Mark Bass hates bass

[Helmholtz]

Also check C32 (100”).

[J M Fahey]

Looks like the layout of this one is a little different.
It's the model CMD 102P.

So you're saying C29 is a bipolar, it had one of those little silver electrolytic caps in there.

Tried a 1uf poly cap with no change.

Also the one end of C29 is not connected to ground, but to the +12 protect line.

Yes, there are Bipolar Electrolytics.
Compare its symbol (same as any conventional cap, parallel plates, no polarity shown) to C32: case and "+" end shown.

For indirect test, lift one end of R68 to momentarily disable DC protection, try your Bass again, any change?

IF DC protection issue confirmed, try doubling C29 to 2.2uF , still bipolar, to mak it less trigger happy.

You may use 2 x 4.7uF regular electrolytics in series, out of phase (positives joined) for testing without waiting.

For an extra test, you may hook up DC coupled scope on a 10V full scale range from Ground to C29/R68 junction, play your Bass, and check what DC appearsvthere.

Any reaching about 2.5V positive or negative will trigger protection.

[drewl]

Cool thanks.
C32 was a 47uf at 25v, bumped it up to 100uf at 63.
It tested tested okay though.

ORiginal c29 flew away somewhere
I'll check it out when I get home.

There is like no room to work on these or measure stuff!

[Helmholtz]

Yes, there are Bipolar Electrolytics.

I think the OP used a film cap ("poly") for replacement. All film caps are "bipolar" and a much better choice in this position than an NP ecap.

[Helmholtz]

Cool thanks.
C32 was a 47uf at 25v, bumped it up to 100uf at 63.
It tested tested okay though.

ORiginal c29 flew away somewhere
I'll check it out when I get home.

There is like no room to work on these or measure stuff!

I don't recommend to increase the capacitance of C32, as that would change the original trigger/release time constant.

[drewl]

I think the OP used a film cap ("poly") for replacement. All film caps are "bipolar" and a much better choice in this position than an NP ecap.

Yes, polyester I believe.

[drewl]

The junction of C29 and R68 sits at about ten vdc.

[J M Fahey]

That is completely wrong, where do those 10V come from?

What do you have at the other end of 1M R68?

Also the one end of C29 is not connected to ground, but to the +12 protect line.

Are we talking about the same cap?
ItŽs C32 (as shown) which has 1 leg connected to +12V

C29 can ONLY get voltage through the 1M resistor.+12V , and its other leg must be grounded.
Sometimes part ID changes in different versions, what matters is function.

This DC protector is quite simple: Speaker out voltage charges a small capacitor (1uF) through a high value resistor (1M)
Part numbers be d*mned, check where to where does each one connect to.

The small capacitor must have the non charging leg connected to ground, period, because itŽs detecting DC relative to ground.

Trigger transistors (in this case MosFets) have 1 leg grounded each, either Source (T19) or Gate (T20) and trigger when they get about +2.5V between Gate and Source is reached.
Might be less, should check datasheet for Turn On voltage but working principle does not change.

[drewl]

As I mentioned, this amp is laid out differently.


The one end of C29 is not ground, but connected to +12 prot.

The rest of the circuit looks the same.

I'll try and map it out, pretty hard due to its construction, but I was studying it through a microscope to see the traces and where they go.

[drewl]

There are no burn marks or detectable damage, I could try lifting the one end of C29 and ground it to see if the circuit functions per the posted schematic.

[glebert]

On mine lifting C29 made the circuit function normally, no more DC on the gate of T19 with no input. I screwed up the SMD pads in the process and haven't gotten around to replacing C29.

[drewl]

It's tricky unsoldering those smd caps!

The proper way is to use solder iron tweezers which don't fit, or hot air.

[Jazz P Bass]

Here is a copy of the Little Mark III.
It has the cap connected to +12V Protect.

[J M Fahey]

Thanks.
I had uploaded a fragment of Mark 2 , which uses a grounded bipolar electolytic.

I see they now downgraded in Mark 3 to a cheap regular electrolytic (hey! ... cents add up!!! ) which is probably leaky and making circuit misbehave.

You can make a quick test, just to check whether problem starts here or lives somewhere else: simply ground the R68/C29 junction with small piece of wire (easier than removing SMT parts) so killing the DC protector.

Try it with your Bass; use lots of slapping and thumping to generate real asymmetrical signals.

Any better?

If so, weŽll troubleshoot that section.

[drewl]

Ok, I'll try that tonight.

Thanks Jazz, I knew I wasn't crazy
C29 isn't grounded.