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Re: Circuit problem...

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5/10/2000 11:24 AM
AMZ	Re: Circuit problem... Could be your guitar picking up hum due to less than complete shielding. Move close to a computer monitor while the guitar is plugged into the amp and if it increases really dramatically, then that is it. Almost all guitars hum when close to a monitor or under fluorescent lights, or if you are using a dimmer in the room. -JACK

5/10/2000 1:45 PM
anonymous	Could be a ground problem. I haven't looked at the schematic for the bluesbreaker, but make sure that everything is grounded correctly, including the pots. On some pedals, one tab of the volume pot goes to ground. Also, try moving some of the wires around a bit.

5/10/2000 1:59 PM
R.G.	Whine is oscillation. Ground problem is a good guess; slight mis-wiring is another one. 25K for 22K is fine. Try reading the DC voltages at all the pins of the transistors and opamp and reporting them back here - that's the best first step to debugging this one. You don't mention - does the guitar sound come through?

5/11/2000 1:10 AM

Re: Circuit problem...revised...

Thanx for all the input ;-)

;-)

Ok.. I found what was causing the whine from the circuit. I had placed a .01uF mylar cap where a .1uF cap should go, replaced it and whine is gone. Got to get my eyes checked for sure ;-)

;-)

Now when circuit is connected and switch thrown, no sound. Tested out all the connections with the multi-meter for continuity and checks out where it supposed to be. Taking RG's advice I've taken voltage readings with circuit under load for the TL072 IC:

Pin 1 = 7.11v
Pin 2 = .6v
Pin 3 = .59v
Pin 4 = 1.15v
Pin 5 = 1.2mv
Pin 6 = 284.9mv
Pin 7 = 1.258v
Pin 8 = 6v

Diodes used are 1N4148 and voltage readings for the lower four on the left of the layout going left to right measured fron the non cathode end:

.46v
.58v
.60v
.96mv

Diode in upper middle postion read:

8.02v

Caps used are all mylar except:

47pF = ceramic
.1uF = metalized film
100uF = electrolytic

I know I've bungled this up somehow but not sure where. All the advice is very much appriciated. If all else fails I will start over from scratch but I would really like to debug this one. Thanks again all...

Charlie

5/11/2000 3:28 AM

Well, good folks, we got trouble,
yes trouble, right here in River City -
That's "trouble" with a "T"
and that rhymes with "B"
and that stands for.... Bluesbreaker...

Pin 4 is where Ground goes. The opamp can't work unless this is zero volts. Likewise, pin 8 is where the power goes. This should be battery voltage, around 9V. An opamp *can't* be operating linearly if all three pins for the opamp (that's +input, -input, and output) aren't all within a few millivolts of the same DC level. In the case of the Blues Breaker, there is a 4.5V DC level generated and applied to one +input directly, and to the other through a 1M resistor.

So - looks like wiring or parts value problems. In any case, nothing will work until you get 9V on pin 8 and 0V on pin4. Use your ohmmeter to see what the DC resistance is from these pins back to the battery clip. I bet they're not 0 ohms.

5/11/2000 4:50 AM
Larry	Re: Circuit problem... Many more learned minds gather here, but here's a thought...how did you read the pin out for the IC? Is it possible you have it "turned over" as you wired it? This only occurs to me because I did that very same thing this past weekend. Larry

5/11/2000 7:39 PM
aron	Really good point. I did this ONCE. Never again!

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