AC30C2 help

[shawn w]

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

[sluckey]

Burned resistors usually indicate something else has failed. Can you provide a schematic? What are the amp symptoms?

[tubeswell]

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?)

[sluckey]

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.

[kagliostro]

Doug has the schematic

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

[PRR]

It is hard to see how this R108 could burn. Some other resistor should burn first.

[tubeswell]

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?

[shawn w]

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

[shawn w]

http://i1297.photobucket.com/albums/ag21/shawn323232/20170503_175109_zps91llcfjb.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 

[shooter]

Someone "flip" J31?, insert tubes 1 pin Off?, spill Beer?

[kagliostro]

...... I couldn't figure out how to get one sized right for here

You can upload it here 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





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





Franco

[shawn w]

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}

[tubeswell]

With the power transformer on, are you getting any AC voltage across each pair of PT secondaries?

[pompeiisneaks]

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

[shawn w]

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

[PRR]

> 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.

[shooter]

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:)

[shawn w]

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

[shawn w]

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

[sluckey]

Replacing the ones that burned up? 1/2 watt will be big enough provided the weird problem that burned then is gone.

[shawn w]

Thanks. 1/2 watt it is.
shawn

[shawn w]

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

[tubeswell]

...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

[shawn w]

...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

[shooter]

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

[shawn w]

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

[shawn w]

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

[shooter]

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)

[shawn w]

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

Hopefully this gets all the pics.
Thanks
shawn

[shooter]

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

[shawn w]

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

[shawn w]

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

[shooter]

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 
the green light might be staying on because the caps haven't discharged 

plug in all the tubes EXCEPT the EL-84's and see if you sacrifice to the fuse god

[shooter]

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

[shooter]

OK, I didn't, looks like R should be B based on the R values below it, go, blow a fuse 

[shawn w]

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 
the green light might be staying on because the caps haven't discharged 

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

[shooter]

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 

[shawn w]

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 

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

[shooter]

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.

[shawn w]

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

[shooter]

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 

[Ed_Chambley]

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 
the green light might be staying on because the caps haven't discharged 

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.

[shawn w]

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 

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

[shawn w]

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

[shooter]

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

[shawn w]

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

[shawn w]

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

[shooter]

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 –    so it's a crapshoot! you should have 2 readings  measuring around 3.3k and 2 readings measuring ????  from each R113 and R116

[shawn w]

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 –    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

[shooter]

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?