So I got my hands on this old Fostex Power Amp, built like a tank, 1.3KVA torroidal power trans, 18000uf cap per rail, huge heatsinks... Initial listen and I reckon it sounds pretty good, serious bottom end, powerfull and controlled,quite detailed without being harsh / grainy / irritating. I think it has potential for sounding really good with a bit of modification. It's getting on a bit in age so new electrolytics are going in, coupling caps are polyester - could replace with polyprop. Transistors are mostly unavailable.. Do any of the resident experts have any ideas on modern replacements or comments on the circuit in general? Any thoughts on worthwhile mods?
Thanks,
John
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Thanks,
John
Please consider replacement of TR607/608 pair and bias pair TR605/606 with
KSA1142/KSC2682 respectively at sight. This is an accident waiting to happen.
Also consider replacing the small TR603/604 (TO92) with KSA1013 transistor.
The clip-on heatsink is also to small and must be upgraded to a proper one.
These amps (300) where designed around available transistors at the time, thus
the very close tolerance to 150V. But today we have 230VAC and thus 160V!
I have repaired several (only 300s) amps with all the same burnt out channels.
(Burnt out PCB traces, drivers and output devices, a lot of work to reconstruct.)
A simple and basic amp of high quality, but bordering on the "voltage" edge.
Well worth upgrading (and please do this) as today high voltage trans are
easily available at low cost.
KSA1142/KSC2682 respectively at sight. This is an accident waiting to happen.
Also consider replacing the small TR603/604 (TO92) with KSA1013 transistor.
The clip-on heatsink is also to small and must be upgraded to a proper one.
These amps (300) where designed around available transistors at the time, thus
the very close tolerance to 150V. But today we have 230VAC and thus 160V!
I have repaired several (only 300s) amps with all the same burnt out channels.
(Burnt out PCB traces, drivers and output devices, a lot of work to reconstruct.)
A simple and basic amp of high quality, but bordering on the "voltage" edge.
Well worth upgrading (and please do this) as today high voltage trans are
easily available at low cost.
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Haha, yes that was a while ago!
Since that post I've had about 20 Fostex Lab300's and 600's through my hands and fixed all manner of little issues (Relays, the odd puffy cap, bias trimmers). They have proved to be pretty damn reliable on the whole.
All of the local ones started life in cinema's and theatres and were run hard and so far I haven't come across any with burned traces or boards!
Even the couple I got with blown outputs just "stopped working"...
I still reckon they sound great!
John
Since that post I've had about 20 Fostex Lab300's and 600's through my hands and fixed all manner of little issues (Relays, the odd puffy cap, bias trimmers). They have proved to be pretty damn reliable on the whole.
All of the local ones started life in cinema's and theatres and were run hard and so far I haven't come across any with burned traces or boards!
Even the couple I got with blown outputs just "stopped working"...
I still reckon they sound great!
John
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