Scratching My Head w/ Crown CDi 4000

[Freecrowder]

So I'll try to keep this brief. I have been trying to fix this amp that doesn't want to power on.. Yes I have read all the posts and seen the most common problem with this series is the cap 196 that is part of the start circuit. this was a design flaw from factory and changing it usually takes care of all the problems. No luck. What is making things the most difficult is that all the circuit is very small surface mount. It seems from measurements that I'm not getting the Kickstart signal that should occur with the SMP power supply. I'll attach schematic. Looking at page 6, power supply. All my measurements are referenced to V_BULKRTN. At power on I get ~168vdc across the main caps after the bridge. At TP31(C8) I get 0vdc. At the top of the resister divider feeding V_KSTART I have 168vdc and at the midpoint of resisters 84vdc. So testing the Zeners, D28, D29 in circuit both the 16v diodes test good .7vdc forward and open on reverse. The D25 tests good also in circuit. I also notice that I never get 30v_CTRL and thus no 15v_CTRL. That goes to alot of stuff. So that is a good start. One question where does the 30v_CTRL originate?? Is it the power transformer circuit at B6?? But that is the primary so not sure?
I have many years of repairing equipment but almost no experience with SMPS supplies. Understand just a little from Youtube. Anyway this is a very common problem with this line of Crown amps. But not the capacitor it usually is.

 

[Freecrowder]

So this is the 2nd site I have posted this to with 0 responses. Is there something I'm not understanding?? Surely there is someone that knows SMPS supplies better than I do. Is there a better site that I should try?

 

[Delta Prime]

Surely there is someone that knows SMPS supplies better than I do. Is there a better site that I should try?

Yes, there is someone but I do not like to be called surely.
https://www.allaboutcircuits.com/

 

[kellys_eye]

So this is the 2nd site I have posted this to with 0 responses.

We actually have lives to live and can't dedicate to this forum 24/7 as a result.....

If you have DC at the main bridge (I'm assuming a 120V input?) then 0V measured at the top of the zeners (TP31) then resistors R370-373 must be open.

What do you measure at the 'top' of R370/R372?

 

[Freecrowder]

At the top I get ~168vdc and at the midpoint, top of 371, 373, I'm getting ~84vdc.

 

[kellys_eye]

I'm getting ~84vdc.

Still getting 0V at TP31?

 

[Freecrowder]

Yes, so logically I would question the two zeners D28, D29. the way I see it is there should be ~32 volts at TP31. I have tested them in circuit and I get .7 forward and open reversed. which I believe is correct for a zener.

 

[Freecrowder]

Again not a lot of experience with SMPS before. But the V_kstart is the kick start that starts up a SMPS supply correct? Without will never come on??

 

[Ylli]

If you are seeing +0.6 volts at TR31, there could be a short on the +30V_CTRL bus.

 

[Freecrowder]

I'm getting 0V on TP31. I get zero at the 30V_CRTL also and therefore no 15V_CRTL. That is one of my main questions is where does the 30V_CRTL originate?? Is it the other section I mention connected at the primary of the XFMR? This section resides on the primary though so do not understand that? If yes than it relies on the V_KSTART signal also which is 0V.

 

[Ylli]

The +30V_CNTL originates at TP31. After the unit is running, it is supplemented by a rectified signal from the transformer (D26, C196). With power off (and all the caps discharged) measure the resistance between TP31 and V_BULKRTN.

 

[kellys_eye]

the most common problem with this series is the cap 196 that is part of the start circuit. this was a design flaw from factory and changing it usually takes care of all the problems.

The only way you can get '0V' at TP31 is with a shorted zener or a shorted C196. You said "no luck" with the C196 answer but never told us what you did. Did you actually replace it? Did you replace it correctly or test it before fitting the new part? Have you retested it?

That said, a shorted C196 (you don't say how C196 usually fails, open or short) could also destroy D27 - check that.

 

[Ylli]

The V_KSTART (TP31) is used directly by one op-amp - U16. If that amp has a VCC/Gnd short it would also pull TP31 to 0 V. If C196 were shorted, I would expect to see 0.6 volts at TP31 (drop across D27).

 

[Freecrowder]

So I am with you guys and have been checking all those things. But I have rechecked all again and will report
1. Resistance TP31 and V_BULKRTN is 7 ohms. I'm assuming I'm measuring the primary coil(5-6) with that reading.

2. Yes I replaced C196 with brand new 220uF 50V cap from digikey. Then later I replaced with another same brand new cap. The one I pulled new measures good still. This is a common fault that causes the same problem I'm having. It was design flaw from factory bc it was only rated to 35v and many people have measured higher volts when primary discharges. Also D27 and D26 both test good in circuit.

3. I have also thought about U16 being damaged but no good way to check unless pulled and replaced. I'm assuming. Might come to that but SMT would be challenge. I have done it before though.

 

[kellys_eye]

What's the resistance from TP31 to ground? (reference ground)

 

[Freecrowder]

If your talking about TP2(B,3) it is open from TP31 to TP2. But I think reference gnd for this circuit is V_BLKRTN. So TP31 to V_blkrtn is 7 OHMS.

 

[kellys_eye]

So TP31 to V_blkrtn is 7 OHMS.

Measured both ways? If so then test again with either of the zeners removed (D28/D29) and again with D26 removed.

 

[Ylli]

7 ohms is a problem. There is no path for V_KSTART through the transformer as long as D26/D27 are not defective. And 7 Ω is way too low even if you are forward biasing the Zeners. The only place I can see that might cause this is a defective U16 or a shorted C61. I understand changing a SM IC can be challenging - perhaps you can just carefully lift pin 8 (solder wick all the solder off the pin, then get your exacto knife blade under the pin, heat it, and lift it just a tiny bit off the pad). Then remeasure TP31 to V_BULKRTN - if it is still low try switching the meter leads.

 

[Delta Prime]

. I understand changing a SM IC can be challenging - perhaps you can just carefully lift pin 8 (solder wick all the solder off the pin, then get your exacto knife blade under the pin, heat it, and lift it just a tiny bit off the pad). Then remeasure TP31 to V_BULKRTN - if it is still low try switching the meter leads

Brilliant! I've never thought of something like that.
My contribution to this conversation is...every doctor needs a nurse you're doing surgery you're going to have to employ someone else to read the meter and change your leads. if you turn your head up to glance at the meter that Chip's lead is going to come right off...

 

[Freecrowder]

Ok out for the weekend. back at it later today. And 7ohms was measured both ways. Looked like it should have been blocked one way. Any way getting ready to order some parts to start replacing if needed. I have done some soldering on SMT before. This is challenging bc of tight component placement. Might have to remove some big electrolytics to make some room. BTW this might be another clue. I lifted the resister string at the bottom of 374-378 and powered on. Then I have .6v across C196. Anyway with all the help I'm sure I can get to the bottom of it. Sure like linear supplies better!