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 Forum index » DIY Hardware and Software » Thomas Henry designs
SN Voice PCB. A question on Vias...
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mubase



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PostPosted: Fri Mar 22, 2013 9:41 am    Post subject: SN Voice PCB. A question on Vias... Reply with quote  Mark this post and the followings unread

Hi !
I've been trying to figure out whats wrong with the LFO on my SN voice board. I have disconnected it from its wiring and it is now sitting on my ETS 5000 digital trainer breadboard station wired up to +/-15V and 5V with a speaker plugged into the triangle wave output.
I have rigged up the LFO rate pot to J3-4 and have a jumper wire between J3-7 and J3-8,9 or 10 for lfo range.

I have plugged pin 21 of the 76477 to an oscilloscope which gives a steady triangle wave which changes frequency according to the LFO rate pot.
However, Upon trying to get an oscilloscope reading from output of IC4a (LF444 LFO output pin.) the oscilloscope produces a flat line. This is reflected when I try plugging in the 1V/Oct to pin 1 of the LF444 also. No LFO. Sad
I have tried a different LF444 and the same thing happens. When I try changing LFO Range from low to mid to high, nothing happens.

I'd like to know if there are any vias on the PCB responsible for the LFO at all and if it could be possible one of those is damaged??
Thanks,
Steve.

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State Machine
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PostPosted: Sun Mar 24, 2013 8:53 am    Post subject: Reply with quote  Mark this post and the followings unread

If I may ..........

Lets first remove the speaker from the equation. None of the outputs are designed to work into the low impedance of a speaker.

You are certain that U4 has it's +15V on pin 4 and -15V on pin 11? lets just double check that. I know it's an obvious but I like to start from the most obvious things. Now I know this may a really dumb question BUT did you also route your ground of the supplies to the PCB? Especially the split 15V supplies ground reference ? This is very important for proper operational amplifier function.

When you say flat line on the scope, does this mean its 0V or is the signal railed out at some other voltage (Offset ??). I know that R31 is connected to pick up the +15V in order to pull the LFO output negative in order to center the waveform about ground.

Make sure U4-1 is not grounded. Shed the power and make the measurement.

Also, do you see a waveform with your scope at TRI OUT?

If you find that a problem exists with your PC board, I would gladly send you another at no charge to you. Its rare BUT boards can be defective. Wink

I sure hope we get this one solved for you, this has been going on for a while. I feel bad man that you cant enjoy this synthesizer Shocked

Bill
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mubase



Joined: Mar 24, 2011
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Location: London UK
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PostPosted: Wed Mar 27, 2013 7:42 am    Post subject: Test results for PCB....
Subject description: Tested tri-wave, voltages for U4, resistors all fine...??
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Hi Bill. Thanks for getting back to me.
OK, I've done some diagnostics...
All grounding is correct. Smile
First off, the triangle wave output looks fine on the scope (see picture.) It also changes frequency with both coarse and fine pots.
U4 +/- 15 V pins were connected to a multimeter. If I connect Black multimeter to earth then pin 4 of U4 reads +15V and pin 11 reads -15V...
Pin 1 (output ica1) is outputting different voltages according to which setting the LFO Range is on (low,med,hi)
These range from -5.38V to 3.12V to 1.28V .....??? Confused
This is reflected on the oscilloscope. The flat line jumps from centre to slightly above ( smaller positive voltages) to below centre ( negative voltage.)
The 75K resistor seems fine.

Pin 21 (the 76477's LFO output produces a nice triangle wave which changes with the LFO rate pot turns as expected.
So the only thing I can see different with your obsevations is the reversal of voltage on U4... Question

The mystery continues...
P.S I am willing to buy another board.


IMG_1268[1].JPG
 Description:
SN voice triangle wave produced from J3.2
No probs there...
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IMG_1268[1].JPG



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PostPosted: Sat Mar 30, 2013 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, the long distance troubleshooting thing sometimes does not work. Wink Two things, if your board has connectors and can be simply disconnected from things, send it to me and I will troubleshoot and repair it for you.

Secondly, I will send you a free PC board long with the "fixed" original.

Just PM me with your address and I will respond with my address to which you can send the sick board. Very Happy

Bill
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PostPosted: Tue Apr 09, 2013 12:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just to keep this thread going for those who may be interested in the outcome of this. The SNV board has just arrived at the sMs laboratory. Very Happy As I work on the issue, I will use this thread as a sort of blog.

First thing I have done is removed the non standard connectors on the board, and installed MOLEX "KK" headers so that I may jack it into my SNV chassis to begin the troubleshooting. I have decided even before that, I will remove all the existing flux on the board then flow all solder to get top to bottom solder flow. Reclean the board, then inspect.

More to come .........

Bill
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PostPosted: Thu Apr 11, 2013 9:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Not much to say. Unit troubleshot, found open from SN76477 PIN 21 (SLF CAP) to rest of circuitry. Added jumper on PCB solder side to repair the circuit. LFO output in all three ranges is now good. Tested PULSE, NOISE, FM SWEEP, LFO LOW MED & HIGH, ENVELOPE, etc. All looked good.

Bill
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mubase



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PostPosted: Sun Apr 14, 2013 3:56 pm    Post subject: Man of qualities fixes board of quality Reply with quote  Mark this post and the followings unread

Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy Very Happy
Thank you sir. Very Happy

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State Machine
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PostPosted: Mon Apr 15, 2013 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Your very welcome. Your board is back on its way to the UK. I did leave the Molex headers I installed on the board but also sent your original headers just in case you wanted to put those back onto the board.

Bill
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