Post Your Repair Tips And Links!

Hi Guys,

Repairing electronics is a great topic and there are tones of benefits when repairing things instead or replacing or throwing them out. So I'm wanting to get a thread with Tips and helpful bits of information for people that are interested in learning how to repair stuff.

If you have any tips, or links to good articles for beginners (or advanced) please post them here.

Regards,
EM

Here is my first link:

Repair FAQ
Features Samuel M. Goldwasser's latest and greatest "Notes on the Troubleshooting and Repair of..." for consumer electronics equipment and other household devices.

Here are also some tips from around the forums:

KeepItSimpleStupid said:

Inspect is always rule #1. A friend found a sandwich or a nest of roaches in two different VCR's that his shop took in for repair.

There is always a rule which states, check the supply voltages. Use identifyable IC's to find possible pins.

Smell. Feel with fingers or temperature probe for high temperatures.

MOS IC's can be non-destructively damaged by static discharges. The charge gets trapped and alters the functionality of the device permanently e.g. a bit stuck on. It may take a while to drain the charge away naturally and shorting discharges it faster. So, potentially the thing that was broken and you took the batteries out, but didn't throw it away might work after a year sitting.

Click to expand...

ljcox said:

You can also look for dry joints and cracks in the PCB tracks.

Click to expand...

BrownOut said:

I've been able to hold the circuit board up to the light and see buried tracks well enough to make my own schematic. I would only attempt to do that for very simple circuit boards however. Last one I tried was a simple switching regulator.

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Do yourself a favor and build a Serious solder sucker. I struggled through Way Too Many years of using (& cursing) Soldapults and variants.

I took a 3" dia 3" stroke air cylinder, put a foot pedal on it and a 3/4" ID thin wall hose to the Soldapult. The stock Soldapult has a great nose & cylinder, just not enough volume. This addresses that. I had to put a piece of cotton ball in the barrel to keep the solder from flying into the tubing.

With a clean nose, this'll pull all the solder from a TO3 case leads. YeaHa... <<<)))

While repairing, dont ever think that fault is complicated and difficult to repair. failures occur due to various factors but mostly it is the design short comings that caused a failure..
Before getting deeply involved with repair, spend some time doing visual inspection of parts and PCB for any clues. Look out for dry solder joints, loose connections, corrosions on PCB, tracks (Especially PTH) etc will tell you a tale. Most of the time, you will be able to cobble together the failure incidents that you see on the circuit for a very good, interesting analysis.

links

Just want to share were to get goodies: Electronic Goldmine.com and Matelectronics.com

I hope it will be usefull.

Multiplexers

If you have a multiplexer which is misbehaving, such as a signal selector to an A to D converter, if only one of the inputs reads correctly and all the others are corrupted, the fault most likely lies with the input which reads correctly.

Strange but true, I have seen it several times in data logging systems.
Usually the relay or switching IC for the working channel is stuck permanently on.

JimB

guitar amps, powered subwoofers.... is this thing working?

you turn an amp on, but hear nothing.... is it actually working?

all DC coupled amps have a very low output impedance. if connected to a speaker this will react to the back EMF of the speaker. it will even react to the current source of a DMM.

speaker method (useful with guitar amps and powered subwoofers):

with the power off, tap the speaker cone repeatedly, noting the sound of the thumps. continue tapping while you turn the power on. if the amp is working, the sound of the thumps changes and the cone motion becomes stiffer.

meter method (useful with most other solid state amps):
connect a DMM across the speaker terminals in continuity test or diode check mode. with no power, the meter should read open, or read as if it's charging a capacitor. with the power on, the meter should read near zero... usually less than 100mV in diode mode. this test will also tell you whether or not a speaker relay in an AV receiver is working.

what makes this work is the feedback loop in the amplifier reacting to changes at the output which are not present at the input. if the input is zero, the feedback forces the output to stay at zero.

this may not work for some solid state amps with a capacitor coupled output. this method may or may not work with some class D (PWM) amps.
tube amps have a transformer coupled output, and so do not react to the above tests. tube amps also MUST have a load connected at all times when powered on!!!!!


simple function check for powered subs and guitar amps (all types):

plug a signal cable into the input of the amp, leaving the other end disconnected.
connect a speaker to the amp output
set all level and tone controls mid-position
turn the amp on
touch the center pin if the open signal cable with your fingertip. you should hear buzzing/hum/popping from the speaker

organizing the contents are in order, yes?

Problem with worn out and burnt plastic tip of your Soldapult?

Cut 1/2" length rubber from Car Carburator small vacuum hose and insert to tip of your Soldapult.
Carburator rubber vacuum hose can withstand the heat of engine in the hood and also the tip of your soldering iron. Hope this help.

