LM13700

kdmr · Joined: Jul 06, 2016 Posts: 18 Location: savannah, ga

So i'm putting together a Soundlab MKII and the ICs in the filter section died due to a dumb mistake on my part. everything else works great.

ANYWAYS...while troubleshooting, I followed the signal from the mixer and it disappears at pin 3 of the LM13700 (junction of the R24/R25 divider). While waiting on a new 13700 to arrive, i'm trying to figure out what would cause the signal to disappear...seems like i should still be getting signal at pin3 and not at the output pin. Not super familiar with the IC so if someone could shed some light on the dynamics of the chip that would be awesome.

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gdavis · Posted: Wed Dec 14, 2016 7:38 pm Post subject:

Are you sure it's actually disappearing completely? The inputs of the 13700 need to be very small to keep the distortion down. The divider is there to attenuate the signal, to about +/-50mV from the +/-5V input. If you adjust the scale on your scope you should be able to see this.

kdmr · Joined: Jul 06, 2016 Posts: 18 Location: savannah, ga

on a related note...can the strange voltages on the TL072 be attributed to a blown LM13700 in this circuit? I'm having a hard time determining flow between both ICs and the 3904/3906 pairs...need to buff up on my semiconductor theory!

gdavis · Posted: Mon Dec 19, 2016 1:48 am Post subject:

It's going to be real difficult to help you if you keep posting in different threads and don't follow up on previous posts. Best way to trouble shoot is to go methodically step by step.

The 13700 won't affect U1 voltages.

Have you read the description that accompanies the schematics on the MFOS site? It explains quite a bit. Of course if you still have questions feel free to ask Smile

I think most people find BJT circuits especially challenging since they're current based which is not as intuitive as voltages. I still struggle with some of the details of these expo converters.

Looking at voltages around the transistors is kind of tricky as this part of the circuit is really about currents. Semiconductor voltages will vary depending on several variables. As long as you're in the ballpark of the expected voltages you're probably good, the trimmers will take care of the rest.

The voltage at U2 pins 1 and 16 should be 2 diode drops above V- which would be about -10.6V for a -12V supply. You can measure the voltage drops across R17 and R18 and divide by 10k to get the bias currents.

kdmr · Joined: Jul 06, 2016 Posts: 18 Location: savannah, ga

My U2 1 and 16 readings are on the money. i've been using ray's doc (the "sanity measurements"...fitting name) thoroughly...thats how i got the rest of the beast calibrated. My U1 pin 1 measurements are also within spec which leads me to rule out the coarse/fine tuners as potential problem areas.

today i'll be investigating everything that has a junction with U1 pin 6 to see if i can get the 072 to behave. i've double checked all the board wiring and reflowed every pad so i'll need to give the panel wiring another once over.

sorry about multiple posts, but i figured since one was asking for voltages and this one is more of a technical questionnaire that theyd be better off separate.