Some comments on electronics repair

[ajay_m]

Recently I posted elsewhere about the sad demise of my Tapco S5 monitors, and the errors I made in an initial repair which led to the destruction of two tweeters, a saga that still embarrasses me when I think about it.
Although I now have a pair of Tannoy Gold 5s, the disassembled carcasses of the old S5s silently reproached me every time I went up to my attic studio.

I could hear a little voice. It said "Wonks would have repaired me. Look at you. You have all the gear and call yourself an electronics engineer?". "Ok, ok", I said "Let's see if there's a service manual or schematic for you". And miraculously, thanks to Electrotanya, there was!. Now this is pretty much the initial touchstone for whether anything's worth repairing, because without a circuit diagram it's a lot harder to figure out what's not working, apart from just replacing components. And I'd already replaced the HF amplifier and that clearly didn't fix the issue.

So I fired up the faulty unit and at least the intermittent fault was present now, with the HF amp output at +32V (one rail). The design is a class AB amp using LM3886s for both HF and LF drivers at 60Wrms each channel, and with an old-school linear PSU with a toroidal transformer. Both the main rails and +/-15V rails to the opamps were fine, which is to be expected since the LF channel is fine. Checking the input to the HF amp, it's being driven to the 15V rail so clearly there's a problem with the opamps driving this channel. These are all dual channel 4560 devices and only U5 and U6 are DC coupled to the main amp, so the fault must lie around this area.

Easiest thing would be to just replace both of these and see what happens. So I ordered 5 devices from China - much cheaper and we're in no rush - for £3 postage included. These are SOIC 8 pin devices about 5mmx5mm so they are about at the limit of hand soldering. Easiest way to get the old chips out is to put a 3mm tip on the soldering station and then use solder braid to remove as much solder as possible and then if the chip doesn't just drop off, gently cut the leads from above with a small pair of diagonal cutters. This is because the pads, not having any plated through holes to secure them, are easily lifted off the PCB otherwise, which we'd like to avoid.

So with the devices replaced... guess what. We still have the HF amp being driven to the rail. Hmm. Well some tracing through shows to my astonishment that U5 doesn't have a -15V rail. In fact it's not even present at the local bypass cap and it looks like the actual PCB trace has failed somewhere. Now this *is* unusual with low-powered circuitry like this, but easily fixed using a small piece of 'pencil wire' (very thin enamelled wire with a special coating that melts when you touch a hot soldering iron to it, making it ideal for this kind of repair work). I run this to a nearby bypass cap on the same rail and now we have -15V.

But the HF amp is still at the rail. So a closer look through U5s pins show that despite the inverting and non-inverting inputs to each opamp being zero, the opamp is driving to the rail. Must have been damaged somehow without that -15V rail, which is unusual but... pull out the replacement and while it's out, let's check the HF amp. You guessed it, still on the rail. But a probe of U6 showed that one of the non-inverting input pins wasn't grounded - it looked like a good joint but obviously wasn't. With this remedied we now had 0V on the HF amp output. Replace U5 and now we had a working HF and LF channel.

Now I'm sure you can guess that in terms of labour cost this is a completely unfeasible repair job. On the other hand, we only have the one planet and it seems such a shame to skip these monitors - they are beautifully built with the rear mounted circuit assembly on a die cast aluminium backplate and combined rear port that must have cost a fortune to tool up for what are budget-price monitors.

Now it's time to think about tweeters. We can purchase a pair of used tweeters from Loud Technologies for 18 euros each which sounds very reasonable until you discover there's a 22 euro 'handling fee' and then about the same in postage as well for a total cost of something like 70-odd euros. And they're not even new units.

So - I'm going to fit replacement tweeters. Now the originals are 4 ohm 1 inch silk dome tweeters and the magnet assembly is a rare earth rather than ferrite magnet with ferrofluid in the gap. You *can* purchase 1 inch silk dome diaphragms by themselves from a bunch of Chinese vendors but they don't quote the power handling and I could only find a minimum of 6 ohm impedance units. But DigiKey stock the BC25SC08-04 Vifa/Tymphany tweeters for around £18 each including VAT and if I throw in a scope probe as well (one of mine is a bit intermittent) this'll comfortably get me above the free postage limit of £33 ex VAT for the order. These tweeters are rated 100W RMS and are 1 inch silk dome 4 ohm units with an integral waveguide. They'll fit just fine but I'll need to design and 3D print a mounting plate for them in black PLA - never much liked the Tapco waveguides anyway, this'll actually look nicer, and I'll put two little holes in for the power and overload LEDs. Crossover freq is around 4KHz (quite high, the main 5 inch driver has a very wide range) for the S5s and these tweeters have a 2.5KHz crossover which is fine therefore as we're well within their design limits. They are also rare-earth magnet units so I expect the sensitivity to be comparable to the original Mackie tweeters and if we lose any more units, they are widely available. And we have a three position HF attenuator/boost switch for -2/0/+2dB anyway if for some reason the replacements aren't absolutely right, but I think we'll be pretty close to be honest.

Of course, as I said, this isn't remotely justifiable on a time and cost basis. I am obviously keeping the actual cost of repairs to a minimum here but you can see why the old-fashioned TV repair shop simply isn't viable these days. You also have to factor in having the kit to even tackle repairs like this. A decent temperature-controlled soldering station with a range of bits is essential, you need a very fine needle nose tip to solder in these SOIC devices, with a pin spacing of just over 1mm. You also need an illuminated magnifying glass or a headband with lenses, like a jeweller, for this kind of closeup work. In this case an oscilloscope wasn't essential but I did connect it up after repair to verify that both units were outputting a nice clean signal on the HF channel (as I don't currently have any drivers yet to connect up).

