Post Your Repair Tips And Links!

Chipset31 said:

1. Whenever taking something apart, put the screws back into the threads after you remove whatever they where holding down. That way you won't lose any screws and won't ever have to wonder which screw went where.

2. After taking the cover/covers off or opening the case of whatever it is you're repairing, take a few pictures with a digital camera/phone for reference later on in case you can't remember how everything fit together or if there's a problem after putting it back together. It helps to take new pictures BEFORE each major step in disassembly.

3. When dealing with manufacturers or products where almost all the components have custom part number labeling, there's almost always spreadsheets available online that translate the manufacturer specific part number to the real part number.

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NICE, good tips, i sometimes have troubles with those screws and that manufacturer-component-name was completely new to me. Do you have any example for an component?

and, welcome to ETO

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such number-monster for such simple IC? man, that was indeed helpful!

I learned this the hard way

Chipset 31 is right about taking pictures. My Ford car radio/cassette player was dropping the left channel when playing a tape. Since I played delayed radio talk show tapes most of the time I lived with it but it ruined music. I took out the radio,got it apart and needed to tear down the cassette mechanism for inspection and cleaning. I even thought about taking pictures but i did not. I said "I will remember..." - I did not. Now I have a wire spring and a small brass pulley thing that I don't know where they go and the radio is still on the 'to be fixed' list. At least the dumpster gods rewarded me with a radio/CD player with an AUX input. But the tapes can only be played at home and not when I want them-on the way home from work during the continuous commercials.

Mains transformers, DMMs, Continuity Tests and Confusion

I just had a problem with my HP Function Generator, it was blowing mains fuses.

So to try and find the problem, I opened the case and started to do some continuity and earth leakage tests on the mains input circuits.

I ended up very confused as the DMM (set to Ohms and Autorange) was flickering wildly and would just not settle down to steady reading.
I was beginning to think that there were problems with burn insulation in the input voltage selector switch in the function generator, but something in my mind told me that I had seen problems like this before.

So, SET THE DMM TO A FIXED RANGE... NOT AUTORANGE...

... end of problem!
Nice steady resistance readings, exactly as they should be.

So, what was the problem with the DMM?
As it changes resistance ranges, so it changes the current flowing through the circuit under test.
This is fine for resistors, but when there is a nice inductive mains transformer in circuit, as the test current changes there is a back EMF from the transformer.
The DMM sees the back EMF and changes ranges, and so it goes on continually changing ranges and displaying nonsense.

(The original problem with the Function Generator turned out to be the mains switch, it was making intermittent high resistance contact when closed.
As there was a spare pair of contacts on the switch, I just moved the wires for the bad contacts to the good spare ones. Job Done!)

JimB

nice tip jim, so there is ''bad'' side when measuring at auto-setting. Does this happen with all transforrmers?
and i dont get it, if resistance gets higher, how come it blows fuse? could you elaborate?

Does this happen with all transforrmers?

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Probably not.
I think it is a combination of the transformer and the DMM. Using a different DMM may not show the problem.
Similarly a different transformer which has a different inductance may not show the problem, but I have no hard evidence or proof either way.
I just know that I remember seeing this problem some years ago.

if resistance gets higher, how come it blows fuse?

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A very good question!
If it was just a case of adding resistance in series then I think that the fuse would not blow.
I think what was happening in my case was that the switch was making intermittent contact and effectively switching the supply on/off.
It sometimes happens with transformers, if you switch off the supply when the core is at its peak magnetisation in one direction, and then switch the supply on again when the supply is of the opposite polarity, a very large current will flow until the transformer core magnetic flux has sorted itself out again.

JimB

A-HA, now it makes sense. i remember when i first made oscillator with '555 that switched relay on/off via fet. He did mention about i should add protection diode in case of inductance/EMF collapsion which would kill the fet. Well, every day wiser, thanks for info .

Mains transformers, DMMs, Continuity Tests and Confusion - Follow Up

Earlier Fezder asked

Does this happen with all transforrmers?

