This almost 50 year old beast has an issue - the output rail of one channel is prone to latchup. Sometimes it works OK but sometimes speaker rail is at +Vcc. Testing board alone on bench with a current limited supply (50v, 100ma limit) it sometimes works, sometimes doesn't (usually the latter). Making threatening gestures with squirty freeze on the semis does not change its mind. Anyone ever met this (and solved it)?.
Problem should be in some of the primary stages, otherwise it would blow in your face. After 50 years some solder might have gone brittle.
Check the condition of the PCB and maybe resolder the heavier parts. For further testing a simple DC protection board on the output would be nice to save your speakers.
Check the condition of the PCB and maybe resolder the heavier parts. For further testing a simple DC protection board on the output would be nice to save your speakers.
I admire your use of the power supply for debug effort and wish more DIY's did the same.
Do you have any sense that the circuit is intermittent? I poke around with a wood stick trying to exercise erratic connections.
My worst war story was a leaky transistor that made huge crashing noise but worked sufficiently the keep the amp biased into operation. But powered probing led me to a suspicious part and my guess proved correct. So I suggest the following: if the amp is reliably stuck on a rail (i.e. you can make measurements without nudging the amp into working--- very frustrating), I suggest taking a series of measurements to find the component that is misbehaving. For example, + input, - input (feedback), amp output, and interstage nodes that that might lend insight. [i.e. base Q541, base Q542, output terminal 19, etc.]
Here's a link to a similiar problem that eventually yielded results.
https://www.diyaudio.com/community/threads/nikko-alpha-220-please-help.396601/
Do you have a link to a legible schematic? My find is barely legible.
Good luck!
Do you have any sense that the circuit is intermittent? I poke around with a wood stick trying to exercise erratic connections.
My worst war story was a leaky transistor that made huge crashing noise but worked sufficiently the keep the amp biased into operation. But powered probing led me to a suspicious part and my guess proved correct. So I suggest the following: if the amp is reliably stuck on a rail (i.e. you can make measurements without nudging the amp into working--- very frustrating), I suggest taking a series of measurements to find the component that is misbehaving. For example, + input, - input (feedback), amp output, and interstage nodes that that might lend insight. [i.e. base Q541, base Q542, output terminal 19, etc.]
Here's a link to a similiar problem that eventually yielded results.
https://www.diyaudio.com/community/threads/nikko-alpha-220-please-help.396601/
Do you have a link to a legible schematic? My find is barely legible.
Good luck!
I eventually narrowed it down to a dodgy MPSA65 Darlington FET (which had been previously exonerated by my Peak DCA75 as I had suspected it early on in the hunt - beware of testgear telling porkies). Anyhoo where have all the A65s gone - there are people selling them on Ebay for far too much money. Ended up rolling my own using a couple of vanilla PNP BJTs and they (it) worked fine.
