Print Page - Total Recall - can't bias despite OK-ish voltages. help!

Title: Total Recall - can't bias despite OK-ish voltages. help!
Post by: axg20202 on October 12, 2017, 09:54:16 PM

Hi all, First post here.
I think my voltages are OK (?), but my audio probe is not picking up any repeats from IC6 pins as per biasing instructions, at any setting of the trimmers. Here are my voltages with all pots set to middle, all trimmers set to middle, EHX power supply reads -33.43 V with no load. IC4 and IC5 values for pins 5-7 are slightly high but not sure if they are the issue. Also IC6 and IC7 pins 3, 4 and 7 are slightly low. Most other values seem to be in the right ballpark. Any help much appreciated.

IC1:
1=-7.53
2=-7.53
3=-7.49
4=-14.86
5=-7.45
6=-7.53
7=-7.55
8=-0

IC2:
1=-13.70
2=-13.05
3=-13.05
4=-14.85
5=-13.05
6=-13.04
7=-4.47
8=-13.06
9=-13.06
10=-7.12
11=-8.83
12=-13.06
13=-0
14=-13.05
15=-13.05
16=-13.82

IC3:
1=-6.09
2=-3.05
3=-3.04
4=-14.85
5=-7.51
6=-7.53
7=-7.53
8=-0

IC4:
1=-7.53
2=-7.53
3=-7.49
4=-14.85
5=-8.51
6=-6.54
7=-6.54
8=-0

IC5:
1=-7.53
2=-7.53
3=-7.51
4=-14.86
5=-7.04
6=-7.11
7=-7.11
8=-0

IC6:
1=-0
2=-7.57
3=-8.71
4=-8.71
5=-15.06
6=-7.57
7=-6.51
8=-14.12

IC7:
1=-0
2=-7.57
3=-8.84
4=-8.84
5=-15.06
6=-7.57
7=-6.55
8=-14.11

IC8:
1=-7.48
2=-7.60
3=-7.51
4=-0
5=-0
6=-0
7=-15.06
8=-15.06
9=-15.06
10=-7.57
11=-7.57
12=-15.06
13=-7.47
14=-0

IC9:
1=varies
2=-7.14
3=varies
4=-15.06
5=-7.14
6=-7.14
7=varies
8=-0

REG:
I= -22.43
G=0
O=-15.06

Title: Re: Total Recall - can't bias despite OK-ish voltages. help!
Post by: axg20202 on October 18, 2017, 11:34:26 AM

Bumpage!

Can anyone shed any light on this? Thanks.

Title: Re: Total Recall - can't bias despite OK-ish voltages. help!
Post by: madbean on October 18, 2017, 11:59:21 AM

Your voltages on IC6 being different is probably due to having your bias trim set in a different place so that's not a big deal. If you have confirmed signal going in pin7 of IC6 and no signal coming out of pins 3 and 4 then the problem is either IC6 or possibly IC8. Are you sure your MN3005 are genuine? Where did you get them? Also, if you audio probe pins 2 and 6 on IC6 do you hear a high pitched whine (you should)?

Title: Re: Total Recall - can't bias despite OK-ish voltages. help!
Post by: axg20202 on October 21, 2017, 06:08:13 PM

Thanks for helping. It appears I have no input signal at pin 7 - I just get a hum at this pin. Pins 2 and 6 are a high pitched whine. Output pins 3 and 4 also have the same whine, mixed with hum.

Title: Re: Total Recall - can't bias despite OK-ish voltages. help!
Post by: axg20202 on October 22, 2017, 09:32:02 AM

Scratch that. Using the schematic I probed along the input path and found a cold solder joint. All working now!

madbeanpedals::forum

Projects => Tech Help - Projects Page => Topic started by: axg20202 on October 12, 2017, 09:54:16 PM