Carvin X100 EQ problem

[KCman]

There is a switch that puts the EQ on either of the two channels. When the EQ is"on" , the volume increases a lot. Doesn't matter where sliders are.

I signal traced the volume increase.. It starts at pin 7 of A2 (op amp). I tried another 4558. It doesn't have the increase with A2 removed. I checked R142 143 144 and 145 and replaced caps C84 and C85.

What are some ideas to check? One 10k slider was bad so I removed it and wired a couple 5k resistors to make it seem like a flat position. I tried removing the filter op amps and still had volume increase. The volume increase doesn't seem like it's frequency specific.

Thanks

[Enzo]

Doesn't matter where they are set? But do they still "work"? I mean if you run the EQ sliders up and down do the appropriate tonal bands still get louder or softer? Even if the whole thing is off kilter

[KCman]

They do change the tone, but it seems they don't change as much as most eqs.

[Enzo]

They may not, but the fact they are trying to do their job leads me to think they are being swamped elsewhere. Just a quick look at the amp schematic leads me to think maybe a stuck/shorted JFET is leaving the bypass mode on. Of course I could be wrong, but I doubt the EQ circuit is at fault.

[KCman]

Thanks, I will look into it. Are you thinking Q3 and Q8? It's taking me a bit to follow the schematic on this one.

[Enzo]

t's taking me a bit to follow the schematic on this one.

You and me both brother...

I am not sure I yet see the whole path to and from the EQ, but yes, those would be my first suspect. But it might be controls. Look at the assign switch on the right, it runs off to H3-2 and H3-3. Those show up all the way over on the left middle, and note they are control signals Q and not-Q, coming from either side of inverter A6 pins 11,12. Those two should toggle from about zero to about +15, though really like up to 3v or down to 12v is OK. But the key is when one is high, the other is low. Ultimately that is how we decide Q3 or Q8. Assuming the two A6 pins are indeed at opposite states, then go to the EQ assign switch and flip it back and forth. As you do, watch the gates of Q3, Q8. They should be toggling high and low. If one is stuck, it could be either the control signal at fault or the JFET itself.

[xtian]

I have a working pull from a Carvin X100, the complete eq board, which you can have for the price of postage. Let me know.

[KCman]

That would be great. I can't seem to find the sliders anyway.

[KCman]

Q3 and Q4 are working. Have gotten further than that yet.

[KCman]

I meant Q3 and Q8

[KCman]

Thanks xtian!! The EQ board fixed it.

Now, what all could cause this? (See the first post).

[xtian]

Happy to hear. Salvaged hardware to the rescue!

[KCman]

Here's what I did before the replacement EQ:

I replaced R142 143 144 and 145
I replaced C84 and C85
I tried a good op amp in place of A2

It seems, since pin 7 of op amp A2 is where the volume increase happens, the only place left is the group of filters.

Am I correct? And how would I find it, pull each slider one by one?

I tried pulling the op amps in the filter section already, with no change.

Thanks

[KCman]

So... The replacement EQ from xtian worked, but... The sliders worked in reverse. If you pushed one up, it brought that frequency down. I first thought I put it in upside down. No, it only fits one way.

A little more digging I found spots on the PCB that had 2 pads that are meant to be soldered or not soldered (like jumpers) one set "A" and one set "B".

I traced them and they change the "direction" of the sliders. Cool, so I changed the new board to be"the other way" now, up is up and down is down. But... Now, the frequencies are in reverse order, cause the board is upside down compared to stock form. Ugg

This led me back to the first board. This had been worked on before me. I found one of those pads that was supposed to be soldered, not soldered. It was the connection of the EQ out back to the bottom of the sliders. Tough find, but fixed it!

It's a universal board meant to be mounted up or down, deepening on amp. There pads are like a horseshoe with a dot in the middle.

I still needed a good slider for the old board and xtian helped out for sure. Thanks