Repair nightmares.
 

I've just tried to fix an old lady's electric organ, fixed it once, then got a phone call to say it was playing up again, this time it has seemed to fix itself, I felt a bit of an idiot and mumbled something about capacitors, but I'd done nothing apart from powering it up and testing a board this time. I'm still expecting a phone call to say it's playing up again.

Has anyone had a similar repair job that made you look like an idiot or had you reaching for the whiskey bottle?

Andy.

Re: Repair nightmare's.
 

Intermittent faults can drive you crazy.

I had a pre-amp in once which would make loud clicking and popping noises (a very bad thing if it's followed by a huge solid-state power amp driving expensive speakers) starting about 15 seconds after switch-on and ending about 30 seconds later. If you switched it off immediately you could quite often get it to do it again. But after that you had to wait hours for the fault to return. I then discovered that if I took it from the workshop into my living room I might have to wait 2-3 days for the fault to re-appear. The circuit was hybrid with a totem-pole triode arrangement and 3 large-ish high-voltage MOSFETs, one of which was in the power supply regulator and the other two of which handled audio signals. The whole thing was built on a pcb.

It turned out that the water-based solution used to rinse the pcb after assembly had somehow been contaminated. When this initially dried out the contamination concentrated itself into a few tiny nearly-colourless dots on the pcb surface. You needed a bright light and a magnifier and, most importantly, the inclination to look for them before you even realised they were there. In this particular pre-amp one of these dots had happened to form bridging the copper pcb islands into which one of the MOSFETs' drain and gate terminals were soldered. There was about 200V between these islands.

When the contamination dot was dry all was well. But it was slightly hygroscopic, and if it was left in a cool environment for long enough it absorbed enough moisture to conduct in a noisy irregular sort of way. This was what was causing the noises. That MOSFET ran hot enought that it could dry the dot out in less than a minute which is why the noises would go away. My living room is warmer and less humid than my workshop which is why it took longer there for the dot to re-absorb enough moisture to start conducting again.

The cure was to take the tip of a cocktail stick and pop the dot off the pcb, which took a few seconds. But by this stage I had had the amp in the workshop for about 6 weeks and I had changed practically every component including, remarkably, the MOSFET with the problem !

Cheers,

GJ

Re: Repair nightmare's.
 

The warning about high power amps and speakers is well worth bearing in mind.

Years ago I was experimenting with a second hand Quad 33 preamp and my hefty Quad 606 power amp. A sudden violent crackling from the 33 selector switch destroyed the tweeters in my KEF 104 Mk 2 speakers!!!!

Impressively, KEF were able to supply replacements and instructions for installation so all was not lost.

Re: Repair nightmare's.
 

I once spent half a day trying to find out why the left channel of an old amplifier had become intermittent when the client only brought it in for a right channel fault which I fixed. Turns out my scope probe had developed a break in the cable.

Re: Repair nightmare's.
 

Try repairing a cracked screen on an Apple Watch!!! They are pure evil! The OLED display is glued to the glass digitizer with some optically clear glue, which you have to cut through with a thin steel wire whilst heating it on a hotplate, you have to try and get it off without cracking the OLED display, or killing it. I killed 2 displays trying this! Got there in the end, replaced the digitizer and now the centre of the touchscreen is very temperamental :(

I gave up and put up with it in the end!
Regards
Lloyd

Re: Repair nightmare's.
 

Hmmm ..... reminds me of the semi-precautionary re-build/re-cap of my Bush VHF 94. It had worked perfectly for thirty years - until the VHF went on strike. Heavy service resulted in rejuvenated VHF ... and a weird distortion that proved totally elusive.

Days went by and having heaved a scope out of its dusty lair, I was on the verge of further surgery when ..... I noted that one of the speaker cones was 'fluttering' in synch with my increasingly unutterable blasphemy! The whole cone had detached from the peripheral speaker frame [a known malady that is invisible if the speaker and/or sound board isn't fully removed]! Duh!!

Re: Repair nightmare's.
 

Like Kevin's experience with scope leads & probes, I also feel a Wally with commercial crock-clip leads at times. Like the actress said to the bishop, give everything a good pull & poke every now & then, just to make sure its working ok.
Talking of feeling a Wally - in one RAF repair section I was in we had three titles for folk who suffered embarrassing episodes :- "***** of the Day", "Wally of the Week" & "Mug of the Month". All used in a friendly joking manner. Roughly speaking - three DoD's automatically resulted in a WoW, & 3 WoW's resulted in a MoM. WoW's & MoM's were expected to cough up for the first drinks at the next social outing.

Regards, David

PS xxxxx = imitation rubber appendage

Re: Repair nightmares.
 

If you take the back off or even look at it in a certain way the owner thinks it's your problem for life !

That's the nightmare :shrug:

Cheers Mike T

Re: Repair nightmares.
 

I've got hard faults on a couple of 70cms transceivers, one ex-PMR, one amateur, which I've never been able to fix.

I might seek advice on here.

Re: Repair nightmares.
 

A few years ago I revived a Dual portable record player for a friend of a friend. The amp only emitted oscillation noise. I changed a few leaky caps and some resistors, cleaned some connectors, put it back together and it was fine on soak. A few days later it was back with the exact same fault as before - this after my having said that I'd overhauled it!

After a lot of messing about, measuring voltages again, etc. etc., it turned out to be an edge connector on a PCB (the sort often seen on interfaces for ZX spectrum computers) that had a very slight warp - when cold, it would 'contract' and lose contact on one of the common grounds. Hard wiring everything in sight finally solved that one!

