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Old 29th April 2008, 04:00 AM   #11
spooney is offline spooney  United States
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still playing around with this old dog.I finally found some original toshiba outputs and installed them and the amp still appears to idle very high and rise slowly.I haven't had a chance to recheck the bias current.If it is indeed still rising would replacing the bias transistor solve the problem?
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Old 29th April 2008, 04:13 AM   #12
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It's possible that replacing the bias transistor could solve the problem but there's no guarantee. It's unlikely that the bias transistor is defective. If the new one has a lower threshold voltage, it would probably solve your problem but there's no way to know if it will be lower.

It would be easier to change the resistors that determine the biasing.

What are the circuit board designations for the output transistors you replaced?
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Old 29th April 2008, 10:53 AM   #13
spooney is offline spooney  United States
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There isn't any circuit board designations on the board of this amp.The outputs that I replaced are in the right channel.
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Old 29th April 2008, 11:28 AM   #14
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The biasing transistor is inbetween the output transistors. There are two resistors directly in front of it. The closest one is R230. The next one is R232. R232 should be marked red, violet, red gold. Change that resistor to 3.3k ohms. That should reduce the idle current. If it reduces it but not enough (particularly at high temps), change it to a 4.7k.
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Old 29th April 2008, 09:15 PM   #15
spooney is offline spooney  United States
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Thanks perry I will give that a shot and let you know how it turns out.
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Old 24th August 2008, 03:24 AM   #16
spooney is offline spooney  United States
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I know this thread is forever old but I did finally get around to swapping those resistors.I first tried the 3.3 k resistor and although it did greatly improve the current draw it still was up over 1 amp and the voltage measured from the emitters of the output transistors was still about .015 volts. I removed the 3.3k and soldered in the 4.7 k resistor and all appears to be well now.It idles very low and there is virtually no voltage from emitter to emitter.I've been playing the amp for about 20 minutes now at very low volume and so far all seems well.nothing overly hot or drawing excessive current.What further testing can I do to insure that this amp is truly good to go this time?
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Old 24th August 2008, 03:32 AM   #17
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You need to run it up to thermal cutout (or until it is FAR too hot to touch for more than a second) and confirm that the idle current is holding. Check it every 2-3 minutes as it heats up so you can catch it if the idle current begins to increase.

If you don't want to wait for it to heat up from the audio, use a heat gun to heat the outside of the heatsink. Use both audio and external heat. Don't use the heat gun alone.
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Old 24th August 2008, 03:55 AM   #18
spooney is offline spooney  United States
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I was running some sine waves through it and checking the output with my oscilloscope and I noticed that at low levels the output from the repaired channel looks "fuzzy" compared to the original channel.This was at well below 1 volt of output it was actually more like .2 volts if that.The unrepaired channel has a nice thin line on the display but the repaired channel has a thick hazy looking line when probed.As the volume/output is increased the line on the display starts to look just like the waveform produced from the original unrepaired channel.Does this indicate any problems?I did not notice any differences in sound between the two channels
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Old 24th August 2008, 04:03 AM   #19
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Some amplifiers will produce low level oscillation on their outputs when they're not biased high enough. This could be the problem with yours.

If you replace the 4.7k resistor with a pot, you could adjust the bias to precisely what it needs to be.
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Old 24th August 2008, 04:10 AM   #20
spooney is offline spooney  United States
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what size pot should I use?Would a 10 K pot work?Does this low level oscillation cause any issues?Is this what makes the amp unstable?If I remember correctly a small trimmer pot has three terminals.How would I integrate this into a spot with only two through holes?
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