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Scratching My Head w/ Crown CDi 4000

Ylli

Jun 19, 2018
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Try lifting one end of D27 (or remove it if it is SM) and see if TP31 goes to +32 volts. If that does not allow TP31 to come up, then it almost has to be a bad U16, and you will have to lift pin 8 (or totally remove it - the unit should actually operate with U16 removed).
 
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Freecrowder

Sep 13, 2023
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Sep 13, 2023
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Yes I agree. Will be working on those things. Stay tuned. Thanks for all the help.
 

Freecrowder

Sep 13, 2023
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So lifted D27 and still 0v on cap 196. Then replaced and removed U16. Now I have my 32 volts at C196 and V_KSTART. But amp does not power up. I only have ~6vdc at the 15 V_CTRL. I do not know if that is bc the U16 is not in circuit or not. Not sure what that is doing. Anyway have new U16 coming. Some progress made.
 

Ylli

Jun 19, 2018
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U16, a LM393 dual comparator, has open collector outputs (pin 1 & 7). They can only pull down, not supply any current. It will pull down and disable the +15V_CTRL line if it detects the AC line is too low (U16B) or V_KSTART is too low (U16A). If U16 is not in the circuit, this fault detection will not function.

Understand you now have +32 volts at TP31.
What voltage do you have at the junction of R378 and D31 (U16 pins 1 & 7 pads)?
How about on the bases of Q39/40/41?

Sometimes, SMPS's are finicky about how the power comes up. Something to try... short D31 (connect the junction of R378 and D31 to V+BULKRTN). Power up, allowing the bulk caps to fully charge (maybe 3 seconds), and then remove the short. Does +15V_CTRL come up to +15 volts then?
 

Ylli

Jun 19, 2018
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Something I should have mentioned earlier: You are working on the AC line side of a SMPS. This circuitry is not isolated from the AC line and there are lethal voltages present. Even the V_BULKRTN node can have a voltage on it as long as the unit is plugged in even if it is switched off. Use EXTREME CAUTION to not come into direct physical contact with any node in this circuit. Also note that the bulk filter capacitors can hold a lethal charge for a while after the unit is turned off.
 

Freecrowder

Sep 13, 2023
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Sep 13, 2023
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Yes thank you for the reminder. I have been exercising caution.
Thank you.
Just a note about where I'm going next. When I was troubleshooting I ended up removing D31 and had to replace with a small glass style standard zener. Made it fit for further testing. It is a cheap bulk chinese version and I'm doubting it's authenticity/quality. I'm getting some more zeners in but am going to pull and test zener voltage on bench. It was a bulk buy of many different zener values and I realize they could be counterfit. I should know by now but all I had around.
 

Freecrowder

Sep 13, 2023
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Sep 13, 2023
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Hopefully a breakthrough. I noticed that even w/o u16 in place I was still measuring ~7ohm to V_BLKRTN at the junction of D31 and Resister string 378/374. Removal of U16 just isolated left part of circuit from right. That is why I got back my 32vdc at C196 but no 15vctrl. All transistors q39-41 measure good and all associated parts on right of circuit. Removed D31 and 378/374 at node point and measured node and still had 7ohm. Long story short low ohm is originating from protection circuit on page 3(B,8). Not that unusual that protect circuit could be culprit, part of crowbar. Anyway going to remove Q15 and Q7 and retest ohm and power up. Stay tuned.
 
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