Before using the switcher on a questionable amplifier, you need to go through all of its features with a known good amplifier. Please ead the NOTES FOR USE at the end of this features list.
This switcher is designed to make initial testing of amplifiers easier, quicker, safer and more convenient than it would otherwise be. This short manual will introduce features and explain why they're important.
Main power switching:
This allows you to control the power for the switcher and the amplifier. When working on any piece of equipment, there's a good possibility that it will draw significantly more current than it should. When initially testing an amplifier, you would connect it to the switcher and (with the remote switched to the 'off' position), turn on the main power. If you found that there was no significant current draw (there should be essentially 'zero' current draw without remote power on the amplifier), you would know that there were no components (FETs, reverse protection diodes, solder bridges...) shorted across the power supply inputs (B+ and ground on the amplifier).
Remote power switching:
The remote switch on the switcher obviously controls the output to the amplifier's remote input but it has a couple of other features. When working on a piece of equipment, you may not notice that the remote lead has become disconnected (or isn't connected because it's not needed). Since the remote circuit isn't capable of handling 30+ amps of current, it would be damaged if it was switched on and was allowed to contact a grounded component. To protect it, there is a current limiting resistor and a buzzer to alert you to the problem. When you initially power up the switcher, you should ground the remote wire directly to the ground wire of the switcher. This will allow you hear what the buzzer sounds like when the remote output is grounded (either accidentally or due to a defect in the remote input of the amplifier). Please note that you must be attentive to the audible warnings from the switcher. If the remote lead is grounded for a period of longer than 5 seconds, the resistor will burn out. It's not, however, a great tragedy if this happens. A replacement resistor is less than $2.
Power selection for the amplifier:
When you initially connect an amplifier to the switcher for testing (especially after replacing all known defective components), you will need to be cautious so as not to damage any of the new parts. To make this easier, I've included a limit/direct power switch. On limit, the main power output passes through a 2 ohm resistor. This will provide enough current to allow most amplifiers to power up but will help prevent damaging newly replaced parts if there is something wrong with the amplifier. Keep in mind that you have to watch the amp-meter of your power supply to determine whether there is a problem. If the amplifier comes on and draws ~1 amp or less, then everything is likely OK. If you see the amperage is greater than 5 amps, you know that there are more defective components in the amplifier.
To see what the buzzer on this line sounds like, switch the main power off and set the direct/limit switch to limit. Connect the main power and ground together (the power and ground leads coming OUT of the switcher -- not the main power supply leads) and then switch on the main power switch. You'll notice that the current draw is ~7 amps if your power supply is set to 14VDC. This is safe to do for a few seconds but shouldn't be allowed to continue for more than that. When testing some amplifiers (especially when they're out of the heatsinks -- as you will need to do when troubleshooting certain problems), the idle current of the amplifier will cause the buzzer to buzz (actually a constant high frequency tone) slightly. This can be used to audibly monitor current draw while troubleshooting. If you hear the buzzer gradually getting louder, you know that there may be a problem (as when the output transistors start to go into thermal avalanche or if you inadvertently short something) and you can quickly switch off main power until you can determine the cause of the increased current draw. This one feature will save you a lot of money that would otherwise be spent on blown P/S FETs. Please note that you must learn the difference in the intensity of the output of this beeper/buzzer. At low current draw, it will be barely audible. When there is a problem, it will be louder.
Note: When the switcher is set to 'limit', the internal fan will run to help keep the load resistor cool. Be sure to keep the area air intake clear.
Switching between an internal dummy load and external speakers for the amplifier:
When testing any type of audio equipment, you'll want to listen to the output to determine whether it sounds as it should. Since it can be annoying to listen to an amplifier at 50-100 watts through your bench speakers as you check each channel with the scope (to make sure the output is undistorted/symmetrical), you can switch the amplifier's output to the internal dummy load.
Note: * The internal dummy loads are rated at 25 watts for initial testing, and are intended to be used only for short term testing. I've tested them with a 50 watt/channel amplifier running up to clipping with music for approximately 1 hour and they did not fail. They can take significantly more than their rated power for 'short' periods of time. The fan cooling helps them handle more than they would with simple convection cooling. For long term/high power testing, you will use an external dummy load. * When the switcher is set to the internal dummy load, the internal fan will run to help provide forced-air cooling of the internal dummy load resistors.
Switching between the amplifier and the bench audio source (receiver or head unit) to the external speakers:
Often, when working on an amplifier, you will want to listen to the radio or CD. To make this easier, there are connections on the switcher for your bench speakers and for the audio source (either a home receiver or a head unit -- whichever works best for you). Since it's inconvenient to have two sets of speakers on the bench or to have to turn down the radio to listen to the amplifier you're testing, the switcher automatically switches between the radio and the amplifier's output. When the main power is off, the switcher defaults to the radio. When the main power is on and the 'speaker' select switch is set to 'receiver' the radio will play through. If the speaker switch is set to 'device under test', the amplifier will play through.
On-board volume control:.
Of course, the main purpose of the volume control is to allow easy access. Typically, the source unit is mounted in an inconvenient location (out of the way) and the on-board control is easier to reach. For those source units that have the push-button volume controls, the on-board potentiomenter allows quicker/more convenient adjustment of the volume.
When working on amplifiers, there will be a few faults that will cause voltage do be applied to the RCA jacks of the amplifier. When this happens, this will typically destroy the shield ground of the audio source (head unit or receiver). To help protect the shield ground, the switcher has a built-in protection circuit. It puts resistors in series with the shield ground between the input and output jacks. To let you know that there is a problem, the buzzer (the same one used with the remote protection) will sound. When you have an amplifier with this problem (typically a shorted transformer), you can use the buzzer to help find the fault. In the case of a shorted transformer, you would twist the transformer (with the amplifier off - generally) to see if the fault is corrected. Twisting the transformer moves the windings and often clears the short temporarily.
