Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shawn w on May 02, 2017, 11:09:03 pm
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Hi,
I was referred here for some possible help. I picked up a Vox AC30C2 amp super cheap, but needs some help. All fuses are good and both the standby and power lights come on. The R108 resistor is burnt. On visual inspection, it appears that this is the onky thing that's wrong. This goes to what i think is the G1 pin. I have limited experience with the hard electronics of a tube amp and diagnosis, but could it be that simple that this one resistor is bad ? Could it be a bias issue?
Thanks in advance for your advice,
Shawn
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Burned resistors usually indicate something else has failed. Can you provide a schematic? What are the amp symptoms?
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R108 is part of the buffer op amp circuit for the FX return. IC101 is probably cooked. If you jumper the FX send-return jacks with a short cable, what happens? (Does the amp work with a purely dry signal - incl reverb switched off?)
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I looked at that schematic. It's highly unlike that R108 in that FX loop burned. I suspect that is either not the correct schematic or R108 is not the correct part designator. Either way, we don't have enough info to do more than guess.
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Doug has the schematic
http://el34world.com/charts/Schematics/files/Vox/Vox_AC30C2.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_AC30C2.pdf)
As far as I can not find R108, in the scheme i see R107 and R109 but not R108
R108 is the grid stop resistor of V7
Franco
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It is hard to see how this R108 could burn. Some other resistor should burn first.
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I looked at that schematic. It's highly unlike that R108 in that FX loop burned. I suspect that is either not the correct schematic or R108 is not the correct part designator. Either way, we don't have enough info to do more than guess.
Ah right. There's one-too-many 'C's. Oops - Talk about Confusing LoL
If it was too much current through the control grid, then there could be something like a plate-to-grid (or screen-to-grid?) short in that output tube, or something messed up/shorted at the grid pins. I wonder if the screen resistor on that socket is also cooked/open?
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I want to thank you all for your responses. You have a great community here. I looked at that first print also, but couldn't see how this tied into the effects loop, so as looking a little more I found the second print posted. I'm sure it's that one.
Looking closer tonight after I took some photos (which I found out are too large to post here) I see that the other four corresponding resistors (R101,R81 and R61) are all bad too.
Thanks again,
Shawn
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http://i1297.photobucket.com/albums/ag21/shawn323232/20170503_175109_zps91llcfjb.jpg (http://i1297.photobucket.com/albums/ag21/shawn323232/20170503_175109_zps91llcfjb.jpg)
http://i1297.photobucket.com/albums/ag21/shawn323232/20170503_174910_zpsiue68bj5.jpg (http://i1297.photobucket.com/albums/ag21/shawn323232/20170503_174910_zpsiue68bj5.jpg)
Here's a couple links to photos. I couldn't figure out how to get one sized right for here.
Thanks
Shawn
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Someone "flip" J31?, insert tubes 1 pin Off?, spill Beer?
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...... I couldn't figure out how to get one sized right for here
You can upload it here http://imgur.com (http://imgur.com)
for free and without registration
then use the <image TAG> and insert the link of the image
the image TAG is the small icon with Mona Lisa you find under the Bolt TAG when you post a message, this icon (http://i.imgur.com/n0WOOIX.jpg)
(http://i.imgur.com/MOBNc4J.jpg)
(http://i.imgur.com/UY53ug7.jpg)
this way the image is shown on the browser correctly also if it is very big
there is also an easy way to show it smaller (amount at your choice), if you want
(http://i.imgur.com/MOBNc4J.jpg)
(http://i.imgur.com/MOBNc4J.jpg)
Franco
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Someone "flip" J31?, insert tubes 1 pin Off?, spill Beer?
Could be? I bought it like this, so who knows what the real story is. I have a grand total of $90 in it, so I figured for that at least I'd end up with two Greenbacks if nothing else!
Thanks for the tips on the photos, too.
Shawn
{edit- dangling /quote fixed --PRR}
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With the power transformer on, are you getting any AC voltage across each pair of PT secondaries?
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It almost makes me think someone did something stupid with the transformer wires and connected them up wrong or accidentally shorted something physically. I agree, looking at that schematic, I have no clue how those resistors would smoke! There are multiple other resistors in line between them and the PI side, and the other side is the tube. Could one tube somehow go bad so that it conducts full current through the input grid and passes that out through all the other input grids I guess? That seems unlikely, but I don't know all the failure modes of a tube.
