Fixed!! Please Help: Crown CTS 4200 Amplifier-Thermistors get REALLY HOT!

[mohare]

Hello Group.
I'm new here...drawing is attached that will help explain below-repaired traces highlighted yellow and thermistors are red (page-3):
I have a small project I'm working on..details:
Crown CST 4200A 4-channel 250W each. Original issue is the unit doesn't turn on. These have a typical issue with their 3-party low voltage PS going bad. I've had success with replacing them with a newer unit made by Mean Well.
I replaced the LVPS and repaired (jumpered) 2 burned-out traces that came from the power switch to the 1) power relay, 2)thermistor.

The unit now turns on but I get a blinking power light which means over/under voltage. I'm assuming over voltage because the 2 thermistors get extremely hot (enough to melt the hot glue off them). I immediately noticed it and turned the unit off. I pulled the LVPS out as-to not damage it, turned the unit back on and they still get very hot...obviously they're on the HV supply side.

There are (2) Y2-type caps in series for overcurrent protection. I pulled one and couldn't get a reading, assuming they're bad (would a 2200pf cap even register resistance on a multimeter?)...I'm assuming I'm getting a short to ground...

Without being able to power the unit up and test at the appropriate points, I'm kinda stabbing in the dark...

Any help/advice of what to check would be greatly appreciated.
Thanks-mohare

[David_AVD]

If the thermistors are getting hot that means they relay that should bypass them just after power up is not activating.  That would tally with the under voltage error.

You may have a short on the output stage, a fault in the power supply, etc.  I doubt the suppression caps are faulty.

[mohare]

Thanks for the reply.
I tested the relay and it works fine...perhaps my rectifier is bad?  I'm assuming it has to be something in-line with the HV section post the thermistors.

[mohare]

mmm...scratch that last comment.  I see your point re-reading your reply...i have to find out why my relay isn't getting the LV to activate.

[David_AVD]

Occasionally a soft start relay can fail, but it's far more likely that you have a fault dragging the PSU voltages down.  The low / missing voltage causes the soft start relay not to engage and the thermistors stay inline for far longer than designed.

[mohare]

Referencing the drawing in this update:

I thought the same thing about the soft start relay.  I have the same amp powering my L/R channels for my home theater, so I swapped the oscillator cards (the power supply changes the 60hz to 130Khz to remove any PS hum) and the one from the broken machine works fine.

I was able to quickly turn the unit on (before the thermistors overheated) and test the 110V line (where they come off at Thermistor RT902) input for the LVPS and that source is good.  The LVPS output is providing the required 15v/5v sources.

So...since the +5 going to the relay was OK, I jumpered pin-1 on the relay to neutral to bypass the SOFT_RLY circuit.  It worked, but almost immediately blew my 15A fuse on the input which is telling me the fault is post relay on the HV side. 

Here's my question/thought: power isn't getting past the relay due to lack of the SOFT_RLY signal from the IC.  That means it's getting a "shutdown" signal from my power supply.  The only place in the power supply that has a "shutdown" command is from the IC oscillator card that feeds the A-B signals into the HV choppers Q93/Q94.  I know the oscillator card is good.  Also, fuse-2 did not blow during my test, which is post choppers...what are the chances Q93/Q94 are bad, shorted?  What could cause that?

[David_AVD]

You most likely have a fault in the amplifier output stage.  Look for shorted output transistors, etc.

[mohare]

I took out those 2 chopper transistors to test them...winds up they are shorted.

[mohare]

This amp actually has a great setup when it comes to the amplifiers.  It has 4 discrete amplifier circuits...actually separated on the board.  If one channel fails, the IC disconnects it from the rail and throws it into fault and the rest of the channels remain live.
That's why it had to be in the PS stage...that is the only common circuit in the unit.
I'll post if the replacement of the IGBT's fixes it...
Thanks for the help!

[David_AVD]

Good find.  Let us know how you go. 

[mohare]

Update: I replaced the IGBT's (Q93/Q94 Choppers) and Q94 immediately popped!!  I was not happy about that because they are about $9 each!
Taking the investigation further, I pulled both choppers (again).  I also pulled the rectifier.  Rectifier tested fine so I put it back in.  Instead of just relying on knowing my chopper signal circuit (on the daughter board) was good, I performed some DSO tests.  TP-11 was seeing the 130Khz signal from the board, but TP-2 was not, Hmmmm....  Backing it up to R939, discovered that resistor was open!  So I bought 2 new choppers (so they're from the same lot) and replaced R939 and cleaned everything up.  Just to be safe, I pulled F2 in-case the failure was past that point (but probably wasn't because that fuse never popped).  Also removed my temporary jumper feeding the soft-start relay.  Then I turned the unit on while hiding behind my beer fridge  ...the power light came on and was blinking again.  That's a good sign!  Turned the unit off and replaced F2, turned it on again and click, click, click...green lights all around!!!  An open gate resistor caused Q94 to blow...always a reason, right!
So here's the full repair:
1) Recapped the LV power supply (only 4 were bad)
2) Jumpered (2) burnt traces
3) Replaced (2) Chopper IGBT's
4) Replaced open 10-ohm/.25W resistor

[mohare]

Thanks for the help and insight @David_AVD and letting me bounce my thoughts off you (so the wife can stop telling me to not talk to myself) 

[David_AVD]

Excellent result! 

When you have a blown (shorted) transistor or FET, it's always a good idea to check the connected parts.  Open circuit base (gate) resistors are not uncommon.

[marvinperry68]

Hello I am looking to replace the Low voltage power supply on a CTS 8200 and I looked at this MEAN-WELL Model PT-65C it is 65 watts 5v@5.5A 15V@2A -15V@0.5A Looks really close to original but with a little more power handling will this work

[mohare]

Hi,
Yes it should work fine.  This is the one I used:
https://www.mouser.com/ProductDetail/MEAN-WELL/PT-65B?qs=pqZ7J9Gt%2FmrUvp2%252BT2RNJA%3D%3D&gclid=Cj0KCQjwqfz6BRD8ARIsAIXQCf0K49TFDNuuT0VYYGFRcgqdKD0iZLktUlSagdOH-iX5T6II8ArBITkaAmR6EALw_wcB

Cheers,
Matt

[marvinperry68]

Thanks Matt Have a great day.

[Essen]

Hey Matt, thanks for sharing all your work here! Curious: how did it hold up since this fix? I'm looking for the power supply module for my 4200 right now. The meanwell one you shared a link to is about a third of the price of the NOS module. Have a great day! Thanks.