CM7777 preamp failure question - AVS Forum
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Old 05-21-2011, 08:42 PM - Thread Starter
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My CM7777 preamp has failed. I want to determine if the failure is due to the power supply or the preamp itself. Does the power supply output a DC voltage on the center conductor? If I measure with a voltmeter, what voltage should I get?
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Old 05-22-2011, 12:01 AM
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Yes, you can measure the DC voltage from the power supply. It should be about 22 volts without the preamp connected, and about 18 volts with the preamp connected. I posted a similar answer here:
http://www.avsforum.com/avs-vb/showt...9#post20227049 post #15
on this thread:
PreAmp location
http://www.avsforum.com/avs-vb/showthread.php?t=1317357

It turns out that the problem that mjdupre was having was caused by a strand of the shield braid wire shorting the DC on the center conductor to ground.

If you can not measure it, you can not improve it.
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Old 05-22-2011, 06:45 AM
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Those power supplies do fail on occasion. If you determine that the PS is the problem, you can order a replacement online.
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Old 05-22-2011, 03:09 PM
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Quote:
Originally Posted by rabbit73 View Post

It turns out that the problem that mjdupre was having was caused by a strand of the shield braid wire shorting the DC on the center conductor to ground.

I ran into a similar problem several years ago. Lost the preamp during the dead of winter. Ordered a new one and installed it the spring. Found out it was not the preamp or power supply at all but a bad coax connection at the grounding block.
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Old 05-23-2011, 05:34 AM
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I had a pair (one VHF, one UHF) of commercial preamps a while back, fed by a dual power supply. The VHF kept flaking out, and would only come back when I pulled the fuse and put it back in.
Ordered a replacement, waited for it, put it in...same problem! The UHF never flinched, though.
Took the power supply apart and discovered it had been built on a Friday afternoon....two of the AC power wires had never been soldered!

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Old 05-24-2011, 05:44 PM
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After looking at this thread again, I realized that some people might have a problem with the DC voltage tests. I made my first crystal set when I was 8, and 70 years later I'm still experimenting with electronics, so I've had a chance to do plenty of testing.

It's easy enough to test the DC voltage of the power supply without the preamp connected:
1. After unplugging the AC, insert a short length of no.18 gauge solid wire (same size as the center conductor of RG6) into the center of the "From Antenna" coax jack on the 0747 Power Supply.
2. Use a clip lead to connect the positive probe of the meter to that wire.
3. Insert the AC plug, and touch the negative meter probe to the outer threads of the power supply coax jack to read the voltage.

It's a little more difficult to measure the DC voltage of the power supply when the preamp is connected, because you don't have access to the center conductor anymore. You could open the power supply to measure the voltage, but that is a lot of trouble, and it isn't easy to get the AC wires back in place to replace the cover. To my knowledge, there isn't a T adapter for F connectors like there is for BNC, so you need to improvise:

1. After unplugging the AC, insert a short length of no.18 solid wire into the center of the "From Antenna" coax jack on the 0747 Power Supply.
2. Insert the other end of that wire into the center of an F-81 coupler that has been put on the end of the coax to the preamp that normally connects to the power supply.
3. Use a clip lead to connect the outer threads of the connector on the power suppy and the F-81 coupler, to connect the grounds (shields).
4. Connect the negative lead of the voltmeter to that gound connection, plug in the AC, and touch the positive probe to the center conductor to read the voltage.

If the voltage for the first test was about 22 volts, and the voltage for the second test is about 18 volts, that's normal. (The preamp could be defective in other ways, but that is rare.)

If the voltage for the first test was about 22 volts, and the voltage for the second test is near zero, then you probably have a short in the coax to the preamp.

If the voltage for the first test was about 22 volts, and the voltage for the second test is also 22 volts, then you probably have an open in the coax to the preamp.

If the tests are inconclusive, then you need to make some ohmmeter tests (no voltage on the coax please; disconnect both ends of the coax before making ohmmeter/resistance tests) of the coax that goes to the preamp to check for opens and shorts as I described in the other thread, post #18.

If you still can't figure it out, then it's time to substitute another preamp.

MikeBiker:
Please let us know how it came out.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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Old 05-24-2011, 09:28 PM - Thread Starter
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I am getting 0 volts at the From Antenna connection, without the preamp attached.
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Old 05-26-2011, 06:57 PM
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If the power supply was plugged in to AC when you made the DC measurement and you were careful not to short the output, then it sounds like a bad power supply.