~Phil
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With the power transformer on, are you getting any AC voltage across each pair of PT secondaries?
I'll have to check that. Maybe tonight I'll dig in a little deeper and see if anything jumps out at me. It almost makes me think someone did something stupid with the transformer wires and connected them up wrong or accidentally shorted something physically. I agree, looking at that schematic, I have no clue how those resistors would smoke! There are multiple other resistors in line between them and the PI side, and the other side is the tube. Could one tube somehow go bad so that it conducts full current through the input grid and passes that out through all the other input grids I guess? That seems unlikely, but I don't know all the failure modes of a tube.
~Phil
I wondered the same thing. My knowledge is a bit limited, so this is definitely a learn as I go experience.
Shawn
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> the other four corresponding resistors (R101,R81 and R61) are all bad too.
Too-too odd.
One end of these goes in the power bottle, and bad things can happen. G2 breaks loose and hits G1, fer example.
However that current has to go TO somewhere. All the paths I see are through 220K or more, or 10K and a cap. Even taking 3K in series with 10K, the 10K will get 3/4 of the voltage and burn-up 3 times faster. That's for same-size parts. I suspect the 10K is a small part and the 3K looks like a bigger part.
I could *imagine* a G2-G1 short in one tube and the usual G1-K conductance in its partner. That could give maybe enough current to toast both resistors (and both G1s). But for this to happen both sides of a quad? Ah, maybe after the first death the user swapped tubes around and invoked the same smoke on the other pair.
I would presume all four EL84s are "sick" until you prove otherwise. Replace the bad resistors and sniff for more. Install known-good EL84, maybe just one pair at a time. Keep the windows and doors open until you are sure the smoke has stopped.
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I could *imagine* a G2-G1 short
Ya, that's where my J31/J33 comment came in, poor design IMO. Crud under, swapped, arc'd, Tube failure that get's shared:)
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Thanks again for all the guidance and direction. I've got a whole set of tubes on the way, will pick up some new resistors a day go from there. I'll let you know if I find anything else.
Shawn
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So I'm going to pick up some resistors this weekend, but I can't seem to find a schematic that recommends the wattage rating? Any suggestions?
Thanks again,
Shawn
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Replacing the ones that burned up? 1/2 watt will be big enough provided the weird problem that burned then is gone.
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Thanks. 1/2 watt it is.
shawn
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Ok,
So I know it's been a while, but i finally got around to replacing those resistors. I put all new preamp and power tubes in it, put it on standby and fired it up. Power light comes on and the power tubes all glowed nicely. No burnt up resistors! So I switched off the standby. The standby light comes on. It made a humming noise and popped the F1 fuse.
So any suggestions as to where to look next?
Thanks
Shawn
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...So I switched off the standby. The standby light comes on. It made a humming noise and popped the F1 fuse...
Probably a shorted filter cap
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...So I switched off the standby. The standby light comes on. It made a humming noise and popped the F1 fuse...
Probably a shorted filter cap
Would that be C10,C20 and C58?
Thanks!
Shawn
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It should be obvious when you look, but if it didn't explode any or all the ones I circled in red. All the things I circled in blue need to be measured VDC to ground once you quit blowing fuses
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It should be obvious when you look, but if it didn't explode any or all the ones I circled in red. All the things I circled in blue need to be measured VDC to ground once you quit blowing fuses
Thanks! I'll check those out tomorrow.
shawn
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So I checked and didn't see anything obviously bad or burnt. I took a few pics. Maybe someone see' s something I don't. Thanks again for the input!
shawn
https://imgur.com/gallery/AKTaRFz (https://imgur.com/gallery/AKTaRFz)
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the pic that shows 3 ecc83, 2 caps and a rectifier, is that just grainy image, or is the board that grubby? If it's that grubby, the dust/dirt can create nice arc potential.
Is it still popping fuses with tubes in?, with all tubes out?, IF so, OHM (NO POWER) each of the blue circles to ground, they each should be many hundred or thousands ohms "charging" (increasing in ohms)
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the pic that shows 3 ecc83, 2 caps and a rectifier, is that just grainy image, or is the board that grubby? If it's that grubby, the dust/dirt can create nice arc potential.
