Vox AC15 TBX - burnt resistor cause?

[Intripped]

Hello,
I'm servicing this amp and I've found one resistor burnt on the output jacks' PCB.
It's the 10 Ohm resistor to ground, in parallel with a 220nF capacitor:
R303 and C301, respectively; please see attached schematic.

I'm wondering what could have been causing this.

The owner reported me volume jumps and some strange random noises, kind of "bzzzzz" that last few seconds and then disappear.
I initially thought the tubes, but then found this burnt res and started mumbling...
Could you please give me some help?

[nevetslab]

R.G might have the best explanation on what causes the Signal Ground to Chassis Ground components to fail. That's what R303 and C301 are between. Signal Ground in this case is common with all of the amplifier's circuit grounds, and Chassis Ground is where the AC Mains Grounding wire attaches. It varies from product to product with how the grounding of the circuits are made to the chassis. Some products just used a 10 ohm resistor, yours adds a 220nF/63V cap across it, other Vox AC15's add a pair of diodes to that network, each in opposing directions, others have used just a 100nF/500V ceramic disc cap. The idea is to provide an RF path between the circuit ground and earth ground (chassis). The idea is to make the metal chassis a earth-grounded shield around the circuit within, while providing a low audio impedance path to that ground, while NOT making a ground loop.

I've seen the 10 ohm resistors burn, seen the capacitors fail, and never fully understood the mechanism that causes the failure. Lightning can do damage to that for sure.

The Signal Ground is a Floating Ground in these cases.....can float above Chassis or Earth ground. The inclusion of the diodes across the network are to prevent that signal ground potential not to exceed one diode drop (0.6V in either direction, after which they conduct). Without the diodes, if the potential floats high enough, it will burn the resistor.

[alexradium]

the second output is a line out or a power attenuated speaker out?

[Jazz P Bass]

R.G might have the best explanation on what causes the Signal Ground to Chassis Ground components to fail. That's what R303 and C301 are between. Signal Ground in this case is common with all of the amplifier's circuit grounds, and Chassis Ground is where the AC Mains Grounding wire attaches. It varies from product to product with how the grounding of the circuits are made to the chassis. Some products just used a 10 ohm resistor, yours adds a 220nF/63V cap across it, other Vox AC15's add a pair of diodes to that network, each in opposing directions, others have used just a 100nF/500V ceramic disc cap. The idea is to provide an RF path between the circuit ground and earth ground (chassis). The idea is to make the metal chassis a earth-grounded shield around the circuit within, while providing a low audio impedance path to that ground, while NOT making a ground loop.

I've seen the 10 ohm resistors burn, seen the capacitors fail, and never fully understood the mechanism that causes the failure. Lightning can do damage to that for sure.

The Signal Ground is a Floating Ground in these cases.....can float above Chassis or Earth ground. The inclusion of the diodes across the network are to prevent that signal ground potential not to exceed one diode drop (0.6V in either direction, after which they conduct). Without the diodes, if the potential floats high enough, it will burn the resistor.

The circuit as described is at the input jack/ preamp schematic.
AC 15 pre.pdf

Alexradium may be correct.

It looks like a line out circuit.

Edit: what a POS, confusing, crappy schematic.

[Dave H]

Edit: what a POS, confusing, crappy schematic

It's not that good

[Jazz P Bass]

Yup.

It's a line out jack.


What took out the resistor?
Maybe something 'bad' was plugged into the jack.

Edit: I pulled up the manual and the jack area was all black.
You could not identify or see the jacks.
(POS manual too.)

[nevetslab]

The circuit as described is at the input jack/ preamp schematic.
[ATTACH]50484[/ATTACH]

Alexradium may be correct.

It looks like a line out circuit.

Edit: what a POS, confusing, crappy schematic.

