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vintage amplifier repair/upgrade manual
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Old 19th February 2015, 01:53 PM   #31
Madaudio is offline Madaudio  India
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Dear SAKIS, that was really comprehensive and authentic. Lot of useful information. Still I would like to arise a doubt. How to figure out the exact bias current of an amp about which no data you have?
Again, what you pointed about the usefulness and importance of schematic is correct. I would like to share my experience. Some 15 years before I was in television servicing bussiness. One day, I recieved a set of which the H and V linearity was totally lost. inspecting, no components were damaged, and I fiddled with it for some days. at last found the schematic and comparing with it, it was found that a hand full of caps and resistors in deflection were replaced with spurious components and offset values. But the chassis was soldered like new and there was no trace of any tampring. may be some deliberate work of some crook. once restoring all components as per the schematic, the set was tested okay without any further allignment.
Thank you for the post.
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Old 19th February 2015, 03:06 PM   #32
east electronics is offline east electronics  Greece
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For class Ab operation there is many ways to go while all moving around the same axis

For starts i don't recall any vintage amplifiers that schematic or recommended bias value cannot be found Obviously i might be wrong .

One way will be to use a scope to see the spot where crossover distortion gets eliminated Often a bit more is always better . Truth is that experience has a lot to do and combined with choice of semis , heatsink , response of Vbe multiplier together with the style of make , power supply , total power of the amplifier , costumer demands and all of it versus safety will help choose a bias point .

For us at east electronics proper bias point is most of the times according to the manufacturers suggestion. In some occasions after costumer demands we may play a bit more ( hardly ever less ) but this is getting verified by spectrum analyzer and scope to ensure that more or less bias doesn't effect original distortion figures for any reason ..

Increasing the bias in any amplifier will most of the times help the sound of the amp especially in low level listening Still this will alter safety characteristics of the amp and is a thing to be done if needed with caution and study .

Kind regards
Sakis
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Old 5th December 2017, 03:57 AM   #33
RobinTW is offline RobinTW  Taiwan
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Quote:
Originally Posted by east electronics View Post
part 2
9- By the same technique (as mentioned above you will be able to notice how well is your outputs matched {especially if more than 2 per channel} by monitoring the voltage drop on every resistor .If voltage drop is equal to all resistors with in a margin of 10% then your outs are very well matched .If voltage drop in one is 10mv and the other 50mv then your outs are very badly selected.

Nice work, Sakis


I really enjoy reading this manual. I got a question about 9. Is there a way to select matched output pairs off the circuit? thanks.
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Old 7th December 2017, 08:10 AM   #34
RobinTW is offline RobinTW  Taiwan
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Found the answer from Google. This may help.


http://www.dragonflyalley.com/synth/...at0011_144.pdf
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