Connections between the switcher and the amplifier:.
To make life easier, you should use a universal connector between the switcher and the test leads. One connector that works well is the molex connector (either a 9-pin or a 12-pin). This will allow you to change between different lead types (bare wire, spade lugs, alligator clips...). You will make up the different test leads (a few shown on the equipment page of the tutorial) and simply use the one that best fits the amplifier you're working on.
Connections between the switcher and the power supply:.
If you're using a 30+ amp power supply (recommended), you will need relatively heavy leads between the power supply and the switcher. Since the terminal block is only designed to handle up to 14g wire, there are multiple power/ground connections. You will need to run two 14g wires each for +B and ground between the switcher and the supply. Be sure to tighten the terminal block screws well (but don't strip them off). If they're loose, they will have undesired resistance and will cause the block to melt when high current is drawn through them.
Testing at high power levels:.
The intended purpose of the switcher is to facilitate safe/controlled/convenient testing at low to moderate power. This is the type of testing that you will be doing most of the time. For high power/long term testing, you will need to use external dummy loads and auxiliary power connections. The dummy loads that I recommend are the tubular ceramic wire-wound type (shown in the tutorial). There is a connection on the switcher to control the fan on the external dummy loads. Whenever the switcher is set to dummy load, the external fan will be activated. I strongly suggest using the external fan. It will not only prolong the life of the high power dummy-load resistors but it will also keep them at a relatively safe temperature (in case some or someone comes in contact with them). The leads going to the external loads will not pass through the switcher (they will be connected directly to the amplifier). The auxiliary power leads can simply be 2 large power wires (8g) connected directly between the power supply and the amplifier.
NOTES FOR USE
Getting used to the operation of the switcher.
Shorted Power and Ground Leads:
After making up all of the cables that go with the switcher, you need to do a few things to see how the switcher responds. Set the main power switch to ON and the limit/direct switch to LIMIT. Now short the 12 volt output wire (from the switcher) and the ground wire. You should hear a loud, high pitched tone. This is what you'll hear if you connect an amplifier to the switcher and it is drawing too much current or has a direct short. If an amplifier is simply drawing a normal amount of idle current, you will hear the same tone but at a much lower level. If you switch the limit/direct switch to DIRECT and short power and ground, you'll hear a high speed chatter from the internal relays. As the voltage is pulled down (due to the direct short), the relays disengage. As soon as the relays disengage, the voltage goes back up and the relays re-engage. This happens at a frequency of 20 or more times a second until the power supply shuts down, the fuse blows or the short is removed. This is a VERY bad situation and should be avoided. There is a power line fuse inside that may blow. If it does, make sure that the replacement fuse is no larger than 30 amps.
Shorted Remote Lead:
Now set the main power and remote to ON and ground the remote output to the ground wire for the switcher. You should again hear a buzzer (slightly differnt than the main power buzzer). This is what you'd hear if you accidentally short the remote lead to ground. Again, immediately remove power until the short in the remote lead can be corrected.
Shorted Shield Ground to +12 Volts:
Set the main power to on and touch the shield ground of the output RCAs to the positive terminal of the switcher. You will again hear a tone (the same one as in the remote lead). This is what you'll hear if you accidentally allow the RCA shield to contact 12 volts or if you have a shorted transformer (only on some amplifiers where the primary shorts to the secondary and the secondary ground is directly connected to the audio shield ground).
Normal Operation with Test Amplifier:
Now connect a source unit, a pair of external speakers and a test amplifier (in proper working order) to the switcher (including the RCA cables - set the volume to minimum). Set the limit/direct switch to DIRECT. Set the source/DUT switch to DUT (Device Under Test). Set the speaker/dummy load switch to SPEAKERS. Switch main power and the remote to ON. Now, slowly increase the volume using the volume control on the switcher.‡ The amplifier should play through the speakers and the output level should be controlled by the volume control on the switcher.
‡: If you're using a head unit for the signal source, you must remember to set the volume on the head unit above zero for the volume control on the switcher to send signal to the amplifier. If you use a home receiver, the tape output will have a constant output signal level. From my experience, an old home receiver is the best for a signal source if you have the required space on your bench.
Now set the direct/limit switch to LIMIT (the remote and main power should also be ON). Set the volume level to its minimum position. You should hear a tone from the warning beeper but it should be at a relatively low level. If you switch the speaker/dummy load to DUMMY LOAD and increase the volume slightly, you will hear that the tone increases in volume as the current requirements of the amplifier increase (on louder passages). Of course, you would never use the dummy load and the LIMIT position at the same time but this shows you how the tone changes when the amp draws more current.
If you had an external dummy load, you would need an external fan to cool them. When the switcher is set to DUMMY LOAD, the external fan will be driven.
NOTE:
You will notice that anytime that the switcher is set to dummy load and/or limit, the internal fan will run. The fan runs to keep the internal limiting resistor and the internal dummy loads as cool as possible.
There is no warranty on the internal dummy loads OR the limiting resistor. They can be damaged by abuse (or inattention to dangerous situations). If you aren't paying attention and they're damaged or if you simply drive them too hard, replacements can be purchased from Digi-Key for ~$6 each.
Listening to the Source Unit:
The signal from the source unit will pass to the external speakers any time the main power switch is set to off. You can also set the source/DUT to source to listen to the source unit while the main power is on.
The interactive demo below shows some of the switching for speaker level audio and power. Click on any of the switches to move their handles to the alternate position. The main power switch (left-most in the demo) must be set to the right (on) position for the switcher to work. The LED next to the main power switch is to let you know that you have power feeding the switcher and is always on as long as the 12 volt power supply is on.
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