Amplifier repairing

[justas12]

Hello,

I found old amplifier, and i want to repair it. I changed pair transistors d718 and b688. The protection light stoped lightning. But in the output the is 30V voltage? How can I solve this problem? Model Fli loaded 250. It is shown in photos.

https://i44.tinypic.com/33nx0xy.jpg
https://i39.tinypic.com/mvhk07.jpg
https://i42.tinypic.com/9h3km8.jpg

 

[Nigel Goodwin]

You need to change all the faulty parts, not just the output transistors - which may now be faulty again - it's rare that output transistors just fail, normally something else makes them blow. Even if the outputs did just blow for no reason, the consequences often blow other components as well.

 

[justas12]

but i don;t see any damaged component. where i should start?

 

[Nigel Goodwin]

but i don;t see any damaged component. where i should start?

Locating (or drawing out) the circuit diagram would help for a start.

Obvious things to check are the driver transistors and the emitter resistors.

 

[alec_t]

But in the output the is 30V voltage? How can I solve this problem?

That voltage is not unusual for a power amp output, so it may not be a problem. What supply voltage is specified for the amp?

but i don;t see any damaged component.

Many components fail with no visible clues as to their faulty state.

 

[justas12]

supply voltage 12-14V

 

[alec_t]

You're getting 30V out from 12V in? Sounds like the amp's oscillating and some inductance or voltage-doubling is doing that. Replace all electrolytic caps for starters.

 

[justas12]

exactly input 12V, output 30V. Ok, i'll try to replace elecrolytic caps.

 

[Nigel Goodwin]

You're getting 30V out from 12V in? Sounds like the amp's oscillating and some inductance or voltage-doubling is doing that. Replace all electrolytic caps for starters.

Perhaps you should have looked at the pictures he posted, before giving extremely bad and totally incorrect advice?.

The amplifier in question is an in-car one, appears to be mono only (for a sub) and uses a switch-mode PSU to increase the battery voltage, and presumably give a split supply - presumably +/-30V and should have zero volts at the output..

 

[alec_t]

Oops, my bad. Delete advice.

 

[unclejed613]

in the second picture, the output transistors are the ones on the left end of the heatsink. the transistors on the right side of the heatsink are the power supply switchers, and if you're getting 30V rails, then the power supply is working. the outputs are likely shorted, as well as the drivers, bias transistor, and possibly all the transistors back to the diff amp.

 

[justas12]

to unclejed613: can you explain simply? I dont understand last sentence

 

[Nigel Goodwin]

to unclejed613: can you explain simply? I dont understand last sentence

If you don't understand what he said you shouldn't be attempting to repair an amplifier.

If it helps you at all?, 'diff amp' was short for 'differential amplifier'.

 

[unclejed613]

the two metal backed transistors below the outputs are the drivers, the two ceramic blocks are emitter resistors for the outputs. the resistors feeding the base lead of the output transistors should be replaced (the one i can see near the left output looks burned). then there are 8 small transistors in a double row , and it's likely that most or all of them should be replaced, and any resistors or diodes in the circuits around them should be checked.

sorry about confusing you, but Nigel is right, it would be a good idea for you to study some amplifier theory. i'm working on a simplified amp tutorial here: https://www.electro-tech-online.com/threads/has-anyone-built-this.473/ , and have posted some other resources for learning amp theory here: https://www.electro-tech-online.com/blogs/unclejed613/129-audio-design-resources-web.html

 

[justas12]

hello, so i replaced 8 transistors in double row, and now voltage in output is 0V. But I haven't tried is it working or not, yet. I found four of transistors not working. Tomorrow I'll try to connect audio signal.

Changed parts shown in red circles:
https://i42.tinypic.com/14tkw5.jpg

 

[unclejed613]

i would also replace the two big transistors on the heatsink, which are 2SA1492 and 2SC3856. also replace the two big ceramic resistors and the small blue resistors that look burned.


you need to replace the parts in the blue circles

correction, after looking closer one of the blue circles is around a green mylar cap, you shouldn't need to replace that, but there's a burned blue resistor somewhere near it that needs to be replaced... maybe next to the larger blue resistor that's next to the right-hand white resistor. in any case, the output devices, the white resistors, and the blue resistors that go to pin 1 of the output transistors, should all be replaced.

 

[justas12]

I checked mentioned details with multimeter and it shows actual values. I think it is not necessary to change it. Transistors 2SA1492 and 2SC3856 I have already changed. I think it would be good to test it.

 

[alec_t]

At the top right corner of the pic in post #16 there seem to be a pair of insulated wires going off the pcb and diving down under the heatsink. Check that they aren't touching the heatsink (I once had to repair a PC power supply which had been badly assembled with one wire touching a heatsink. Of course, the insulation had melted and the magic smoke escaped).

 

[unclejed613]

i'm pretty sure those wires are for a thermal sensor. since it's on the power supply side of the board, and the OP was getting +/-30V, i think it's safe to say the power supply section is working

 

[ectt]

start the measurement from protection light, if it is LED, then make sure it still function; trace back what connected to this light, what component connected to it then measure that components functionality ok or not.