What are the most common failing parts on old(er) circuit boards? - Page 1
Forum section: electronics / beginners

author: nsummy
author_rank: Contributor
timestamp: September 28, 2022, 08:44:48 pm
content: I'm sure this has been asked and answered a million times. Its such a broad question that google searches provide either little or repetitive info. I am a beginner with electronics (probably considered intermediate compared to average population). I know the basics, can follow simple schematics, light arduino programming, etc. I have taken up an interest in vintage electronics and while most of the stuff I buy has easily detectable problems, I have been working on 80's era Akai linear turntable. I've seen a few posts about this turntable elsewhere and the overwhelming responses seem to be that this is an overly complicated nightmare that isn't worth troubleshooting. Looking at the service manual and numerous circuit boards confirm that the amount of engineering and circuitry that went into building something to play a record is overkill. This is probably the worst or best thing to learn on, but I digress.I've tested the voltages at various parts of this thing, and so far, everything seems to be in spec. Beyond on the electrolytic capacitors (which the esr all seem to be within in normal limits), what are some good components to check? There are a good number of ceramic & film capacitators, lots of resistors and quite a few transistors, not to mention logic gates and other ICs. I suspect the problem lies with the old Fujitsu Microcontroller but I'd like to rule out anything else obvious. Long question short, beyond electrolytic capacitors, what are some other components that can commonly fail? As I go through this I'd like to initially skip over the stuff that is the more reliable. Thank you.

author: Benta
author_rank: Super Contributor
timestamp: September 28, 2022, 09:21:50 pm
content: Apart from the electrolytics, the main failure point is the solder joints (especially in high-temperature areas, but I don't expect those in a turntable).PCBs back in those days were wave-soldered, and this poses a problem.If your PCB was at the beginning of a production run, the solder bath would be eutectic (37/63). Later in the run, "solder depletion" happened, where the balance between tin and lead changed due to the copper's affinity. This led to mediocre solder joints.It's really tedious, but if you want to get this up and running for real, there's not a lot you can do, except remove the old solder (pump or wick) and resolder with quality eutectic solder by hand,

author: james_s
author_rank: Super Contributor
timestamp: September 28, 2022, 09:34:49 pm
content: The primary one is aluminum electrolytic capacitors, beyond that anything can fail. I've seen bad resistors quite a few times, particularly in higher voltage applications like CRT monitors. Semiconductors can fail, especially higher power stuff. I've had crystals fail a few times too. Ceramic and film capacitors are extremely reliable.

author: Benta
author_rank: Super Contributor
timestamp: September 28, 2022, 09:36:56 pm
content: Quote from: james_s on September 28, 2022, 09:34:49 pmExcept the RIFA suppression caps...

author: MrAl
author_rank: Super Contributor
timestamp: September 28, 2022, 10:47:46 pm
content: Quote from: nsummy on September 28, 2022, 08:44:48 pmHi,I've seen electrolytics fail badly in power supplies in several different power supplies. The ones that do the filtering. They not only develop high ESR but also the capacitance gets very low. I have scope pics somewhere that clearly show this to be the case when they were removed from the device and tested. Replacing them with good quality caps always fixed the power supply.Transistors in high voltage circuits, like CRT television HV circuit.Had one rectifier diode fail in the full wave bridge rectifier circuit of a TV once too, only time i ever saw a diode fail in that kind of circuit. I think that may be because they used to think you could get by with a 200v diode in a 120vac bridge rectifier circuit, but not sure about that. Not sure if they improved on that yet either with newer designs.Here's one from a longer time ago: The non volatile memory could go bad meaning some bits will flip, and since the programming depends on the memory (typical microcontroller chip or other) even one bit flip could crash the system every time it start up, and memory longer ago was not as good as it is too they had less retention time back then. Luckily flash memory and the like improved a lot over the years. However, the guarantee may be 10 years (check that) so 20 years down the road there could still be a failure and again one bit flip could cause the entire system to stop working or start working in a very non typical way with certain features not functional anymore, and unfortunately the only way to fix it is to get a new chip with the memory programmed as it was needed in the old chip.I had a high quality CRT monitor fail dead one time but never looked into what caused it. I suspect the high voltage circuit transistor. That was surprising because it was a good make and model but it only lasted a little over a year. The monitors of today are made entirely different without a CRT so we probably get much better reliability.In some cases you just never know what can go bad. The rule of thumb is that the more external connections there are the lower the reliability, and that means external to the components like IC chips and the like. That's why IC chips that do more inside the chip are considered more reliable in general, because most of the connections are inside the chip not outside.For a bit of a laugh, i've seen very large electrolytics blow up like a cannon fire. The tops would blow right up and out of the body, and would bend the 1/4 inch heavy copper buss bars connecting them into a bank of several capacitors. Quite a site to see and what more what a SOUND to hear when they blow, sounds like a cannon too.That is regular failure mode for caps that have to put up with ripple current that is too high for them, but it's mainly due to bad design. The incident quoted above was not like that though it was because a technician had a three phase heavy duty variac set to the MAXIMUM voltage when he turned it on, and so the caps blew out immediately. This variac was something like 3 feet high and 1.5 feet diameter, and the shaft was motor driven, so it could put out 10's of kilowatts probably at 300 volts line to neutral with 3 phases.It's kind of funny because he thought he had the variac turned all the way DOWN to 0v, but he turned it the wrong way up to the MAXIMUM voltage and that was too high for the rectifier capacitors to take.

