Hi all, bit of a newbie here but hoping I can make some progress with the right guidance.

I recently picked up an old Technics amp that will not power up properly. I get the initial orange light, but the (speaker?) relay is staying open.
I'll admit to only seeking Chatgpt's guidance so far. In the process I have identified 3 burned out resistors and some unusual voltage values.

I've asked Chatters to summarise what we have measured so far:

Initial symptoms

• Orange power/voltage-control indicator illuminates.
• Red/current-drive indicator does not illuminate.
• Speaker protection relay does not click.
• Speakers and all inputs have been disconnected during testing.
• The amplifier is connected directly to mains, not through a dim-bulb tester.
• Speakers have not been connected while DC is present at the outputs.

According to the service manual, the relay and red indicator should activate after the initial protection delay. The amplifier appears to be remaining in protection.

IC501 – SVI3204 measurements
All DC readings are relative to confirmed circuit ground.

IC501 pin	Measured
1	−40 V
2	−6 V
3 – channel output	−3.7 V
4 – positive main rail	approximately +40 V
5 – negative main rail	approximately −40 V
6 – channel output	−3.7 V
7	−3.7 V
9	0 V
10	−6 V
11	0 V
14	0 V

Pins 11 and 14 confirmed that the meter ground reference is correct.

Both amplifier outputs, pins 3 and 6, are sitting at approximately −3.7 V DC. This appears to be why the protection relay is not engaging.

The main rails are present and reasonably balanced at approximately ±40 V, although the service manual shows nominal voltages closer to ±48 V.

IC401 – AN7062N measurements

IC401 pin	Measured
1	0 V
2	−2 V
3	−5.6 V
4	−2 V
8	−1.8 V
9	−0.8 V
10	−12 V
11	−1.2 V
15	0 V
16	−11.3 V
17	−4.5 V
18	−10.3 V

The negative supply-related readings at pins 10 and 18 are present, although pin 10 appears lower than the service-manual value.

The expected positive voltages around pins 4, 8, 9, 11 and 15 appear to be absent or collapsed. Both channels of IC401 appear to be biased abnormally negative.

Resistor observations
I attempted to locate and measure R701 and R702. The components I identified from the PCB markings gave:

• One resistor: approximately −37 V on both sides.
• Other resistor: approximately −39 V on both sides.

These readings do not match the schematic expectation that R701 should be associated with the positive rail and R702 with the negative rail.

I have also noticed that R438 and R439 appear visibly heat-damaged or burned. I have not yet lifted a leg or conclusively measured their resistance out of circuit.

The service information appears to show R438 and R439 as low-value resistors in the voltage-control/output-stage circuitry. I am concerned that replacing them without identifying the cause could result in the replacements burning again.

Current working possibilities
The fault appears to involve a shared part of the voltage-control circuitry, because:

• Both output channels have almost identical −3.7 V DC.
• Both main supply rails are present.
• Both halves of IC401 show abnormal voltages.
• Positive voltages expected around IC401 appear to be absent.
• R438 and R439 may both have overheated.

Possible causes I am considering include:

• Failure of IC501/SVI3204.
• Failure of IC401/AN7062N.
• A missing positive supply or open PCB connection feeding IC401.
• Failed transistors, diodes or capacitors around the voltage-control stages.
• R438/R439 being open or damaged due to a downstream short.
• Cracked solder joints or damaged PCB tracks.

I have stopped powering the amplifier until I can check the suspect resistors and surrounding semiconductors with power removed.

------------------------------------------------------------------------------------

Chatgpt seems particularly puzzled that both R701 and R702 appear to have negative voltage; to the point that I don't think it trusts my abilities to test them properly.

If anyone could provide some further guidance it would be much appreciated. As I mentioned, I'm not particularly experienced, this is my first amp repair job, but I have a multimeter and a soldering iron and a bit of perseverance..

Thanks,

Tig

 

These SVI "chips" are common failure points with little to do but replace them if available. Check for poor solder connections at the regulators, replace the damaged resistors and hope for the best. But, you'll likely need to replace the output pac (SVI3204) at the very least.