Is it still popping fuses with tubes in?, with all tubes out?, IF so, OHM (NO POWER) each of the blue circles to ground, they each should be many hundred or thousands ohms "charging" (increasing in ohms)
I didn't try it with tubes out. So I should take all the pre and power amp tubes out, turn it on and turn off the standby and see if the fuse goes?
I'm having trouble posting the photos. Yes , it is dusty lol.
http://imgur.com/a/AKTaRFz (http://imgur.com/a/AKTaRFz)
Hopefully this gets all the pics.
Thanks
shawn
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and see if the fuse goes?
yup. the tubes are the main source for current draw, without them in the fuse should live long time.
IF the fuse blows without tubes, unplug, NO power, and ohm to ground the blue circles
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and see if the fuse goes?
yup. the tubes are the main source for current draw, without them in the fuse should live long time.
IF the fuse blows without tubes, unplug, NO power, and ohm to ground the blue circles
Perfect. I'll give that a shot when the new fuses show up. Thanks again for your help.
shawn
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and see if the fuse goes?
yup. the tubes are the main source for current draw, without them in the fuse should live long time.
IF the fuse blows without tubes, unplug, NO power, and ohm to ground the blue circles
Ok, so new fuse in it, pulled all the new tubes and powered it on. Got the red on light. Turned the standby off and got the green light. I left it on for quite a while without any fuse failures. With the standby off, you can hear a slight hum from the speaker. Very slight. Turn it on standby and the hum stops. Interesting note, when I turn it on standby, the green light stays on. When it's initially powered up, the green standby light is only on when the standby is turned off.
So where to now? Lol
Thanks!
shawn
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So where to now? Lol
I'll take a look at the pwr/stby, it would help me saying something like standby switch open, (or closed), standby on/off messes me up :laugh:
the green light might be staying on because the caps haven't discharged :dontknow:
plug in all the tubes EXCEPT the EL-84's and see if you sacrifice to the fuse god
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Homework, on page 3 of my schematic just below D13 is a reference to R+2, try and find the other end
of R+2 on your copy, I gave up
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OK, I didn't, looks like R should be B based on the R values below it, go, blow a fuse :laugh:
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So where to now? Lol
I'll take a look at the pwr/stby, it would help me saying something like standby switch open, (or closed), standby on/off messes me up :laugh:
the green light might be staying on because the caps haven't discharged :dontknow:
plug in all the tubes EXCEPT the EL-84's and see if you sacrifice to the fuse god
You're correct on the standby light. If I left it sit a bit, the caps discharge and the light goes out. I know what you mean about standby on/off. I was trying to figure out the best way to word it lol.
So with all 3 preamp tubes in, no blown fuse. I powered it up 3 times and nothing blew, caught fire or exploded.
shawn
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go look at reply 24, measure the blue circles, clip your - lead to chassis, put one hand in your pocket, measure, record and post the VOLTS DC.
there's not much load, but enough to spot something :dontknow:
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go look at reply 24, measure the blue circles, clip your - lead to chassis, put one hand in your pocket, measure, record and post the VOLTS DC.
there's not much load, but enough to spot something :dontknow:
So I'm a bit of a newb at this. Am I following the trace on the board to where the plug is and.checking there? What kind of voltage should I see? I'm probably going to need a new multimeter.
shawn
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you might want to hunt up an experienced buddy to help out then, your DCV should be stable, north of 300vdc at B+1, dropping as you move down the line. the caps circled in red, positive side, is a great place to get each voltage.
and ya, make sure we don't whined up chasing a bad meter.
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you might want to hunt up an experienced buddy to help out then, your DCV should be stable, north of 300vdc at B+1, dropping as you move down the line. the caps circled in red, positive side, is a great place to get each voltage.
and ya, make sure we don't whined up chasing a bad meter.
Ok. New multimeter will be here tomorrow and I'll check those. I do have some experience, I just want to make sure I'm checking the right spots. Getting to the back side of that board is going to be a pain, lol.
Thanks!