author: Haenk
author_rank: Super Contributor
timestamp: September 29, 2022, 11:11:53 am
content: Also things to fail: socketed ICs (black legs = bad), voltage regulators (78xx, 79xx), plastic trimmers, plastic mechanical parts (not really electronic parts, I know...), EPROMs/EEPROMs/FlashHowever - don't replace them or consider them damaged unless diagnosed. Very few parts need immediate action. Cold solder joints and RIFA caps certainly do - before switching it on.

author: james_s
author_rank: Super Contributor
timestamp: September 29, 2022, 08:30:38 pm
content: And leaky electrolytics. Always check to see if something is full of those surface mount ones especially before switching it on. A sniff test is often sufficient, if you smell fish, investigate before powering it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 30, 2022, 12:57:31 am
content: The electronics are usually the most reliable part of these old electro-mechanical reproducers of music. I don't have experience with the AKAI, but do have another brand of linear tracking turntable. These have a servo loop to keep the arm perpendicular to the record groove. There are at least two non-electronic points of weakness in the design of my turntable. First, as with many devices of this era the power to move the belt is transferred by a rubber belt. Even if the belt is there and appears to be working it may have stretched enough to ruin the control dynamics. In all of these linear tracking turntables have some means to measure the angle of the tone arm relative to the track, and it must be both sensitive and very low friction. In mine it is optical and open and thus suffers from dust and other contaminants. A third weak point is the sensor which finds the entry track of the record. On mine it is also optical and sensitive to dust, oil and other things. Finally, the lubricant in the mechanisms used to move the tone arm may have dried and interfered with motion.It is likely that your machine has been through many hands, and not all of them may have been familiar with the foibles of linear tracking turntables. AKAI may have imperfectly armored against inept operators, and someone may have forced the tone arm to move, disrupting the mechanism.Rather than looking for the type of components that fail I would trace the problem functionally. Does the turntable turn? Is the speed control correct? Does it respond to any or all of the buttons?. Does the arm lift on command? Does it return to the home position? Does it search for the record start? Then use the answers to those questions and others to narrow down the problem area.

author: radiolistener
author_rank: Super Contributor
timestamp: September 30, 2022, 03:19:47 pm
content: electrolytic capacitors and powerful mosfets

author: HalFoster
author_rank: Frequent Contributor
timestamp: September 30, 2022, 06:34:42 pm
content: Connections. The vast majority of errors in old equipment comes down to flaky connectors - card edge, wire and socketed ICs. Old electrolytic and dipped tantalum capacitors are right up there, of course, but a surprising number of problems can be fixed by just reseating connectors.Hal

author: free_electron
author_rank: Super Contributor
timestamp: September 30, 2022, 07:57:33 pm
content: Quote from: Benta on September 28, 2022, 09:36:56 pmThe PIFA capacitors in the PAFFNER power filters... pif-paf

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 30, 2022, 08:28:04 pm
content: Quote from: Benta on September 28, 2022, 09:36:56 pmAlthough as they're normally just for filtering, when they blow, the equipment often carries on working unless the cap took something else out with it.