10 - Checking the circuit
barry (Administrator) 12-31-2013 at 05:47 AM.
Post: #1
Previously we've tested the high-voltage power supply and the low-voltage heater supply. Since then we've added the signal path circuit and connected it to both.

Before we power up again, it's worth doing some basic checks to catch many simple mistakes that could cause the amplifier not to work. We'll check these before applying power to the amplifier.

The first of these are ground continuity checks. Get your amplifier chassis on the bench and set all controls to minimum and set the Boost switch to the centre position and the Bypass switch up. If your DMM has an audible continuity mode select that, otherwise choose the lowest DC resistance setting. In these tests, we're looking for a DC resistance of 1Ω or so (very few DMMs can accurately display resistance under 1Ω, so you will never get a perfect continuity reading of 0Ω).

Touch one of the DMM's probes to the chassis grounding lug close to the Input socket (J1) and then check the points shown in the diagram and check that all of the points with a green X in this diagram show continuity to ground. If any of them fail this test, check the solder joints and other connections.

[Image: n5xcont1.jpg]
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barry (Administrator) 12-31-2013 at 05:51 AM.
Post: #2
Checking the Boost and Bypass switches
The Boost and Bypass switches both alter the grounding of certain components and connections and you can check them, too. First, change the Bypass switch and check lug 3 of the Middle pot. It should show no continuity to ground.

The Boost switch has two active positions. Up is the 'Bright' mode and in this position the two points shown with red 'X' marks should show continuity to ground, but the two points shown by the blue 'X' marks should not.

[Image: n5xcont2.jpg]

Change the switch to the Down position and the amp is in 'Fat' mode. Now the opposite should be true.
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barry (Administrator) 12-31-2013 at 05:54 AM.
Post: #3
Checking the Earth connection
Plug a guitar cable (but without guitar) into the Input, and then plug a speaker cable (but without speaker cabinet) into any of the outputs. Now plug a mains (IEC) lead into the amplifier BUT DO NOT PLUG THE OTHER END INTO THE MAINS SOCKET (because at this stage, we’re just checking connections, and we don’t want any voltage inside the amplifier)

[Image: n5xps17.jpg]

Now use your multimeter to check for continuity for each of the following:

1) Between the Earth (or ground) pin of your mains plug and the shield of the plug at the end of the guitar cable

2) Between the Earth (or ground) pin of your mains plug and the shield of the plug at the end of the speaker cable

3) Between the Earth (or ground) pin of your mains plug and the two M4 grounding lugs that are fixed to the chassis

4) Between the Earth (or ground) pin of your mains plug and the two M3 grounding lugs that are on the main turret board

5) Between the Earth (or ground) pin of your mains plug and the central lug of the 3-way tagstrip

If you read anything other than good continuity for any of these tests, go back and check your wiring. Contact Amp Maker if you cannot locate the problem. Your amp is NOT safe to switch on until this is fixed.
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barry (Administrator) 01-20-2014 at 04:50 AM.
Post: #4
Direct links to reference diagrams
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