Shawn
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I've worked on 2 of those, one I fixed, one was to damaged to even bother, working the amp is fine, but it's a POS from a quality assembly point of view.
you should be able to find a place on the parts side to measure, look at the schematic page 3, find the Resistor that goes to the cap, like R120, that gets you both B+1 and B+2, R74 gets B+3............
moving the board a lot will only invite gremlins in, we want to avoid them, or chase them away :icon_biggrin:
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So where to now? Lol
I'll take a look at the pwr/stby, it would help me saying something like standby switch open, (or closed), standby on/off messes me up :laugh:
the green light might be staying on because the caps haven't discharged :dontknow:
plug in all the tubes EXCEPT the EL-84's and see if you sacrifice to the fuse god
I prefer the references AC switch and DC switch when Standby is present. This eliminates me trying to clarify.
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I've worked on 2 of those, one I fixed, one was to damaged to even bother, working the amp is fine, but it's a POS from a quality assembly point of view.
you should be able to find a place on the parts side to measure, look at the schematic page 3, find the Resistor that goes to the cap, like R120, that gets you both B+1 and B+2, R74 gets B+3............
moving the board a lot will only invite gremlins in, we want to avoid them, or chase them away :icon_biggrin:
By looking at the schematic, that's what I was thinking so thanks for confirming my thought. I'll let you know what I find.
shawn
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So with new tester I hand I checked for voltages at the resistors listed below. It changes slightly from one side of the resistors to the other, but nothing more than a couple volts. This is what I saw-
R74 - 385v
R22 - 380v
R15 - 379v
R106 - 28-34v (left and right sides)
R103 - 29.5-35v (left and right sides)
R120 - 385v
It took me a minute to figure out R120. There's a callout spot on the board, but nothing there. Not even holes where it would be. I figured out the resistor was on the other side of the board, with just the solder points showing lol.
Thanks,
shawn
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stay on page 3, verify that to 50 ohm cathode R is close to 50, ohm out the 470 R's that go to the screen grid r79, r95..... If they ohm good, pick your favorite fuse, put it in, put in the PA tubes and knock on wood, throw the switch
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stay on page 3, verify that to 50 ohm cathode R is close to 50, ohm out the 470 R's that go to the screen grid r79, r95..... If they ohm good, pick your favorite fuse, put it in, put in the PA tubes and knock on wood, throw the switch
Perfect! I'll give that a shot and let you know what I find.
shawn
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stay on page 3, verify that to 50 ohm cathode R is close to 50, ohm out the 470 R's that go to the screen grid r79, r95..... If they ohm good, pick your favorite fuse, put it in, put in the PA tubes and knock on wood, throw the switch
So I checked the resistors. Here's the results-
R79 - 466
R95 - 466
R104 - 466
R117 - 465
R119 - 51.5
So I put a new fuse and lit the power switch. All tubes glowed nicely. I moved the stand by switch to rock n roll and...... loud hum. Popped the fuse.
These are new power tubes. I haven't tested them individually. Should I try one tube at a time to see if I can isolate the issue?
Thanks again so much for your time and help!
shawn
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Look on page 1, see the “points” OP + and OP - (top of R113, and bottom of R116)
Go to page 3, see where they come in, through J31, where your original R’s were toast.
Got it, pull all the power tubes, clip 1 meter lead at R113, then, measuring ohms (NO power), put the other probe in pin 2 socket, tube side of V4, record value, repeat for V5, 6 and 7
Then repeat using R116, all 4 tubes pin 2
NOTE the schematic, page 3 shows 2 OP – :cussing: so it's a crapshoot! you should have 2 readings measuring around 3.3k and 2 readings measuring ???? from each R113 and R116
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Look on page 1, see the “points” OP + and OP - (top of R113, and bottom of R116)
Go to page 3, see where they come in, through J31, where your original R’s were toast.
Got it, pull all the power tubes, clip 1 meter lead at R113, then, measuring ohms (NO power), put the other probe in pin 2 socket, tube side of V4, record value, repeat for V5, 6 and 7
Then repeat using R116, all 4 tubes pin 2
NOTE the schematic, page 3 shows 2 OP – :cussing: so it's a crapshoot! you should have 2 readings measuring around 3.3k and 2 readings measuring ???? from each R113 and R116
So here's what I measured, going left to right (4-7)
R113 - 3.2, 3.2, 5.5, 5.5
R116 - 5.5, 5.5, 3.2, 3.2
Thanks,
shawn
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forgot last night to have you ohm all the PA tubes pins to ground also, so put the - on chassis, and ohm each socket pin, tube side, to ground. something in your PA is drawing way more current than design spec'd.
