DVR Troubleshooting Techniques

Draft Document 6/26/02 Use at your own risk.

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First things first.When trouble shooing any electronic device, the first thing you check is power.All Advanced Computer Controls equipment has 2 grounds, one analog, one digital.BOTH MUST BE TIED BACK INDIVIDUALLY TO THE POWER SUPPLY.This is a ďNoise ThingĒ and helps prevent the digital noise from the processor from getting into the audio circuits.Make sure both pins are grounded!

 

IMPORTANT, always be careful when probing on a circuit board with the power on. One slip can ruin the entire board.It is best to have help, one person to find the test point and hold the probes steady, the other to read the meter, record the reading and find the next test point on the diagram.

 

+12 volts Ė Should appear on CR1, cathode.That is the side toward the edge of the board.

 

+5 volts Ė There are 2 five volt regulators on board.Both have their own heat sink.They are 3 legged devices.The leg closest to the center of the board is the 5 volt output.

 

-4.5 volts -Can be checked at U83 pin 1.

 

The next thing to check will be the audio levels into the board.TP1, 2, & 3 should be set with a voltmeter to 2.7 volts AC.Have your helper hold a touch-tone digit while you adjust the levels on each channel.Remember, you have to go from repeater to repeater or to the phone line to send the test tones depending on where the DVR is connected.

 

This next test is very important, it is the touch-tone decoder test.If the touch-tone decoder doesnít decode, then itís all over, nothing will work.If your DVR had the adapter boards plugged into the main board, set your voltmeter to DC, 5 volt range, and watch pin 15 of the chip on the adapter board. (U84 for Ch 1, U67 for Ch 2, and U72 for Ch 3)If your board does not have the adapter board, then watch the SSI202 IC pin 14.See above for IC and channel numbers.Have your helper hit touch-tone digits rapidly on the repeater.Watch the IC pin go high (5 volts) and low as the tones are sent.Fine-tune the audio level so that the touch-tone decoder decodes the tone quickly and reliably.If you have a problem with this, you could have a problem with ďTwistĒ.Check ALL digits on the touch-tone pad.Bad twist will cause some digits to decode reliably and others not.To fix twist, you will need to pick the audio from a different point in your receiver.Bad twist is caused by some tones being louder than others that make up a touch-tone digit. This causes the decoder to miss the digit.

 

Next, we need to verify that the repeater COR signal is working properly.Without COR, the DVR does not know when to start and stop recording.With your voltmeter on DC, 25 volt range, Watch U6 pin 2 for channel 1.(U6-8 for Ch 2 and U7-3 for Ch 3)Have your helper key the repeater and watch the voltmeter.The meter should follow the your helpers signal into the repeater.If the signal goes high during the test signal and low when there is no signal, make sure DVR Dip Switch 2 position 1 is on.If the signal works the other way, low when your helper is keying his radio and high when not, then turn Dip Switch 2 position 1 off.

 

So far we have checked the power into the DVR and itís on board power supplies.We have verified that audio is getting to the DVR and set its level.We have also checked to see if the touch-tone decoder is working and if twist is a factor.At this point, you should be able to record a signal into the DVR and have it record it.I like to hook a Radio Shack audio amplifier to the DVR and listen for the test signal.Page 257 of the 2002 Radio Shack Catalog shows it for $11.99.It is part number 277-1008.You will also need something like 42-2421 1/8 mini jack to stripped wire and some clips or probes to make a nice tester.This amp and probes is really useful for all kinds of repeater projects.

 

Hook the ground side of the audio amp to DVR ground, either one of the grounds will be OK for this test.You can clip it to the bolt on the heat sink for the 5 volt regulators, but not to the heat sink itself.Hook the positive side of the audio amp to the DVR output for the channel you are testing.Turn the audio amp about half volume and set the output pot on the DVR to half volume as well.Have your helper step away from the DVR so you can hear and have them key up and enter 4***.This is the default audio test code.They should un-key after entering the code, then re-key and say something really important like ďTEST 123456789987654321ĒOnce they un-key, you should hear the important message in the test amp.If you donít hear the message, have them try again, check your connection to the DVR output.If you still donít hear anything, try clipping the test audio amp to pin 2 of U81 (Ch 1) or U82 (Ch 2) or U83 (Ch 3).If you still donít get anything, clip the test audio amp to TEST POINT 1 up by the pots. Listen to the touch-tones and repeater audio there.The audio must sound clear and undistorted.There should be no squelch noise once the user un-keys from the repeater.At this point if you still donít hear audio on the output of the DVR with the audio amp you could have DVR problems and will probably need to send it off for repairs.

 

If you now hear audio in the test amplifier you only have 2 more steps to complete the test.We need to verify that the DVR is holding the repeater up while it is talking.When your helper records a test message, time the length of the message.If they record a 10 second message, the repeater should hang from the time they un-key for 10 seconds before you hear the courtesy tone.If you have that delay, this portion is working, if not, take your voltmeter on DC 5 volt scale and measure on resistor R3.It is down by the 25 pin connector on the corner of the board.Measure for channel 1 on the side away from the 25 pin connector.It should go to 5 volts while the DVR is playing back recorded audio and low once complete.If you donít see any level changes during playback, unhook the busy output from the DVR to see if what it is connected to is loading down the signal.The busy signal is a very weak output and does not have the ability to drive heavy signal loads.You may have to buffer the signal if you are driving something heavy like a relay as a transistor is about all it can drive.If you disconnect the busy output and you still donít see the level change, you may have to send the DVR off for repairs.Channel 2 can be tested on R2 the side away from the 25 pin connector, Channel 3 should be tested on R1 on the side toward the 25 pin connector.

 

By now you should be hearing DVR audio on the repeater. If not make sure the audio output is turned all the way up on the DVR for the channel you are testing.Take your test audio amp and listen to the signal as it enters the repeater from the DVR to test your cables.Make sure the input pots on the repeater are adjusted properly to allow the DVR to be heard.Listen with your audio amp as you connect and disconnect the DVR audio from the repeater to see if the repeater is loading down the audio.

 

Take each test a step at a time.Donít make any assumptions; prove to your self that you believe the reading that you see.I canít say enough about how important the test audio amp is in figuring out where the problem lies.

 

 

If you have any questions, email me at paul@accrepairs.com or check www.accrepairs.com for more information.