CR6, CR7 are clamps, just as CR13, CR14 at the input stage, and CR25, CR26 at the power amp input. They don't do anything under normal conditions. But in the case of large signals or unwanted voltage on those jacks, they shunt anything beyond the 15v rail voltages, which would have the net effect of LOOKING like reverse voltage to the IC. You don't want voltage on your IC that exceeds its power rails. These clamp diodes are quite common onb PV circuits, especially their mixers.
Well I took some readings, so here we go.
U1 Pin 1: 7mV
U1 Pin 7: 19mV
U4 Pin 1: 2.5mV
U4 Pin 7: 1.3mV
U5 Pin 1: -3.3mV
U5 Pin 7: 1.6mV
U6 Pin 1: 6.8mV
U6 Pin 7: 29mV
U7 Pin 1: 9.6mV
U7 Pin 7: 9.8V YES, volts, NOT mV
U8 Pin 1: 1.7mV
U8 Pin 7: 3.9mV
U9 Pin 1: 6.8mV
U9 Pin 7: 6.5mV
DC Offset with preamp disconnected: 10mV
DC Offset with preamp connected: 10mV
DC Offset from the preamplifier: -9.7mV
U1: TL082CP
U4: TL082CP
U5: TL082CP
U6: RC4558P
U7: RC4558P
U8: TL082CP
U9: TL082CP
U1 Pin 1: 7mV
U1 Pin 7: 19mV
U4 Pin 1: 2.5mV
U4 Pin 7: 1.3mV
U5 Pin 1: -3.3mV
U5 Pin 7: 1.6mV
U6 Pin 1: 6.8mV
U6 Pin 7: 29mV
U7 Pin 1: 9.6mV
U7 Pin 7: 9.8V YES, volts, NOT mV
U8 Pin 1: 1.7mV
U8 Pin 7: 3.9mV
U9 Pin 1: 6.8mV
U9 Pin 7: 6.5mV
DC Offset with preamp disconnected: 10mV
DC Offset with preamp connected: 10mV
DC Offset from the preamplifier: -9.7mV
U1: TL082CP
U4: TL082CP
U5: TL082CP
U6: RC4558P
U7: RC4558P
U8: TL082CP
U9: TL082CP
Last edited:
That would be the reverb driver opamp U7b.
Do you have the reverb unit plugged in?
What are the voltages on pins 5&6 of that chip?
That is high,disconnect the reverb if it is not and see if your noise goes away.
jer
This is with the reverb disconnected.
That would be the reverb driver opamp U7b.
Do you have the reverb unit plugged in?
What are the voltages on pins 5&6 of that chip?
That is high,disconnect the reverb if it is not and see if your noise goes away.
jer
U7 Pin 5: 18.7mV
U7 Pin 6: 19.4mV
Little correction.. I said that the amp was dead silent with the preamp disconnected. Well, not completely true. If you put your ear to the speaker and listen carefully you can hear the hum very faintly, and hear the scratchy sound. The hum and scratching is a bit louder with the preamp connected.
I'm starting to suspect it may be the filter caps in the preamp section.. I know that when filter caps are failing, you get a but of a hum.
The scratchy noise sounds like a dirty pot or like I'm brushing the preamps output to the input on the power amp. A scratchy static noise. As stated, all pots were cleaned and have no effect (well the lead gain post knob lowers the hum when it is set to the 12:00 position, but not by much.) All jacks were replaced, and even when directly soldering the preamp to the power amp, the hum and scratchy noise still remains. All jacks are properly grounded, and all board solder joints were reflowed.
Last edited:
Okay it could be be high becuase it is disconnencted.
I would have to model that circuit in Circuitmaker (spice) to be sure as pins 5 and 6 appear to be normal.
I would tear mine apart to check as well but I don't have the room at the moment.
If you reconnect the reverb unit it may return to normal as it has a lower resistance than the 4.7k resistor that you are measuring across but I still think it should be close to 0V any how.
