BAJA 59 Tweed Bassman debugging help needed

[holm]

Hello

I've attempted to build the BAJA 59 Tweed Bassman: https://www.freestompboxes.org/viewtopic ... 13&t=28915


I get some weird oscillating sound/noise from the output. In the first place I'm not sure I did the off-board wiring correct, I've done it as in the following drawing:



Here's some pictures, don't know if they'll aid in sorting out anything. Some of the resistor values I had to match by putting 2 in series.




Also I been debugging using an audioprobe, and everything is good up till the output side of the treble wiper. So if I deattach the wire from wiper lug on treble pot, it sounds ok measuring at the wiper lug. But measuring at the 220nF cap going in to the non-invert i/p at IC2b, this is where the weird sound originates.

input side:


output side:




Also, the 180K resistor situated between the 2.2nF cap and the non-invert i/p at IC2b, is missing on the layout, and I'm not sure whether it's crucial or not so much? I've added it, but doesn't do any difference with regards to the issue.


Finally, I have measured the voltages, which reads the following:
+9V battery: 8.19V

Q1 (from top and down as in the layout): 1(+9V): 8.19V
2: 3.73V
3: 4.44V

IC1a:
OUTPUT: 4.09V
INVERT: 3.45V
NON-INVERT: 3.32V

IC1b:
OUTPUT: 4.08V
INVERT: 4.08V
NON-INVERT: 4.08V

IC2a:
OUTPUT: 4.08V
INVERT: 4.08V
NON-INVERT: 4.05V

IC2b:
OUTPUT: 6.97V
INVERT: BACK AND FORTH BETWEEN 4.08V AND 4.13V!
NON-INVERT: BACK AND FORTH BETWEEN 4.08V AND 4.33V!

IC3a:
OUTPUT: BACK AND FORTH BETWEEN 4.08V AND 4.34V!
INVERT: BACK AND FORTH BETWEEN 4.08V AND 4.34V!
NON-INVERT: BACK AND FORTH BETWEEN 3.94V AND 4.24V!

IC3b:
OUTPUT: BACK AND FORTH BETWEEN 7.38V AND 7.45V!
INVERT: 0.37V!
NON-INVERT: BACK AND FORTH BETWEEN 3.59V AND 3.85V!

Any help is greatly appreciated
Erik

[tonyharker]

The 180k resistor is needed to bias the op-amp otherwise the output of the op-amp will sit at a voltage determined by the internal leakage. It will not amplify properly. The voltage readings on the inputs of the op amps will be affected by the internal resistance of the measuring device and should no be relied upon.
Check the soldering on the 18k resistor connected to IC3a the soldering looks like a dry joint - similarly the 1M on IC3b.

[holm]

Thanks

I'll have a look at it.

\Erik

[holm]

hi again

So I looked at the much appreciated suggestions by tonyharker, added the resistor and I've resoldered the 18K and 1M resistors, but I still get an output like a gang of biker elfs is residing the IC. So in my frustration I tried to rule out some of the components. It's obvious that I don't know what I'm doing here, but I removed the bottom IC and added jumpers at ins and outs, like depicted below. The "oscillating" sound is absent when circumventing the bottom IC, but what should I gather from this, if anything, it's not the IC that's bad, I tried several others. So the obvious thing would be a bad soldering job, which is likely, but I'm curios if there's anything else I can do before starting from scratch?
Any help is highly appreciated - Erik

[sixthfloor]

Erik, the sound samples you provide ring a bell... It might be some cold solder on a ground connection. Did you check the voltages at the ICs power supply pins (4 and 8 ) ?

Try re-flowing all your solder joints, starting with the ground connections. It might sound daunting but it's still quicker than to start over again

[holm]

Thank you for your reply. The voltages at 4 was 0 and at 8 it was identical to +9V (is that to be expected?). I started re-flowing the solder joints, and in the process I discovered that I had the diodes positioned wrong, not sure that was the reason for the chopper sound though. Moving them in the correct positions gave an output without the chopper sound, but the output was weaker at the output of IC3b compared to IC3a. So I started messing with the 18K, 330pF cap, 2k2 resistor and 1uF cap, and somehow I managed to get the chopper sound back, and now I can't fix it!!! The soldering is a mess now, so I think I'll start over anyways, but thanks for the input.

\Erik

[sixthfloor]

Yes, pin 4 at ground and pin 8 at +9V is expected.

Weaker output at IC3b compared to IC3a is normal as far as my knowledge goes, since some signal is lost through the clipping diodes present between them.

Depending on how messy the solder is... I sometimes use a saw to cut solder bridging the rows. Kind of overkill, but works wonders

[holm]

Sorry for starting a new post, but once again I accidentally hit the solved button on the last post https://www.freestompboxes.org/viewtopic ... 73&t=29002

Yes, pin 4 at ground and pin 8 at +9V is expected.

Weaker output at IC3b compared to IC3a is normal as far as my knowledge goes, since some signal is lost through the clipping diodes present between them.

Depending on how messy the solder is... I sometimes use a saw to cut solder bridging the rows. Kind of overkill, but works wonders

Thanks, I'll try that
I've put it aside for now and I'll have a look at it again tomorrow.
\Erik

[holm]

So, just a quick update. Started all over, I circumvented the pots and stuff for starters and got signal all the way through! But something i off at the IC1a or the transistor or the power row, 'cause first I got signal and suddenly it got messed up. So measuring the voltage at the battery it reads 6.72V, but on the power row it reads 6.17V! is that to be expected?
Also, I have signal at the IC1a non-inverting input but it's really noisy at the inverting input and the output!!!

Any input is highly appreciated
Erik

[yiyorebel]

Hi, I do not know if you solved the problem, but at first glance I see that your diodes are not good look well where the two higher diodes go, should not go on the same track of the resistance of 2.2k. I hope you help my observation, greetings

[holm]

Yeah, thanks, I noticed, and corrected it, but still haven't made it work. I'm realizing that I might need some more experience before jumping into these large builds.

\Erik