DThomasC
Senior Member
Nuuska could be on to something. I still want to verify that one of the output tubes isn't pulling much more current than it should be.
MEDIC ! My Carvin just crapped out on me
- Thread starter JohnW63
- Start date
JohnW63
Enlightened Member
I have some measurements, but, I found the amp shut it's self down, while I was taking them. I haven't had the speaker plugged in, while doing this. I hope the lack of an output load didn't fry the amp.
JohnW63
Enlightened Member
V4:
2 = ( pin 3 to 2 = 0 )
7 = ( pin 3 to 2 = 290 )
9 = ( pin 3 to 9 = 290 )
V5:
2 = ( pin 3 to 2 = 0 )
7 = ( pin 3 to 7 = 0)
9 = ( pin 3 to 9 = 0 )
Oddly, with tube 4, simply touching the pins with the positive lead changed the hum amount. I have looked at the solder spots and don't see any cracks. It changes even if I lightly touch any of the three. I also get higher values if I use chassis ground rather than pin 3.
2 = ( pin 3 to 2 = 0 )
7 = ( pin 3 to 2 = 290 )
9 = ( pin 3 to 9 = 290 )
V5:
2 = ( pin 3 to 2 = 0 )
7 = ( pin 3 to 7 = 0)
9 = ( pin 3 to 9 = 0 )
Oddly, with tube 4, simply touching the pins with the positive lead changed the hum amount. I have looked at the solder spots and don't see any cracks. It changes even if I lightly touch any of the three. I also get higher values if I use chassis ground rather than pin 3.
JohnW63
Enlightened Member
Nuuska,
That point starts at 399 vdc and drops as the tubes warm and ends up at 308 vdc
That point starts at 399 vdc and drops as the tubes warm and ends up at 308 vdc
DThomasC
Senior Member
Well, one doesn't need to be a genius to see that not much about that is right. With the grid (pin 2) at ground, V4 is passing an Amish ton of current drawing the plate voltage from 360 or so down to 290. The British call them valves because that's sort of what they are. This one is wide open. Meanwhile, V5 isn't doing anything. With push-pull amps (two output tubes) each tube makes a certain amount of hum. But, if both tubes are idling about the same then the hum more or less cancels. In this case we have one tube making a lot of hum and the other one isn't canceling it at all.
It could be caused by a lot of things, some worse than others. It could be that some of the connectors and tube sockets are corroded. You might try getting a can of Deoxit D5 and cleaning everything. Start by cleaning all the connectors inside the amp. One at a time, unplug a connector, squirt some Deoxit on both parts, then plug and unplug a few times to clean the contacts. Move on to the next connector. Do them one at a time so they don't get mixed up. When you're done with the connectors, flip the amp over and do the same thing with each of the tubes. Pull a tube, spray some Deoxit in the socket, plug the tube back in, then move the tube around as if it had a tiny hula hoop around it.
The Deoxit will get all over the place and make a mess and stink, but it will mostly evaporate and not cause any harm. Just the same, avoid getting it on the covering of the amp as it might make a discolored spot that will be hard to clean.
JohnW63
Enlightened Member
I already had the circuit board out and sprayed DeOxit in the tube sockets and the Amp mode switch connector, the amp mode switch, all the control knobs and the power input connector. The tubes don't change the issue by tapping them or the area around them. The board looks clean as a whistle. No dark colors, no stains.
I was thinking the tall tubes, V1 and V2 were the output ones and the three short guys, V3-V5 were preamp and "soak". Still, for some reason one tube is not getting signal, when they all should be doing something.
Why is the voltage different from the tube ground vs the chassis ground ? With Chassis ground as a reference, the lazy tube has some voltage, but only in the low 2 digit range.
I was thinking the tall tubes, V1 and V2 were the output ones and the three short guys, V3-V5 were preamp and "soak". Still, for some reason one tube is not getting signal, when they all should be doing something.
Why is the voltage different from the tube ground vs the chassis ground ? With Chassis ground as a reference, the lazy tube has some voltage, but only in the low 2 digit range.
JohnW63
Enlightened Member
Another thought.
The amp runs in "Pentode" mode at 16w and "Triode" mode at 5w. What 2 tubes are turned off in Triode mode ? A quick check of the slider switch shows it is at 5w mode. I slide it up and down a bunch with the DeOxit spray and didn't think about where is landed.
The amp runs in "Pentode" mode at 16w and "Triode" mode at 5w. What 2 tubes are turned off in Triode mode ? A quick check of the slider switch shows it is at 5w mode. I slide it up and down a bunch with the DeOxit spray and didn't think about where is landed.
