Vox AC15C1 blowing HT fuses-need official schematic and advice

[cgoepel]

I have a customer's VOX AC15C1 which blows the HT fuse. I have the Inkimodo.Dragon 5 page schematic for reference. There is a 37 ohm short (read on my Fluke 77 meter) from B+ to the 6 volt heater circuit. The ground referencing resistors, R90 and R91 are burnt. I have removed the tube board and also the main board and I am checking everything I can think of with no success.
I temporarily disconnect R90 and R91, add a current meter to the HT fuse slot and read about 350 mA. I measure from the ground connection of the tube board to chassis ground and get 400+ volts. If I attempt to ground the tube board to the chassis as intended with a mounting screw, R11 and R12 start to burn. BTW, I have removed all tubes from the circuit, 3-12AX7 and 2-EL84's. The unintended connection or short between B+ and the grounded side of the 6 volt transformer connection reads 37 ohms. I have checked all diodes and capacitors for proper function. What little hair I have left is slowly dropping to the floor. I could use a real Vox schematic with all the connector numbers, and wire colors. Any help from the seasoned members is much appreciated. Thanks in advance

[g1]

Not sure what you mean by "grounded side of the 6 volt transformer", as neither side should be grounded?
Have you checked for shorts in BR1 ?
The first thing I would probably look at is disconnecting the 6V transformer winding and checking for shorts from it to the outside of the transformer or to the HV winding.

[Jazz P Bass]

Quote: "There is a 37 ohm short (read on my Fluke 77 meter) from B+ to the 6 volt heater circuit. The ground referencing resistors, R90 and R91 are burnt"

I would look for a bunt output tube socket.
It is one of the places a B+ to heater short circuit can occur.

Quote: "I measure from the ground connection of the tube board to chassis ground and get 400+ volts. If I attempt to ground the tube board to the chassis as intended with a mounting screw, R11 and R12 start to burn. "

The B+ to heater short may be causing this.

Be aware that once R11 & R12 burn open, the issue may now be masked.

[cgoepel]

G1 - Thanks,
1.The schematic shows one side of the heater winding of the power transformer being grounded.
2. Yes, I checked the Bridge Rectifier and it is good.
3. I agree with you, I also checked each individual winding of the PT. Each is functioning properly. HT winding is 630 VAC, Heater Winding is 6 VAC and the Op Amp supply is 30 VAC. There is no connection between any of the secondary winding.

[cgoepel]

Jazz P Bass - Thanks for your input.
1. I will check the tube sockets again. The "short" seems to be on the main board.

[pdf64]

There is a 37 ohm short (read on my Fluke 77 meter) from B+ to the 6 volt heater circuit.

Was that with the tubes in place? I'm thinking that an EL84 may have developed a short.

[Jazz P Bass]

Yeah, an EL84 B+ short to the ground pin would duplicate the symptoms.

[Tom Phillips]

...The schematic shows one side of the heater winding of the power transformer being grounded....

One side of the low voltage heater winding for the IC supply circuit is grounded but neither side of the heater winding is grounded. Just making sure you keep that in mind while you troubleshoot so it doesn't confuse your troubleshooting process.

[cgoepel]

Sorry guys, the 37 ohm reading is with all tubes removed from the amp.

[Jazz P Bass]

Then you likely have a short on the printed circuit board.

[cgoepel]

Sorry folks, I misstated something. I said that one side of the heater circuit is connected to ground. I was wrong. As the schematic shows, there is no connection of the heater circuit to ground. The 37 ohm reading is from B+ to the non-fused side of the heater transformer connection. Also, the 37 ohm reading is with all transformer windings disconnected and both boards are out of the amp.

[Enzo]

OK, let's isolate the problem. You have 37 ohms from "B+" to "ground". Do you mean the B+1? The first filter cap node? And where are you making the ground connection with your meter?

Usually we probe with the pointed meter probes, but it is preferable to use clip wires. I often just clip wires to my probe tips if my dedicated clip wire probe wires are not handy. We want to get a real stable reading that is consistent. So we want to note if it is 37.1 or 37.9 ohms. Both are "37" but even half an ohm might help us. "ground" is all over the amp. So is B+. B+ might be at the center tap wire of the OT, or it might be at the filter cap lead, and so on.

When you got 37 ohms, assuming it was at the first filter, you needed to move the probe around and see if it was higher or lower at the plates of the power tubes. Is it higher at B+2? It looks like B+1 goes to the OT CT, R80, Four caps and two diodes. SO we want to go to the other end of each of those items and see if the resistance is higher to ground - it ought to be, but might not be.

SO let me assume the 37 ohms is lowest on B+1. clip a probe to B+1, now move to different grounded places all over the board. We probably get about 37 ohms. But look carefully, do we get a half ohm or an ohm lower or higher as we move around? The lower the resistance reading, the closer we are to the short. Likewise, select a good firm ground spot and clip a probe to it, now probe around the B+1 nodes, and along the traces, especially any point they come near the heaters - like the power tube sockets. Is there any spot that has a little lower resistance.

Hey, it may do nothing, but it MIGHT point us to an area for closer examination.

But before rushing off to do that, remember we are having a short through the heaters. So while we have 37 ohms to ground, what is the resistance from B+1 to any points on the heater string. And that list includes either end of the heater winding (Yes I know the transformer is disconnected), the heater pins of the power tubes, and also the ungrounded ends of R11, R12. We may have 37 ohms to ground, but that 37 ohms might be in the heater circuit rather than the B+ per se.

[cgoepel]

I finally found the problem. The trace which conducts the heater passes directly underneath C62. C62 leaked and caused a charred, yet conductive path from B+1 to the heater circuit. Thanks everyone who helped me solve this problem. Charlie