Location: maryland
Posts: 32
I'm jim.
I tried using the end of R20 - same readings from that point as well.
I also took that 4.7uf tant out of circuit and confirmed about -4V on the plus side were C12 had been, NO voltage on the other side on R13. Circuit is definitely broken with it out so that confirms no board trace leak. Put (another) fresh 4.7uf back in, just in the one in a million chance the new one was bad somehow, same readings once back in. About -2V on the R13 side. How is that possible?? It just doesn't make any sense. With R13 no going anywhere (disconnected from the selector switch) the ONLY other is through R20 to ground. the 4.7uf removal established there is absolutely no voltage present on that side. Perhaps I should replace R20 anyway, even though it measures fine? But I thought the Cap did all the DC blocking anyway?
Location: maryland
Posts: 32
I'm jim.
Location: North Island New Zealand
Posts: 1,757
I'm Chris.
So it looks like there is just some residue DC voltage contained in the capacitor
Using a parallel 1k resistor across resistor R20 which is 100k, dissipate that voltage
from C 12 it should take 30 seconds and not return once the 1k resistor is removed.
You could load the phono board output slightly by changing R20 for 22k, the effect will
be to achieve lower noise
Did you replace the capacitors at the high gain op amp - which I advised earlier ?.
Cheers / Chris
Location: maryland
Posts: 32
I'm jim.
i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there. The original, and now two new ones. How can there be residue voltage on a brand new capacitor? Also - why isn't that same level of residue voltage present on the other channel?
I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.
As for the 1uf tants on the main board, they came in Sat but I have not put them in yet since I have been chasing down this phono board voltage problem. Are you saying that they are in any way responsible with the voltage problem we are experiencing on the phono board? If so i don't see how. Or are you just saying that because of the voltage problem on the phono board, those main board caps aren't handling things well. If so that's a symptom, good to fix, but shouldn't the priority be to fix the source of the problem and not just mask it? Maybe I'm just not understand your intent clearly.
Location: North Island New Zealand
Posts: 1,757
I'm Chris.
The original issue was popping noise, now you are chasing DC voltages from coupling capacitors, they are related,
so let the coupling caps do their job. The Crown has intent to load extremely lightly hence it may be intentional to have
what are called offset voltages present. Most circuits today do the opposite - they load circuits down to reduce noise.
A capacitor blocks DC from passing, but in the Crown we see the ground plane is itself capacitively coupled
so you are unlikely to ever rid the circuit of offsets unless measures are taken to bleed DC voltages continuously
The best method at lower safe voltages that the Crown uses is to use back to back diodes, these afford connection and also
a defined ground lift from earth much along the same reasoning why Crown use a capacitor placed to the chassis,
I would try a parallel diodes back to back around C40 which is a capacitive connection to ground - the capacitor is useless
on its own ,as it is then frequency dependent passing AC but not DC.
You can use 1N4007 or UF 4007 types. This should instantly remove the popping noise or any DC on the board
sitting around latently on coupling capacitors causing the problems.
You should notice the Crown is then extremely quiet, and will sound much better too.
So one diode is placed anode ( from the roman word meaning way in ) to the ground plane and cathode ( way out ) to
the chassis. The other diode is placed anode to the chassis and cathode to the ground plane..... hence back to back
You could strap these across C40 if there is room.
This will afford exactly the same isolation that Crown desired, but allow proper DC connection as well
A measurement will show a semiconductor lift of approximately 0.6v.
The more complete analysis of this circuit also involves a resistor which sadly will not give the isolation
Crown wanted, but adding 2 diodes will.
Hope this helps
Cheers / Chris
Last edited by Light Dependant Resistor; 19-04-2017 at 03:58.
Neither do I.
Seems sensible to me.
I suggest you measure the voltage on the other side of C12 where it meets R19/R16 and the emitter of the transistor. Then do a little mock-up of that bit of the circuit by having a capacitor and 100k resistor (like C12 and R20) and putting DC on one side of the capacitor, then measure the voltage on the other. This should at least verify that your meter is making sensible readings.
Location: North Island New Zealand
Posts: 1,757
I'm Chris.
Location: North Island New Zealand
Posts: 1,757
I'm Chris.
How wrong you are, 0v in the Crown iC150 is not zero volts as stated, as 0v is always in series
with C40. Connect 2 capacitors in series you get voltage at their junction... no mystery to that.
C40 is on the schematic is a capacitive connection to chassis ground,
hence a zero volt reference up to the currents involved with a preamp.
Crown's mistake was to take the floating ground concept a bit too far by relying on capacitors
alone. Two diodes should retain Crowns concept and allow connection at a semiconductor junction
at the same time, allowing residual voltages to properly dissipate.
Cheers / Chris