JohnW63
Enlightened Member
Measurements in 16w mode:
V4:
2 = 0
7 = 306
9 = 308
V5:
2 = 0
7 = 0
9 = -26
V4:
2 = 0
7 = 306
9 = 308
V5:
2 = 0
7 = 0
9 = -26
Nuuska
Enlightened Member
next one is easy - switch tubes V4 & V5 - measure those points again - if the numbers follow the tubes - get two new ones. If the numbers stay put in location - it is the surrounding components - wiring - trafo etc
DThomasC
Senior Member
The tubes in the V4 and V5 positions should be EL84 / 6BQ5. The other three should be 12AX7 / ECC83. The EL84 are always taller than 12AX7.
The tube numbers are labeled on the PCB near the sockets.
JohnW63
Enlightened Member
Nuuska,
I already swapped the tubes, one at a time, with the stock tubes that came with the amp. There was no change in the symptoms.
DTC,
Yes. I can see the tube numbers. Otherwise I could not have posted the voltage measurements. The tubes and the sockets are pointed away from the circuit board when I use the multimeter on the topside of the board.
I already swapped the tubes, one at a time, with the stock tubes that came with the amp. There was no change in the symptoms.
DTC,
Yes. I can see the tube numbers. Otherwise I could not have posted the voltage measurements. The tubes and the sockets are pointed away from the circuit board when I use the multimeter on the topside of the board.
DThomasC
Senior Member
John, at this point I'm not sure how to advise you. The usual troubleshooting technique is to start at one end and work your way towards the other until you see something change. Then you have at least isolated where the trouble is. In this case, if I had the amp in front of me, I would start at the power supply and work my way towards each tube. For this, you can pull all the tubes out.
1. Measure the AC voltage across the two red wires from the power transformer. There's a connector that plugs into the PCB. It looks like part of the crimped connectors are exposed so you should be able to poke your meter probes in there. I expect 260 VAC
2. Do the same for the two green wires. You should see 6.3VAC. Probably closer to 7VAC, but that's OK.
3. Measure the DC voltage between the junction of D4 and D6 and ground. That's the output of the rectifier. With no tubes installed I expect at least 360 VDC.
4. Measure the voltage between connector terminal QC2 and ground. That's connected directly the point we measured in step 3, so you should get exactly the same reading.
5. In the video on page 1, it looks like the red wire from the output transformer can be unplugged from the PCB. If so, then unplug the red wire and measure the DC voltage between the PCB where the wire was and ground. You should get the same reading as steps 3 and 4.
6. If possible, unplug the blue and brown wires. With the red, blue and brown wires all unplugged, measure the resistance between the red wire and blue wire. Also measure the resistance between the red wire and brown wire. I expect them to be a few hundred ohms each, but not exactly the same. EDIT: we want to measure the resistance of the output transformer. So put the meter probes on the wires, not the spots on the PCB where the were plugged in.
7. Plug the red, blue, and brown wires back in. I posted a screen shot from the video in post #37. Make sure the wires go to the same places as in the photo. Now measure the DC voltage between pins 7 of both V4 and V5 and ground. Without any tubes installed you should see the same reading as steps 3, 4, and 5.
I expect that by now we would have found something amiss.
1. Measure the AC voltage across the two red wires from the power transformer. There's a connector that plugs into the PCB. It looks like part of the crimped connectors are exposed so you should be able to poke your meter probes in there. I expect 260 VAC
2. Do the same for the two green wires. You should see 6.3VAC. Probably closer to 7VAC, but that's OK.
3. Measure the DC voltage between the junction of D4 and D6 and ground. That's the output of the rectifier. With no tubes installed I expect at least 360 VDC.
4. Measure the voltage between connector terminal QC2 and ground. That's connected directly the point we measured in step 3, so you should get exactly the same reading.
5. In the video on page 1, it looks like the red wire from the output transformer can be unplugged from the PCB. If so, then unplug the red wire and measure the DC voltage between the PCB where the wire was and ground. You should get the same reading as steps 3 and 4.
6. If possible, unplug the blue and brown wires. With the red, blue and brown wires all unplugged, measure the resistance between the red wire and blue wire. Also measure the resistance between the red wire and brown wire. I expect them to be a few hundred ohms each, but not exactly the same. EDIT: we want to measure the resistance of the output transformer. So put the meter probes on the wires, not the spots on the PCB where the were plugged in.
7. Plug the red, blue, and brown wires back in. I posted a screen shot from the video in post #37. Make sure the wires go to the same places as in the photo. Now measure the DC voltage between pins 7 of both V4 and V5 and ground. Without any tubes installed you should see the same reading as steps 3, 4, and 5.
