Vox VT50 - digital board power cycling

[jondoe]

Hi all,

I have an intermittent fault with a Vox VT50 whereby the digital PCB doesn't initialise, although sometimes it does . Sometimes it will drop out when it's working and powering it off and on again causes the power cycling problem to return.

The +-15 and +-34 rails appear to work fine, no signs of sagging/changes during malfunction. The 2.5 and 3.3v rails for the digital board are fed by the DC6.3v supply, measuring the voltage on the connector CN12 pin 8 I can see 7.9v, when the failure mode happens it drops to 6.3v. Occasionally when it's powered up and working a hum/buzz will start rising up and the observed reading on that pin drops down to around 6.5v, often restarting the unit keeps it stuck in this looping startup mode.

Is this likely to be a problem on the digital PCB voltage regulator pulling too much current causing the sagging? If I remove the 12AX7 which shares the circuit the voltage never sags and the digital board always initialises. As far as I can see the "6.3v" rail carries the heater for V2, the fan and the 2.5/3.3v supply, so one of these must be at fault.

Any advice gladly taken

Thanks.

[Jon Snell]

The main thing to find out is what voltages are on pins 4 & 5 of IC9. Check to see if the supply side is allowing the regulator to drop out, if it is I would look at C120 as if low value the DC voltage can have lots of ripple and upset the regulator.

[jondoe]

The main thing to find out is what voltages are on pins 4 & 5 of IC9. Check to see if the supply side is allowing the regulator to drop out, if it is I would look at C120 as if low value the DC voltage can have lots of ripple and upset the regulator.

Hey John,

At one point I did have the power board and digital board out and I checked C120 in my ESR meter, it was the correct capacitance and the ESR was low, so I popped it back, in hindsight it might have been a good idea to change it for a new one regardless. I checked for dry joints at the time, touched up any that looked rough. It sat on the bench all afternoon working fine, so I partially rebuilt it, tested some more, still fine, after 4 hours testing it went back together, at which point it started to malfunction again.

I'll extract the digital board tomorrow and check the outputs on the regulator and update this thread with what I find, hopefully it will malfunction and not keep working for hours again

[Jon Snell]

If it was fine out of the box, as it were, then it is probably a bad connection on one of the connectors.

[jondoe]

If it was fine out of the box, as it were, then it is probably a bad connection on one of the connectors.

I did suspect that, I've been over it with chop stick, everything has been given a firm nudge, poke, tap, components and connectors, I can't make the fault appear. I've tried some IPA here and there to cool and a hair dryer to heat it, still no dice. It's had a few firm thumps on the desk next to it and as last resort a few mid air jiggles, but it seems to be a schrodinger's fault, this morning it's working as it should.

I'll leave it on for a few hours, if it continues to work its going in the boot of the car for a long drive for maximum jiggles, don't worry I'm not going to leave it by the side of a busy A road

[jondoe]

Not observing the amplifier for some time has had the desired effect of bring the fault back.

In the fault condition the 2.5v rail of the IC9 voltage regulator is rock solid at 2.50v, the 3.3v fluctuates between 3.19 and 3.28 as the digital board constantly reboots itself and fails to start. The 5v rail on the other voltage regulator is also rock solid. The supply voltage to IC9 fluctuates between 5.7 and 6.9 during the fault condition.

Could this be caused by over current on the 3.3v rail? I guess I could lift the leg of the 3.3v rail and see if the supply fluctuates when left for a while.

[Jon Snell]

On some Yamaha digital mixing desks they use a regulator similar to the one in question and it fails due to being rated at 400mA with a 440mA current drain. The replacement regulator is rated at 500mA but I can't even find a manufacturers data sheet on your regulator so replacing the regulator may fix your issue or it may not.
I don't think the the supply dropping down to 5.7volts is an issue but I would put my external DC supply on the feed to the IC and see what happens. That will rule out the supply side.

[jondoe]

I did some digging, the current part number for the voltage regulator is UR13325, data sheet attached. It should be good for 1A and looks like it can go quite low on the input voltage before dropping out. Pulling the tube always sees the voltage levels stabilise at 5.6v on the input side of the voltage regulator and the 3.3v output becomes rock stable, I've tried another 12AX7 and the same fault comes back.


If the voltage regulator functions normally when the tube is pulled would that indicate the problem is elsewhere?

[Jon Snell]

Put an external 6.5volt DC supply on R205 and monitor what happens. That will remove any issues before the regulator.

[jondoe]

Splendid idea, as a terribly equipped home tinkerer I don't have a dedicated bench supply, but I do have a buck/boost converter somewhere that will probably do the job, it's in a box somewhere in the shed, I just need to find it

Just looking over the schematic again I can see the 3.3v rail leaves the digital board and ends up back on the power board, it's labelled +3.3 [F] on page 5, maybe there is a problem in that area upsetting the 3.3v rail?

[jondoe]

Ok I found my little boost/buck converter. Isolating the digital board and powering with 6.5v alone shows it working first time every time. If i remove the external power, plug it back into the power board and power the whole unit from the mains it constantly resets. Turning off the mains, removing the power board connector again and powering from an external supply shows it booting first time every time. The 3.3v on the regulator sits at about 3.25.

With the main digital connector plugged into the power board and mains off, applying external power sees the tube LED light and the FAN spin up and the digital board boots. The total draw is about 0.6A.

The power draw on the digital board alone is approx 0.2A, which I think is about right.

At the moment the fault is constant, so at least it's not sometimes working

[Jon Snell]

In that case you have determined the regulator is fine and it is the low DC supply causing the issue. Find out why the supply is dipping low.
Difficult to do without test gear but you are getting there.

[jondoe]

Might it be an early sign of the bridge rectifier failing? Could I isolate the AC input and feed 6.3v straight at the bridge rectifier +/- legs with some hook clips and observe the operation of the amp, or would I need to remove the bridge rectifier completely?

[g1]

Yes, if you disconnect the transformer winding from the bridge, you can attach your supply without removing bridge. And this test needs to be done as it will put the tube heaters back in the picture.
I think there must be a spike, or bigger drop out causing the reset. A little fluctuation between 3.3 and 3.2V should not cause it. The meter is probably not fast enough to catch the spike (or maybe large drop-out), a scope would catch it.

[g1]

Also should mention, you have not looked at the 'sys reset' line. It could be that the supplies are ok and something is wrong in the reset circuit.
pg.4 of schematic, lower right, CN16 pin 3.
IC1 pins 2&6, IC15 pin 19.