the schematic?
It's not a switch line goes straight. This is a sum of 2 different papers alligned somewhat poorly, there isn't any other schematic in internet.
Do you have a multimeter with a diode check function?
I do.
Do you have access to an oscilloscope?
Unfortunately.
I should have... Too much confidence bad for me...
Still hearing what buzz?
Hum maybe? When I plug the cab I am hearing bassy square wave-ish noise, it also used to be like that before I change the transistors, but I could hear tiny signal from the guitar getting amplified with that noise. Now input signal is not getting amplified at all, I only hear deep hum, which I think the sound of that 33 DC volt I measured at speakers output with speaker attached.
What resistors burnt?
First R16, then R25, R15, R32, R11.
Which wire to which transistor was detached?
Q6's collector, R16 was the first one to burn, and was the hottest, melted some of pcb plastic.
Why did you replace the old transistors? Did you measure them and find a short or an open?
I thought, I could solve the problem if I change them. Then looked for 'superior' parts... After I replace them, I checked them and two of them where short, D381 K78C and BC 207B.
Now I am comparing the datasheets between D381 and tip41c, they kinda look different, however AI told me that I could replace them, I might be tricked.
Did you look around for any electrolytic capacitors that appear to be bulging and or leaking?
There wasn't any, but replaced em all anyways, all the volts, values and polarities are double checked.
Any signs of corrosion, localized heating of the PCB, cracked solder joints, etc.?
None expect the ones that burned, and I replace them with same values, but there is one burnt resistor that reads around 10-11k ohm, but there isn't any 10k value in schematic, maybe feedback resistor R11 is that one? Just speculations.
Where is the switch or relay contact shown in series with L1 on the schematic?
It's not a switch line goes straight. This is a sum of 2 different papers alligned somewhat poorly, there isn't any other schematic in internet.
Do you have a multimeter with a diode check function?
I do.
Do you have access to an oscilloscope?
Unfortunately.
Thread moved to Instruments and Amps
About my new BC557, this could be the wrong one for old bc204b (Q3, Q5)
This might silly but here is what I find from AI;
also bc207b has less hfe compared to bc547 (Q4), now I am realizing my hfe's are all over the place, this might be a bad thing, should have kept the transistors the same...
This might silly but here is what I find from AI;
| Parameter | BC 204B | BC557 | Difference & Impact |
|---|
| Gain (hFE) | 40 - 250 | 110 - 800 | BC557 has a much higher gain, which can affect biasing and increase DC offset. |
| Max IC | 500mA (0.5A) | 100mA (0.1A) | BC557 handles much less current, which can cause overheating or failure if the circuit requires more. |
also bc207b has less hfe compared to bc547 (Q4), now I am realizing my hfe's are all over the place, this might be a bad thing, should have kept the transistors the same...
Is the DC on the output +33v or -33v?
Could be one of the power supply rails has gone bad or been damaged somehow along the way. If you can leave it on without anything burning for a little while you could do some quick voltage measurements of the power supply rail voltages, and of the voltages on the opamp power pins. Maybe also the opamp output voltage.
Another approach to troubleshooting in a power-off state would be to test every transistor in the amp in forward and reverse bias conditions, and also for a C-E short even though the base junctions may seem normal. Also pull on wires, push them around a little and make sure all connections are intact. If you still have any original transistors left and if they measure okay then you might try putting them back.
At some point it may be possible to do some more investigation of signal level voltages when the unit is powered on, but first the schematic copy we can see would need to be more legible. Or maybe you could find a better copy online? As it is now I can't read some component values, and I can't always tell if there is a dot showing two crossing lines on the schematic are connected or not. Maybe you could make the connection dots more legible somehow?
Also, we probably should be able to see the power supply part of the schematic, and any changes to the circuitry not shown in the schematic.
At some point maybe breaking the feedback loop might also be a possible strategy to simplify analysis of static voltage levels.
Regarding using possibly incompatible transistors, the risks are that they might fail, and or that the amplifier might turn into an oscillator. In the latter case, there may be unexplained heating but a scope would really be needed to see what is going on. Oscillation could be at a very high frequency where you can't hear it and where your AC voltmeter can't measure anything.
Could be one of the power supply rails has gone bad or been damaged somehow along the way. If you can leave it on without anything burning for a little while you could do some quick voltage measurements of the power supply rail voltages, and of the voltages on the opamp power pins. Maybe also the opamp output voltage.
Another approach to troubleshooting in a power-off state would be to test every transistor in the amp in forward and reverse bias conditions, and also for a C-E short even though the base junctions may seem normal. Also pull on wires, push them around a little and make sure all connections are intact. If you still have any original transistors left and if they measure okay then you might try putting them back.
At some point it may be possible to do some more investigation of signal level voltages when the unit is powered on, but first the schematic copy we can see would need to be more legible. Or maybe you could find a better copy online? As it is now I can't read some component values, and I can't always tell if there is a dot showing two crossing lines on the schematic are connected or not. Maybe you could make the connection dots more legible somehow?
