Hifi amp has stopped.

[keeney123]

Re: Hifi amp has stopped.

So I am wondering if your speakers are causing your problem? Are they the correct ohmed speakers for that unit? Or could something in the speaker causing a short out that takes out the output transistor?

[Portishead]

Re: Hifi amp has stopped.

Just to note - Both speakers read 05.4 on the cable terminal ends.

Well, I did test them earlier.
The system was put together by the rega dealer some time ago. Haven't changed it.

[keeney123]

Re: Hifi amp has stopped.

Well, I did test them earlier.
The system was put together by the rega dealer some time ago. Haven't changed it.

So these speakers with this amp has been working for some time with no problem?

[Portishead]

Re: Hifi amp has stopped.

14 years or so.

[budm]

Re: Hifi amp has stopped.

Put the new parts in and install all the hardware back in place, and use the 60~75W lamp to power the unit, but do not have anything else hook up to the unit.
Turn the unit on and you will see the lamp lights up brightly for seconds due to the in rush current of the TOROID power transformer and the charging of the main filter caps, then measure the DCV (it should be in the mV range, may be about 5~10mV range) across the EMITTER resistors, the bad channel should be about the same as the good channel. Leave the unit on while keeping an eye on the lamp to see if it starts getting brighter by itself. The amp will be sitting at idling stage so the lamp should barely come on.

[Portishead]

Re: Hifi amp has stopped.

Ok. Should I replace the trimmer also? I can't find out how to test / check them both. Should both channels be the same? It may have failed but I don't know how to set it accurately. Is it critical to get it accurate if I swap it??

[budm]

Re: Hifi amp has stopped.

You can change the TRIMMER, set it to 12'o clock as how it shows in the picture.
The trimmer is used for setting the bias current for the output transistors by reading the Voltage drops on the Emitter (the ref diagram that we have shows the resistors to be 0.22 Ohm), since we can find out what the idling current by calculation based on Vdrops and the value of the resistor, most amp set the idling to around 25~30mA range, correct way to set is to used scope to look for crossover.

[rievax_60]

Re: Hifi amp has stopped.

The trimmers should be set to result in minimum idling current through the output transistors. Faults in the circuitry associated with the trimmers usually results in the output transistors drawing too much rail to rail current and failing.
The small transistors under the heatsink are part of this idle current setting circuit. Look for cracked joints as well as checking them.

[Sparkey55]

Re: Hifi amp has stopped.

the trimmers should be set to result in minimum idling current through the output transistors. Faults in the circuitry associated with the trimmers usually results in the output transistors drawing too much rail to rail current and failing.
The small transistors under the heatsink are part of this idle current setting circuit. Look for cracked joints as well as checking them.

+1

[rievax_60]

Re: Hifi amp has stopped.

Also, if doubt still exists, there is the option of powering up the amplifier with just the 2 output power transistor removed. This will minimize the damage if something is still wrong. If there is no smoke, some voltages can then be measured to confirm all is well before the power transistors are fitted.
If the primary cause of the problem is an unknown intermittent one, this option will fail also.

[Portishead]

Re: Hifi amp has stopped.

I've fitted it all back together as you say budm and the new large transistors and hooked up to lamp.
Will the using the lamp protect them now as well.
Don't really want to plug it in now.

[mmartell]

Re: Hifi amp has stopped.

Think of it this way.

If you plug a lamp into the wall it will burn brightly. If you cut the wire after the lamp it will of course not burn at all.

Now connect those wires to your device. If your device has a short that would be similar to touching the two wires together, causing the lamp to burn brightly. If the device is working normally there will be a low current draw through the device (and the lamp!), causing the lamp to glow dimly.

The genius in using the lamp this way is that as the higher current gets pulled through the device it gets consumed by the lamp, giving you a visual indicator that there is still a problem and keeping your device from pulling the full and harmful current through it.

So watch the lamp but don't leave a brighlty lit lamp on for too long.

[Portishead]

Re: Hifi amp has stopped.

So, I plugged it all in. Turned the unit on. The lamp does momentarily flash bright on then dims to nothing.
Testing the emmitor resistors -
The good side read 1.2 & 1.3 putting the probes on each end of each emmitor.
The bad side reads 16.2 & 10.1 but they appear to be rising as I test ???

Turned it off now.

[budm]

Re: Hifi amp has stopped.

So, I plugged it all in. Turned the unit on. The lamp does momentarily flash bright on then dims to nothing.
Testing the emmitor resistors -
The good side read 1.2 & 1.3 putting the probes on each end of each emmitor.
The bad side reads 16.2 & 10.1 but they appear to be rising as I test ???

Turned it off now.

OK, now adjust the Trimpot to see if you can make it read the same as the good channel. As the idling current goes up the lamp should also get brighter.
BTW, the reading also show imbalance between the Positive and Negative side of the outputs so it does look like you still have problem in the circuit some where.

[Portishead]

Re: Hifi amp has stopped.

So, do you suggest I turn it left or right ? Really best that I ask.

Happy with the 60 watt bulb?

[Portishead]

Re: Hifi amp has stopped.

Imbalance?
Still want me to turn it on and adjust the trimmer? I am unsure how much to turn and if too much turning will further damage?

[budm]

Re: Hifi amp has stopped.

Find the way to keep the meter probes on the Emitter resistor so you will have free hands to adjust the Bias pot, turn it Counter clock wise slowly to see if the Voltage starting to drop, if not turn it clockwise and see.
Once you see it starting to drop then keep on turning to see how low it will go down to.
60W is good to use.

[Portishead]

Re: Hifi amp has stopped.

Ok, I kept testing and turning. Perhaps I had the probes the wrong way round previously but now all four Emitters read 01.5

I have done it a few times now. Is it suppose to move or go up or down? They all read the same but maybe the longer its switched on the higher it goes slightly???

[Portishead]

Re: Hifi amp has stopped.

After 5 minutes all four are at about 02.1

[budm]

Re: Hifi amp has stopped.

Yes the Voltage drops on the Emitter should change as you turn the trimpot, if they are reading the same then leave it on the way it is and keep an eye on the meter to see if it will drift upward, it will drift a little bit, the small transistor that is under the heatsink will sense the temperature rise and it will try to maintain steady idling current, if you look at the diagram, that transistor is part of the active bias circuit.
You may want to tap on the board to see if the lamp will get brighter which will indicated bad connection.
BTW, the speaker relay should be on right now so you should also check the DCV between the Red and Black speaker terminals to see what kind of DC Offset you have.