do you have a 5k 10 watter, maybe 2 10k 5watters you can put in parallel laying around? how bout 20 - 100uf 400+vdc caps?
once you get those measurements, put all 4 tubes back in (NO power) and ohm from B+1 to ground (right side of R120 pg3)
***EDIT*** make sure you indicate ohms, K ohms, Meg ohms, guessing your 3.2 is really 3.2K?
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forgot last night to have you ohm all the PA tubes pins to ground also, so put the - on chassis, and ohm each socket pin, tube side, to ground. something in your PA is drawing way more current than design spec'd.
do you have a 5k 10 watter, maybe 2 10k 5watters you can put in parallel laying around? how bout 20 - 100uf 400+vdc caps?
once you get those measurements, put all 4 tubes back in (NO power) and ohm from B+1 to ground (right side of R120 pg3)
***EDIT*** make sure you indicate ohms, K ohms, Meg ohms, guessing your 3.2 is really 3.2K?
Yes, measurements are in K. I'll check all those pins and let you know what they read. I don't have any resistors oe caps like that, but I'll pick some up.
shawn
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I'll pick some up.
everything we've done so far is "on the cheap" :laugh: If you have a local place, try for a 5K 10 to 20w, 20w preferable (2 10k 10watters works also). I use one for PS load testing.
hopefully you're having fun :icon_biggrin:
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I'll pick some up.
everything we've done so far is "on the cheap" :laugh: If you have a local place, try for a 5K 10 to 20w, 20w preferable (2 10k 10watters works also). I use one for PS load testing.
hopefully you're having fun :icon_biggrin:
I ordered (2) 5k 10 watters and (6) 100uf 600v capacitors. Should be here Weds. Yes, i am having fun and learning a lot. I'm a bit of an information junkie, so this is right in my wheelhouse. Thanks again for your patience and expertise.
shawn
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forgot last night to have you ohm all the PA tubes pins to ground also, so put the - on chassis, and ohm each socket pin, tube side, to ground. something in your PA is drawing way more current than design spec'd.
do you have a 5k 10 watter, maybe 2 10k 5watters you can put in parallel laying around? how bout 20 - 100uf 400+vdc caps?
once you get those measurements, put all 4 tubes back in (NO power) and ohm from B+1 to ground (right side of R120 pg3)
***EDIT*** make sure you indicate ohms, K ohms, Meg ohms, guessing your 3.2 is really 3.2K?
I checked those pins on all four tube sockets and here's the results-
Pins 1,6&8 i didn't get any reading from at all
Pins 7 & 9 the results were moving all over. I couldnt get a steady reading
2 - .186
3 - 51.9
4 - 108.5
5 - 108.5
Thanks
shawn
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2 - .186
:dontknow: Ohms, kilos, psi ?
Re-measure 7 & 9, and watch close, the #'s should be raising, then maybe max out at something close to open.
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2 - .186
:dontknow: Ohms, kilos, psi ?
Re-measure 7 & 9, and watch close, the #'s should be raising, then maybe max out at something close to open.
Checking today #2 settled in about .256m ohms. All #7 pins went to ol, except tube #6. It stopped at about 111k ohms. Pin number #9 did the same. All stopped at about 111k ohms.
shawn
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measure;
pin 3 to ground on all 4 ( I know, we did it, just once more )
Find J29, 35, and 36 connected to OT, take a pic, draw a pic, then unplug 'em, and ohm the CT (where j35 went) to each "end" (where 29 and 35 36 went)
did your resistor come in?
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measure;
pin 3 to ground on all 4 ( I know, we did it, just once more )
Find J29, 35, and 36 connected to OT, take a pic, draw a pic, then unplug 'em, and ohm the CT (where j35 went) to each "end" (where 29 and 35 36 went)
did your resistor come in?
Pin 3 measured 51.5-51.6 ohms. Just to be clear, I'm unplugging those terminals at the board and measuring resistance from one terminal on the board to the other? So clamp on J35 spade and touch on 29 and 36?
Thanks
Shawn
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Just to be clear
you're measuring the transformer those wires go to, one lead on CT, then the other measures each end, in a good world they should be about equal in value.
