Hi everybody,
this Coustic Logic series amp lied down along over 20 years by an installer having been pulled out of an install for sound output defect... tested and amp turns on regularly and current draw is less than 1a, left channel is ok right one no sound.
i pulled out the m.b. and checked for bad soldering first, and yes i found more than some, plus a short solder between gate and source of Q40 (irf540),
After removing the bad solderings i checked all mosfet resistance and they were all fine (83ohms at g-s), output tips too were all ok (1,6-1,8k), I also checked for gain pot and switch boost too, and applied anti-oxidation spray, so i was confident to re-assemble chassis amp (...) and crank it up again.
Turned on regularly and sound from both channels came out! But, as goin'up with volume, let's say 15-20w, sound disappear even if amp still on & no protection.., lowering volume (signal in) are back playing both channels!
Did resolder output caps and some driver stage caps (100uF ones), the amp goes a bit higher watts but same sound disappearing at 25w now and same reappearing by lowering signal in.
Checked again for signal path caps' defects and found a 10uF of the right ch input (c31) defective, replaced and now right ch doesn't play anymore!
I measured all small caps in signal path and did refresh soldering, no results..
Left ch, on the contrary, still there (even if bass reproduction is not that for a 98w.ch rated one & ol' school)...Measured rail voltage they are regularly about +/-37v on both output tips pairs (12,8v in),,,
Yesterday I've checked this forum for any new on that amps and found some interesting threads&post but not the kind of defect I'm facing, while today I finally found one more cap C63 is defective as touching it music was again in right ch!
So, I've pulled it out and found it being a 0,33uF/50v, got surprised of this low capacitance as I see also C61 is 3,3uF/50v being close to C63.. finally my query is could be its correspondent for ch left and an inserting error has been done in assembling process?
This doubt of mine arises also for filtering caps in +/- input are (4) 2200uF/35V 105°, while output power cps (4 again) are 2200uF/35V 85° only...?
Hope have not annoyed with this long of mine, and hopefully look for any help from any of you...
thanks in advance 🙂
this Coustic Logic series amp lied down along over 20 years by an installer having been pulled out of an install for sound output defect... tested and amp turns on regularly and current draw is less than 1a, left channel is ok right one no sound.
i pulled out the m.b. and checked for bad soldering first, and yes i found more than some, plus a short solder between gate and source of Q40 (irf540),
After removing the bad solderings i checked all mosfet resistance and they were all fine (83ohms at g-s), output tips too were all ok (1,6-1,8k), I also checked for gain pot and switch boost too, and applied anti-oxidation spray, so i was confident to re-assemble chassis amp (...) and crank it up again.
Turned on regularly and sound from both channels came out! But, as goin'up with volume, let's say 15-20w, sound disappear even if amp still on & no protection.., lowering volume (signal in) are back playing both channels!
Did resolder output caps and some driver stage caps (100uF ones), the amp goes a bit higher watts but same sound disappearing at 25w now and same reappearing by lowering signal in.
Checked again for signal path caps' defects and found a 10uF of the right ch input (c31) defective, replaced and now right ch doesn't play anymore!
I measured all small caps in signal path and did refresh soldering, no results..
Left ch, on the contrary, still there (even if bass reproduction is not that for a 98w.ch rated one & ol' school)...Measured rail voltage they are regularly about +/-37v on both output tips pairs (12,8v in),,,
Yesterday I've checked this forum for any new on that amps and found some interesting threads&post but not the kind of defect I'm facing, while today I finally found one more cap C63 is defective as touching it music was again in right ch!
So, I've pulled it out and found it being a 0,33uF/50v, got surprised of this low capacitance as I see also C61 is 3,3uF/50v being close to C63.. finally my query is could be its correspondent for ch left and an inserting error has been done in assembling process?
This doubt of mine arises also for filtering caps in +/- input are (4) 2200uF/35V 105°, while output power cps (4 again) are 2200uF/35V 85° only...?
Hope have not annoyed with this long of mine, and hopefully look for any help from any of you...
thanks in advance 🙂
The left and right circuits may differ because one channel is inverted to make the amp bridgeable.
The primary caps are stressed significantly more than the secondary filter caps so they often have 105C caps. The 85C secondary caps are perfectly OK in most amps.
The primary caps are stressed significantly more than the secondary filter caps so they often have 105C caps. The 85C secondary caps are perfectly OK in most amps.
Hi Perry,
thanks your suggestions and so prompt,
I've only 1uF/100V replacement for C63 and I see in the circuit negative pins of both C62 (22uF/16V) and C63 are common while C63 positive pin goes to Q18 mpsa56 through r116 22k and it is closed there by d17.
