FIXED Recalcitrant Pork Barrel...

[RobA]

Trace the input path now.
Start at the emitter of Q1,
Then the output side (+) of C2,
Pin 1 of IC1 (output of the buffer stage),
Junction of C8 and R14,
Base and emitter of Q2,
Pin 3 of IC3, (the input to the MN3007)

If something fails at any of these stages, narrow it down further by looking at the parts on the schematic between the last good location and the one that failed to have audio. You could reverse the order I listed above to and maybe find it faster. If you have audio all the way to pin 3 of the MN3007 and nothing at the output (7 and 8 ) and nothing on the base of Q3, then you need to start looking at the voltages around the MN3101 and the MN3007 again. Start with trying to rebias the thing and looking at the level of VB (should be measurable at the + side of C23).

Edited: I noticed that my 8 followed by a ")" got changed into a , so I edited to make it more clear what I was actually trying to say.

[wilsona]

Trace the input path now.
Start at the emitter of Q1,
Then the output side (+) of C2,
Pin 1 of IC1 (output of the buffer stage),
Junction of C8 and R14,
Base and emitter of Q2,
Pin 3 of IC3, (the input to the MN3007)

If something fails at any of these stages, narrow it down further by looking at the parts on the schematic between the last good location and the one that failed to have audio. You could reverse the order I listed above to and maybe find it faster. If you have audio all the way to pin 3 of the MN3007 and nothing at the output (7 and and nothing on the base of Q3, then you need to start looking at the voltages around the MN3101 and the MN3007 again. Start with trying to rebias the thing and looking at the level of VB (should be measurable at the + side of C23).

Thanks - great info. I worked backwards as suggested. Here are the results:

Audio is present on PIN 3 of MN3007 and on base and emitter of Q2. Nothing at the outputs of MN3007 or the base of Q3. I tested the latter two at all points in the range of the bias pot.

I cannot get at the plus side of C23 or the legs of the trimmer from the component side - is there another point I can use? e.g. one side of R42, R43?

I can fairly easily replace C23 or the trimmer.

Here are re-measured voltages on the MN3007 and MN3101.

MN3007
1 8.7
2 4.75
3 3.74
4 0.6
5 0
6 6.7
7 7.07
8 7.07

MN3101
1 9.14
2 4.68
3 0
4 4.75
5 0.36
6 8.02
7 2.99
8 0.6

[wilsona]

Voltage at the right side of R21 varies between 2.19V and 7.01V (same as left sides of R42/43 respectively) as the trimmer is turned. I take it from this that VB is in an acceptable range.

[RobA]

I wish I had I Pork Barrel build that I could check to compare your voltages with, but I've converted my build over to an MN3207. VB should be around 4.5V (half supply really). So, the range you have there looks good and should be plenty to dial in the level that your BBD needs.

The levels on the IC's look mostly OK to me, but pins 6, 7, 8 on the MN3007 look high. Other builds I've seen have the voltage at 7&8 closer to the half supply point. Pin 6 of the MN3007 should be the same as pin 2 of the MN3101 and yours are different. I don't know how much time lapsed between your readings on those pins and if anything else changed that would explain the difference, but it's suspicious and worth looking into further.

When I looked at the jumpers in your photo, I first thought they seemed suspect, but when I followed the trace in the doc, it seemed to me that the solder bridging that was possible there was just going to a pad that had a trace anyway. But, since we can't see the top of the board for those jumpers, maybe there is something that could be wrong in that area. Do the jumpers go through to the other side of the PCB? It does seem that there is possibly something wrong in the are of the MN3101. It would be worth reflowing those joints and checking for continuity on the pins that should be connected there and testing for bridging on pins that shouldn't be connected.

[wilsona]

Thanks - They do go through to the other side. I will have a look but the IC socket hides them. I think I might cut the socket out, clean up and redo the jumpers and surrounding area and replace the socket. The readings for both ICs were taken at the same time.

[wilsona]

Removed the M3007 socket, cleaned it all up and and redid the jumpers. Biased VB to 4.5V. No change. I've ordered a new MN3007 and MN3101 and will report back when I have replaced them.

[wilsona]

Replaced the main chips - dang! No change. Don't know whether that's good or bad.... Probably means there is not much wrong but it's a b*gger to find. I'll stare at the schematic for a while and see what pops into my head. All suggestions gratefully received. Last thing I did was measure VB at R21 which is 4.49V.

[wilsona]

Thought I'd check the diodes first. For D3 I am getting reading both ways on the diode check setting on my DMM: 0.465 and 1.955 - Is this correct? I was expecting to see a reading one way only. Checking a new 1N4001 gives 0.496 one way only. The other two diodes 1N914 give readings one way. D4 is omitted for this config.

[RobA]

For a 1N4001 you should only get a forward voltage drop in one direction. But, you can't check the reverse direction in circuit because there are parallel resistive paths to ground in the circuit and that will change the reading. If you were actually getting a 2V drop in the reverse direction, I think your power rail wouldn't be good and it does seem to be fine.

Have you found known good voltages for the IC's anywhere? I did a bit of a search around the forum for them, but I didn't see any that looked to from a verified working pedal.

It definitely seems like the issue must be around the MN3101 or the MN3007, or possibly the parts the connect the LFO to the MN3101. Have you checked for stray continuity between the pins of the two IC's and checked that the pins that should be connected are?

[blearyeyes]

I have a working Pork Barrel (not boxed) and will post the IC voltage readings tonight....

[wilsona]

I retested the voltages and I think they are OK.

Regarding continuity between the 3101 and 3007, I think its OK -

Pin 2 of 3101 -> Pin 6 of 3007
Pin 3 of 3101 -> Pin 5 of 3007 -> GND (?)
Pin 4 of 3101 -> Pin 2 of 3007

Not sure about pin 5 of 3007 marked 'Out 1.1' on schematic, this is definitely grounded.

No other pins on 3007 and 3101 are connected together.

[wilsona]

I have a working Pork Barrel (not boxed) and will post the IC voltage readings tonight....

Thanks, I will look forward to it.

[blearyeyes]

This is with a bench supply exactly at 9v. I built a charge pump and am going to run it at 15v..lots cleaner.

IC1
4.64v
4.64v
4.63v
.2mv
4.637v
4.64
4.63
8.98

IC2

vary 1.8-8.2
vary 4.46-5.46
vary 2.6-5.6
0v
vary 4.46-5.46
vary 3.0-5.8
9v

IC3

8.9v
4.55v
0v
4.55v
300mv
8v
vary 2.8-3.2
597.5mv




IC4

8.89v
4.56v
3.91v
597mv
0
4.54v
4v
4v

[blearyeyes]

Google the chip numbers and get the spec sheets they will shed light on how they are integrated into the circuit.  The power pins and the grounds etc..

[wilsona]

IC3 (3101) my results look almost identical.
IC4 (3007) all except 7 and 8 (4V) which at my last reading were both 6.63V

Any ideas what could cause the difference?

I will compare IC1 and IC2 later today.

Forgot to add my readings taken with both pots max.