I assume that you checked the meter to be sure that it was reading DC on another source of voltage.

If you can not measure it, you can not improve it.
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Old 05-26-2011, 07:24 PM - Thread Starter
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I found out that the wire I was using to do the measurement was not making contact. I'm now getting 22V. When the storms clear, I'm going to re-hookup the preamp and see if I just had a flaky connection.
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Old 05-29-2011, 06:16 PM
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Measuring the output voltage of the CM0747 Power Supply without the CM7777 connected:



Measuring the output voltage of the CM0747 Power Supply with the CM7777 connected:



I used 3/8" ring terminals on the F connectors because the black clip leads kept popping off the threads. They were attached using extra nuts from F-81 couplers.

The preamp current is about 125 mA (0.125 A).

Attachment No. 1 is the top of the PS board; No. 2 the bottom. I think C4 is not a capacitor, but a resettable fuse (more exactly, self-resetting) with the markings D2R2 CM 75 (with the C encircling the M), which protects the power supply if there is a short in the coax. I haven't figured out the specs, or who makes it yet. Note that there is a voltage drop of 0.3 VDC across it during normal use. No. 3 is a closeup of the resettable fuse. No. 4 & 5 repeat above image host photos.

More info about resettable fuses here:
http://en.wikipedia.org/wiki/Resettable_fuse
http://e-gizmo.com/wordpress/?p=290
LL
LL
LL
LL
LL


If you can not measure it, you can not improve it.
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Old 06-01-2011, 06:26 PM
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When I was helping a friend figure out why his preamp wasn't working, I discovered that he had a short in the coax to the preamp. The manager of the local Radio Shack had put a twist-on connector on the coax.

I told my friend that I didn't like the twist-on connectors, and he understood why when it easily came off the end of the coax. Also, when he replaced the connector, he screwed it on the preamp, slid the boot over the connector, and then tightened the connector through the boot with pliers! I told him that was a bad idea because he couldn't see what was going on under the boot; he should tighten the connector and then slide the boot over it. We tried the preamp; it still didn't work.

I removed the coax, opened his 7775 UHF preamp, and inspected the preamp "OUTPUT/POWER" connector. He had turned it so hard that it broke off all 4 of the connector ground lugs that were soldered to the circuit board and lifted the copper traces off the board, so that even if I had a replacement connector it wasn't possible to make it work. I put a wrench on the inside of the connector and tightened it on the outside with another wrench. I told him that his preamp was now in backup status with the ground being made through the metal plate to the other connector. He was now in the market for a replacement preamp; he needed a 7777 because some of his channels are VHF.

Since his power supply had survived a short, I decided to risk mine for some tests to see how the resettable fuse worked. I used an adjustable load resistor that I had made for testing batteries to check their capacity in Ah.

Code:
    Load      Output      Output
  Resistor    Current     Voltage
    Ohms       Amps            

    none       none        23.29 
    100        0.160       17.59
     50        0.290       15.03
     33        0.380       13.43
     25        0.460       11.97  voltage beginning to fall during measurement
                                        as resettable fuse heats up
     20        0.510       10.70  at first
               0.500       10.50  after 10 seconds
               0.250        4.21  after 20 seconds
               0.070        1.66  after 30 seconds; transformer got very warm,
                                       but the cover was off to help it cool

And to simulate a coax short:
      3        0.070        0.31  after 15 seconds; fell very rapidly
After making the above tests, I discovered that some F-81 couplers aren't very good. Using my no.18 solid wire for checking them, I found that it wouldn't even go in some of them, and it was very loose in others. I suggest that you check yours this way. It is difficult to check them with the connector on the coax because you have to insert the center conductor at the same time as you tighten the threaded collar. That is why you need a little of the center conductor sticking out, so that it starts into the jack before you start to tighten the threaded collar.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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Old 06-01-2011, 07:08 PM - Thread Starter
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Quote:
Originally Posted by rabbit73 View Post

When I was helping a friend figure out why his preamp wasn't working, I discovered that he had a short in the coax to the preamp. The manager of the local Radio Shack had put a twist-on connector on the coax.

