Anushri issue, possibly LM311?

Evening all,
Troubleshooting my new build of anushri, getting constant 4.52v at point E, ie pin 7 of LM311 is this correct?
pin 2 1.22v
pin 3 2.49v
pin 7 -4.52v

Also, the LM7905 giving -5.7v at point 3, compared with 7V at point 2, does this difference matter?

Whole thing behaving quite oddly, getting a quiet tone but much louder if fuzz turned on. Presumably something wrong somewhere, not got any further than this!
Thanks, Si

  • You shouldn’t be getting 5.7V at the 7905 output to start with.
  • I cannot tell anything from the voltages you measure around the LM311 - the actual “correct” value depends on power supply rails, and it’s a bit strange anyway to do DC voltage measurements for points that are supposed to carry signals!

What is important is whether or not the signal at IC7 pin 7 is a sawtooth wave - and for that you need a scope or at least a cable connected to your amp to “listen” to the signal at this point.

What makes you think there’s a problem with the LM311 anyway?

Only because the troubleshooting section suggested it should be at 0V. Will have a listen at that point tonight.
Swap the 7095? The negative rails are at a happy -5v at the green points.

Can you post some hi res pictures of both sides of the PCB?

Will do tonight. Have run the jack sockets out to quarter inch jacks instead, but that’s on control board so hoping you won’t need to see the rats nest of wiring that created!

Some of the signals are routed via the switching jack back to the Motherboard - so w hope your wiring just looks like a rats nest…

Have had a poke in the rats nest & tightened a couple of cables. All looks OK to me - working on assumption that of the two parallel inputs on PCB for each jack input, top one (near the other) is ground (1), vertical one near it is switched input (3) and opposite & on its own is pin (2). without a jack in socket 2 & 3 should connect, with a jack in it connects to 2. Sorry, hard to describe…

Main issue is synth very quiet compared to drums - faint sound if fuzz on, even fainter without.

Yes there’s an audio signal at pin 7 on LM311. no scope available, sadly.

Still leaves the -5.7v issue at the 7905.

pics here:
wired to fit in modular rack so might look weird: midi connects directly to pins on PCB, & not using main in & out sockets, just front panel connectors.

changed the 7905 & getting a more respectable -6.8v now.
sound still very quiet unless fuzz turned on, but that’s a problem for another night.

a 7905 should deliver more or less exactly 5.0v as implied by the name…

I am not sure I understand the problem with the 7905. Is -6.8V the input or output voltage?

Sorry, not being clear, output voltage is -4.99v exactly which is fine, getting same readings at green points so all OK. Just not sure why original input voltage was -5.7v when troubleshooting guide suggests should be similar number to positive input voltage. Taking out all the ICs got it to -6.8v input, but voltage reduced as ICs put back in. Maybe this is how it should work, but replacing the 7905 has stopped this so maybe a faulty regulator or bad soldering in first place. Anyway, all good now, except signal very quiet when fuzz not turned on, but I’ll work through schematic tonight and see if I can find the issue. thanks.

If the Voltage of your PSU Drops this much under Load (if you can talk of load with this few chips…) its way to weak… i suggest you get a bigger one with at least 300mA. -5,7V is hardly the Minimum for the 7905 to generate a stable -5V Rail as it has about 2V of Voltage Drop. MAybe your problem disappears when your unit is properly powered.

Brilliant, thanks, will try. I think it’s rated higher but will try a different PSU.