Electronics > Beginners

Beginners Road To Repair Part 2

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don@cascadefellowship.org:

--- Quote from: Shock on October 03, 2018, 01:07:41 pm ---I recommend you move this post to the repair section of the forums.

Obviously be careful with a live circuit could be any voltages present. In the manual it states that the constant flashing is a amplification self test failure. It puts the unit in some kind of safe mode. I would test the op amps and transistors. Won't hurt to check the different power rails though especially if there is +/- for opamps and the logic supply.

There is a gain control pot as well you might want to measure where it was set at and then lower it so see if it makes any difference as well as apply an input signal. If you need a cheap floating audio signal source you can buy cheap Chinese function generators on ebay starting at about $10-$20.

--- End quote ---

Wow I completely missed that!  Section 4.1.  Thanks!!! 

I did adjust the gain and drive pots to see if there was any change, which there was not.  Power LED continues to blink and the Compression LED's are in Overload.

Instead of checking the voltage regulators first I think I'll start with op amps and transistors.  Or perhaps just getting into the habit of checking voltage regulators first and them op amps and transistors would be better?  Just looking for a general order of basic component checks, that sort of thing.

don@cascadefellowship.org:

--- Quote from: Shock on October 03, 2018, 01:07:41 pm ---I recommend you move this post to the repair section of the forums.

Obviously be careful with a live circuit could be any voltages present. In the manual it states that the constant flashing is a amplification self test failure. It puts the unit in some kind of safe mode. I would test the op amps and transistors. Won't hurt to check the different power rails though especially if there is +/- for opamps and the logic supply.

There is a gain control pot as well you might want to measure where it was set at and then lower it so see if it makes any difference as well as apply an input signal. If you need a cheap floating audio signal source you can buy cheap Chinese function generators on ebay starting at about $10-$20.

--- End quote ---

I have a signal generator that I can use for input and I don't know how to move this thread, yet.

don@cascadefellowship.org:

--- Quote from: Shock on October 03, 2018, 01:43:46 pm ---When I see big wattage resistors the first thing I look for is dry solder joints and I can see there is a possible dry joint there. If in doubt give them a reflow and ensure you have a low ohm path on any trace that connects to them.

The transistors and regulators on the heatsink can have insulated screws so make sure you keep track of what one was used where. That is a good early lesson to learn that because something is on  a heatsink doesn't mean it's meant to be electrically bonded to it. Heatsinks on many devices are a touch hazard and aren't always safety labelled.

That heatsink looks like it might be attached to the chassis. You can test this though to be sure and if other parts of the circuit that look like they should be are in fact properly connected.

The two electrolytic filter caps have plastic disc covers that often warp as the plastic shrinks a little. Nothing too concerning if the feel flat when you push them down.

--- End quote ---

I can see that identifying a dry solder joint is going to take some practice.  It's as if there's a small circle around the dry joint and nothing but a smooth slope of solder on a healthy joint.

don@cascadefellowship.org:

--- Quote from: tpowell1830 on October 03, 2018, 04:51:16 pm ---
--- Quote from: don@cascadefellowship.org on October 03, 2018, 04:42:39 pm ---
--- Quote from: tpowell1830 on October 02, 2018, 08:35:15 pm ---Hi Don, the big electrolytics on the lower right in the first pic seem to have bulges on the top, can you verify?

--- End quote ---

Sure.  It's possible that the one furthest to the right might have a minute bulge.  Very, very small though.  Here's a pic with the straight edge of the frame as a reference.

--- End quote ---

From that angle, doesn't seem to be bulging at all. If they were bulging, wouldn't mean that is your problem, only time to change them out with your maintenance. There have been thoughts that the firmware is telling you something with the blinking LED, do you have error codes or manual?

EDIT: I see you already posted that you don't have good information.

--- End quote ---

I overlooked the error in the manual as it was under installation and not troubleshooting.  Here's what it says:

4. OPERATION & SYSTEM SET-UP
4.1 Start-up Procedure
An internal self test system has been incorporated in the equipment. When switching on, the power LED will flash for some 5 seconds, during which time the amplifier is tested for correct operation. If correct, the unit will then switch to an operational mode, and the LED will remain on without flashing. However, if an amplifier or loop fault exists, or develops during operation, the equipment will return to a safe mode, and the LED will flash continuously.

don@cascadefellowship.org:
Testing IC 1 in circuit.  TL074N

No pins connected to ground.  Pins 6 and 7 are shorted together (continuity test then visually confirmed by looking at the trace)

Pin 1   Out 1   4.053-4.058 volts (fluctuates)

Pin 2   In 1-   3.072-3.078 volts (fluctuates)

Pin 3   In 1+   1.507 volts (steady)

Pin 4   Vcc     15.00 volts (steady)

Pin 5   In 2+   1.513-1.517 (fluctuates)

Pin 6   In 2-   2.536-2.539 volts (fluctuates)

Pin 7   Out 2   2.527-2.529 volts (fluctuates)

Pin 8   Out 3   2.087 volts (steady)

Pin 9   In 3-   2.077 volts (steady)

Pin 10  In 3+   1.651 volts (steady)

Pin 11  Vee     .738 volts (steady)

Pin 12  In 4+   1.644 volts (steady)

Pin 13  In 4-   2.076 volts (steady)

Pin 14  Out 4   2.084 volts (steady)

Some questions:

Shouldn't the + and - pins of each input read the same voltage?  Is it correct to assume they should be equal?  I'm an audio guy so I guess I may be assuming that a positive voltage should have an equal and opposite negative voltage for a single input.

How steady should the readings be when testing op amps or other electrical components?  I guess in theory I would be comfortable in saying the more steady the better but are fluctuations in the order of thousandths of a volt a problem or to be expected?

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