My alpine amp makes a high pitch squeaking sound

[Perry Babin]

Is there any significant DC across either channel (meter probes on speaker/output terminals) when it shuts down?

Does it appear to be drawing excessive current when it shuts down?

Is anything getting hot?

 

[weesatron]

when it cuts out it was reading about .11v the transistors get a bit warm but they don't seem abnormally hot got a 10a fuse in so i don't think its pulling much current

 

[Perry Babin]

You need to measure the DC before it shuts down.

Did you measure the DC on both channels?

Which transistors are getting hot?

Were they clamped to the heatsink?

 

[weesatron]

before the amp cuts out it reads about .01v on all channels when it cuts out
the good channel stays at the same voltage but it has no transistors in that channel

the output transistors are the ones getting hot but i don't know how hot they normally run so i will start up one of my other amps and run it for a while and compare the temperature between the two

yes i have always had the transistors clamped down when running it

 

[Perry Babin]

The output transistors should not heat up at idle (amp on, no audio). If they're running hot, you may have another defective driver transistor or the 1225 driver IC may be defective.

Are there any potentiometers in the area of the output transistors?

 

[weesatron]

there is 1 near the 2 big capacitors and 2 between the two driver ic's

 

[Perry Babin]

The 2 between the driver ICs may be the bias pots.

Connect your meter to one center leg and one outer leg of an emitter resistor in the working channel. Measure the DC voltage. Turn the pots to see if either affects the voltage across the legs of the resistor. Adjust it for something near 0.001v DC.

Turn the pots slowly. In some amps, a very small change can make a large difference in current draw.

 

[an2ny888]

i have the same amp, should i be at all concerned that my amp makes the squeaking noise for an instant when i turn on the radio? aside from that it works fine, i looked at all the caps and none appear to be leaking

 

[weesatron]

do i need to put the transistors back in the working channel to do this

 

[Perry Babin]

weesatron:
The 'working' channel is the one with the output transistors in the circuit.


an2ny888:
It would probably require an experienced tech to find a problem like that.

 

[I Am An Idiot]

An2ny You should not be concerned about your amp. I am sure that it is just charging the rail caps.

 

[weesatron]

found the problem i hand another blown transistor the only thing that concerns me is that it heats up fast even did before it blew up do you think if the the previous owner messed around with the 3 potentiometers that it would cause this

 

[Perry Babin]

If the pots in the audio section are bias pots and someone set the bias too high (happens often, people turn all pots that they find fully clockwise for some reason), that could cause the amp to fail prematurely and would cause it to run hotter than normal.

 

[weesatron]

just checked them they were reading .12v set them down runs cool now is there any way to check if the one near the power supply is set correctly if some one has set the bias right up wouldn't surprise me if the have messed with that one too

 

[weesatron]

how would i go about finding equivalents for 2sc3421 2s3423 2sa1358

 

[Perry Babin]

I'm not sure what the pot in the power supply is used for. It may be used to set the rail voltage. If it's fully clockwise, set it back to the middle of its range for now.

You'll have to find those parts locally. They all appear to be available in the US. I'm not sure what's available in NZ.

 

[weesatron]

do you know any good websites that ship worldwide

 

[Perry Babin]

http://octopart.com/
http://www.findchips.com/

I don't know which ones ship internationally. Use the search engines above to find the parts. Check with them to see if they will ship to NZ.

 

[weesatron]

I got all the parts today i have put them all in and amp is now working. The thing that has me worried is one of the channels is heating up fast

 

[Perry Babin]

Did you try reducing the bias current with the bias pot?

Turn it very slowly while watching the current draw from the DC power supply. Set it at the point where it just barely causes an increase in idle current.

If you want to set it by the voltage across the emitter resistors, connect your meter (set to the lowest range of DC volts) to the center and one of the outside legs of one of the emitter resistors in the channel in which you're adjusting the bias current. Set the bias pot so that it reads between 0.001 and 0.005 across the emitter resistor. Let the amp idle (no audio) and recheck it after a few minutes.

Readjust it if it drifts. After you get the idle current right when the amp is cool, you'll have to drive it hard enough to get it hot (too hot to hold your hand on continuously). Recheck the idle current every few minutes by removing the audio signal and measuring the DC voltage on the emitter resistor. You can leave your meter connected during testing. If it drifts upward, reduce the biasing. If it remains OK when it's hot, the bias current is properly set.