Re: Repair nightmare's.
 

Yes! there are some particularly cheap-and-nasty short croc-clip leads (Maplin used to sell them) where the wire is 'attached' to the clip by being folded-back over the insulation then feebly crimped into the plated-metal of the clip.

If you put more than an amp or so through these you get heat created between the clip and the wire, causing the insulation to soften and contact to become intermittent or lost. Cue another frustrating hour of fault-investigation until you realise that part of the test-setup now also has a fault!

Another fault I had years ago was on a BT leased-line between Tyneside and Edinburgh (9600 bits/sec modem link) which would occasionally show errors, then run-clean for a week before faulting again.

Much complaining to BT, getting them to run tests, me replacing the modems (several times - with similar ones and with an entirely different brand) and racking up hundreds of miles driving between the two sites with my test-gear - eventually I tracked-down the problem as being a 'crossover' between the pairs within the grey BT box at the tyneside end.

So it was wired:

tx-------------------------tx
tx-------------------------rx
rx-------------------------tx
rx-------------------------rx

which of course didn't reveal itself when the line was either in local or remote-loop.

That it ever worked at all amazed me! I got a year's free line-rental out of BT for that.

Re: Repair nightmares.
 

Many years ago I had made up some cables to interconnect a source with a stereo amplifier.
There was an unexplained difference in gain between the two channels, which varied as I moved the cables about.
Close inspection with a lens suggested the connections were faultless, with no stray strands of braid etc.

What it turned out to be, was the black covering over the inner core of the screened lead was not the insulating dialectic as I thought, it was an additional conductive layer designed to reduce microphony and needed dressing back slightly.

Re: Repair nightmare's.
 

I bought a load of these Chinese banana plugs, they are not fit for purpose.
The springy "cage" is a lose fit over the pin.

To make them usable I have to run a fillet of solder at the place arrowed.

Re: Repair nightmares.
 

Surprised at those 4mm plugs! I have several Chinese-lantern style, with a loose sleeve, but when plugged into a socket the sleeve is compressed onto the shaft and everything firms-up.

I did have a fault some years ago - an indicator LED flickered intermittently when supposed to be 'on.' No problem - replace the LED - cured. Then that one started going intermittent, so did a couple of others.

It turned out the contact between the internal LED wiring or limiting resistor, to the external tab terminal, was formed by the wire being pressed against the brass tab, in the housing's plastic moulding. No spot weld or solder joint at all. When soldering the wire to the tab, the moulding relaxed a bit, the contact pressure went to zilch, and current flowed mainly by luck. So these indicators are now on my personal 'banned' list.

Re: Repair nightmares.
 

Where's the like(or hate?) button. :-D It so often happens!

I have seen so many recall issues over the years but the one that still amuses me was that when an engineer repaired a TV, returned it to the customer then after a couple of days or so she called reporting everything was backward. It transpired that he had dis-connected the horizontal scan coils and re-connected wrongly resulting in a flipped picture which she only noticed when text was displayed. An easy result of using a mirror to see the screen when fixing the set ;D.

Re: Repair nightmares.
 

That catches a lot of people out. I have had to fix many of those over the years, including some commercially made ones. The problem is often unnoticed until they get plugged into a high impedance circuit, or a guitarist starts wondering why an amplifier is sounding "dull".

And as mentioned further up the thread, the terminations on clip leads and croc leads can lead to hours of wasted time and effort.

Re: Repair nightmares.
 

It plagues the whole industry. I got involved in the early 90's with a hifi amplifier company that started out in NZ, but went bust and started up again in the UK. Reason was the MOSFET's of the power amp were screwed into blind tapped holes, which weren't quite deep enough - so their entire first production run all blew up.

Turned out the co-founder, who also did the engineering drawings, was number blind. So he transposed two numbers in the tapped depth, with disastrous results. That was one of several assembly issues with the same root cause.

Craig

Re: Repair nightmares.
 

I think I have probably mentioned this one on here before but I wasted hours trying to find why one channel of a home constructed phono preamp/tone control was low gain. It turned out after much head scratching that the fault was actually in the "good" channel, which had far too much gain due to an open circuit negative feedback loop. Of course I just poked a tone in to the circuit without actually checking the level, merely backing off the input level for no clipping on the output. I think I changed just about every component on the low gain channel, totally ignoring the faulty one. You're never too old to learn!
Alan.

Re: Repair nightmares.
 

Now it's funny no-one has talked about a problem that is common to may types of equipment. Oxygen starvation. It causes all manner of odd faults. However, universally, when you take the lid off to investigate the problem goes away, and only comes back after the lid is back on, proving my point.

I did once have to investigate a car based problem, that only ever re-appeared after the car had been out for a test drive, even though it was an electrical issue absolutely unrelated to the engine.

Re: Repair nightmares.
 

When thinking about this topic it’s hard to know we’re to start having been in the industry for many years.
One problem I do remember is my first encounter with a pcb.
We had just delivered a beautiful Radiogram, it had a lovely wood cabinet and the sound was outstanding, after installation the customer was delighted.
A few months later we had a call from the customer saying one speaker didn’t work and there was a funny smell.
When I visited and removed the back it didn’t take long to find the fault. The pcb had burnt up around the output stage valve base.
We had no option but to order a new pcb, it was a long tricky process, and yes a bit of a nightmare.
John