For those who was new to repair of PC Switching Power Supply ( PWM). Always check the primary filter capacitors for high dc voltage charge before start to repair. Especially if defective power supply fuse was not open or blown. DC charge voltage can still exist in the capacitor almost same with its labeled working voltage. This indicate that the PWM IC was not oscillating and Power Fet was not shorted or busted.
You can use your soldering iron to discharge the charged voltage or as "bleeder" resistor by placing its ac plug to terminal of filter capacitors for 30 seconds. Check again charge voltage using Multimeter if dangerous voltage still exist before start of diagnose or repair.

At the end of days hobby or while pursuing hobby, always protect the unacquainted and over enthusiastic, from getting shocks or burns.
1. Use an outer double shielded metal jacket for soldering iron resting. This especially so, where we have to work with children around, as they don't imagine the danger of soldering iron.

2. While working with mains powered supplies like SMPSs, better to wire a 220K resistor in parallel to electrolytic capacitor s that it would discharge soon after power down.
Otherwise we are likely to forget and face the shock risk, while trying to handle after shutdown.

3. Always work the soldering iron at a little lower voltage than full230 /120as it is likely that the bits wear off faster. While placing a new bit, smear a little pencil carbonon its surface before fixing. it is easier to handle its removal later.

Upgrading power handling, or not.

Many times I see the advice "use a higher watt resistor." This may be very good advice, however it may also be a very inappropriate, and even costly move. Let me explain.

I see a copy the same switch mode power supply on my bench from time to time, and the problem is almost always the same. There are two, two watt wire wound resistors which have gone open circuit. I only ever replace them with the original value and power rating, and would never upgrade these gems, because they are cheap and easy to replace (and easy to trouble-shoot.) They are a favorable failure point, and if I were to 'gust them up' then perhaps next time the switches, or some other thing, would blow.

So the next time you think about putting in a higher power rated component, spend a thought on how it works in the circuit before you 'slam in a big one that's NOT gonna blow!' You just may be doing yourself a disservice.

This isn't an electronic tip, but it's one that took me nearly 50 years to figure out (and now I have a flat forehead because of multiple slaps ). When you use a tube of silicone sealant and you're finished with it, take the cap off and squeeze the tube to draw the stuff slightly back into the tube. Then take a dab of Vaseline and seal the top of the tube with it. This will ensure the sealant won't dry out while it's in storage. Before you use the tube again, squeeze out a small amount of the stuff onto a paper towel to clear the Vaseline. This works with a variety of things in tubes -- I've used outdoor caulking from my caulking gun a year later and it came out again as if it was a new tube.

A corollary is to put Vaseline on the threads of a jar. For example, the PVC cements love to dry out after opening and the cap's threads jam so that you need to find a pair of pliers to open the can. Wipe some Vaseline on the threads of the can and cap and these problems disappear. I learned this trick from my wife, who told me most women know to put Vaseline on the threads of their nail polish bottles.

Another heavily-used tool in my shop is a 10 cc syringe filled with Vaseline and a 14 gauge needle (grind off the sharp tip). It lets me put Vaseline exactly where it's needed (e.g., the rollers on kitchen drawers or down into a hole before I tap it). Thin the Vaseline with a little naphtha or mineral oil for easier use in the winter. Note: for tapping, I instead usually use a mixture of lard and mineral oil, but the Vaseline is occasionally used in a pinch.

I got one of those $10 Cen-Tech IR thermometers from Harbor Freight a number of years ago and it is a heavily-used tool on my bench. It's great for measuring temperature rises in conductors and finding hot ICs.

Just a few thoughts on repairing big power amplifiers in the 500 - 1500 W per channel range.

When repairing power amplifiers that have blown any of the big transistors, I change all of the power transistors on that channel. The others are likely to be damaged and certainly won't match the new ones at all well.

I also test all the emitter resistors, and all the semiconductors around the power transistors. There are usually a few burned resistors and failed smaller transistors, and missing one can lead to the power transistors failing again, usually explosively.

When powering the amplifier up again, I add a 25 W 470 ohm resistor in series with the power supplies. That limits the current if there is a shorted component that I have missed, or something else that has caused both sets of power transistors to turn on at the same time. I've blown a couple of these safety resistors, but they are cheap compared to a set of power transistors.

The amplifiers should always be tested under load. Amplifiers that have switched power supply voltages should always be tested with a large enough signal that the higher voltages come into play. I have an old frying pan with three flat 100W power resistors bolted to it. At full load on some of these amplifiers it gets too hot to touch within a few seconds, and the hanging loop on the end of the handle is useful.

I've downloaded a sinewave test tone mp3 to use as the test signal. It makes fault finding with a 'scope far easier than with music.

