Carvin V3 - perplexed

[lowell]

Hey all,
This thing isn't channel switching. Opened 'er up and immediately saw an unterminated/floating ribbon cable. It's a plastic clip header with 4 pins. I've searched and searched the chassis and boards for where it goes...and am at this point perplexed. I cannot find it. Anyone have experience with these or a layout? Here's a pic. You can see the white plastic header floating near the upper left of the chassis.

[lowell]

I do see 4 solder pads called H6A next to the yellow cap near the left side of the middle board. There is no header clip there...I even checked under the board. It's possible the clip broke off the board. I shook the amp around to see if it was in there but nothing. It's the only sense I can make of this. I checked the schematic but don't see H6A anywhere. Here's a schematic. http://carvinimages.com/schematics/3...ocked_full.pdf

[lowell]

Called Carvin. Their tech is very nice. He said that ribbon is for the grill backlighting...which is an optional add-on. So that's not the problem. Gonna check the +5v supply and the main switching chip U1.

[Richard]

Seems odd that they would put the ribbon in there if it's not used.

[lowell]

Ok so on to troubleshooting the channel switching. I've tested the following and they are all ok:

5v supply is good
U1 pin15: 5v
U1 pin5: 0v
U2 pin10: 5v
U2 pin16: 5v

When I use the manual front panel momentary switches, the channels do not switch, however when monitoring voltage on U1 pins 18-20 I do see that the switches are indeed shorting those pins to ground. Those pins have +5v on them, when the switch is toggled that voltage goes to 0v.

pin2: +2.5v (changes to 0v when channel 2 or 3 momentary switch is toggled)
*NOTE - pin2, when touched with my VOLT meter, switches the SMART to ON. Not sure why this is...and I can't find the SMART on the schematic. Guess I'm not smart enough.

U2
pin3: 0v
pin8: +5v


This is an extremely complex repair for me. I began reading the 87LPC764 datasheet and my brain hurt. I don't know anything about digital. How would you go about this repair?

[lowell]

I realize this repair might require some time on folks' parts to help me out. But I do need your help if you can find the time. Thanks!

[MarkusBass]

This is an extremely complex repair for me. I began reading the 87LPC764 datasheet and my brain hurt. I don't know anything about digital. How would you go about this repair?

Let's face it: 87LPC764 is a microprocessor. All you need to know about it is that it gets correct power supply, it gets input signals CH1SW, CH2SW and CH3SW and it generates output signals on other ports of the microprocessor. It means that you don't need to read the datasheet. I don't know which output ports are used but it seems to me that the switching signals finally appear on TRG 1-2 and TRG 3 pins of U2 and these signals are passed to the preamp board to switch K2 and K3 relays. So I would start from checking whether CH3 TRG signal is present on the base of the Q3 transistor on the preamp board. If not, go back to the interface control board and check where the signal disappears.

Mark

[nickb]

You have to treat the 87LPC764 micro as a black box. It's pre-programmed to work in a certain way and it either does, or doesn't. If it doesn't check it's got power and a 12Mhz clock.

It receives certain switch inputs and outputs signals that control the LEDS and relays. It also handles the MIDI I/O. The way it generates all its control outputs has a twist to it. It only has a few pins and there are many outputs required. The get around that they send the the information out in a serial stream. The serial stream is turned back to parallel by the HCT595 serial-in parallel-out shift registers. The point of all this is you wiggle a switch input and one or more of the 595 outputs should change in sympathy so that is where to take a look to see if the micro is doing anything first.

[MarkusBass]

That's correct. Serial data from the microprocessor is converted to parallel data on HC595 outputs. So the easiest test is to switch the channels and see what happens on TRG 1-2, TRG 3, LED CH1, LED CH2 and LED CH3 outputs on U2 pins.

Mark

[nickb]

Simulpost strike again! At least we're saying the same thing...

[lowell]

Thanks for the replies. I will follow your advice. So do you guys have an formal training/education in audio or computer electronics? I ask, because I learned everything I know through doing, reading books etc.. When I come upon a repair that has ICs I usually get REALLY slow and it takes forever to figure it out. Sometimes I don't figure it out.

[mhuss]

From an engineering point of view, it's much easy to monitor a set of inputs and control a (potentially larger) set of outputs using a microcontroller, like you have here, so that's become commonplace. As Nick says, you really have to treat these as a black box and just check that the inputs and outputs make sense. Trying to figure out how and why it's doing what is does is beyond the ken of most of us.

Note also that most microcontrollers have their programming 'burned in' - so reading the datasheet allows you to see a huge range of possibilities, but is not very useful in figuring out how this specific microcontroller has been programmed to do what it does.

[olddawg]

You have to treat the 87LPC764 micro as a black box. It's pre-programmed to work in a certain way and it either does, or doesn't. If it doesn't check it's got power and a 12Mhz clock.

It receives certain switch inputs and outputs signals that control the LEDS and relays. It also handles the MIDI I/O. The way it generates all its control outputs has a twist to it. It only has a few pins and there are many outputs required. The get around that they send the the information out in a serial stream. The serial stream is turned back to parallel by the HCT595 serial-in parallel-out shift registers. The point of all this is you wiggle a switch input and one or more of the 595 outputs should change in sympathy so that is where to take a look to see if the micro is doing anything first.

And reset! The first thing I would do is a hard reset, or at least scope the reset pin in DC mode and look for the reset pulse.

[nickb]

Thanks for the replies. I will follow your advice. So do you guys have an formal training/education in audio or computer electronics? I ask, because I learned everything I know through doing, reading books etc.. When I come upon a repair that has ICs I usually get REALLY slow and it takes forever to figure it out. Sometimes I don't figure it out.

I have a Masters in hard knocks, from the school of the same name.

[lowell]

Ok so what do these signals on these pins look like? I'm guessing the static voltage changes from some positive voltage to 0v, or vice versa, when a channel switch is thrown? Therefore turning the transistors on/off that control the relay coils. I get that when a positive voltage is applied to the transistor base, the transistor saturates therefore pulling current through the relay coil.