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Cleaning the input jacks |
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Check to see that the jack nuts are tight and
making good contact with the chassis. Insert a rolled
up piece of emery cloth or a metal brush into each
jack and rotate it until the jack barrel looks clean
and free of corrosion. Take a piece of emery cloth
and insert it between the tip and switch on the jack.
Pull the emery cloth so that the connection between
switch and tip is cleaned. After cleaning the barrel
and switch, spray some tuner cleaner into the jack.
Insert a plug and rotate it a few times.
Check each jack to make sure the jack is grounding with no plug inserted. With the amplifier on, touch the tip of each jack with your fingertip. If the amplifier hums or makes noise when you touch the tip, the jack tip is not shorted to ground. Try putting some more spring into the switch so that it touches the tip more firmly. If this does not work or the switch returns to it’s old location, replace the jack. The resistance of the jacks can be checked by using a multi-meter. Insert a dummy 1/4 inch phone plug into jack # 1. Connect the meter between the tip and ground of the plug. Read the resistance. Jack #1 should read 1 meg (+-10%). Jack #2 should read 136k (+-10%). Now measure the resistance between tip and the end point where the two 68k resistors are soldered together. Jack #1 should read 34k (+-10%). Jack #2 should read 68k (+-10%). If the resistance readings are not correct, check the following.
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(1) A switch may not be closed and may need
cleaning or replacing. (2) The jack barrel may be corroded and may need cleaning. (3) Check all solder joints on and between the two jacks. (4) Check to see that the jacks are tight and there is a good connection between the jacks and the chassis. (5) The resistors attached to the jacks rarely need to be replaced because they do not see high current and do not heat up. They may crack from vibration or from being bent, but this is rare. |
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| If a jack needs replacing I usually replace both jacks and all the resistors. It is very difficult to unsolder the old components without overheating them. It is easier to start from scratch and build the inputs. Label the jacks one and two so you don’t forget how they were installed. Remove the old jacks. Insert the new jacks. Line up the new jacks so that the tabs touch just like the old jacks. Tighten the new jacks. Add the resistors to the new jacks by copying the old jacks. |
| Cleaning the pots |
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The pots need to be cleaned if they produce
a scratchy noise when rotated (except a presence control).
I clean all the pots when servicing the amp. Using
an electronic tuner cleaner, put the plastic tube
on the end of the spray can and insert it into the
top opening of the pot. Spray a tiny amount of cleaner
into the pot and rotate the pot from zero to ten,
five times.
Do not spray the cleaner so that it runs out the bottom of the pot or sprays all over the outside of the pot. Some cleaners contain a lubricant that does not dry. If the amp has cloth wires, the wires under the pots will absorb the lubricant and remain wet, this can cause problems with cloth covered wire. On plastic wires this is usually not a problem, but the lubricant will collect dust over time and this can also cause problems. You can save some time by spraying all the pots at once and then use the side of your hand to rotate all the pots by sweeping back and forth across all the pots. If a pot is still scratchy after cleaning, spray it once again using enough cleaner to rinse out the pot. First pull out any cloth wires that are under the pot and put an absorbent cloth or paper towel under the pot to soak up the excess cleaner. Rotate the pot several times again. If the pot is still scratchy sounding the pot may need replacing or it is possible that there is direct current (dc) running through the pot, as there is in some presence controls. See the scratchy pots and direct current on pots section on the Amp noises page. |
| Clean and retention the tube sockets |
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It is a good idea to check the tube sockets
for sloppy clamps that are not gripping the tube pins
properly. A pointed instrument like a dental pick
or small flat head screwdriver can be used to reach
into the socket and squeeze together any sloppy tube
socket clamps. Remove all the tubes and keep them
in the order they were removed, unless they were wrong
to begin with.
First look at the nine pin sockets. Gently press together any clamps that look like they are too far apart. After retentioning all the 9-pin sockets, spray a little contact cleaner into each socket. Take a paper towel and wipe out any excess spray that may be sitting on top of the socket. Take each pre-amp tube and spray a little contact cleaner on all the pins. You may have to clean the pins with a metal brush or emory cloth if they appear dirty or corroded. Insert each 9-pin tube back into its proper tube socket. Later, after turning the amp back on, you can wiggle these tubes and listen for any scratchy noises which indicates that the tube still is not making good contact. If a pre-amp tube refuses to make good contact with the tube socket after cleaning and retentioning the tube, it may need to be replaced or swapped with another tube. Next move to the 8-pin power tube sockets. These sockets take more abuse than the 9-pin sockets and this is where retentioning really helps. These sockets have a circular piece of metal that is supposed to wrap around the tube pins. When power tubes are removed and replaced the metal clamps lose some of their tension and don’t grip tight enough to make a good connection. Heat from the upside down tube does not help the situation either. The only thing to be careful about here is that you do not close the socket clamp so much that the tube will not fit into the socket. I do not recommend using a cleaner on the power sockets because of the high voltage present on these sockets. The cleaner can conduct electricity and arcing may occur. When arcing occurs a very high voltage will seek a lower voltage or ground and jump across that point leaving a burnt channel of carbon between the two pins that it jumped. The carbon is difficult to remove and the pins will probably arc again if all the carbon is not removed. I have seen a channel cut right through the tube socket when two pins arc. After retentioning the power tube socket clamps, insert each power tube with a small amount of rotation so as not to stretch out the socket clamps again. After the power has been turned on to the amp you can gently wiggle the power tubes and listen for any noises that may indicate that the socket is still not gripping the pins properly. If a socket is just plain worn out and refuses to grip the tube pins, it will need to be replaced. If you ever hear a distorted, low volume tone coming from your amp, first reach back and see if all the power tubes are hot. If a tube or tubes are not hot, it usually means that the heater pins, two or seven are not making good contact or are corroded. Pins two and seven are the 6.3-volt ac heater pins and the tube will not operate without a heater. Go to the Heater system page and read about the heaters. |
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