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Old 22nd November 2009, 09:29 AM   #1
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Default help with repair of DC300A series II

i've recieved an amcron DC300A series II with some issues. but, i think i've made it worse then it was...

problem 1: channel B gets very hot
problem 2: channel A doesn't work properly sometimes. very loud pink-noise like sound, cone sucking in, etc etc. other times it works fine.

first: i don't know wich channel is wich, so i named the right channel (amplifier towards you, normal orientation) B, and left A.

then the schematics and layout (see attachments) (pre)driver board components, layout and complete schematic.

so, i started with problem 1, channel B, first. this since problem 2, channel A, didn't happen the first few test runs.
i noticed one of the driver transistors (Q209,Q214) from channel B was a MJ15003, instead of the original 2SD555. the other wasn't replaced. there's your oscillation. so i replaced both drivers with new MJ15003 (these are suitable, but not perfect. shouldn't be a problem tough). oscilation gone

then, i fired it up, it ran good for an hour or two. but no celebration yet because it overheated. turns out the bias of channel B isn't stable. after a minute it's over .1 volts higher (measured acros R232) then it should be, and raising. blowing cool air at the bias transistor brought it back to normal, so there's something wrong there.

after discovering this problem, channel A suddenly started acting weird. loud noise, peaks to VSupply, fluctuating DC offset, agressive speaker movement. channel B still 'normal'.


i turned the amplifier off, on, off, on.. sometimes normal, sometimes weird behaviour. i cound't measure for more then a few seconds because channel A heated up very quickly and i was sure my test-speaker would be destroyed.

I suspected the main amplifier board A. maybe a leaky power/driver transistor. so i disconnected the yel/wht coming from Q106 (pre-driver) to Q109 (driver) and vio/wht from Q107 to Q114 (ditto). same problem. reconnected the wires. same problem.
then i suspected the main opamp (uA739). swapped with another DC300A II, same problem. then i suspected the famous zeners+10uF 250V cap. replaced the cap, zeners were OK and besides should also influence channel B, not only A. still the same problem, only more frequent. could be because i've turned it on and off quite some times now.


went to sleep.

the next day, i turn it on and it works like a charm. same bias problem in channel B, but when the bias pot was set to minimum bias would just be stable enough to continue measuring channel A. turned it off, desoldered the red wire going to both output boards. i thought to do so to protect the output boards from the weird behaviour by not giving it any voltage.

turned back on. seems stable but music is still coming through quite clear, but low volume. strange. must get power from somewhere else?

i start tapping around on various component to check for loose connections. nothing triggers the fault. all transistors measure ok. while tapping around on the channel B-half of the board suddenly a spark flies from somewhere. unplugged the amp, and saw that Q220 (2n4125) was in two halves. measured around that transistor, no problems, inserted a BC557 for replacement (and doublechecked the orientation )

turned it on, acting as before with some music.

BUT THEN. channel A does the same trick as before, voltages, DC, everything. with the red V+ wire disconnected!!

stupid as i am, i turn of the amp, and reconnect the red wire (this was at 0:00, stupid stupid stupid. i should've slept) _only_ to channel B. turned it on, and Q106 (the pre-driver of channel A, D40P3) burns out.

the following questions where dominant:

- what now?
- was the BC557 a bad move?
- what common transistor can i use for replacement for the D40P3?
- how do i measure what is going on in channel A? i can't leave it on for more then a few seconds.. and i'm pretty sure i can't turn it on at all now.
- does anyone have a clue what could be wrong?

things i do know
- when you switch to mono (i've discovered this before things started blowing up) the noise etc is in both channels!!
- all the power transistors, resistors and diodes in both output boards are OK
- the uA739 is OK
- the power supply for the uA739 is OK


then i decided it was time to ask you folks
Attached Images
File Type: jpg drivercomponents.jpg (156.8 KB, 251 views)
File Type: jpg driverlayout.jpg (68.4 KB, 239 views)
File Type: jpg completeschematic.jpg (815.8 KB, 257 views)

Last edited by powerflux; 22nd November 2009 at 09:52 AM. Reason: forgot something
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Old 22nd November 2009, 09:37 AM   #2
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oh, i see the complete schematic is badly readable.

here you can find the whole service manual
http://www.crownaudio.com/pdf/legacy...viceManual.pdf

