Hi Everyone,
I've nearly completed restoration work on my dumpster-bound AU-9500, so far so good - it's now sounding great and looking better doing it. But, I have come across a few complications that I'm hoping to discuss with you all. This is also my first post here.. so please do advise if this is better posted in a different section of the site.
This unit has been quite the project and my first real foray into proper transistor amps. I'm a valve amp guy mostly..plenty of experience with simple transistor circuits, but I struggle to make heads or tails of these more "complex" designs with lots of DC coupling etc. All of the Electrolytic and tantalum caps have been replaced, as well all the BJT's excluding outputs, drivers and a handful of "uniwatt" types. All the small signal transistors were replaced with the commonly recommended KSA992/KSC1815 type and film caps were used for audio coupling wherever it was economical to do so.
I'm not usually one to shotgun components in this manner, but due to the poor storage conditions the amp had endured (stored in a damp garage since approximately 1987...) I felt this was wise as to avoid opening the thing up every few months when something else inevitably fails. Really, it was that bad! None of my local techs wanted to deal with it, despite great interest in the model. Understandable as it took me about 8 hours just to strip the rust and repaint the transformer, internal shields, chassis, external case etc. Not to mention scrubbing all the old flux, rust particles and miscellaneous hairy detritus off the boards.
The 2sa706/2sc1124 pair on the F-2030 supply board were replaced with KSA940/KSC2073 - I figured the substantially higher dissipation rating can't hurt here, no operational faults I can discern from the lower gain.
This is where I'm a bit lost:
I suppose KSA940 would be a decent sub for the 2SA706 highlighted in green - as far as I can tell, the lower gain and fT of the 940 should not be too much of a problem here as it is only really affecting the DC conditions of the drivers and outputs?
However, the pair highlighted in purple are proving to be a bit more of a challenge to procure considering the oft recommended KSA1220 is obsolete, and TTA004b / TTC004b are somewhat hard to come by at the moment.
Now... I have KSC3503 / KSA1381 on hand, which seem like reasonably good replacements bar the collector current. These units are rated at 0.1A, whereas the originals are 1A rated. Realistically, how much current can be expected in this position?
Perhaps there are moving parts here that I'm not considering, but my best guess is that when we consider the 7.9w dissipation rating of the 2SA706, the most current it could possibly pull from its 49v rail before blowing up would be about 160ma. And for the 5w rating of the 2SC1124, about 100ma. Surely they are not operating anywhere near this for reliability sake.. so perhaps these "uniwatts" were not selected for this position based on their Icc rating, and the 0.1A rated KSC3503/KSA1381 will suffice?
Anyway, what say ye? would love some input here - needless to say I have been scouring datasheets and forum posts but haven't come up with anything yet.
Unfortunately I've also happened upon a few odd issues, no doubt I'll figure these out eventually... but I'd love to hear if anybody has experienced something similar.
- No output from the left channel until volume hits a certain threshold, it then "switches on" and functions as expected. I have isolated this to either the output transistors or preamp board F2019. More testing required.
- Also getting a bit of high frequency white noise in both channels coming through, possibly from preamp board F2019. Most prominent at 10 o'clock on the dial, dies down at 12, and then louder from there - though still quiet enough to not detract from listening too much. A bit of a mystery as nearly all transistors and caps are fresh. Time to break out the scope I think!
And, of course, the amp in question.
Would love to know what you all think.
cheers!
I've nearly completed restoration work on my dumpster-bound AU-9500, so far so good - it's now sounding great and looking better doing it. But, I have come across a few complications that I'm hoping to discuss with you all. This is also my first post here.. so please do advise if this is better posted in a different section of the site.
This unit has been quite the project and my first real foray into proper transistor amps. I'm a valve amp guy mostly..plenty of experience with simple transistor circuits, but I struggle to make heads or tails of these more "complex" designs with lots of DC coupling etc. All of the Electrolytic and tantalum caps have been replaced, as well all the BJT's excluding outputs, drivers and a handful of "uniwatt" types. All the small signal transistors were replaced with the commonly recommended KSA992/KSC1815 type and film caps were used for audio coupling wherever it was economical to do so.
I'm not usually one to shotgun components in this manner, but due to the poor storage conditions the amp had endured (stored in a damp garage since approximately 1987...) I felt this was wise as to avoid opening the thing up every few months when something else inevitably fails. Really, it was that bad! None of my local techs wanted to deal with it, despite great interest in the model. Understandable as it took me about 8 hours just to strip the rust and repaint the transformer, internal shields, chassis, external case etc. Not to mention scrubbing all the old flux, rust particles and miscellaneous hairy detritus off the boards.
The 2sa706/2sc1124 pair on the F-2030 supply board were replaced with KSA940/KSC2073 - I figured the substantially higher dissipation rating can't hurt here, no operational faults I can discern from the lower gain.
This is where I'm a bit lost:
I suppose KSA940 would be a decent sub for the 2SA706 highlighted in green - as far as I can tell, the lower gain and fT of the 940 should not be too much of a problem here as it is only really affecting the DC conditions of the drivers and outputs?
However, the pair highlighted in purple are proving to be a bit more of a challenge to procure considering the oft recommended KSA1220 is obsolete, and TTA004b / TTC004b are somewhat hard to come by at the moment.
Now... I have KSC3503 / KSA1381 on hand, which seem like reasonably good replacements bar the collector current. These units are rated at 0.1A, whereas the originals are 1A rated. Realistically, how much current can be expected in this position?
Perhaps there are moving parts here that I'm not considering, but my best guess is that when we consider the 7.9w dissipation rating of the 2SA706, the most current it could possibly pull from its 49v rail before blowing up would be about 160ma. And for the 5w rating of the 2SC1124, about 100ma. Surely they are not operating anywhere near this for reliability sake.. so perhaps these "uniwatts" were not selected for this position based on their Icc rating, and the 0.1A rated KSC3503/KSA1381 will suffice?
Anyway, what say ye? would love some input here - needless to say I have been scouring datasheets and forum posts but haven't come up with anything yet.
Unfortunately I've also happened upon a few odd issues, no doubt I'll figure these out eventually... but I'd love to hear if anybody has experienced something similar.
- No output from the left channel until volume hits a certain threshold, it then "switches on" and functions as expected. I have isolated this to either the output transistors or preamp board F2019. More testing required.
- Also getting a bit of high frequency white noise in both channels coming through, possibly from preamp board F2019. Most prominent at 10 o'clock on the dial, dies down at 12, and then louder from there - though still quiet enough to not detract from listening too much. A bit of a mystery as nearly all transistors and caps are fresh. Time to break out the scope I think!
And, of course, the amp in question.
Would love to know what you all think.
cheers!
Thank you, I thought as much - good to have someone else chime in.
I’ll fit the a1381/c3503 and see how we go.
I’ll fit the a1381/c3503 and see how we go.
How can it be that way? The amp has a pre-out/main in jumper - you should be able to find out easily which end of the signal path is problematic.
Ah - I misspoke sorry, and I believe it is now too late to edit my post. Meant to say either the output transistors or driver board F2029. Issue does not follow the driver boards when swapped, so I suppose it must be the outputs.