Did the reverb work?
Don't connect it yet though.
jer
I would have to model that circuit in Circuitmaker (spice) to be sure as pins 5 and 6 appear to be normal.
I would tear mine apart to check as well but I don't have the room at the moment.
If you reconnect the reverb unit it may return to normal as it has a lower resistance than the 4.7k resistor that you are measuring across but I still think it should be close to 0V any how.
Did the reverb work?
Don't connect it yet though.
jer
Reverb works great. I just didn't want to unscrew it from the cab.
Should I just pull U7 and see what happens, or is U7 needed to keep everything happy? In some amps (like Hafler) removing the MOSFETs cause the voltage to shift wildly and burn resistors, so they need to stay in to keep it working.
Should I just pull U7 and see what happens, or is U7 needed to keep everything happy? In some amps (like Hafler) removing the MOSFETs cause the voltage to shift wildly and burn resistors, so they need to stay in to keep it working.
Last edited:
An analog meter would probably have been better, but at the noise and scratchieness changes, the AC voltage goes wild. I'm measuring in reference to ground (between the two main filter caps) and I get readings of 8V and jumping to 50 and 120V and back down. Pretty odd.
I'll make a quick video.
I'll make a quick video.
I just turned mine on and it isn't making the scratchy sound as it comes and goes.
And my controls have been cleaned yet so the are a bit dirty.
You will get some hum in the preamp if the gains are up and there is nothing plugged into the input as it is open in that condtion and it as tremendous gain.
As I leave it on some times it will start a loud scratchy sound all by itself.
I have left it on for days and forget about it and then it will start up and I then say to my self what the heck is that and discover that I left the amp on.
I am expecting this is what you are experiencing.
I really would like to know what is causing this as well.
Past experience has showed me that cold solder joints and bad caps can cause this as well as a noisy transistor.
The main issue is to determine if it is in the preamp stage or is it the power amp.
I have seen controls go bad as well where they would open on one side of them giving them the appearance that the are working,sorta.
The only way to check these it to take it out or isolate it from the rest of the circuit by lifting on end of any resistors connecting to it and then checking its function with an ohmmeter.
Sometimes it can be done in circuit depending on how it is configured.
jer
And my controls have been cleaned yet so the are a bit dirty.
You will get some hum in the preamp if the gains are up and there is nothing plugged into the input as it is open in that condtion and it as tremendous gain.
As I leave it on some times it will start a loud scratchy sound all by itself.
I have left it on for days and forget about it and then it will start up and I then say to my self what the heck is that and discover that I left the amp on.
I am expecting this is what you are experiencing.
I really would like to know what is causing this as well.
Past experience has showed me that cold solder joints and bad caps can cause this as well as a noisy transistor.
The main issue is to determine if it is in the preamp stage or is it the power amp.
I have seen controls go bad as well where they would open on one side of them giving them the appearance that the are working,sorta.
The only way to check these it to take it out or isolate it from the rest of the circuit by lifting on end of any resistors connecting to it and then checking its function with an ohmmeter.
Sometimes it can be done in circuit depending on how it is configured.
jer
Cool thanks for the Video.
I don't think that the caps are bad on the 55V supply because the amp is quiet when it is disconnected from the preamp.
I would check the 15v Rails and zener's.
You can try just tacking in a filter cap with some clip leads and see if the hum goes away.
But make sure that both the amps cap, and, the spare cap, is fully discharged before you add it.
Because the voltage from the spark from connecting the cap while the power is on could be high enough to take out an op amp and create more problems.
jer
I don't think that the caps are bad on the 55V supply because the amp is quiet when it is disconnected from the preamp.
I would check the 15v Rails and zener's.
You can try just tacking in a filter cap with some clip leads and see if the hum goes away.
But make sure that both the amps cap, and, the spare cap, is fully discharged before you add it.
Because the voltage from the spark from connecting the cap while the power is on could be high enough to take out an op amp and create more problems.
jer
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.