Crestron AV2 Processor: Need help identifying a suitable substitute for a part

[plat27265]

*** Warning: Long read ***

I am currently repairing a Creston AV2 Control System. For those who are unfamiliar with this device, it is a microprocessor-based remote control system that can communicate with intelligent devices capable of one or two-way communication via RS232/422/485 serial, relay contact closures, I/O logic ports, and Infrared serial signals. I have acquired this control processor and it was dead when I picked it up from my friend.

First thing I did when I got home was plug it into an AC outlet to verify it was indeed dead. Then, I took off the top cover and unplugged the SMPS from the main bus board that hold the main processor and control board together. Measured the output of the SMPS and got +24V DC. Plugged it back in and the voltage goes to zero. This tells me that there is a shorted component somewhere in one of the boards. Disconnect the SMPS again and took a resistance reading between the power rails. It read less than 1 ohm. So, near shorted which confirms my suspicion.

Next, I was able to identify that the bad component is on the main board by unplugging the other board from the bus board. With only the main board connected to the bus board, I connected my programmable DC power supply to the power terminals and proceeded to inject voltage and current into it. Frist, I started with 1.5V. Felt around both sides of the main board to find out which component is very hot to touch. Didn't find anything. I proceeded to increase the voltage to 3V, at this point, the current went up to nearly 2A. I know this is excessive. After a few minutes, I smelled something burning. I narrowed it down to the area around the black phoenix-type connected called the CRESNET port. Felt around and found a component labelled Z3 on the board that is very hot to the touch. Nailed it!

The CRESNET port has 4 terminals. The center pair is for the proprietary control signals for other Crestron devices like touchscreens and button control panels and the outer pair which is the 24VDC +/- supply. The built-in SMPS has a 50-watt capacity. The CRESNET power output pins take its power from the SMPS along with the main board and the control board.

Component Z3 has a label, AD13 MM. I looked everywhere on the web for a data sheet for this component but I could not find it. When I contacted Crestron, they refused to give me any information about it. They said to send the whole unit back to them for repair service. I am in no position to pay for expensive shipping to NJ (from BC Canada) and back just so they can replace a component that's worth $0.10. Also, this AV2 processor is more than 13 years old and already an obsolete product.

I would like to seek the help of knowledgeable people in this forum to identify Z3 for me. I suspect that the AD13 part number is not real. It is a proprietary part number for a common part that Crestron ordered from a semiconductor company.

Z3 straddles the 24V power rails. When I measured it in-circuit, I get 3 ohms across regardless of polarity. I have included photos in this post to show you the main board and where this Z3 component is located. The large fuse beside it sits in series to the +24 VDC line. I have confirmed that one end of Z3 sits on the (-) side of the power rail and the other is on the (+) side.

I suspect that Z3 is a zener diode but not sure. I suspect that it is meant to protect the main board from someone connecting something with 24VDC into the green port that is outputting 24VDC. Or, prevent damage by accidentally connecting a load in reverse polarity.

I am seeking some thoughts and recommendations for a substitute part. Thanks! I know this is a long read but I think sharing this much info is essential.

[Whales]

If there is a Z3 then there might be a Z1 and a Z2. 

Desolder one and do some tests with a DC power supply,  a resistor and a multimeter 

[plat27265]

If there is a Z3 then there might be a Z1 and a Z2. 

Desolder one and do some tests with a DC power supply,  a resistor and a multimeter 

Z3 is the only component of its kind anywhere in this device.  I've been looking all over for a Z1, Z2, etc. And, could not find any.   I will look again especially in the SMPS. 

[SpecialK]

I see Z1 and Z2 right next to the suspect Z3 in the second picture.  They are those through hole diodes.

Really, I don't suspect Z1 is bad.  It's probably getting hot because there's a dead short downstream and it's having to handle unusually large current.

[Manul]

If you say that this diode is connected across ground and +24 DC input, just remove it from board for now. It is not needed for a device to function, it is just for protection. Why I say like that is because maybe you will find that the short (3 ohms) is somethere else and not actually this diode. And you will be able to test a device to see how if works or what other problems it has. You can always put a zenner in that place later. It is not at all critical.