Also, we probably should be able to see the power supply part of the schematic, and any changes to the circuitry not shown in the schematic.
At some point maybe breaking the feedback loop might also be a possible strategy to simplify analysis of static voltage levels.
Regarding using possibly incompatible transistors, the risks are that they might fail, and or that the amplifier might turn into an oscillator. In the latter case, there may be unexplained heating but a scope would really be needed to see what is going on. Oscillation could be at a very high frequency where you can't hear it and where your AC voltmeter can't measure anything.
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Last time it was DC +33v with speakers connected. Now DC +40v without speakers connected, I am not going to expose my speakers to DC.
Also checked with AC I read 86 volts without speakers connected.
DC power:
I read at + DC 40v , - DC 45v
Opamp:
V+ +4.2v
V- -1v
Output: 4.2v
Nothing is burning so far, only Q1 and Q7 getting hot like 170 fahrenheit in 45 seconds.
This is a viable option, will try that.
Should have provided it in first place, attached it as PDF file.
I am all ears.
Looks like there is a problem with the opamp power. The PL16Z zener diodes should be regulating the opamp power pin voltage to +-15v, or pretty close to that.
Could be various problems. Capacitors C3, C5 may be partially shorted, or opamp might be bad. Something is wrong right around in there if there is really +4.2v and -1v on pins 11 and 6. When electrolytic caps go bad often the top of the can may be a little swollen out, and or some signs of leakage of electrolyte.
Could be various problems. Capacitors C3, C5 may be partially shorted, or opamp might be bad. Something is wrong right around in there if there is really +4.2v and -1v on pins 11 and 6. When electrolytic caps go bad often the top of the can may be a little swollen out, and or some signs of leakage of electrolyte.
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I removed opamp, and measured power pins +-15v.
Placed another opamp and got -14.5v +14.7, nothing heatling slightly, only opamp is lukewarm but I read DC -44v at speaker output, something perhaps still wrong? Also checked each diode and transistörs B-E&B-C I did measure voltage drops and no shorts. Measured about 44 negative and positive volt through all transistor legs expect Q1's base -3.5v and emitter -1.5v.
Don't wanna plug and perhaps damage speakers.. I read speaker should have almost 0 dc but mine is somehow -44v ...
Any lead?
Opamp appears to be better. Now it would be helpful to know the voltages on the opamp input pins, and the output pin.
Regarding Q1, its possible for an NPN transistor base to be more negative than the emitter. It just means the transistor is being turned off hard. Please note that most transistors can't tolerate more than a few reverse volts on the B-E junctions. However, there are two transistors labeled Q1. There is one at the output of the opamp, and the other one is an output transistor probably on a heat sink. So which Q1 are you talking about?
The output voltage of the amplifier at the speaker should be pretty close to zero volts in the absence of any guitar input signal, so looks like something is still wrong.
Looking at the output of the amplifier, output transistor Q1 must most likely be off if the amplifier output is -44v, as the purpose of that Q1 when turned on is to pull the output in a more positive direction.
Similarly, the purpose of output transistor Q2 is to pull the output negative as needed. If the output is at -44v then likely output transistors Q2 is being turned on somehow, or else its shorted.
Regarding measuring transistors with a diode checker, B-E and B-C junctions should be measured in both the forward bias and reverse bias directions. Also, even if a transistor looks okay measured that way, there is still a rare but not impossible failure mode where the only thing that measures wrong is C-E. It can be shorted while both of the base junctions measure as normal.
If you don't measure the transistors fully with a diode checker when they are disconnected from the circuit, then you may miss some failure mode.
Okay, moving along with power-on measurements, we need to see if the opamp is responsible for turning on output transistor Q2. First thing is to make sure output transistor Q2 is being turned on with base current and not shorted. Also need to know both opamp input voltages and its output voltage.
Very basic normal operation of the circuit is that when the opamp goes output goes + enough to turn on Q1 on the PCB, that should then turn on Q2 on the PCB. Turning on PCB Q2 then turns on Q6 more and tend to turn off Q7 (really Q6 and Q7 may not normally be all the way on nor all the way off, rather they may be each partially on and in some balance controlled by the opamp, or else one may be fully off and the other partially on). Q6 and Q7 then control the output transistors. When the opamp output voltage is exactly right, the output transistors should produce about zero volts at the amplifier output in the absence of any guitar input voltage.
Regarding Q1, its possible for an NPN transistor base to be more negative than the emitter. It just means the transistor is being turned off hard. Please note that most transistors can't tolerate more than a few reverse volts on the B-E junctions. However, there are two transistors labeled Q1. There is one at the output of the opamp, and the other one is an output transistor probably on a heat sink. So which Q1 are you talking about?
