Hartke HA3500 engaging eq causes reduced sound

[KCman]

When engaging the eq switch, the sound gets very faint.

I've disconnected the ribbon cable with the eq slider pots (cn201 {located in the lower left section of schematic}) then engaging eq works, has the same sound either way. (If I'm correct, that means IC1-C{op amp 1 channel 3}) is working.

I tested the slider pots and they seem to be fine. 10k instead of 100k like schematic though?)

The 3 quad op amps (tl074cn) are getting + & - 15 volts.

I reflowed the solder for components in question.

What i notice is, the first 2 bands(op amp 1) cut in and out with noise and pops. Band 2 makes all sound silent when raised all the way up.

The next four bands (op amp 2) get hissy when raised above 0, but change tone.

The last four bands (op amp 3) are quiet but don't seem to change tone.

Does it seem like a bad op amp?

Is there a way to test them?

Could it still be the eq pcb?

I've heard of freeze spraying for shorts, and this sounds like a short??

Thanks for any help or insight.

[Jazz P Bass]

Does it appear that some tech was in there before.

Kind of odd that all of the sliders are 10K instead of 100K.

Even full on, the 10k's are going to attenuate the signal.

[KCman]

Maybe I tested them wrong?

I left them connected.
I found the two contacts that tested 10k and didn't change. Then found the one that did change.
The wiper changed as it should, but only up to 10k at the top.

I don't have to take those out to test them do I?

I don't think anyone has been in here. It's not that old and there is a rubber seal that couldn't help being destroyed upon opening.

I will try to test the sliders again.

[Enzo]

Look at the schematic, you have ten 100k sliders all in parallel, that makes a 10k total. This is a normal reading then.

The ends of this parallel strip are on the inputs of an op amp, IC1C. Then each slider wiper is a selective filter. The slider setting determines the amount of positive or negative influence for each.

Maxing slider two killing the sound tells me either the slider shorts to frame at that position, or the associated op amp may be screwed. Or maybe C221 is shorted.

The last four share an op amp and don;t work, I'd replace the IC.

I did this professionally, and parts are a lot cheaper than labor. If I suspected an IC, I'd throw a new one in it. They are less than a dollar.

[Enzo]

Look at the schematic, you have ten 100k sliders all in parallel, that makes a 10k total. This is a normal reading then.

The ends of this parallel strip are on the inputs of an op amp, IC1C. Then each slider wiper is a selective filter. The slider setting determines the amount of positive or negative influence for each.

Maxing slider two killing the sound tells me either the slider shorts to frame at that position, or the associated op amp may be screwed. Or maybe C221 is shorted.

The last four share an op amp and don;t work, I'd replace the IC.

I did this professionally, and parts are a lot cheaper than labor. If I suspected an IC, I'd throw a new one in it. They are less than a dollar.

[Jazz P Bass]

oops.
100k x 10 in parallel.

Thanks for clarifying that, Enzo.

[KCman]

Don't know where my brain was, I think the first thing I learned about electronics was resistance in parallel and series. Thanks for the great info.

I was going to order the op amps when ordering whatever else I needed anyway. Like you said, cheap. Probably cheaper than the postage.

If I could get your knowledge on this Enzo;

At first while troubleshooting this, I unhooked all wires at the connector cn201 except 8 and 9(going to IC1-C). No change, the sound was still faint.

Next I measured resistance between 8 and 9(10k ohms). I pulled 8 and 9 connector and added a 10k resistor in its place. This worked fine.


Now, does that mean there IS a problem with the eq board since all 10 channel op amps were disconnected while the eq board was still hooked up to IC1-C ? (Cause I pulled the board and replaced with a 10k resistor)

I'm not saying the other parts aren't toast also.

Thanks again

[Enzo]

Don't know where my brain was, I think the first thing I learned about electronics was resistance in parallel and series.

We all do it, but the lesson here is not parallel resistance, the lesson is always look at things in their context. Look at the system rather than a bunch of individual parts.

New op amps? Wouldn't break my heart to put sockets under them just for future fun.

Again, thinking like a system: you have the row of sliders measured at 10k, and the amp doesn;t work. You unplug all that and put a 10k resistor in its place and it does work. Presumably that resistor was on the main board side of the connector. With the connector back in place and power off, measure resistance between pins 9 and 10 of IC1. Do you still see the 10k of the slider board? Or did you already do that and I misread?

With your resistor sub in place, and sliders disconnected, measure to ground from pins 9 and 10. What do you see? Now remove resistor and plug in sliders, does either end look grounded? Especially with that #2 slider all the way up? ANything look different?

[Enzo]

Don't know where my brain was, I think the first thing I learned about electronics was resistance in parallel and series.

We all do it, but the lesson here is not parallel resistance, the lesson is always look at things in their context. Look at the system rather than a bunch of individual parts.

New op amps? Wouldn't break my heart to put sockets under them just for future fun.

Again, thinking like a system: you have the row of sliders measured at 10k, and the amp doesn;t work. You unplug all that and put a 10k resistor in its place and it does work. Presumably that resistor was on the main board side of the connector. With the connector back in place and power off, measure resistance between pins 9 and 10 of IC1. Do you still see the 10k of the slider board? Or did you already do that and I misread?

With your resistor sub in place, and sliders disconnected, measure to ground from pins 9 and 10. What do you see? Now remove resistor and plug in sliders, does either end look grounded? Especially with that #2 slider all the way up? ANything look different?

[J M Fahey]

It looks like Powers That Be / God / Fate / whatever has decided that Enzo´s voice is worth twice as much as before, so it automatically doubles all his posts for a couple days now

[Enzo]

I see it happening to others lately too. Every now and then this forum does this, then tboy resets something and it goes back to normal.

I like to think one is for the original poster, then there is an extra in case someone wants to take it home with them for later.

[J M Fahey]

You should pack the second one in a small styrofoam box so it arrives home warm enough for dinner

[KCman]

I thought the double posts was my phone. I was at the store today and every text I sent my wife was doubled or tripled.

Anyway, I was checking that eq board again and I pulled the two wires for the LED lights (built into the sliders) and that solved my volume drop issue. Pin 2 on the connector is the one.

There is definitely more than one problem. While it fixed the volume problem, the last four sliders still don't change the tone. Hopefully fixed with op amp change.

Also, I pulled slider 2 and subbed another one in for it. That didn't fix the 'sound off when slider maxed problem'. Then I pulled c221 and that fixed it.

So far, 3 op amps and 1 cap.
As for the LED power problem, I didn't get time to look into it.

Thanks again. I must not have ever pulled those first two wires on the connector. Just talking it out has helped me rethink some things.

[KCman]

I'm thinking the LED power problem fried these other components.?.?

[KCman]

An update;

I replaced the 3 op amps and c221. That fixed the slider killing all sound when maxed. (Not sure exactly what fixed it, but don't care. About 3 bucks worth)

I figured out the other problem;
- I socketed the op amps(Thanks for the tip)

- when I pulled ic-2, the problem stopped

- I then unsoldered all 4 sliders on that ic, one at a time. One of them ended up bad. It tested fine as a potentiometer and the LED worked, but it must of shorted the voltage from the LED to the eq circuit. (I never tested it to confirm, but when i pulled the main power from the LEDs, that fixed it also so....)

I still think the slider might have caused the other problem?.?

Now that it is fixed, I think I like the amp better with the eq bypassed anyway, seems cleaner. It just bugs me to have broken stuff. Ha ha

Thanks again Enzo for taking time to reply and give me reason to keep screwing with this!!!