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Old 28th April 2010, 12:46 AM   #1
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Default Remote Control Problem

I have an Audio Research Preamp with remote control.
I can make the volume go up and down, but the mute button actually causes the volume control to turn.
I've attached the schematic. As you can see there is an Inhibit pin on U8. This never goes low. I can force it low and the volume works, but it doesn't answer the remote. It needs to go low for anything to function.
I have replaced every IC on the board. As a result I had to increase R1 to 22k. Any other value and nothing works, even with the inhibit forced low.
I'm out of ideas.
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Old 28th April 2010, 06:08 AM   #2
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hmmm... a discrete digital logic design, takes me back a few years.

U5 and U6 are shift registers that capture the address and command. U2B, U2D, and U7 decode the device address and unused bits in the command. U8 decodes the binary command to cause the motor to turn the volume pot or toggle the mute logic. The Inhibit pin needs to be low for the decoder to activate one of the outputs. See datasheet here: http://www.onsemi.com/pub_link/Colla...MC14514B-D.PDF

If U8-23 is never going low then you need to look at U7 and its inputs. U7 is an OR gate, the output will go high (and inhibit the decoder) if any one of its inputs is high. You can measure this with a voltmeter, anything below about 0.7V is a logic "0", anything over 2.0V is a logic "1". If an input is somewhere around 1 - 1.5V then it is not connected and the result of any logic will be unpredictable. Check the outputs of U5 and U6 to see if they make sense.

The decoder responds to commands 13 (mute), 16 (vol up), and 17 (vol down) according to the RC5 command tables (see RC-5 - Wikipedia, the free encyclopedia) for address (or system number in RC5 terminology) 16 or 17 (preamp or tuner) depending on the jumper at R2.

So a mute command should result in the pattern 10000001101 (with the leftmost bit being U6-Q2, then U6-Q1, U6-Q0, U5-Q7, ... U5-Q0). The 5 bit address part should be the same for all commands (volume up, volume down, mute) and only the 6 command bits should change.

I would check R2 and the jumper at U2D-12, also look for any open/short traces between U5, U6, and U2D, U2B, U7, and U8. Note that U2B and U2D are exclusive-OR gates, U2D will either pass or invert the least significant bit of the address depending on if the jumper is in place or not.

If you don't find anything broken there, and the address/command data captured in U5 and U6 is not consistent from one press of the remote button to another, then you need to work back upstream. U3A (and several other parts) is a one-shot with the timing controlled by an external RC, if the time constant is wrong for the timing of the stream of bits coming from the IR receiver then you won't capture the right bit stream. Or it might even be the IR receiver is flaky. To figure out if either of those is a problem you'll need a storage scope.

Shouldn't be too hard to figure out what's wrong.

Last edited by mightydub; 28th April 2010 at 06:24 AM.
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Old 29th April 2010, 03:40 AM   #3
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This is exactly the kind of information I needed. I'm not used to this kind of remote control circuitry, and it had me somewhat perplexed.
I'll be going over it in the next few days. There's got to be something silly I've overlooked.
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Old 29th April 2010, 05:07 AM   #4
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Glad to be of help. Data sheets for all the logic parts are available at Onsemi.com - MC is the prefix used by Motorola, and the 74HCxx stuff is all generic. I started my career many years ago doing complex designs with logic like this, so this is all easy for me to understand. Any more questions, just ask away.
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