When I try to test the amp with scope and sine wave generator, the relay clicks on and off and dim bulb tester flashes with each click. With no input all measurements on the amp board are correct; resistors in spec, caps all new. Power transistors not shorted testing them with diode setting on meter..
I am new to this hobby, learning from you tube and some textbooks, and just starting to read boards and post. So, is this the way to find technical answers?

 

Download the service manual here..

https://elektrotanya.com/pioneer_sa-7500.pdf/download.html

The relay is to protect the speaker outputs and is on page 33 of the manual.

It would help you better if you describe the problem without the testing.

 

Download the service manual here..

https://elektrotanya.com/pioneer_sa-7500.pdf/download.html

The relay is to protect the speaker outputs and is on page 33 of the manual.

It would help you better if you describe the problem without the testing.

Thank you for your reply.
When I turn on the amp, relay click and voltages are normal (idle). When I input a signal thru aux or thru tuner inputs, I get abnormal behavior. The relay clicks on and off and the dim bulb in series to the power cord, which detects high current, also flashes on and off. The DC measurements on the speaker terminals, which are protected by the relay as you point out, indicate negligible voltage.

What is the next step?

 

Remove the bulb and power it up normally.

 

Remove the bulb and power it up normally.

The bulb is in series on the AC power. It is used to indicate excessive power draw by the unit being tested. It is protective, like a fuse, and is recommended in testing a unit which may be faulty. Mine IS faulty and I am seeking advice on further testing. You may be interested in the descriptions of "dim bulb tester" on the net.

 

Dodgydave is correct. While an incandescent light-bulb may act as an over-current indicator, it may also prevent the unit from operating correctly.
If the unit draws too much power the input fuse would blow. Diagnosing such a problem is a difficult task. In such cases, I have an electronic fuse which is simply a circuit breaker that trips when the current exceeds a set limit. Without such test equipment one has to resort to clever or brute force techniques for finding the problem.

Having a large supply of fuses may not sound like an attractive strategy.
The first step would be to locate the power supply circuitry and to disconnect the output of the power supply from its load.
Another step would be to disconnect the output amplifier circuitry or remove the power transistors in the output stage.

At the very least, you have to get the circuit schematics and look over the power distribution. The most common components to fail are the output transistors and power supply smoothing capacitors.

 

Here is a partial schematic for Pioneer SA-7500 KCU showing the power amplifier stage.

Disconnect the signal driving the inputs of the Left and Right Channel power amps at C1-R3 and C2-R4.
Proceed to test using the signal generator and oscilloscope.
Don't be afraid to ask questions for further guidance.

 

Here is a partial schematic for Pioneer SA-7500 KCU showing the power amplifier stage.

Disconnect the signal driving the inputs of the Left and Right Channel power amps at C1-R3 and C2-R4.
Proceed to test using the signal generator and oscilloscope.
Don't be afraid to ask questions for further guidance.


View attachment 134097

Thank you for giving me direction. Is there an electrical explanation why the dim bulb interferes?.
I will report back...

 

Thank you for giving me direction. Is there an electrical explanation why the dim bulb interferes?

What is the voltage and wattage rating of the light bulb?
Let us assume for the moment that the bulb is rated at 100W @ 120V.
The amplifier is rated at 190W. With the bulb in series with the amplifier AC mains input, the voltage to the amplifier cannot reach above 60% of the required input voltage. We don't know how the power supply and the rest of the amplifier will behave at reduced voltage input.

 

What is the voltage and wattage rating of the light bulb?
Let us assume for the moment that the bulb is rated at 100W @ 120V.
The amplifier is rated at 190W. With the bulb in series with the amplifier AC mains input, the voltage to the amplifier cannot reach above 60% of the required input voltage. We don't know how the power supply and the rest of the amplifier will behave at reduced voltage input.

The bulb is 60 watt, 120 volt, but I have all wattages if another size is preferred. With that bulb and the volume about 25%, I got oscillation with the relay which I could hear in synch with the bulb flashing which I could see. It was a total sensory experience--just waiting for fumes which did not happen.

BTW, I took the bulb out of the circuit and turned on the amp with very low volume on the knob. Now the amp will not turn on or at least the front panel bulb will not light. But the 3 fuses are ok.

I assume I lift the resistor to cut off the signal to the power transistors on the amp. What am I measuring? My original attempt was to play thru
8 ohm dummy load and measure clipping.
I

 

I assume I lift the resistor to cut off the signal to the power transistors on the amp. What am I measuring? My original attempt was to play thru
8 ohm dummy load and measure clipping.

You input a signal source (sine wave or music) into the input stage and check that the signal gets to the output of the preamp stage, both left and right channels.

Then you assume that one of the power amp channels is causing a problem. You assume that only one channel is causing the problem. Connect one channel at a time and find which channel is bad.

 

I will do. as soon as new caps come for the other boards (power board already finished).
Will check in towards the end of the week, I hope.