I expect that by now we would have found something amiss.
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JohnW63
Enlightened Member
Volts across the red wires, unplugged from board = 168 VAC. Plugged into board = 157 vac
Volts across the green wires, unplugged from board = 6.25 VAC , Plugged into board = 2.45 vac
DCV at D4/D6 junction = 407
QC2 = 407
Red wire to output transformer plugs onto QC2. = 407
Red to brown = 85ohm
Red to blue 93 = ohm
Pins 7 on both tube connectors is 407 volts, either from pin three or chassis ground. However, measuring with my Fluke meter, it is unhappy and just flashes like some odd over range is happening.
Volts across the green wires, unplugged from board = 6.25 VAC , Plugged into board = 2.45 vac
DCV at D4/D6 junction = 407
QC2 = 407
Red wire to output transformer plugs onto QC2. = 407
Red to brown = 85ohm
Red to blue 93 = ohm
Pins 7 on both tube connectors is 407 volts, either from pin three or chassis ground. However, measuring with my Fluke meter, it is unhappy and just flashes like some odd over range is happening.
DThomasC
Senior Member
Hi John, Sorry about the late replay. I've been buried with work for the past few days.
Not sure what to make of that. In order to get the 407 VDC you report below, we would need about 290 VAC here.
6.25 VAC is OK. The fact that it dropped to 2.45 VAC when plugged in should be disconcerting. That's the heater supply. The fact that you get hum suggests that the tubes are probably heating up. But they wouldn't with only 2.45VAC. Was there hum with the tubes pulled?
All of that looks good. Honestly, I don't see low hanging fruit here, so I'm not sure where to take this adventure.
Not sure what to make of that. In order to get the 407 VDC you report below, we would need about 290 VAC here.
6.25 VAC is OK. The fact that it dropped to 2.45 VAC when plugged in should be disconcerting. That's the heater supply. The fact that you get hum suggests that the tubes are probably heating up. But they wouldn't with only 2.45VAC. Was there hum with the tubes pulled?
All of that looks good. Honestly, I don't see low hanging fruit here, so I'm not sure where to take this adventure.
Nuuska
Enlightened Member
Hello - back from lake . . .
"Volts across the green wires, unplugged from board = 6.25 VAC , Plugged into board = 2.45 vac"
Besides obvious tube filaments you have C39 & C40 plus R43 & R44 before rectifier D2-8-9-10 - after that C37 & C38
I would first disconnect one or both end - whatever is easiest - of C39 & C40 - measure filament voltage - if no progress - remove rectifier D2-8-9-10 - if progress - check wether it is the rectifier diodes or the following caps C37 & 38
A remote possibility remains with resistors in filament circuit
Over and out
EDIT - before you start pulling components out - one more thing - plug in ONLY the filament voltage green cables to see if the 6,3V crashes to 2,4 - if yes - then remove all tubes and measure again - if yes - then it must be where I wrote - if no - one of the tube filaments is shorted partially.
Volts across red wires 168VAC worries me - in schematics it says 260VAC - and I'm using this:
"Volts across the green wires, unplugged from board = 6.25 VAC , Plugged into board = 2.45 vac"
Besides obvious tube filaments you have C39 & C40 plus R43 & R44 before rectifier D2-8-9-10 - after that C37 & C38
I would first disconnect one or both end - whatever is easiest - of C39 & C40 - measure filament voltage - if no progress - remove rectifier D2-8-9-10 - if progress - check wether it is the rectifier diodes or the following caps C37 & 38
A remote possibility remains with resistors in filament circuit
Over and out
EDIT - before you start pulling components out - one more thing - plug in ONLY the filament voltage green cables to see if the 6,3V crashes to 2,4 - if yes - then remove all tubes and measure again - if yes - then it must be where I wrote - if no - one of the tube filaments is shorted partially.
Volts across red wires 168VAC worries me - in schematics it says 260VAC - and I'm using this:
Last edited:
fronobulax
Bassist, GAD and the Hot Mess Mods
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I am just lurking here but I am sure capnjuan is watching this thread with approval.
JohnW63
Enlightened Member
I am wondering about the health of the power transformer, if it shows low levels at the plug without having a load on it.
JohnW63
Enlightened Member
No tubes = no hum
JohnW63
Enlightened Member
The green and red wires are all in one plug. I don't have the little tube like tool to slide in and depress the tang to remove the wires from the plug. I have one at my Dad's. I can get it on Saturday and see what those voltages are.