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Old 23rd June 2006, 09:48 AM   #11
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I now have a multimeter that i can trust.

Just noticed the bias seems to creep up from less than one millivolt up to around 8mV then i turned it off, is this normal behaviour for this amp? it suggests to me that the transistors in the bias network are getting warm fast and that this may cause thermal runaway.

Open to suggestions from everyone regarding this.

Heres a link to the schematic in another thread:

Comments on Fostex Lab 600 Amp circuit?


Thanks
Craig
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Old 23rd June 2006, 02:10 PM   #12
thejohn is offline thejohn  South Africa
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Craig, I'll pull the lid off my amp this weekend and give you some feedback...
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Old 23rd June 2006, 08:47 PM   #13
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Hi John,

That would be great, like i have said my main concerns are the bias drifting (it does stabilise after about 15 minutes) and the transistors with the small clip on heatsinks heating up. If you could check your amp for the same things it would be very interesting.
Also what volatge are you running it off?
with 220v i get +/-75vDC rails as per the schematic but with my pure mains voltage ( not through a variac 240V) the rails are 85v dc which is quite a lot and is a danger to the transistors used in the circuit.
Check out the other thread i started :

amplifier bias drifting

Regards
Craig
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Old 28th December 2015, 10:41 PM   #14
CAVU is offline CAVU  Canada
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Cool Fostex 600/300 Power Amplifier brochure

Find attached the original brochure with description and specs for the Fostex 300 and the Fostex 600 power amplifiers from 1980.
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Old 23rd January 2016, 11:35 AM   #15
Ardenza is offline Ardenza  South Africa
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Hi I am glad to have found this post on this forum.
Found the 300 at a second hand shop for about the equvalent of $60. My eye catched on the name Fostex and I was so exited that I didnt even asked the busy shopkeeper to test the unit.
Now for a good cleaning and sound test. Thanks for posting the spec sheet.

Jan
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Old 23rd January 2016, 12:11 PM   #16
d a o is offline d a o  Denmark
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Fostex 300 Manual - Professional Two Channel Power Amplifier - HiFi Engine
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Greetings Hans
www.diyaudio.dk/
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Old 23rd January 2016, 04:40 PM   #17
thejohn is offline thejohn  South Africa
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Hi Jan,

If you ever want to re-home it I'm looking to expand my collection...
I also have loads of parts if you need anything

John
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Old 23rd January 2016, 09:56 PM   #18
Ardenza is offline Ardenza  South Africa
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John, you have turned the Fostex amp collection world on its head .
Quite and impressive collection you have there. What was their main application in the 80's? movie cinemas? Is it class AB? Do the 600 run hot?

What speakers do you drive with the 300 and do you recommend any modifications, upgrades to old and weary parts?

Sorry a mouthful of questions . and the discussion is open to anyone interested in this solid Japanese amp.
I like the heatsink structure.
At the moment Im testing the 300 on a ancient pair of TEAC sx-500 speakers. Straight from the cd player to amp. Find the sound to be impressive.. fast and clear.
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Old 23rd January 2016, 10:39 PM   #19
CAVU is offline CAVU  Canada
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Quote:
Originally Posted by Ardenza View Post
Quite and impressive collection you have there.
I too have a large collection of these wonderful amplifiers plus about 37 years of experience with them being the person who introduced these products in about 1978.
Quote:
What was their main application in the 80's? movie cinemas?
I don't know the South African market but, in North America these amplifiers were and continue to be used almost exclusively in recording studios and broadcasting.

These are the recommended amplifiers for use with the RWO/Fostex Laboratory Series Studio Monitors.



Quote:
Is it class AB? Do the 600 run hot?
The amplifier is Class AB and runs very cool.

Quote:
do you recommend any modifications, upgrades to old and weary parts?
The phone plugs can become intermittent. The level controls and switches should be cleaned. Beyond that, there are no recommended mods.
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Old 23rd January 2016, 10:42 PM   #20
CAVU is offline CAVU  Canada
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Second page of brochure ...
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