[plat27265]

Thanks!  I will do just that.  Actually, I thought of doing that while having lunch this afternoon.  I am almost sure that Z3 does not really need to be in place for the whole thing to come back  to life.  You are correct in saying that it might be something else. 

I suspect, based on my observations of this one-of-a-kind unknown value component, that it was placed in there so in case of failure, the Owner will have no choice but to either send it back to the manufacturer for service or just buy a new one.   I also say this because, as a diode, it should have a polarity marking.  Not having any polarity marking on it is highly suspicious which just reinforces my opinion that Z3 is not an essential part in the circuit and it was just planted in there to promote future sales of new product.  That's my conspiracy theory :-D

[plat27265]

If you say that this diode is connected across ground and +24 DC input, just remove it from board for now. It is not needed for a device to function, it is just for protection. Why I say like that is because maybe you will find that the short (3 ohms) is somethere else and not actually this diode. And you will be able to test a device to see how if works or what other problems it has. You can always put a zenner in that place later. It is not at all critical.

I must be going blind!  I totally missed Z1 and Z2 right beside it!      I was looking all over several boards for them!   But, there is still nothing like Z3 anywhere else, for sure.  This is the only part that  got very hot to touch when I pumped in 5V, 2A current from my DC PSU.   The built-in SMPS is only 50-watts at 24V, so at 5V, 2A this thing should not get very hot to the touch but it did.  Can you imagine if I injected a full 24VDC, that thing would have fried.   

This is one effective way of finding out a shorted component in a motherboard or processor-based board that is dead due to a shorted part.

[SpecialK]

I would say that Z3 is a Fairchild diode for reverse voltage protection. Or maybe flyback. I'm not sure the particular model.  It's probably nothing special.

 The more I think about it, the more I think you're right that it is at fault.  Desolder it and test the board, just make sure not to get your polarity reversed. 

[Manul]

If it is on +24V rail and have no cathode marking, I bet it is a TVS diode. Just to surpress overvoltage from sparking or other events. Not critical. In fact you can choose one for replacement just by voltage and footprint and it will be just fine.

[plat27265]

Thank you for the advice, Manul and SpecialK.   

I took out 'Z3' from the main board this afternoon and confirmed that it is indeed causing the short circuit.  Not a complete short but near short at 3 ohms even when the probes are swiched around.  Tested the 24VDC power rail and no more shorts.  Connected its SMPS to it, the Green LED power light lit up.  All is well.

Coincidentally, I have some Crestron branded touchscreen interfaces lying around in my office.  Opened one up and at the CRESNET port of these devices, I found a similar diode but this one has 'AK27 MM' and cathode markings.  Did an in-circuit resistance test and it tests like a diode.  Forward bias, needle deflects to around 50 ohms.  Reverse bias, no needle movement.  It is a polarized diode of some sort. 

Both diodes have the stylized 'F' logo which is most likely Fairchild, as SpecialK mentioned.

I did not desolder the 'AK27' diode to find out what type it is (by taking a forward bias voltage reading) since I don't plan to replace the Z3 (AD13) diode from the main board.  I could have done it since my vacuum desoldering gun was still hot but just felt lazy.  Also, I think that this Z3 'AD13' is different from 'AK27' since it does not have cathode marking on it.  I think it may be a bi-directional TVS diode like Manul suggested.   The Z3 (AD13) diode straddles the 24 VDC power rails.  The 'AK27' from the other Crestron device is used and oriented differently.

Also, I have no intentions of using the +24VDC power from the CRESNET port.  Modern input devices such as Crestron touchscreens communicate with a control system, like this AV2, via Ethernet Network and rely on PoE power. 

I might do it tomorrow when I fire up my soldering and desoldering irons to repair a busted channel (Shorted Power MOSFET) from a LabGruppen power amplifier and blown in-circuit current limiter (thermistor) from a QSC audio power amp.

Thanks everyone for your input!