Hi all,
Very short version:
If your 192 has problems from topic - replace component R1 (NTC thermistor) on PSU board.
R1 component name and value - NTC thermistor, resistance 10R @ 25C, 3A
Long version with detailed description and background:
If your 192 has power up issues like power up delay, no power or popping when turned on, this is step by step procedure on how to repair. This is moderate level repair with required minimal soldering skills and tools. Switching power supplies contain some high voltages, so please skip this if you are not familiar with basic repair knowledge. Parts needed for repair cost around 0.5USD. Tools needed - Philips screwdriver, a decent soldering iron.
Now, let's get to the point:
Most common symptom is power up delay. When 192 is turned on, nothing happens, and it takes some time (from 15 seconds to more than 30 minutes) for it to turn on, relay clicks and from that point all works fine. When it's turned off and on again, problem repeats. Usually just before it starts, popping can be heard from inside device or even some burning smell out of fan outlet. Before 192 develops thiss issue it can power on correctly but with slight delay and popping from inside the unit. This condition will most likely deteriorate and lead to longer power on delays and finally end with consistent no power issue. In case of complete no power on issue, this service procedure will only repair cases which developed from popping and power up delay condition. It will not repair no power on issue caused by some other malfunction on PSU board (procedure step 6 will decide to continue or not).
The problem is in SMPS NTC thermistor (component R1 on PSU board). NTC thermistors are temperature-dependent resistors that employ special semiconductor ceramics with a negative temperature coefficient (NTC). They have a high resistance at room temperature, and when they are energized, they generate heat by themselves and the resistance falls as their temperature rises. With this property, they are used as current protection devices for electrical and electronic devices which easily and effectively limit abnormal currents including an inrush current at the time of powering on. NTC thermistors used as current protection devices are also called power thermistors.
At the time of powering on an SMPS, the device is charged with an inrush current with a high peak to charge a smoothing capacitor. Because this inrush current may negatively impact the service life of the capacitor, damage the contacts of the power switch, or destroy a rectifier diode, it is necessary to take countermeasures - NTC thermistor.
So, what happens in failing 192 - if NTC thermistor is burnt, device will not turn on. However, depending on how thermistor is burnt, ambient temperature and moisture, it will leak some voltage, it will charge smoothing capacitor little by little and when it's full, PSU will start. 192 has very fine PSU that takes this application to the next level - Once PSU starts up and self-test pass, relay is engaged to bypass NTC resistor. Bypassing NTC resistor nullify power loss and a decrease in performance caused by NTC itself.
Repair and test procedure:
1. Take apart 192 (remove top cover)
2. Take out PSU assembly (6 screws)
3. Take out PSU board (6 screws)
4. Locate component R1 (NTC thermistor) next to relay
5. Do visual inspection of NTC. Does it have burnt holes or any cracks and marks?
6. Measure NTC resistance in circuit. Is it around 10 OHM?
7. If NTC is visibly burnt or measure high resistance (defective NTC usually measure few KOHM or MOHM) - replace NTC thermistor
8. Test 192 with PSU outside the chassis. Observe visually and take measurements with DVM
9. Check all output voltages (voltages are written on PSU PCB)
R1 component name and value - NTC thermistor, resistance 10R @ 25C, 3A
You can use any component brand as long value is original. I found it in a nearest local shop for 0.5USD. It's very common component used in almost every decent PSU.
Now, if you didn't do it in a long time - it's good idea to do 192 internal cleaning and do some maintenance.
1. Remove all cards
2. Clean all dust
3. Clean all connectors and headers
4. Clean (or even replace) cooling fan
I also recommend the following:
1. Check analog cards (especially input cards) for discolored or burnt MELF resistors (visual inspection with magnifying glass)
2. Check analog cards in place capacitor values and ESR
For capacitor test you will need ESR tester. This test can show how healthy is your 192. This 22uF SMT capacitors gets hot and with age capacitance decrease and ESR increase. Good condition 192 has around 28uF with ESR around 2R. My other 192 that is much older has ESR around 20R that is 10 times higher. Don't replace these capacitors unless totally open or shorted. They are not critical and just use this test to see overall condition of cards and hours of use.
Last thing I suggest is - check small capacitors around PSU oscillator. ...and of course take a look at output filter caps for visible defects or leaks. These I never seen drifting or failing, even on my oldest 192 (I have five 192 units).
Check attached pictures for details.
That's it! Reassemble and enjoy new life of your 192
Cheers!
Suad
PS: I don't know about new AVID HD I/O (never seen it open), but surely it also has similar PSU and NTC thermistor, so this symptoms and repair procedure apply to any SMPS, including HD I/O.