Dirtbag MN3005 delay very low level issue

[djaaz]

Hi Guys,

Second Dirtbag build, second sets of issues. This thing is a beast to build.
I have delay on IC outputs but only "dry" & "noised" signal on the board output
So far, what i've done:
- Debugged a few thing (2n5087 was dead and led was always on for instance)
- IC voltages checks ok compared to my first build.
- Audio probed up to the compander (tested & working), delay input level on pins 14&15 is ok, outputs on pins 10&11 is audible but barely.

R11 & R12 are 68k according to the MN3005 BOM.

What should i check next?
I was planning on removing & socketing R11 & R12 but before unsolder these i would like some feedback.

Thanks in advance for your help guys.

[djaaz]

Been audioprobing some more and made some comparisons with my first build:

Levels seems ok all the way trough the delay and up to the part of the schematic where the delayed signal gets back into the compander.

So i don't see the point of changing R11 & R12.

Main difference between the two builds is the Pin 16 voltage which is 0,8V when it is 0V on my fits build.

[djaaz]

This one behave a tiny bit differently tonight.
Same low level at the output of the ne570. Same voltages.
BUT the led is continuously on with a 2,50v getting to it. Don't know if it is related.

[djaaz]

Next move i'm thinking of is socketing C27, C28 R42 & R43 but not getting how the compander works i would gladly have some advice before trying to unsolder these from the board as it already had felt a lot of heat.

Anyone?

[jkokura]

Wish I could help. I know how frustrating it is to be the only one responding to your own tech help thread.

Keep with it. I'm sure you'll get it, or someone with more knowledge of the Dirtbag circuit will pipe up.

Jacob

[madbean]

It seems as though the compander is the issue. Have you tried swapping the one from your other build into this one?

You should expect some voltage on the 2N5087, so I do not think that is the problem.

[djaaz]

Thanks for the kind message Jacob! It felt lonely but i am patient and i understand that the dirtbag is not the easiest build to assist on.
Brian,
I swapped the compander already between the builds. I'll try with a SA571 i have lying somewhere and let you know.
Thanks for the help.

[djaaz]

No Joy.
I swapped the compander. I resoldered the socket in between and just in case.

For those who remember my first build troubleshoot, i even tried the faulty distorting compander just for fun and it was loud enough: distorted and noisy.
I changed C28 as a "lottery troubleshoot", no luck.

Something in the circuit is preventing the compander to do its job.

[madbean]

K, try this: socket R42 and try higher values there. 15k, 20k and upward.

[djaaz]

Ok, tried socket R42 & R43 (just because i misread the layout).
Going up to 18k helped but above, no change and still clearly not loud enough.

So i probed my first build and i think i may have an issue before the compander: when i probe pin 15& 16 i have wild level and the feedback somehow kicks in until oscillation.

Nothing comparable to what i have in this second build.

So i'll audioprobe a bit more later and let you know how it goes..

[madbean]

Gotcha. Yeah, dig some more in the audio path before the compander to see where you are losing signal. It should be quite loud until the 571 attenuates it before the output.

[djaaz]

Solved.
Don't ask me how. My mother would say "the devil was sitting on it".
I probed the circuit before the compander, toying with the gain trimmers and it was like something was stuck in those trimmers. I made a tiny bit of change and boom! there it was.

Anyway, thanks Brian. I really needed the help, otherwise i would have stayed on the compander.

Thanks!

[madbean]

NP. You were right on top of it all the way through actually---you identified the problem, thought of potential causes and eliminated possibilities one by one. Pretty much the essence of all debugging