Forte Model 1a Class A amp runs ice cold - diyAudio
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Old 27th November 2008, 05:10 PM   #1
rsquare is offline rsquare  United States
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Default Forte Model 1a Class A amp runs ice cold

An earlier post by someone with the same Forte amp mentioned one side being cold and a response suggested the problem might be the bias. Well, my unit runs cold on all sides, but it didn't used to. Brought it to a local audio store and they checked it out, said it sounded good to them, and had no idea why it would run cold.

Any thoughts much appreciated, and thanks in advance.
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Old 27th November 2008, 06:57 PM   #2
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Since the channels are independent of each other, the likelihood
of identical failures is pretty small. You will want to measure the
bias across the emitter resistors.

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Old 28th November 2008, 11:06 PM   #3
rsquare is offline rsquare  United States
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Thanks, Nelson, appreciate the reply. I don't work on my own equipment so am not sure what to do here. Should I just bring it to another audio shop and mention what you said? Thanks again.
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Old 30th November 2008, 11:52 PM   #4
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Find somebody confident with a DC voltmeter, and see what the
voltages and resistances are on the Emitter resistors.

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Old 1st March 2010, 07:33 AM   #5
codname is offline codname  Romania
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Hello , i have the same problem mentionet earlier , my forte audio 1A is cold after runing for 1-2 hours , i have measured the bias in this way : the minus of the voltmeter i put it to the base of the transistor and the plus on the resistence conected to the emitor of the transistor . my readings where initialy 460 mV on left and 126 mV on the right then i have ajusted them to 500 mV aprox.

it is correct how i measured the current of the transistors ? if yes why it is still cold at 500 mV.

thank you
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Old 1st March 2010, 01:45 PM   #6
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You have to measure the voltage drop across the emitter resistors. You can then calculate the current. Afterwards you compare this value with the value given in the service manual.

Note: I guess, the service manual gives the current summed over all devices...

Good luck.
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Old 1st March 2010, 04:09 PM   #7
codname is offline codname  Romania
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thank you ,

I have only a scheme of the amplifier uploaded in another thread , but the scheme has no voltage markings , after looking more careful i think that i have measured correctly - (minus) of the voltmeter at the base (b) and + (plus) of the voltmeter after emitter resistor of each transistor .

actual current at the transistors is 500 mV , but the amplifier is cold, no shadow of warm

one more mention is that the right channel at initial current (126 mV) the sounds where distorsioned , but when i rised the current (bias) the distorsions disapeared almost 100% but not totally .. it is normal ?
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Old 1st March 2010, 04:42 PM   #8
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You have to measure across the resistor. Not the base pin. R=U/I, I=U/R. Who knows the correct idle current???
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Old 1st March 2010, 04:49 PM   #9
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Attention: Current is in amps, not in mV. Of course you'll hear bloody distortion: 126mV means the silicon is switched off. Don't turn the pot, if you do not understand what you're doing. Otherwise you'll blow your amp..

You measure a voltage, because you know the resistor value, you calculate the current through it...

Please ask BEFORE killing Nelsons baby...

Dirk
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Old 1st March 2010, 05:40 PM   #10
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Quote:
Originally Posted by HVfanatic View Post
idle current???
40W in Class A for the Model 1 usually corresponds to 1.6A idle current, divided by 5 output devices per rail is 320 mA.
50W in Class A for the 1A is 1.8A, divided 5 makes 360 mA.
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