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TR-9090 (909) noise circuit not working, 4070 getting hot
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author TR-9090 (909) noise circuit not working, 4070 getting hot
sake
Finished my (trevor) 9090 but unfortunately noise circuit isn't working.

4070 (and 7815) is getting hot, also when both 4006s (U68 and U69) are removed. I did renew the 4070, 4060s and also C84 . Any clues?



MickMad
maybe posting good photos of the actual build would help in figuring out the issue.

anyway, the "getting hot" would mean there's a short to ground somewhere. double check the electrolytic caps polarity, then check for correct orientation of the ICs too.
sake
All the electrolytic caps polarity (especially C76 and C82) seems to be allright. IC orrientation also

Hereby a photo of the noise section build.

guest
remove the 4007, and gently bend up all the pins on the right side except for the top pin (bend up 8-13 so they dont go in the socket anymore), then reinsert it and see if it gets hot. if it still gets hot, then gently bend up the left side pins, except the bottom (bend up pins 1-6), then reinsert it. if its still hot its most likely a bad chip.

there is sometimes an issue with floating (disconnected) pins on CMOS ICs, in that they can float to the middle state and turn on both high and low output drivers, shorting power to ground. so this isnt the most conclusive test. as a side note, this could also be happening as a result of the 4006 being absent. you can sometimes feel which quadrant of the IC is hottest, and narrow it down that way as well.

once you narrow down which side is making it hot, you can then reinsert all but 1 XOR from the chip and see what happens. its most likely a solder short to ground someplace, or perhaps D48 is bad.

if you dont want to bend the pins, you can also turn the board on and quickly measure all the voltages on the 4070, and then shut it off. pins 2,3,6 should be midrail (as they should be oscillating); 1,14 should be Vcc; 5,7,11,12,13; 8,9,10 are unclear, as their inputs are floating, but your meter will most likely pull them to ground, so they will all be 0V.
makhho
Scope pin 9 on the 4006 ICs -- if you aren't getting a ~15v p-p noise signal, then it is likely bad chips.
sake
Thank, will check that. Just blew up the pcb fuses so have to fix that first :(
sake
Well i tried two 4070's:

Seems to be that both of them are getting hot when XOR C (pin 8, 9, 10) is connected.

When pin 8,9,10 are disconnected:
Pin 1, 14: 15V
Pin 2, 3, 4, 6: ~ 15V p.p signal

4060 pin 9: no output

I checked for solder grounds in that area but everything seems to be fine. Even D48 is fine.

Any idea what to do next? Could both 4070's be faulty?
guest
glad youre narrowing it down. with power off, measure the resistance to 0V and then to Vcc from pins 8,9,10. then measure resistance between adjacent pins. ive seen problems where there are shorts in the socket.

another approach is to just insert all pins except pin 10, and see if it overheats. then check the voltage on pins 8,9 and the uninserted 10.
sake
Well, i found another 4070 (and my last one) and two fresh 4006's. I tried that one (pin 8,9,10 connected) and now everything seems to be fine It's peanut butter jelly time!

Measured pin 9 on both 4006's with my scope and i think this signal is okay (~15V p.p).

Although the 4070 gets a bit hot i think it's okay.

guest
glad its working. having 2 bad chips in a row is a real pain when debugging.
sake
Thanks. Also glad that it finally works.

Will test the 9090 in a couple of weeks and hopefully all the drums are working correctly then smile
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