Last edited by powerflux; 22nd November 2009 at 09:57 AM.
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Old 22nd November 2009, 02:30 PM   #3
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please? at least any ideas for the d40p3?
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Old 24th November 2009, 11:14 AM   #4
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here are AMCRON/CROWN DC300A II transistor replacements i've found.

original -> preferred
------
original specs
------
other options

when i've recieved the replacements; the measuring can start again

D40P3 -> MJE344 / MJE340
--------------------
Si NPN Power BJT
Various
V(BR)CEO (V)=180
V(BR)CBO (V)=250
I(C) Abs.(A) Collector Current=500m
Absolute Max. Power Diss. (W)=1.67
I(CBO) Max. (A)=10u
@V(CBO) (V) (Test Condition)=250
h(FE) Min. Static Current Gain=40
@I(C) (A) (Test Condition)=80m
@V(CE) (V) (Test Condition)=10
f(T) Min. (Hz) Transition Freq=50M
@I(C) (A) (Test Condition)=80m
@V(CE) (V) (Test Condition)=10
t(r) Max. (s) Rise time=2.5u
t(s) Max. (s) Storage time.=2.5u
C(obo) (Max) (F)=6.0p
@V(CB) (V) (Test Condition)=10
@Freq. (Hz) (Test Condition)=1.0M
Package=TO-202AC
Military=N
--------------------
3DA87D BD410 BF381 BF382 BF461 MPSU10 MJE344



2n4929 -> 2n2216
--------------------
Medium Power, High Voltage, General Purpo, 2N4929, Silicon, PNP, 1W, 150V, 150V, 4V, 500mA, 200°C, 100MHz, 10,

25/200, MOT, TO5
--------------------
2N2216 2N3413 2N3634 2N3743 2SA1573 2SA698 2SA781 2SA793 2SA922S 2SB736 BC303
BC303-4 BC303-5 BC303-6 BC304-4 BC345 BC396 BC461 BC640 BFR23 BFR79TO5 BFS93
BFS98K BFT19 BFT37 BFT37A BFT44 BSV11 BSV17 BSV17-10 BSV17-16 BSV17-6
BSW40 GES4929 KT3108A KT313A KT505A KT639E RCA1A02 2N3636..37 2SB606 BFT44
BFT45 2N3634 2SA698 BFT44 KT505A


NSD128 -> MJE350 (?)
--------------------
Si NPN Power HF BJT
National Semiconductor
V(BR)CEO (V)=180
V(BR)CBO (V)=180
I(C) Abs.(A) Collector Current=250m
Absolute Max. Power Diss. (W)=1.7
I(CBO) Max. (A)=50n
h(FE) Min. Static Current Gain=50
h(FE) Max. Current gain.=500
@I(C) (A) (Test Condition)=10m
@V(CE) (V) (Test Condition)=5.0
f(T) Min. (Hz) Transition Freq=100M
Status=Discontinued
Package=TO-202AA
Military=N
--------------------



2n4125 -> BC558 / 2n2907A
--------------------
Low Power, General Purpose, 2N4125, Silicon, PNP, 310mW, 30V, 30V, 4V, 200mA, 135>C, 200MHz, 5, 50/150, F, TO92-2
--------------------
242760 2N2906 2N2906(A) 2N2907 2N2907(A) 2N2907/A 2N3209 2N3903 2N3905 2N3906 2N4126
2N5139 2N5141 2N869 2N869(A) 2N995(A) 2SA1055 2SA709 2SA741(N) 3CK3F A5T4125 BC205
BC213 BC252 BC308 BC350A BC513 BC558 BFV81A,B BFV82(A...C) BSW24 BSY40 BSY41 GET2221
KT3107A KT361B LY2894 MPS4123 MPS4124 MPS4125 MPS4126 MPS6517 XGFP2894


2n4250A -> BC206
--------------------
Low Power, Low Noise, General Purpose, 2N4250A, Silicon, PNP, 200mW, 60V, 60V, 5V, 100mA, 125°C, 100MHz, 6, 250MIN,

