Vox VT120+

[Mick Bailey]

Anyone got a schematic or service notes for this amp? Got 2 failed from the same rehearsal room on the same day. DC checks out but no output at all. Headphone socket picks up the Aux signal, but nothing else.

PCB is distorted around the output device but this could be 'normal' for this amp - can't believe 150 watts with hardly any heatsink. Looks like a fun amp to work on.

[Jazz P Bass]

Here is a link to the VT80+schematic. http://www.sound-pros.com/schematics..._schematic.pdf
It may help.
You may want to read the complete post, as Stan knows his stuff.
http://music-electronics-forum.com/t33710/

[Mick Bailey]

Thanks - that's invaluable. Looks like it could take a while.......

[Mick Bailey]

Fixed one amp - O/C heater on one triode half, but it needed a factory reset to get it working again. Totally silent, no hiss, hum or anything else until reset.

The other one has me slightly baffled; with a test signal into the guitar socket I wasn't getting any signal to the headphone socket. I carried out the audio test and it began working. Assumed there could have been a software fault (never assume, it makes an Ass of U and Me.....). Powered on and off quite a few times and still ok, but no power amp output.

The signal also gets to the TAS5615 output device on INPUT_A and INPUT_B.

The amp uses the RESET control on the TAS5615 to mute the amp via the AMP_MUTE control generated by the R8C/38 processor; low=mute. At present there's no voltage on the RESET pin. According to the device spec sheet it needs a 47k pull-up resistor to VREG, but I can't see this anywhere on the schematic or PCB.

Now I need to find why the control is permanently low.

My question is this; does the AMP_MUTE signal toggle between high and low under control of the R8C/38, rather than just going low and relying on a pull-up resistor to set it high? The simple answer would be to check with the working amp, but I don't want to risk pulling that one apart now it's working.

[Jazz P Bass]

If there is not a pull up resistor on the NOT/RESET pin, then it must be pulled up elsewhere.
If the only connection is to (R8C/38), then that is either the fault or the NOT/RESET pin is damaged low.

[Mick Bailey]

The absence of a voltage is probalby indicative of a fault within the RBC/38. It isn't logic 0, so looks like an O/C. I've now way of knowing if any firmware condition could cause this, but I can't clear it. Looks like a replacement board, or send back to Korg (where they'll still probably replace the board because the factory gate price in China for this is probably $15).

The other option is a bit of a bodge - cut the control line and install a pull-up resistor to permanently un-mute the amp.

Unless someone else with experience of these amps has any other ideas.

[Jazz P Bass]

Stupid question: have you checked that IC 3 (RBC/38, Pin 9) has a valid high signal on the NOT /RESET Pin?
If it is high, then cut the Amp Mute buss (Pin 7)
Now you can fully monitor RBC/38.
If there is still no voltage on the pin, it's either staying in mute for a reason or it's dead.
And what is the actual part # of IC3?
As to manually pulling up the Amp Mute pin, that's up to you.
The datasheet indicates a 47K R to Pin 9 (VReg)

[teemuk]

My question is this; does the AMP_MUTE signal toggle between high and low under control of the R8C/38, rather than just going low and relying on a pull-up resistor to set it high?

I assume the VT80+ service manual is for the part a reasonably comparable reference...

...and it doesn't show any pull up resistors in I/O ports so probably the processor handles the task internally.

Which begs a question, is the mute function on for a reason? For example, could there be a contact problem in the headphone jack, which also could force the amp to mute mode (see phone_dt input @ pin 37).

[Mick Bailey]

The VT80+ looks a good reference far as I'm able to tell. The resistor to ground R164 is missing on 'my' amp (which would be a pull-down resistor), as are a few other components here and there. Possibly revision changes or just differences between the models. Pin 39 and 40 on IC3 are not connected on this amp, either.

The IC is an R5F21388AN. The spec sheet shows pin 7 as a programmable CMOS I/O and mentions that if configured for input can optionally be programmed to use a pull-up resistor, but doesn't mention using a resistor for output. I'm making an assumption from the schematic that the output can be switched high or low.

The logic condition on pin 7 is slightly confusing as it isn't 0. Perhaps R164 was added after this amp was made to ensure the line pulled down cleanly. The condition may be leading me to a wrong conclusion - perhaps the port is operating correctly and, as you say, something else could be causing the absence of a high on pin 7.

I've just checked PHONE_DT and this is toggling fine.

Also checked the SW_BYPASS/TUNER input. This is held high and toggles low when the amp 'All Mute' button is pressed. Makes now difference to pin 7, though.

Are there any other inputs which will mute the amp?

[Mick Bailey]

Had another thought on this - isolate the power amp RESET pin and temporarily install a 47k pull-up resistor. Nothing. So the AMP_MUTE control wouldn't have done anything even if it did go high. Re-checking AMP_MUTE now with nothing connected confirms that the pin is floating. Suspect that the absence of any output on IC3 and the loss of function on the power amp RESET is a cascaded failure between the two ICs.

Changing the output IC is one thing, but IC3 is custom programmed.

[Jazz P Bass]

As a last final check , I would verfy pin 15 (shutdown ) & pin 19 (ready) are working.

[Mick Bailey]

Good call - pin 19 READY is high, but pin 15 SD is low. Checked the resistors and cap around that line and they're good. It doesn't say in the spec sheet, but I'm guessing if there's a fault detected by the internal protection which would cause SD to go low, then RESET will be ignored. I wouldn't expect READY to be high at the same time SD is low, though.

Tricky little number, this one.

[Jazz P Bass]

IC 3 microcontroller Pin #18 is held high.
That is the ShutDown pin.
The Class D IC has overtemp pins also.
Pin 16 & 17.
So you are back to the controller or the Class D chip.

[Mick Bailey]

I did some resistance checks with the output filtering removed - the outputs should be high impedance, but these read 56 ohms and 4 ohms. Just in case I was missing something with the internal architecture in doing unpowered resistance checks, I confirmed my readings with TI and the suggestion is that the outputs are shorted, as you might expect.

The power amp is NRND and no longer supported.