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VCO issue !

by biggiesmalls23 on Tue Sep 25, 2012 7:15 am

Hey guys,

So i completed the SAW tooth osc, didnt have an oscilliscope so i measured the 8.5VDC on R105 and moved onto the Square. I installed the parts for the Square and went to measure PIN 3 on the wave for switch only to find i have pretty much exactly have the volts required, i get 3.6VDC instead of the 7VDC mentioned in the manual. I went back through and checked all solder points, they were fine, i then went back through and checked that all the parts were correct, they were fine, still no luck. I still get the correct voltages going to D31 (11.82VDC) and the IC9 (5.33VDC), i checked as much as the path as i could and cant seem to find where its dropping half the voltages.

What is my course of troubleshooting for this? I checked with headphones for a square, i get a faint click but no square wave, obviously im not generating enough voltage.

Thanks in advance.
biggiesmalls23
 
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Re: VCO issue !

by MMM on Tue Sep 25, 2012 4:48 pm

Did you connect the 3V to R90?
Do you get a good audible sawtooth in the headphones?

Without the 3V at R90 the frequency will be very low. For the square wave this could mean that you're only hearing the "clicking" of the flanks because this clicking is too far apart to be heard as an actual tone.
MMM
 
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Re: VCO issue !

by biggiesmalls23 on Tue Sep 25, 2012 6:08 pm

I did connect the 3V to R90, tested that its there, tested pin 3 of IC11 to see im getting 3V, its there.. Does it matter what side of the resistor im latching the 3v to, i orginally had it to the south side and then swapped that resistor out for a new one to make sure it was north side like the manual, did the same with R105 as well..

Im wondering if i should move to the VCF or debug this now before going on?
biggiesmalls23
 
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Re: VCO issue !

by biggiesmalls23 on Wed Sep 26, 2012 2:12 am

Tried again and again to debug it today, still no luck, the oscillators are not oscillating !!!
Replaced Q8 thinking it was my issue, its didnt change anything, thought because i was only getting 3.5VDC out of Q8 and following its path to SW1 where i get 3.5VDC i would swap it out but nupe, no luck.

what should i try next?
biggiesmalls23
 
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Re: VCO issue !

by MMM on Wed Sep 26, 2012 2:43 pm

I think it's important that we use a very "clear" approach here. I simply don't know which steps you've taken, so we need to check and doublecheck everything.

I take it that you're also not gettting a sawtooth output, right?

biggiesmalls23 wrote:I did connect the 3V to R90, tested that its there, tested pin 3 of IC11 to see im getting 3V, its there..


However you should theoretically get 5.33v at pin 3 of IC11; you should get 3v on pin 5 and pin 7 of IC11.
MMM
 
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Re: VCO issue !

by biggiesmalls23 on Sun Feb 10, 2013 1:57 am

Update:

So i continued on with the build, finished everything today. When powering on i get a flash of LED's, then the track 1 led lights up and stays lit. I try to put it into random mode but it doesnt seem to do anything, i can press run and it seems to be running although i get no sound and i dont see a moving pattern (like my other x0x). A couple of things ill mention, like i said in my first post i get exactly half of the required voltages from R105 and pin 3 on the selector switch, 4VDC and 3.5VDC there abouts. I backtracked again and checked my parts over and over, 1 mistake i had was K118 not being in Q29, it had the K30AY in there instead. I thought here is my lucky day, swap it out for K118 and ill be right.. nope, it didnt change a thing. So continuing on with the build and troubleshooting from forum posts i saw someone had an issue with C11, it seemed to be similar to what i was getting so i swapped out C11 (what voltage should i be getting across C11 btw?) and i actually lost the 4VDC on R105 and its now down to 1VDC. Its exactly the same cap, a 1uF electrolytic.

So after continuing each step of the build all the way to the end and powering on the machine i find that my VCO problem seems to be having repercussions on the proper operation of the CPU, i should atleast be getting random mode running shouldnt i? I remember when i built my last x0x i powered up and switched to random and voila it started playing a random pattern. Or was i mistaken and it was just playing that in the headphones without any leds moving?!

A couple more things, i went through with my multi-meter and checked a few IC pins on the digital section, i seem to be getting 5VDC from them all as well as from the CPU. I reckon im that close to finding the issue, i just need a little push over the line. MMM or anyone if your still out there please give me a minute !!!

Many thanks in advance.
biggiesmalls23
 
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Re: VCO issue !

by MMM on Sun Feb 10, 2013 4:06 am

Let's do this one step at a time. Since you've completed the digital side it makes sense to debug this first (if it is even not working) because I find troubleshooting the analog section easier if the digital section already works reliably.

The analog and digital side are usually completely separated (unless there are major problems with the power supply) so problems in either section don't affect the other one.
I think the LED for Pattern 1 lighting up when being in Pattern Play mode is a good sign. Does the Tempo LED blink in this mode (or in Random mode)? If you switch to keyboard mode do the LEDs over each key light up when pressed?
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Re: VCO issue !

by biggiesmalls23 on Sun Feb 10, 2013 5:43 am

Actually i cant remember looking at the tempo light, i do remember trying to speed it up and down but didnt see the light speed up/down. When grab the 1/16 patter encoder and move it, it corresponds with the 1-16 step leds, not sure what mode im actually in tho. Does the random mode play notes with corresponding leds if i recall correctly ? I have tried every mode but nothing really makes a difference.

Id love to start checking back at the VCO tho, like i said before ive rehit all the solder joints just to make sure, checked parts over and over. Logically, if i keep testing from the capacitors (C11, C31, C32) onwards, i should find where it is loosing voltage?

No the keys in keyboard mode leds dont light up, thats also why i think im in some half working digital stage
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Re: VCO issue !

by MMM on Sun Feb 10, 2013 7:53 am

I'd stick with the digital side for now because the VCO is easier to troubleshoot once we can "play" the xoxbox from the front panel.

In Random mode the only LED lighting up is the Tempo LED.
Pattern play mode is with the flat end of the mode switch toward the top (or toward the other controls). You should be able to select different patterns then; the corresponding LED should light up.
Keyboard mode is three clicks to the left; please check this as well. It could be that the digital side is working just fine already.

What I'm trying to determine is if perhaps the mode switch is malfunctioning. The Bank switch seems to work ok.
MMM
 
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Re: VCO issue !

by biggiesmalls23 on Sun Feb 10, 2013 5:16 pm

right, looks like the digi section is fine, keyboard mode when hitting keys corresponds to the leds, tempo light flashes in random mode and speeds up and down based on the pot.

1 issue i noticed was in my kit (chipforbrains) that Q5 was both a 733/536 - i had a 733 in there first but swapped it out just now for a 536, this seemed to align up better for the digital section as the top row of leds/switches for the keyboard started to work.

so now onto the VCO i guess.
biggiesmalls23
 
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