PT-80 Debug- Enormous volume boost, squeals.

[sjaltenb]

Hello all,

Unfortunately I must request ya'lls help. I just built my first PT-80. I was very careful when building and I am nearly 95% sure there are no cold solder joints. I was also very careful in preventing any bridges. Im pretty sure all the component values are right...

I used silver mica for the 47 and 100pf, sockets for all transistors and ICs. The grey box caps are all 1uf. I am getting an ENORMOUS volume boost and squeels from it when engaged. I am using my test rig with 9v coming from a OneSpot power supply. All offboard wiring has been checked over several times.

The PCB is mounted on a metal plate for my board but I used insulated material so I know the solder side never touched the plate. Sorry either my DMM is broken or I dont know how to properly take readings...

*** I found the first mistake: the lefthand 100uf electrolytic was reversed. I flipped it, and the same problem. Basically goes into instant feedback, very loud squeels- cant even play through it. It does, however, pass guitar signal, but its so boosted that it comes through very distored. the problem seems like it started bad and simply got much worse as time passed.

Im guessing the reverse polarity on that 100uf cap killed the PT3299 IC since its connected directly to it...

Here are some pics:

[oskar]

But still you need to post voltages!    really...
You might want to check the feedback of the op-amps.
Audioprobe. Where in the chain does it freak out?

[sjaltenb]

yeah i know, but like i said, either im not doing it right or my DMM is broken 

Would the backwards electrolytic  damaged anything else, or woiuld it just have slowly fried itself and need to be replaced?

come to think of it, I think they are all 16v electrolytics thats not good, as it is running at 18v........maybe they are all fried?

[oskar]

yeah i know, but like i said, either im not doing it right or my DMM is broken 

The odds your up against without a working DMM are... high!

Would the backwards electrolytic  damaged anything else,

No. The cap is part of a reference voltage network only. It would mess up the biasing of the op-amps and you would probably get gated sound or no sound but I don't think it would give you distortion in the way you describe.

or woiuld it just have slowly fried itself and need to be replaced?

I don't know. But if you can't take voltages, changing that cap is one of only a few things you can do...

come to think of it, I think they are all 16v electrolytics thats not good, as it is running at 18v........maybe they are all fried?

But there are voltage regulators on the input so none of them caps see more than what they're rated for.


If you get strange measures then try measuring known voltages/ resistances.

[sjaltenb]

thanks Oskar!

Yeah as soon as i posted i went back and looked and realized that those caps are after the regulator 

I guess im kinda in an advanced "paint by numbers" stage.... would you mind quickly describing the best way to take a voltage....say im just trying to measure each voltage on the PT... whats the best way to do that?

I really need to learn this anyway...i've never gotten good solid voltage readings and i wonder if i didnt screw up the DMM when i first got it doing something dumb...

[oskar]

There use to be fuses in DVM's so it's sort of protected.
If your build is all completed and hooked up don't forget to insert a cable in the input.
hold the black/com to ground ( I use a guitar cable for this. I loosen the handle of the plug, stick in the probe tip and tighten the handle again. With the plug in the input the unit is on and the DVM com probe is grounded ) pooke around with the other one...
Not much more to it   
Put it in mode to register VDC and in an apropriate range.
Start by getting a reading from a battery.

All links you need should be in the sticky... "what to do when it doesn't work..."
If you scroll down you'll find a template to follow as to what voltages to provide.
Test the DVM first to se if it works and to get used to it. Measure resistances and voltages that are known.


Good luck


Oskar

[sjaltenb]

thanks, I am on the road for the next few days but when i get back I'll take some readings...