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Just to be clear
you're measuring the transformer those wires go to, one lead on CT, then the other measures each end, in a good world they should be about equal in value.
So measuring from the J35 wire to J36 wire, get 17.3 ohms. From J35 wire to J29 wire I get 17.5 ohms. I haven't been to the post office, but I'm sure my caps and resistors are there.
shawn
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get 17.3 ohms
I went hunting for "typical DCR" because 17.3ohms seemed low, about right for hummm pop, :dontknow:
from these 2 threads and notes I have 100 to 400 seems a "normal" range, anyone wanna weigh in?
http://el34world.com/Forum/index.php?topic=14479.msg137295#msg137295
http://el34world.com/Forum/index.php?topic=18909.msg194096#msg194096
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get 17.3 ohms
I went hunting for "typical DCR" because 17.3ohms seemed low, about right for hummm pop, :dontknow:
from these 2 threads and notes I have 100 to 400 seems a "normal" range, anyone wanna weigh in?
http://el34world.com/Forum/index.php?topic=14479.msg137295#msg137295
http://el34world.com/Forum/index.php?topic=18909.msg194096#msg194096
I think you may be correct. I did some looking, too and most are 100+ ohms. That and the variance from one side to the other seems to be larger than a couple tenths. Maybe a dead short in it?
shawn
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Try this...
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Try this...
I build one of those and give it a shot.
Thanks
shawn
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So I built the test light. I went from J35 to J29, J35 to J36 and even J36 to J29 and didn't get any flashes or light out of the bulb. I did get a noticeable pop out of the speaker each time it was connectected. So bad transformer?
Thanks,
shawn
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So bad transformer?
Until proven otherwise, I think you're correct.
popin fuses means "short" They can be easily found with enough current and voltage :icon_biggrin:
the more better way is to use ohms, not enough ohms indicates lots of current, without really plugging it in, make sense ? You happen to have an inductor problem, which doesn't play quite the same, so extra testing
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So bad transformer?
Until proven otherwise, I think you're correct.
popin fuses means "short" They can be easily found with enough current and voltage :icon_biggrin:
the more better way is to use ohms, not enough ohms indicates lots of current, without really plugging it in, make sense ? You happen to have an inductor problem, which doesn't play quite the same, so extra testing
Awesome! So a new transformer it is. Thanks to everyone for working through this with me.
So the next question is, what brand of transformer? There seems to be quite a few different ones available. If I want that "older" A.C. chime, is there one preferred?
shawn
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So I built the test light. I went from J35 to J29, J35 to J36 and even J36 to J29 and didn't get any flashes or light out of the bulb. I did get a noticeable pop out of the speaker each time it was connectected. So bad transformer?
Read the instructions for that tester. You cannot have speakers connected while doing this test!
Keep in mind there are a few sneaky things that can invalidate your test. If there is a load on any
winding, those loads will eat the inductive kickback and the neon will not light up. You’ll think you
have a bad transformer but you might not. If the transformer is a power transformer, watch out for
connections to filament windings or solid state rectifiers. If it’s an output transformer, make sure the
speaker is not connected and there is no minimum load resistor on the output jack or diode from plate
to ground, as is found in some amps.
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noticeable pop
:think1: :BangHead: :cussing:
It was late, I was ................. :cussing:
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noticeable pop
:think1: :BangHead: :cussing:
It was late, I was ................. :cussing:
So I did the test again, with no speakers hooked up. Same results. No flash, no light.
Thanks
Shawn
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Same results
Cool, then you gotta decide where next. there are still a few "free" things, like verifying preamp DC volts and AC signal, if that plays out it makes the tranny cost/benefit scale tip :laugh:
(btw, I was kicking myself for missing a nugget in reply #68)
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Same results
Cool, then you gotta decide where next. there are still a few "free" things, like verifying preamp DC volts and AC signal, if that plays out it makes the tranny cost/benefit scale tip :laugh:
(btw, I was kicking myself for missing a nugget in reply #68)
My inclination is that it's the tranny. Low ohm reading and no light from the test would indicate to me that it is bad. I'm going to order a Classic Tone and put it in. If you feel that we should check the other things, too great. I'm all for learning more!