I told my friend that I didn't like the twist-on connectors, and he understood why when it easily came off the end of the coax. Also, when he replaced the connector, he screwed it on the preamp, slid the boot over the connector, and then tightened the connector through the boot with pliers! I told him that was a bad idea because he couldn't see what was going on under the boot; he should tighten the connector and then slide the boot over it. We tried the preamp; it still didn't work.

I removed the coax, opened his 7775 UHF preamp, and inspected the preamp "OUTPUT/POWER" connector. He had turned it so hard that it broke off all 4 of the connector ground lugs that were soldered to the circuit board and lifted the copper traces off the board, so that even if I had a replacement connector it wasn't possible to make it work. I put a wrench on the inside of the connector and tightened it on the outside with another wrench. I told him that his preamp was now in backup status with the ground being made through the metal plate to the other connector. He was now in the market for a replacement preamp; he needed a 7777 because some of his channels are VHF.

Since his power supply had survived a short, I decided to risk mine for some tests to see how the resettable fuse worked. I used an adjustable load resistor that I had made for testing batteries to check their capacity in Ah.

Code:
    Load      Output      Output
  Resistor    Current     Voltage
    Ohms       Amps            

    none       none        23.29 
    100        0.160       17.59
     50        0.290       15.03
     33        0.380       13.43
     25        0.460       11.97  voltage beginning to fall during measurement
                                        as resettable fuse heats up
     20        0.510       10.70  at first
               0.500       10.50  after 10 seconds
               0.250        4.21  after 20 seconds
               0.070        1.66  after 30 seconds; transformer got very warm,
                                       but the cover was off to help it cool

And to simulate a coax short:
      3        0.070        0.31  after 15 seconds; fell very rapidly
After making the above tests, I discovered that some F-81 couplers aren't very good. Using my no.18 solid wire for checking them, I found that it wouldn't even go in some of them, and it was very loose in others. I suggest that you check yours this way. It is difficult to check them with the connector on the coax because you have to insert the center conductor at the same time as you tighten the threaded collar. That is why you need a little of the center conductor sticking out, so that it starts into the jack before you start to tighten the threaded collar.

What resets the fuse? Does it reset when it cools down or is other action needed?
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Old 06-01-2011, 07:13 PM
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It resets itself; that is why it would be better to call it "self-resetting." As it gets hot, its resistance increases to limit the current. When it cools, its resistance decreases, which takes less than a minute. The advantage of being self-resetting is that it can be inside a piece of equipment that doesn't need to be opened to replace a fuse. I quote from the wiki link that I gave in post #10:
Quote:


A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse) is a passive electronic component used to protect against overcurrent faults in electronic circuits. They are actually non-linear thermistors, however, and cycle back to a conductive state after the current is removed, acting more like circuit breakers, allowing the circuit to function again without opening the chassis or replacing anything. These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult.


If you can not measure it, you can not improve it.
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Old 06-02-2011, 11:25 PM
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For anyone who is wondering what the schematic diagram of the power supply looks like, you will find it in the attachment.

The power injector works like this:

Inductor L passes DC but blocks RF; capacitor C2 passes RF but blocks DC. This makes it possible for the RF signal and the DC power for the preamp to share the same coax.
LL

If you can not measure it, you can not improve it.
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Old 06-12-2011, 08:49 PM
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Electrolytic capacitor C1 is capable of retaining a charge that can shock you even if the AC plug has been removed. You should be able to see that charge on the voltmeter after removing the AC plug after test No. 1 Without the Preamp Connected.

You can remove that charge by carefully shorting the "From Antenna" output of the power supply, making sure the AC plug is pulled out. The meter reading should drop to zero.

After test No. 2, with the AC plug out, the voltage should drop on its own without shorting the output because the preamp will drain the charge on the capacitor.

If you can not measure it, you can not improve it.
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Old 07-04-2011, 05:52 PM
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Just found this posting while trying to confirm that "C4" is a PTC.
A minor matter, but I find capacitor C1 in the CM0747 I just bought to be directly connected to the D3/D4 output, with the "C4" PTC between C1 and C3/L1.
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Old 07-04-2011, 09:40 PM
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BrsEE:

Welcome to the forum.

I looked at my circuit board again and you are correct. Good catch; thanks.

If you can not measure it, you can not improve it.
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Old 07-06-2011, 10:59 PM
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Here is the revised diagram for the CM0747 Power Supply. The attachment is the same diagram in case the link to my image host is broken.


LL

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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