Author Topic: Magnus Modulus troubleshooting  (Read 3026 times)


Magnus Modulus troubleshooting
« on: July 20, 2009, 06:37:29 PM »
I'm very excited to get this beast working!

I used the vero layout found here:

Initial fire up gave me all three leds. All four lfo controls have a visible effect on the led indicator. So far, so good right? However, when the modulation toggle is engaged, the led stops flashing. That can't be good.The tremolo switch does not have this effect.

I decided to hook up a guitar and see if anything was working. The booster works, but that's about it.

Back to the bench. I fed a signal into the circuit and followed it with the scope. The signal is present at the junction of C19 and R14, but not on the other side of R14.  I suppose I'm looking for a short somewhere around this area? I'll check that more after I clear my head.

Time to check the voltages, and the first thing I notice is that my 4.5v is actually about 4v. The two 47k resistors are from the same batch and are supposedly 1% metal film. They do look a little chinsey compared to other resistors I have. Would the cap have any effect on the voltage divider? This doesn't seem to affect the opamp too much since it seems to be doing it's job (signal is boosted). Could this be preventing the fet from working correctly? It seems that if the fet were working I would at least have tremelo.

Here is the entire list of voltages:

IC 1 (buffer/booster):
1. 4v
2. 4v
3. 4v
4. 0
5. 4v
6. 4v
7. 4v
8. 9.5v

IC2 (PT2399):
1. 5.1v
2. 2.56v
3. 0
4. 0
5. 3.5v
6. 2.6v
7. 4.7v
8. 4.7v
9. 4.9v
10. .374v
11. .385v
12. 4.8v
13. 2.9v
14. .115v
15. 5v
16. .115v

IC3 (lfo):

1. fluctuating
2. fluc.
3. flur.
4. 0
5. fluc.
6. fluc.
7. fluc.
8. 9.5v


G .637v
S 0
D .003v

Thanks for the awesome circuit Nathan! And thanks for the vero layout Lars!


Re: Magnus Modulus troubleshooting
« Reply #1 on: July 21, 2009, 01:43:06 PM »
Any thoughts on the voltage divider? Is this just a case of low quality parts or should I be looking for a different problem here? The other resistor in the divider is dropping 5.5v, so the voltage isn't going anywhere unexpected.

I'm guessing the modulation LED stopping when the mod switch is thrown is probably due to a short somewhere between the lfo and pt2399?
Likewise, the input signal disappearing after the buffer is probably another short?

I did try two pt2399s from two different batches with no change in the behavior, so I don't think the chip is the culprit, but I have no experience with this chip so I don't know.

I've had a day away from the pedal, so I am going to dive back in for a few hours to see what I can come up with.
In the mean time, if I'm missing something obvious, please clue me in.



Re: Magnus Modulus troubleshooting
« Reply #2 on: July 22, 2009, 12:28:36 AM »
Two tiny shorts have been tracked down and the pedal works!

While I had the pedal taken apart I also replaced the resistors in the voltage divider with resistors that I matched and it's still setting VB at 4v.  ???  Everything works, so I'm guessing that's fine.

This thing is amazingly fun!