RCA RT2500 with bad caps?

[rumblefish73]

I have a RCA RT2500 that doesn't power on sometimes, after some Googling, i found a few people that have replaced the two 6800uf caps "between" the heat sinks, they don't appear to be bad on mine...

There's a couple of caps in the "power supply" section that i'd like to run by you guys and see if any of these are known bad caps?

2 YEC 6800uf 16v SM 85c
2 YEC 2200uf 35v SM 85c
2 United Chemi-con AHS 12000uf 63wv 85c
1 NOVA 1000uf 25v

Thanks!

[AlmightyOatmeal]

Re: RCA RT2500 with bad caps?

None of those caps look blown, cracked, or leaking.

Usually caps don't fail unless there is a manufacturing defect or over-current. Over-current will happen due to a power surge or voltage regulator malfunction. If a surge were the case, you would most likely have damage to the power transformer and possibly the rectifiers. A small surge could damage to the components enough to operate outside of their designed ratings which could lead to erratic behavior.

Most likely there is a problem with the main power relay or the voltage going to the relay. That could be caused by a malfunctioning voltage regulator or transformer (one of the secondary windings could be shorted).

Without being there or using an oscilloscope, testing capacitor function/stability is going to be impossible. You can use a component tester to test the cap to make sure it can charge to its rated capacity and voltage, but those cost around $100.

What can you do? First, "reflow" or fix cold solder joints if you're daring. Unhook the wires going to the power supply board (or remove it enough to flip it upside down but be careful not to spill solder on any other PCB or bad things will happen when it's fired up). Take your soldering iron and melt the solder on each of the major components on the power supply. Be sure to let one pad solidify before moving on to the next component to prevent excessive heat transference or components falling out. You can try removing the old solder via a suction tool or soldering wick, or just add a tiny bead of fresh solder too. This will make sure that each component has a good solid connection to the traces on the board. This fixes 3/4 of the problems that I run into.

Next, you can unscrew the transistors in the power supply, carefully unsolder them and pill them from the circuit. Use a DMM to test to see if they're malfunctioning (refer to YouTube for videos on how to find the collector, base, and emitter of a transistor as well as determine NPN from PNP and check for a open/closed fault with the prior types). If your DMM has a transistor test function, use that as well (Again, videos from YouTube will show you how).

If you need to replace a transistor with more than one in the same function of a circuit, replace both or all. This will ensure that the function uses the same valued transistors to eliminate voltage discrepancies that can lead to premature component failure again (a common mistake I've seen). You can use a different transistor than the one you pulled *if necessary* but it needs to match the specs of one(s) you removed or rated slightly higher (too high will cause it to operate ineffectively). You can google the transistor code to find a datasheet containing specs. Be warned though, not all transistors are created equal. Just because a spec sheet says they have one rating doesn't mean there isn't a defect causing them to operate under-spec, a component tester or DMM with transistor test functions will be helpful.

If the transistors are good or you just want to reinstall, do so but remember how you pulled them out. I recommend cleaning the old thermal compound off with an alcohol prep pad or a q-tip with isopropyl alcohol (shouldn't leave a residue) then apply a nice silver based (radioshack sells silver based thermal compound with 99.9% concentration for around $12) thermal compound. Remember, a small dab goes a long way; a little tube should last this project and many more.

If you need to continue to troubleshoot, check the transformer. I recommend pulling it out of circuit but due to the number of soldered pins, I would use a solder wick to remove the solder. Once the transformer is removed, take a DMM and check the output pins for a direct short. A transformer's windings are simply wires directly connected but in a functioning transformer they offer resistance. A failed transformer will offer little to no resistance on the output windings for a particular voltage.

An example of this was a project I recently did that needed a new transformer. This transformer only had one output, so two pins, which I used the DMM's diode check function. It went from OL (infinite resistance, basically no resistance) to around 1ohm. That's a failed transformer with a near 100% direct short on the secondary winding. I also checked the input winding the same way, but that offered several thousand ohms of resistance; the input windings were okay (I would have tripped a breaker at some point if the input windings were bad too).

You can also check for proper transformer voltages as well but that starts to become dangerous as it is testing with live power. I won't go into detail but you can apply AC to the input winding and measure the outputs in or out of circuit, depending on your comfort level. You could run into some seriously high voltage or cause damage to the transformer if you aren't careful.

** I DO NOT RECOMMEND THIS FOR INEXPERIENCED PEOPLE ** THERE IS A SERIOUS RISK OF ELECTROCUTION, FIRE, OR COMPONENT DAMAGE. DON'T BE A DUMBASS IF YOU DON'T KNOW WHAT YOU'RE DOING.

If the reflow doesn't help and the transistors are okay and the transformer isn't shorted, it's time to call in someone experienced. Advanced troubleshooting requires live power to a circuit and/or advanced diagnostic equipment such as component testers, DMM experience, oscilloscope, etc.