Re: Ampeg VH140C

Posted by mnemonic on Dec 23, 2018; 8:15pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Ampeg-VH140C-tp30301p45170.html

Hey everyone, today I finished building the above layout (Anders layout directly above my post), as this one is labelled 'verified'.  Though I'm having trouble getting it to work, I was wondering if someone more knowledgeable than me can give me some pointers on where to start.  All my previous pedal builds I've been very lucky and I haven't had to do any serious troubleshooting.

All the controls appear to work, the problem is it just sounds really bad, loose, woofy, dull, not that much gain on tap.  My voltages are as follows, with nothing plugged in pedal input or output:

Power Supply: 9.05v at input

my meter reads around 2mv to -2mv in open-air so I'm not sure if those mv readings ought to be considered 0v.  

IC1 (top left)
RC4558p
1. 242.9 mv     8. 15.48 V
2. 41.8 mv       7. 0.86 V
3. 41.9 mv       6. 26 mv
4. -14.48 V      5. 21.9 mv

IC2 (bottom left)
RC4558p
1. -240 mv      8. 15.48 V
2. 1.3 mv        7. 1.7 V
3. 1.3 mv        6. 0.9 mv
4. -14.53 V     5. 1.2 mv

IC3 (bottom right)
RC4558p
1. 9-14 V*      8. 15.48 V
2. 1.5 mv       7. 10.7 V
3. 0.66 V        6. 1.2 V
4. -14.53 V     5. 10.3 V

IC4 (top right)
LT1054ip
1. 1.57 V       8. 8.75 V
2. 4.09 V       7. 1.4 V
3. 1.7 mv      6. 2.56 V
4. -4.23 V      5. -8 V



pin 1 of IC3 (bottom right) is kind of erratic, it won't fix on one voltage, its kind of bouncing between 9 and 14 volts.  I don't really understand why as it is connected to pin 5, so it ought to be the same voltage, I would think.

Pins 5 and 7 of IC3 (bottom right), theres definitely something wrong there, I shouldn't be getting 10ish volts should I?  There is no continuity between them, so they must independently be getting voltage from somewhere.  

I've checked and double checked my board, no solder bridges, misplaced parts or cuts that I can see.  I've also checked the layout against the hand-drawn schematic posted above, and it follows it correctly as far as I can see (with the exception that the two halves of the opamp in IC3 are switched).  But I'm pretty new to tracing these things out so maybe I missed something.  

the chips all seem to be getting the right voltage, and apart from sounding wank, the pedal works, so I'm hoping that whatever the issue is, is centred around IC3.  

I did try subbing in a different opamp, and there was no change.  

Does anyone have any tips on things I should check?  


thanks!