I had the AC 15 Service manual out, and the other side of the network, here labeled W7.....is Chassis Ground. That network and many like it are typically the RF bypass network between the 'floating Ground and Chassis/Earth Ground, assuming the other end of the power cord gets to earth. And yes....the one side where R303 and C301 are connected to ground at IS Ground....the system ground, but not tied to Chassis/Earth ground.

[Intripped]

Thanks for the second output clarification, I was wondering the exact function, since there are no labels on the amp.

In the meanwhile I've replaced the resistor and tomorrow I'll test the amp.
Anyway, it was working (almost) well without that resistor... do you guys know if this missing ground reference could have been the cause of the volume jumps and random buzzzz? Or should I make further investigations? (Worn out tubes, oxidized sockets, ...)
I've already made a careful visual inspection on all the amp - components, solder joints, connectors... everything looks ok.
I've also cleaned all the pots since they were crackling a bit.

[nevetslab]

The RF Grounding network, to which the 10 ohm resistor is associated, wouldn't be related to the random volume jumps. Has the random buzz occurred at the same time as the volume jumps? When I've chased random volume changes on the Vox AC15 or AC30 models, it usually turns out to be a solder connection, sometimes well buried from access just to spite ya.

What caused the 10 ohm resistor to burn? Significant voltage across it....AC voltage. Did the two diodes survive, or were there diodes across the network? We've seen a couple different variations of this circuit, one with, one without them. Are you using a pedal board? Those are normally powered with a small AC-powered DC supply to run the effects pedals, and could perhaps be the source of a different voltage sitting on signal ground, which then gets plugged into the amp, where there is the network from it's signal ground to earth ground. Not really sure...just thinking aloud.

[Intripped]

No diodes on this AC15.
Not sure about the buzz and volume loss simultaneity, I've only reported what the owner told me; when I tried the amp, I did not experience any problem, but I played it for 10 minutes only.
Thanks for all the hints, tomorrow I'll try the amp and report back.

[g1]

The diodes they are talking about are at R25 on the preamp schematic.
There is some confusion here about which resistor burnt. From post #1 it is R303 on the schematic in post #1. (left hand side labelled jack pcb).
I'm pretty sure W302 & 301 are hot and common of OT secondary, but am not sure what W304 connects to.

[nevetslab]

Floating Ground to Chassis Ground network on Vox AC15 Schematic

I've attached the schematic from an AC15HTVH which shows the RC & diodes network that we've been discussing. It shows the Signal Ground side that we've all seen, and the other end is what I'm used to seeing with a network like this.

AC15HTVH Schematic from Service Manual.pdf

Here the schematic designation #'s are R40, C27, D1 & D2

[alexradium]

since the amp has no feedback,i would omit that resistor/capacitor network and connect the speaker straight to ground.
Give it a try,if it doesn't buzz its okay.
Otherwise put a 5w resistor and a couple back to back 1N4007 and you're safe.

[Enzo]

Those two parts are the common to earth connection. What probably happened was that something with a hot chassis was plugged into this amp, and the 10 ohm resistor was the path to ground for that hot chassis current. The remote unit with current on its chassis came through the shield of a cord, then found its way to earth through the resistor.

It is drawn with the speaker connections, but it really isn't involved with the speaker.

[Intripped]

The diodes they are talking about are at R25 on the preamp schematic.
There is some confusion here about which resistor burnt. From post #1 it is R303 on the schematic in post #1. (left hand side labelled jack pcb).
I'm pretty sure W302 & 301 are hot and common of OT secondary, but am not sure what W304 connects to.

Yes, the burnt resistor is R303 on the schematic in post #1; W302 & W301 are hot and common of the OT secondary.
W304 is the connection to chassis/earth. So this res + cap network is the earth connection for the OT sec and the speaker ONLY.

I've checked in the amp, and found ALSO the other network with R25, C24, D1 and D2. This network is placed immediately after the input jacks, and it's the earth connection for all the amp circuit (except the OT sec and speaker). It has no signs of damage.

I've not tried the amp yet. I'll let you know later