JohnW63
Enlightened Member
DTC,
Pulled tube 3. Hum cam up, then went back down, then a little pop and it was louder ,scratchier, hum.
Pulled tube 3. Hum cam up, then went back down, then a little pop and it was louder ,scratchier, hum.
MEDIC ! My Carvin just crapped out on me
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JohnW63
Enlightened Member
Nuuska,
I need to verify where point C is on the circuit board.
I need to verify where point C is on the circuit board.
JohnW63
Enlightened Member
If I'm not supposed to have AC at end of R34, then I have a problem. I have 600v AC and 320 DC. The cap look fine, visually. No bulging or leaking.
DThomasC
Senior Member
I agree with Nuuska that 600VAC is not possible.
He and I are looking in different directions. That's ok, but it keeps John busy. My suspicion right now is that one of the EL84s is conducting while the other isn't. There's still lots of possible causes, that's just where I would look first if the amp were in front of me. If your amp looks like the one in the video, then you should be able to measure the grid, screen, and plate voltages on the output tubes pretty easily. The cathode is grounded, that's pin 3, so put your black probe on pin 3. Then measure the pins 2 (grid), pin 9 (screen), and pin 7 (plate/anode) of both output tubes (V4 and V5.) I expect around -17VDC, 360VDC, and 360VDC respectively.
He and I are looking in different directions. That's ok, but it keeps John busy. My suspicion right now is that one of the EL84s is conducting while the other isn't. There's still lots of possible causes, that's just where I would look first if the amp were in front of me. If your amp looks like the one in the video, then you should be able to measure the grid, screen, and plate voltages on the output tubes pretty easily. The cathode is grounded, that's pin 3, so put your black probe on pin 3. Then measure the pins 2 (grid), pin 9 (screen), and pin 7 (plate/anode) of both output tubes (V4 and V5.) I expect around -17VDC, 360VDC, and 360VDC respectively.
Nuuska
Enlightened Member
A wild thought just popped into my tiny mind . . . Thanks to DThomasC
WHAT IF ??? There´s that switch to change operating mode. DPDT switch. One position connects grid 9 to anode 7 - the other position connects grid 9 to 360V DC And it has to switch both tubes the same. What if there's a problem with this switch - and that causes the imbalance in output tubes - causing the hum and gritty sound.
Have you been using that switch? If it has been sitting in same position for years - chances are, that it might be partially oxygenated - leading to imbalance. If this is the case - then first aid is to switch it back and forth about 10 times - then POWER ON - if good - repeat once or twice a year.
WHAT IF ??? There´s that switch to change operating mode. DPDT switch. One position connects grid 9 to anode 7 - the other position connects grid 9 to 360V DC And it has to switch both tubes the same. What if there's a problem with this switch - and that causes the imbalance in output tubes - causing the hum and gritty sound.
Have you been using that switch? If it has been sitting in same position for years - chances are, that it might be partially oxygenated - leading to imbalance. If this is the case - then first aid is to switch it back and forth about 10 times - then POWER ON - if good - repeat once or twice a year.
JohnW63
Enlightened Member
I measured the volts at either and of the resistor and got 600 VAC. It's right next to the cap. There are only 3 that are 400v caps.
DThomasC
Senior Member
Are you sure it's not 600mV? (That's millivolts.) I've made that mistake plenty of times.
JohnW63
Enlightened Member
Nope. My meter has 200 and 500 vac ranges. Nothing small on the VAC side. 200mv to 1000 v on the DC side.
Today, with no speaker and fully warmed up, I get 250vdc on one side or R34 and 266vdc on the other. The schematic shows it would be different values. For ACV on the same resister, I get 660vac and 680vac when I first power it up and it goes down, to 550vac and 570vac.
I get 320vac coming in from the input ( power ) transformer.
Today, with no speaker and fully warmed up, I get 250vdc on one side or R34 and 266vdc on the other. The schematic shows it would be different values. For ACV on the same resister, I get 660vac and 680vac when I first power it up and it goes down, to 550vac and 570vac.
I get 320vac coming in from the input ( power ) transformer.
JohnW63
Enlightened Member
Can anyone guess at the proper input voltage ? The schematic is chopped off at the bottom.