I have briefly (30sec.) tested this 1uF in place of the 0,33uF and it works good seems no problem.. (m.b. unplugged, no part is heating abnormally) do you think I can leave this one instead of original 0,33uF?
thanks your suggestions and so prompt,
I've only 1uF/100V replacement for C63 and I see in the circuit negative pins of both C62 (22uF/16V) and C63 are common while C63 positive pin goes to Q18 mpsa56 through r116 22k and it is closed there by d17.
I have briefly (30sec.) tested this 1uF in place of the 0,33uF and it works good seems no problem.. (m.b. unplugged, no part is heating abnormally) do you think I can leave this one instead of original 0,33uF?
oops,
I forgot to mention right ch outputs 270mv on speaker terminal while left one has 47mv only.
By the way the mono/bridge connection IS NOT shown on terminal output panel and polarity sequence of speakers are left ch: blue+ black-, right ch: yellow- black+.
inside wiring measure both blacks are common, so guess inverting is already made on board ics, why put + - - + sequence wiring if absolute polarity is carried by left ch blue wire + and right ch yellow wire -?
Following the polarity on panel speaker terminal should result in out of phase wiring, that's why i felt lacking bass freq? or am i wrong on this thesis?
I forgot to mention right ch outputs 270mv on speaker terminal while left one has 47mv only.
By the way the mono/bridge connection IS NOT shown on terminal output panel and polarity sequence of speakers are left ch: blue+ black-, right ch: yellow- black+.
inside wiring measure both blacks are common, so guess inverting is already made on board ics, why put + - - + sequence wiring if absolute polarity is carried by left ch blue wire + and right ch yellow wire -?
Following the polarity on panel speaker terminal should result in out of phase wiring, that's why i felt lacking bass freq? or am i wrong on this thesis?
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Hi Perry,
yes in a first moment me and even installer we did try to connect every speaker positions to find out why right channel was mute, but this was before C32 replacing and -of course- no sound came out wiring blue to yellow, while sound was in wiring opposite + + as well as + and - of left ch.
My sensation of lacking bass came while further testing m.b. unplugged and only ch left, as switching the sound boost slider (+6db and +12db) in both positions mid highs were increasing but not bass (that's sure). Even at low power output (5~10w) in a silent room easy understand if bass is in or not...
Anyway, just on the theme of inverting signal, yesterday evening I have replaced both ics LF353n and re-tested m.b., so while testing (1uF/100v in C63) in the low-light corner of my room I detected a small spark coming from the collector area of Q32 (tip142)! I checked with a magnifier and realized the smallish black wire carrying signal (B to B) from R83 to the tip was detached from its hole!
After soldering this B to B wire the right channel works again steadily and problem is overcome. I've tested again for dc out and found 61mv in right ch, while left one has 47mv.
Do you think these numbers are too high?
Should I absolutely replace C63 with the 0,33uF?
Can you -or any other respectful member of this forum- specify if that value is the correct one?
thanks®ards
p.s.: probably mid-highs predominance at turning on of the amp is even due to "cold transistors and circuitry".. this phenomenon is well known between audiophile-people as to be waiting at least 10-15 minutes before judging a system timbre, as in this time (ol' school) electronics get warm and warmish music timbre too!!
yes in a first moment me and even installer we did try to connect every speaker positions to find out why right channel was mute, but this was before C32 replacing and -of course- no sound came out wiring blue to yellow, while sound was in wiring opposite + + as well as + and - of left ch.
My sensation of lacking bass came while further testing m.b. unplugged and only ch left, as switching the sound boost slider (+6db and +12db) in both positions mid highs were increasing but not bass (that's sure). Even at low power output (5~10w) in a silent room easy understand if bass is in or not...
Anyway, just on the theme of inverting signal, yesterday evening I have replaced both ics LF353n and re-tested m.b., so while testing (1uF/100v in C63) in the low-light corner of my room I detected a small spark coming from the collector area of Q32 (tip142)! I checked with a magnifier and realized the smallish black wire carrying signal (B to B) from R83 to the tip was detached from its hole!
After soldering this B to B wire the right channel works again steadily and problem is overcome. I've tested again for dc out and found 61mv in right ch, while left one has 47mv.
Do you think these numbers are too high?
Should I absolutely replace C63 with the 0,33uF?
Can you -or any other respectful member of this forum- specify if that value is the correct one?
thanks®ards
p.s.: probably mid-highs predominance at turning on of the amp is even due to "cold transistors and circuitry".. this phenomenon is well known between audiophile-people as to be waiting at least 10-15 minutes before judging a system timbre, as in this time (ol' school) electronics get warm and warmish music timbre too!!
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