The output voltage of the amplifier at the speaker should be pretty close to zero volts in the absence of any guitar input signal, so looks like something is still wrong.
Looking at the output of the amplifier, output transistor Q1 must most likely be off if the amplifier output is -44v, as the purpose of that Q1 when turned on is to pull the output in a more positive direction.
Similarly, the purpose of output transistor Q2 is to pull the output negative as needed. If the output is at -44v then likely output transistors Q2 is being turned on somehow, or else its shorted.
Regarding measuring transistors with a diode checker, B-E and B-C junctions should be measured in both the forward bias and reverse bias directions. Also, even if a transistor looks okay measured that way, there is still a rare but not impossible failure mode where the only thing that measures wrong is C-E. It can be shorted while both of the base junctions measure as normal.
If you don't measure the transistors fully with a diode checker when they are disconnected from the circuit, then you may miss some failure mode.
Okay, moving along with power-on measurements, we need to see if the opamp is responsible for turning on output transistor Q2. First thing is to make sure output transistor Q2 is being turned on with base current and not shorted. Also need to know both opamp input voltages and its output voltage.
Very basic normal operation of the circuit is that when the opamp goes output goes + enough to turn on Q1 on the PCB, that should then turn on Q2 on the PCB. Turning on PCB Q2 then turns on Q6 more and tend to turn off Q7 (really Q6 and Q7 may not normally be all the way on nor all the way off, rather they may be each partially on and in some balance controlled by the opamp, or else one may be fully off and the other partially on). Q6 and Q7 then control the output transistors. When the opamp output voltage is exactly right, the output transistors should produce about zero volts at the amplifier output in the absence of any guitar input voltage.
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To follow up a little on the above comment. Is the 10R resistor R16? Is the 1k resistor R11?
According to schematic it's 10R. Could not read the ex resistor, it was obliterated with a burnt crater beneath it. R11 is 1K.
The one next to opamp.
By the way thanks for the feedback, appreciate it. Will do readings&checks tonight.
Here is my voltage readings regarding output transistors Q1 and Q2, ratings are sorted as BCE
B C E
Q1: +45v -43.9v -43.9v
Q2: -44v -44.5v -44.5v
Voltage readings for onboard transistors, ratings are sorted as BCE once again:
B C E
Q1: -1.5v +45v 0v
Q2: +45v -43.6v +45v
Q3: +45v +45v +45v
Q4: -43.6v -43v -43.7v
Q5: +43.8v -42.6v -43.7v
Q6: -43.7v +45v -43.7v
Q7: -44v -44v -44v
Once I accuired my new transistors I did check them both ways before installing, If something bad happened since, then I might need to recheck them. From one of the old power transistors I was getting low resisstor readling like 13k ohm's, can't remember the pins. So altough bias direction is there the other one had much more higher resistor so I ordered new pair of MJ150003G's along with other transistors.
Inverting (-) -10.8v
Non inverting (+) 0v
Output: -1.2v (Kind of tricky to measure, once I turn on the amp starts from -6v then gets lower to -1.2v in 10 seconds and stabilize there)
Supply power is very close to +/- 15v
My opamp output is constant DC -1.2v so there might be a problem?
Q1 shouldn't have -43.9 volts on the Collector. The schematic shows it is connected to the +48v rail. Did the lead melt off again or something? Maybe there is also a C to E short?
Also, Q2 shouldn't have the same voltage on the Collector and Emitter. Even when transistors are driven into saturation they still don't have exactly 0v between C and E.
I suggest you remove carefully investigate both output transistors and their wiring with the power off. Q2 should be removed from the circuit and thoroughly tested with a diode check function on your DVM. It may have a C-E short even if B-E and B-C look normal. Same for Q1.
Once those things are sorted out, we should discuss next steps before turning on the power again.
My bad, of course it doesn't have negative volts on the collector, readings supposed to be in order of CBE, ignore the past BCE order for power transistors.
BCE readings only apply for onboard transistors, power transistors follow CBE so:
Q1: Collector: +45v Base -44v Emitter: -44v
Q2: Collector: -44v Base: -44.5v Emitter -44.5v
I removed all 9 transistors, looked for C-E short as well after that checked for hfe value. Everything is fine, expect Q3 'PNP' one. It was short all pins. I don't know what happened there. I have to look for a replacement for it.
Q3 should limit current through Q2. When Q2 draws enough current then the voltage drop across R16 will get up to be .6v which is enough to start turning on Q3. Q3 should then pull up on Q2 base to limit Q2 current to a safe value.
Since this is a guitar amp it may intentionally be operated in a heavily distorted mode. Q2 might be driven hard enough to pass too much current if not restrained by Q3.
Once Q3 is fixed then things may start looking closer to normal.
Since this is a guitar amp it may intentionally be operated in a heavily distorted mode. Q2 might be driven hard enough to pass too much current if not restrained by Q3.
Once Q3 is fixed then things may start looking closer to normal.
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