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Old 19th June 2007, 09:35 PM   #1
steevo is offline steevo  United Kingdom
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Default Quad 306 Rebuild

I have just repaired 1 of the chaneels on my Quad 306. I have replaved all of the resistors with 1% metal film version, and all of the semiconductors except the output transistos which were o.k. In place of the MPSA43 I have used MPSA42 which is a higher voltage version. And for the BC413C I have used BC239 but made sure it had gain of about 350 using my transistor tester. The bc214C I have replaced with 214L but made sure it had a high gain of 350. The channels checks out o.k when I test it with a dummy 5 onhm load, at varying frequenices and power all the way upto full power, but there appears to be a high pitched noise eminiating from 1 or more of the components as I vary the frequecy, something oscillating and its quite loud compared to the other channel. In comparison to the other channel its about twice as loud. Could the channel be oscilating somewhere. It appears to funtio ok however.

Can anyone Help.

Thanks
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Old 19th June 2007, 11:03 PM   #2
Zen Mod is offline Zen Mod  Serbia
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Default Re: Quad 306 Rebuild

Quote:
Originally posted by steevo
I have just repaired 1 of the chaneels on my Quad 306. I have replaved all of the resistors with 1% metal film version, and all of the semiconductors except the output transistos which were o.k. In place of the MPSA43 I have used MPSA42 which is a higher voltage version. And for the BC413C I have used BC239 but made sure it had gain of about 350 using my transistor tester. The bc214C I have replaced with 214L but made sure it had a high gain of 350. The channels checks out o.k when I test it with a dummy 5 onhm load, at varying frequenices and power all the way upto full power, but there appears to be a high pitched noise eminiating from 1 or more of the components as I vary the frequecy, something oscillating and its quite loud compared to the other channel. In comparison to the other channel its about twice as loud. Could the channel be oscilating somewhere. It appears to funtio ok however.

Can anyone Help.

Thanks
did you replace all electrolytic caps,too?

besides that- check all solder points,and also check all resistor values.......mebbe you ( or seller) made mistake with values
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Old 19th June 2007, 11:16 PM   #3
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Default Quad 306

What you are hearing probably is not any type of oscillation, but elctrons traveling from one juntion to the other in the output transistors. As you apply a signal electrons flow and under high current will cause the case to vibrate at the frequency being amplified high frequency are easy to hear, also the type of heat sink material density ect. will increase or decrease this effect.
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Old 24th June 2007, 09:56 AM   #4
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Never heard an "audio transistor" !!!

Apart from jokes, the most likely culprit is an electrolytic. Now those used to be built with sometimes loose foils, and the magnetic effect of the current (and voltage stress) can cause them to vibrate (just as in speakers).

If you didn't change your caps, try doing so and my guess is that it would be an output or power cap (did the 306 have an output cap?)

cheers
John
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