JohnW63
Enlightened Member
D6/D7 =390 AC
D4/D5 = 370 AC
D4/D5 = 370 AC
Nuuska
Enlightened Member
I did not make it clear - sorry for that - but my previous numbers mean that you measure between those two points - so there's supposed to be 260VAC between D4/D5 & D6/D7
When you measure floating AC voltages against ground you get whatever - is this maybe the reason you got that 600VAC earlier.
When you measure floating AC voltages against ground you get whatever - is this maybe the reason you got that 600VAC earlier.
JohnW63
Enlightened Member
297 AC from one junction to the other.
Yes, I was measuring from a point to ground. Voltage measurements are always with reference to ground, unless looking for the difference between two points. I wanted to test to power transformer input to see if it was sending the wrong voltage to the circuit board, so I would really like to pull that plug from the board and test the input. I can simply check the leads on the wires while plugged in, but that might change the values.
In your previous request, you wanted me to check voltage at point C for 325 V. Given only one point, I would have to go to ground to test it. Are you saying I should have connected to a different spot ?
Yes, I was measuring from a point to ground. Voltage measurements are always with reference to ground, unless looking for the difference between two points. I wanted to test to power transformer input to see if it was sending the wrong voltage to the circuit board, so I would really like to pull that plug from the board and test the input. I can simply check the leads on the wires while plugged in, but that might change the values.
In your previous request, you wanted me to check voltage at point C for 325 V. Given only one point, I would have to go to ground to test it. Are you saying I should have connected to a different spot ?
DThomasC
Senior Member
What kind of meter are you using? Not all meters can correctly measure AC volts with a DC bias. You could setup a cap and resistor to block DC, but it might be cumbersome to use.
BTW: I'm still interested in those DC volt measurements at the pins of V4 and V5
EDIT: looking closer at the photo, I realized that what I thought were pin numbers might not be numbers. In any case, the pins are numbered clockwise starting from the pins spaced farther apart than the others. In the photo, pin 1 is roughly at 1 o'clock while pin 9 is at 11 o'clock.
BTW: I'm still interested in those DC volt measurements at the pins of V4 and V5
EDIT: looking closer at the photo, I realized that what I thought were pin numbers might not be numbers. In any case, the pins are numbered clockwise starting from the pins spaced farther apart than the others. In the photo, pin 1 is roughly at 1 o'clock while pin 9 is at 11 o'clock.
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JohnW63
Enlightened Member
The V numbers are the tubes, as in "valves", as they call them in England. Do you want voltages of all the tube socket pins ? I have to assume you want from each pin to ground, otherwise I need a pair of measure points.
"What kind of meter are you using? " Beckman DM5XL. Looks like I see more Wavetek branded ones of these models now.
I could borrow a Fluke, from work, if you think what I have isn't good enough.
"What kind of meter are you using? " Beckman DM5XL. Looks like I see more Wavetek branded ones of these models now.
Wavetek XL Series Multimeters
Industrial Electronics By Ross LLC, distributors of Wavetek: XL Series - DM5XL, DM10XL, DM15XL, and DM16XL Have Large LCD Digits. The largest in their class for easy viewing.
www.industrialelectronics.biz
I could borrow a Fluke, from work, if you think what I have isn't good enough.
DThomasC
Senior Member
Yes, V is for Valve. They're labeled on the top of the PCB and you can measure from there. See photo above.
I want measurements from 3 different pins to ground for both tubes.
Pin 2 (grid) should be around -17 volts DC
Pin 7 (anode, aka plate) should be around 360VDC
Pin 9 (screen) should be around 360VDC
It turns out that pin 3 (cathode) is grounded, so you can put your black probe there and your red probe on the other three pins one at a time. You can use any ground that's convenient, but I would use pin 3 myself.
I'm suspicious that the weird AC readings you're getting are because the meter fakes AC voltage measurements in a way that works with pure AC, but doesn't work in this application. Making the same measurements with a Fluke should verify if that's the case or not.
Nuuska
Enlightened Member
hello again
Since your x-former delivers 320VAC instead of 260VAC as in schematic - PLUS - the DC-voltage at R34 is 266/250 - and it should be 335/325 - I would check the rectifier diodes - You should have 360VDC at junction D4/D6/R36/C29 - measured against ground/chassis.
I say this because if the rectifier diode fails - no short, but breaks - there will not be enough current floating thru - thus leading to lower anode voltages - and at same time the x-former output voltage raises, because there's not enough load.
DThomasC ???