07-12-2016, 02:00 PM
When asking for help, it's surely good etiquette to provide (links to) service data where available?
http://www.hifiengine.com/manual_library...1400.shtml
(you have to register, but it's free and well worth the effort).
This amp is conventional in design, runs from modest supply rails, and lends itself to logical fault finding. Is it worth repairing? Only the OP can answer this. How much is the customer willing to pay, how long will it take, how long is any guarantee, will you cover consequential damages (as there appears to be no DC offset protection to save the loudspeakers), etc, etc... We can't tell you how to run your business.
Forgive me while I climb on my soapbox
I've never understood why people get so nervous around audio amplifiers.
Sure, if you just plug it in and hope for the best, you're likely to waste transistors. But why would anyone do that? It just baffles me...
Quite simply, a pair of bench PSUs (or a dual model) with variable current limiting is all you need. Most people already have these, but if not, they are cheap and plentiful second hand. Something like a Farnell L series, or the classic Thurlby-Thandar PL320 or similar.
If your bench power supplies aren't current limited, or - inexplicably - you don't have bench power supplies, then build a couple of 50mA current sources. Just 2 transistors and 2 resistors per rail, plus a scrap heat sink. Or, a pair of 24V light bulbs. Even a pair of 100 ohm resistors! Most output transistors are rated to at least 10 amps - usually more.
At the very least, a lamp limiter on the mains would save the new output devices. Many folk have no qualms using these on old radios, so why wouldn't you use them on an amplifier?
Once you can power the thing up without fireworks, you're then able to apply logic. The circuits are very straightforward to understand, so after suitable measurements and a bit of thought, you've found the root cause - usually in less time than it takes to research and source and fit substitute transistors. And by avoiding the "blind" blanket-change approach, you've avoided the risks of introducing new faults along the way.
Beyond a quick check for shorted transistors, I hardly ever bother with "cold" checks. Get the thing safely powered and follow your nose. Simple
http://www.hifiengine.com/manual_library...1400.shtml
(you have to register, but it's free and well worth the effort).
This amp is conventional in design, runs from modest supply rails, and lends itself to logical fault finding. Is it worth repairing? Only the OP can answer this. How much is the customer willing to pay, how long will it take, how long is any guarantee, will you cover consequential damages (as there appears to be no DC offset protection to save the loudspeakers), etc, etc... We can't tell you how to run your business.
Forgive me while I climb on my soapbox
I've never understood why people get so nervous around audio amplifiers.
Sure, if you just plug it in and hope for the best, you're likely to waste transistors. But why would anyone do that? It just baffles me...
Quite simply, a pair of bench PSUs (or a dual model) with variable current limiting is all you need. Most people already have these, but if not, they are cheap and plentiful second hand. Something like a Farnell L series, or the classic Thurlby-Thandar PL320 or similar.
If your bench power supplies aren't current limited, or - inexplicably - you don't have bench power supplies, then build a couple of 50mA current sources. Just 2 transistors and 2 resistors per rail, plus a scrap heat sink. Or, a pair of 24V light bulbs. Even a pair of 100 ohm resistors! Most output transistors are rated to at least 10 amps - usually more.
At the very least, a lamp limiter on the mains would save the new output devices. Many folk have no qualms using these on old radios, so why wouldn't you use them on an amplifier?
Once you can power the thing up without fireworks, you're then able to apply logic. The circuits are very straightforward to understand, so after suitable measurements and a bit of thought, you've found the root cause - usually in less time than it takes to research and source and fit substitute transistors. And by avoiding the "blind" blanket-change approach, you've avoided the risks of introducing new faults along the way.
Beyond a quick check for shorted transistors, I hardly ever bother with "cold" checks. Get the thing safely powered and follow your nose. Simple