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Since then, I wondered if it happens with all DMMs, and, can we display the problem on the oscilloscope?

So as a test, I connected the original cheap DMM to the mains input of the function generator to see that the effect was still there (it was), and then connected the scope across the DMM and found the waveform shown attached here as Cheap DMM.

I then connected my (expensive) Fluke 8840A bench DMM.
When the DMM was set a fast sample rate there was a fair amount of jitter on display, but the autorange set the correct range.
Slowing down the sample rate, the display was nice and steady.
Looking at the waveform Fluke 8840A we see that there are still large spikes, but the DMM just settles down and gives a steady reading.

With the Cheap DMM we see that the spikes are of both polarities as the autorange switches up and down.
With the Fluke, the spikes are all of the same polarity as the autorange switches down to the lowest range and stays there.

JimB

wow, interesting measurements yet another difference between cheap and expensive dmm's.....at what price were these around?

The Cheap DMM is probably not as cheap as I make it sound, it is 15 to 20 years old and probably cost £30 when I bought it new.

The Fluke was second hand when I bought it, about 3 years ago. From poor memory I think it cost something like 150Euro (I bought it in Germany).
When new it would have cost a lot more than that.

JimB

ah okay, makes sense now i'm looking to buy new meter, i'm thinking between those brymen's models, and one that is used at school, finest 703. price tag is around 100-200€

Earlier in this thread Fezder wrote:

if resistance gets higher, how come it blows fuse?

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And I replied:

A very good question!
If it was just a case of adding resistance in series then I think that the fuse would not blow.
I think what was happening in my case was that the switch was making intermittent contact and effectively switching the supply on/off.
It sometimes happens with transformers, if you switch off the supply when the core is at its peak magnetisation in one direction, and then switch the supply on again when the supply is of the opposite polarity, a very large current will flow until the transformer core magnetic flux has sorted itself out again.

Click to expand...

But that is not the end of the story.
I wanted to use the function generator earlier this week, it switched on OK, the power indicator was illuminated but there was no output.
Diving inside and measuring the supply voltages showed that there was a short circuit on +17v supply line.
Gloom! the problem could be one of many decoupling capacitors.

I found the problem after an hour of various tests.
It turned out to be a 2.2uF tantalum capacitor at the output of the voltage regulator section.
Replaced the capacitor with something similar (not tantalum) and all is well.

JimB

i have pretty much same problem, posted about it if someone could help out. Mine gives at least something out, but only positive dc...

Keep your solder sucker lubricated by giving it a squirt of P19 PTFE plumbers lubricating spray, which is super slippery and safe to use on the rubber O rings. My cheapo sucker works twice as well when it's had a clean out and fresh lube!

Good tip throb

All the best,
tvtech

P19 PTFE plumbers lubricating spray

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I have never heard of this before.
If I wander into the local plumbing suppliers and ask for "P19 PTFE spray" will they know what I am on about, or will I just get blank looks?

JimB

JimB said:

I have never heard of this before.
If I wander into the local plumbing suppliers and ask for "P19 PTFE spray" will they know what I am on about, or will I just get blank looks?

JimB

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If i'm not terribly wrong, any lube' for o-rings is suitable. I've used basic vaseline spray, (can't recall name) with good results. The main thing in all these are that o-rings don't dry out!

or was it silicone spray?

P19 is made by Everbuild. I use it in preference to other things on the general principle that petroleum based products might do Bad Things to the rubber. But it's good stuff for all sorts of things where you might not want to use some other sort of lubricant - eg on plastics, or where it might contact drinking water. But fezder is right - slippy stuff is slippy stuff and that's the main thing!

Ok, thank you for that info.

One thing that grips my bits about solder suckers is when solder sticks in the PTFE nozzle.
I was thinking that a good clean out of the nozzle and a mild application of the PTFE slippy stuff would help to improve the situation and lower my frustration.

JimB