Learn from this forum

My tip: Read the repair forum right here. It helped me fix a Denon Surround Sound receiver that had no output. I found a thread regarding a different brand receiver with the same problem and the answer was in the reply threads. One reply said something like "...this is like the Denon receivers of the mid 1990's which were known for blowing regulators...". Sure enough the +15V reg had gone open which shut down the entire audio chain on all 5.1 channels right up to the power amps. A new regulator brought the receiver back to life for a little cash and and not much time - and I get to keep the receiver!

when checking transistors on a circuit board with a DMM, unsolder the B and E leads and float them in the thru-holes. this eliminates false readings from other components in the circuit.

in stereo and multichannel amps, if you have a good channel, you can double check your measurements (voltages, etc in powered testing, and resistance readings across components in unpowered testing) against the known good channel. sometimes the channels are laid out identically, or mirror-imaged.

when powering up a suspected shorted amp, or powering up anything after any power supply or amp repairs, an incandescent bulb in series with the power line will act as a ballast resistor if something is drawing too much current. the nonlinear temperature vs resistance properties of the incandescent bulb will allow the device to operate normally if everything is ok, but will drop enough voltage if there is a problem to protect everything from further damage, as well as operate at reduced voltage and current long enough to do some troubleshooting. with audio amps, this test is done unloaded.. choose a wattage for the bulb of about 5-10 times the IDLE wattage of the device under test. the wattage of the bulb is correct for the unit under test, when with the unit idling, the bulb glows with a barely visible reddish orange glow. the initial inrush current of power supply capacitors will light the bulb up bright, but it should dim down within a short time (usually 5-10 seconds or less).

when troubleshooting an audio amplifier, you can in most cases replace the output transistors with 1500 ohm/2watt resistors across the B-E connections. this will close the feedback loop and allow the amp circuitry(before the outputs) to operate normally. never attach a speaker or dummy load to the amp when it's in this condition, this is only for checking voltages and waveforms. shorted drivers will smoke these resistors in some cases. with the feedback loop closed, you can troubleshoot many offset and gross distortion problems, as well as bias circuit problems, etc... one amp manufacturer i worked for, used a pair of TO-3 transistors with the top cut off, the B-E bond wires removed, and the 1500 ohm resistors soldered across the B-E posts, so we could troubleshoot an amp without removing the PC board from the heat sink, and without the danger of ruining a pair of new output transistors (resistor $0.20, power transistor $10.00).

Simulate circuits you are unfamiliar with: I see wise ETO members do this all the time. Very sophisticated simulators are available for free download. Nonworking circuits can be simulated to gain insight into how it should operate.

you can also simulate a failure to see what symptoms it causes, for instance you can simulate an amplifier circuit, and see how it fails if you remove one of the power supply rails from the diff amp portion of the circuit (in amp circuits, some of the failure modes aren't always what you expect to happen). quite often in commercially made receivers, you have one set of supply rails for the outputs, and another set of supply rails for the input stages.

as an example, run the included files in LTSpice. with R3 open (i changed the value from 1k to 1G), which removes negative rail voltage from the diff amp, the output latches to the negative rail. the reason for this is that with R3 open (there is often a transistor here as a current source), there is no base current for Q3. with Q3 shut off, the current source Q9 pulls the output stage negative.

I am fairly new to this also but I do recognise when I need an extra hand or two to hold stuff. I use those hair clips that the hairdressers use to hold hair. They are easily straightened and come as aluminium or thin guage steel. They work well as a heatsink also.
When trying to find components you amost always find equivalents on the following site. The equivalents all have data sheets so you can check that the component is suitable.
https://alltransistors.com/crsearch.php

a lot of newer stuff is made with a minimum of screws. it often requires three or more hands to hold all of the plastic clips open while removing a front panel from a chassis. i keep a supply of small pieces of circuit board material on hand to wedge into the clips once they have been released. that way i only need two hands.

a caveat on transistor replacements.... when replacing audio output transistors and drivers, be sure to replace transistors with, not only equal voltage, current, beta, and Ft specs, but the junction capacitances need to be pretty close to those of the original. if not, the amp could become unstable and oscillate.

10 turns of magnet wire on a plastic form can be connected across an oscope probe to be used as an RF sniffer. the sniffer can be used to detect transmission from RF remote controls, and other devices (not outside the scope bandwidth). i once used such a device to find RF leakage from a loose coax fitting inside a malfunctioning spectrum analyzer.

for adjusting FM tuners, etc, that use "squeeze" tuning on air core coils, a tuning tool made from a worn out "tweaker" can be made. on one end, glue a small piece of copper foil, and glue a small piece (both about 1mm in size) of ferrite to the other. if you are tuning a coil, and are close to having the tuning correct, and the signal improves while moving the copper foil near the coil, the turns need to be spread out. if the signal improves with the ferrite near the coil, squeeze the turns closer together.