Oh, don't kick yourself. I read the instructions, but in my limited experience misinterpreted what it said. It's why I always try and provide as much information as I can. So someone can catch me! (;
shawn
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New transformer in. Same problem. I lit the main power, all power tubes glow lightly. Turn the standby switch to rock n roll, light comes on, hum from the speakers and it pops the fuse. Anymore ideas?
Thanks again for all the help.
shawn
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It's been a while on this thread, but to me that sounds like something is drawing too much current to ground. You've got some kind of short. Have you tried connecting a multimeter to ground on continuity mode and touched all of the power rail and tube anode areas to ensure nothing in the power side has a direct route to ground?
also photos of things as they are now will make some of this troubleshooting a bit easier.
~Phil
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Same problem
sucks!
did you get a 5k 10w or bigger resistor?
If so;
remove R120 in the PS, page 3. you want to put the 5K in parallel with c72. You can do this by soldering a temp wire in the "right" hole were R120 came from, then clip one side of the 5k there, the other to end clipped to ground.
set your meter to Volts DC, clip the leads across the 5K. when you're sure of the setup, power up, get a quick reading, don't leave power on for more than a minute.
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It's been a while on this thread, but to me that sounds like something is drawing too much current to ground. You've got some kind of short. Have you tried connecting a multimeter to ground on continuity mode and touched all of the power rail and tube anode areas to ensure nothing in the power side has a direct route to ground?
also photos of things as they are now will make some of this troubleshooting a bit easier.
~Phil
We checked all the tube posts to ground. Those results are posted earlier. I'm not sure if we checked the power rail? What would you like to see photos of? I'd be happy to take some more.
Thanks
shawn
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Same problem
sucks!
did you get a 5k 10w or bigger resistor?
If so;
remove R120 in the PS, page 3. you want to put the 5K in parallel with c72. You can do this by soldering a temp wire in the "right" hole were R120 came from, then clip one side of the 5k there, the other to end clipped to ground.
set your meter to Volts DC, clip the leads across the 5K. when you're sure of the setup, power up, get a quick reading, don't leave power on for more than a minute.
Yes, I did get some resistors and capacitors you mentioned earlier. I got 10watt 5k and 100uf 400v. I'll give that a shot and report the results.
shawn
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Ok here's an idea, it will cost another fuse, but put your multimeter on volts dc, and then connect to one of the resistors that burned, like R101, that you've replaced. Set the DMM to 'max' so it remembers the max voltage, and turn it on. If the problem is still there, it may show you if that area is still getting a voltage level it shouldn't. It should have 0 dc as it's a cathode biased amp, and won't have negative bias. If it has anything there, something is leaking voltage towards it. Also another idea would be to put a second one on the same 'max' setting but VAC and see if there's any ac on the line, (there should be some in an active amp, as signal, but this is test mode, so I'm guessing you don't have any.) If you have a variac, it would make this a bit easier as you could keep the voltage lower so it's not drawing enough current to burn things up and blow the f use, but test for voltages around the board and report back what you're seeing.
If that still doesn't help, you may have to think about gutting and building a hoffman AC30 in there :D
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Hi,
It's been a minute or two. I stuck this project in the corner and left it there for a bit. About a month ago I thought I should do something with it, so I took it to Matthew Dawson in Portland,OR to have him look at it. What an amazing guy. He found a trace on the board that was arcing across, causing the whole issue. Some Dremel work to cut it out, little old school hand wiring and its works better than new! Thanks to everyone who contributed with ideas.
Shawn
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It would be helpful for everyone if you could get a final picture of where the trace arc'd across and then made a note on the schematic and post that too. This would help everyone to see/know what happened where plus this might just happen to more of these amps down the line and could be a real time saver? I know I'm curious...
ps - these are some of the things that just can't be seen here troubleshooting remotely. But in the hands of many here likely it would've been spotted (perhaps almost immediately)? Visual & manual inspections cannot be overstated enough to the trained eye.
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(https://imgur.com/gallery/KwNPcnL)
I agree totally. According to Matthew, this "could" be a common problem because the traces are so close together. Hopefully my photo shows enough to make heads or tails of it.
Shawn
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Did you forget something? :icon_biggrin:
https://imgur.com/gallery/KwNPcnL
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I've been trying to get it to post lol. Thanks!
Shawn
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