F, TO106-1
--------------------
2N4248 2SA1049 2SA1136 2SA1137 2SA970 BC206 KT361K


2n3859A -> BC546
--------------------
FRH
Various
FRH, 2N3859A, Silicon, NPN, 360mW, Vcb 60V, Vce 60V, Veb 6V, 100mA, 125°C, 90MHz, 10, HFE 100MIN, GEN, TO98-1
--------------------
2N2926-1 2N3793 2N3794 2N4256 2N4409 2N5209 2SC1815 2SC1815-BL 2SC1815-GR 2SC1815L-B
2SC1815L-G 2SC1815L-O 2SC1815L-Y 2SC1815-O 2SC1815-Y 2SC2109 2SC2111
2SC4444 2SC4446 2SC4449 2SC4485 2SC689 2SD474 2SD474K 2SD591 2SD592 2SD592A 2SD592ANC 2SD592NC

2SD599 2SD601A 2SD602 2SD602A 2SD603 2SD636 2SD637 2SD638 2SD639 2SD655 2SD661 2SD671 2SD734
2SD754 2SD767 2SD769 2SD770 2SD771 2SD778 2SD779 2SD786 3DG121B BC168 BC169 BC174 BC182
BC182B BC182L BC184 BC190 BC337 BC546 BC637 BC682L BC817 BC846 BCW98 BCW98A BCW98B
BCW98C BCW98D BCW99 BCW99A BCW99B BCW99C BF299 KT3102A KT3102B KT3117A KT3130V KT315E



MPSA93 -> MPSA92
--------------------
Si PNP Power BJT
Zetex Semiconductors
V(BR)CEO (V)=200
V(BR)CBO (V)=200
I(CBO) Max. (A)=250n
h(FE) Min. Static Current Gain=30
h(FE) Max. Current gain.=150
@I(C) (A) (Test Condition)=30m
@V(CE) (V) (Test Condition)=10
f(T) Min. (Hz) Transition Freq=50M
Package=Chip
Military=N
--------------------
2N5401 2SA860 BF423A BF491 BF492 BF493 BFP23 BFP26 ECG288 MPSA92 SO693 2N5415 2N6432
2SA840 2SA860 3CK02 BF435 BSS74(S)
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Old 24th November 2009, 06:08 PM   #5
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i think anyone who would want to repair this will need it on the bench in front of them and start from basics, ie remove all the transistors and check them, along with most if not all the rest of the componants
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Old 26th November 2009, 06:07 PM   #6
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nvm

Last edited by powerflux; 26th November 2009 at 06:07 PM. Reason: nvm
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Old 27th November 2009, 06:41 PM   #7
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so many questions so little time!

the red wire to o/p board is the pos, the blue was still connected (neg) hence half an output signal!

the 4 pots are dc input and output offset NOT bias. / bias is set be the resistor on the board that pushes into the two metal pcb stand offs.

d40p3 = d40v2

check ALL small signal transistors esp the 2961 and 2n4125. (all of them!)

like said above by other people you need to be checking components not doing wierd things with the amp, otherwise it will go DC BIG! BIG! time!!!!!!!!!!!
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Old 27th November 2009, 06:46 PM   #8
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Quote:
Originally Posted by krmaudio View Post
so many questions so little time!

the red wire to o/p board is the pos, the blue was still connected (neg) hence half an output signal!

the 4 pots are dc input and output offset NOT bias. / bias is set be the resistor on the board that pushes into the two metal pcb stand offs.

d40p3 = d40v2

check ALL small signal transistors esp the 2961 and 2n4125. (all of them!)

like said above by other people you need to be checking components not doing wierd things with the amp, otherwise it will go DC BIG! BIG! time!!!!!!!!!!!
i realized the first point very quickly after the first blown transistor.. i never work late on this amp anymore

i've desoldered all transistors, found 3 blown ones. 2x 4125, 1x bc639 (replacement voor the 3859a).

also found a dead small-signal diode and a keramic missing one leg

replaced most transistors now, i'm trying to get a big variac before firing it up again.
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Old 28th November 2009, 04:29 AM   #9
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Quote:
i'm trying to get a big variac before firing it up again
Just put a 60W light bulb in series with the Mains AC. This should give you time to do some "live" measurements without the amp goes into
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Old 28th November 2009, 06:50 AM   #10
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Quote:
Originally Posted by Jan Dupont View Post
Just put a 60W light bulb in series with the Mains AC. This should give you time to do some "live" measurements without the amp goes into
yeah, i used that trick, but that didn't help enough to give me time to measure, nor protected the poor pre-drivers

i gotta be patient, get the variac, pop in some new transistors and then
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