[Lostgallifreyan]

...easily fixed using a small piece of 'pencil wire' (very thin enamelled wire with a special coating that melts when you touch a hot soldering iron to it, making it ideal for this kind of repair work).

Thanks for mentioning that. I didn't know about it. I replaced caps on a Mini-ITX board several times, but one board makes occasional slips of the least significant bit in data transfers. I even have some idea of which tiny joint caused a track failure. This pencil wire might let me fix it.

Now I'm sure you can guess that in terms of labour cost this is a completely unfeasible repair job. On the other hand, we only have the one planet and it seems such a shame to skip these monitors...

It is worth it. The fact that work is devalued, is skewing perceptions in general. It's why we have a tory party that wants 'to make work pay', but does not want to pay for work! Absurdities like this arise when the real value of work is ignored. The 'real world' does not dictate these terms. Greed does. In the real world, not everyone is greedy. Repairing stuff is good. I knew a guy with a Cuban girlfriend. She was very reserved when she first met me but warmed up a lot when she learned I repaired and maintained my own stuff. That's a BIG thing in Cuba, it really matters apparently. They respect it.

I am obviously keeping the actual cost of repairs to a minimum here but you can see why the old-fashioned TV repair shop simply isn't viable these days. You also have to factor in having the kit to even tackle repairs like this. A decent temperature-controlled soldering station with a range of bits is essential, you need a very fine needle nose tip to solder in these SOIC devices, with a pin spacing of just over 1mm. You also need an illuminated magnifying glass or a headband with lenses, like a jeweller, for this kind of closeup work. In this case an oscilloscope wasn't essential but I did connect it up after repair to verify that both units were outputting a nice clean signal on the HF channel (as I don't currently have any drivers yet to connect up).

Learning the minimum kit is useful. I used to get by with less, when doing repairs for a second-hand shop before Ebay put paid to that whole business. It was run by a guy who had escaped from Dartmoor prison. Interesting guy.. I wanted to do repairs, and did them as a freelancer, so long as I didn't do this for rival shops, he was happy. In many cases the task wasn't that sophsticated, it became like triage, looking at a row of stuff on a shelf, asking him what he knew of its faults, then going for a few devices that looked like being worth the time, not needing lots of exotic test gear, etc. I ended up being able to afford some better gear because of what the crude stuff could do for me.

Sometimes the cheapest gear is still the best. A pin vice with a 0.7mm drill is perfect to clean holes in solder without damaging pads and tracks. It puts much less demand on the quality needed for a soldering iron, because the degree of heating of parts can often be based on timing, rather than temperature control. Not that I'd be without my Magnastat iron (and a backup) now.. Having a fine tip never really worked for me, it tends to fail in thermal coupling too often. A 2mm wide tip with a fine edge works better with less wiping. Solder can flow up across the surface so it's good for desoldering small SOIC's. Big ones, that's very different, but gravity and a feeler gauge can work wonders, even with a standard iron. Lots of flux needed though, when soldering them. I hate SMT though, I don't have the vision needed to cope with that anymore.

[ajay_m]

Along with pencil wire, another useful product for board rework is uv-curable transparent resin. This comes in a little 60ml bottle and is the consistency of thin honey. A tiny drop will hold down an SMT component and cures instantly with a low-power 405nm laser pointer.

This can be very handy as you can stop 'tombstoning' where the component has the infuriating tendency to stand on one end. Luckily in this case the SMT components were all fairly large, the ones on some modern PCBs are less than half the size of a grain of rice, and as you say the TSSOP packaged chips have a lead spacing of around 0.5mm which is truly challenging.

Unfortunately of course many manufacturers refuse to make service manuals available and then there's the challenge of spare parts. We do need to see more initiatives like the US "right to repair" to avoid this ever-growing mountain of e-waste, but even so I'm lucky if I can manage a 50% success rate on repairs. For example recently I had a 27 inch flat screen monitor fail where the monitor would just turn off spontaneously. The problem gradually worsened and I replaced the caps around the invertor but this didn't resolve the issue; it's something to do with the backlights behind the flatscreen display, which is old enough not to use LEDs so these are effectively tiny little fluorescent tubes, hence the invertor. With the screen panel disconnected the fault goes away, but whatever's wrong I can't go in any deeper and I would never be able to get spares anyway, so unfortunately that'll have to go down as a failure.

[Lostgallifreyan]

Along with pencil wire, another useful product for board rework is uv-curable transparent resin. This comes in a little 60ml bottle and is the consistency of thin honey. A tiny drop will hold down an SMT component and cures instantly with a low-power 405nm laser pointer.

Very nice idea. I can think of lots of uses for that. Got any names to drop? I'd try anything you know well enough to name. (Dentists use some interesting light-cure stuff, but probably fairly specific to dentistry, so maybe expensive and hard to get.)

About those inverters and cold-cathode tubes, I have the same problem in some used 17'' ELO touchscreen monitors I bought on Ebay several years ago. There are kits for putting LED backlights in, or were, maybe no longer, if the monitors became too rare for the maker of the LED kits to make them. Replacing inverters and tubes together was always intended as a fix, so LED's can make a lower maintenance and higher brightness alternative. I balked, mainly because I hate messing with displays, there's always some nasty bit of dust or airborne fibre that wants to get in there the first chance it gets. I'm actually considering making my own lightbox with matt polyester film as a screen for a tiny projector! This has the advantage of huge dynamic range, and a I could make an arbitrary screen size more easily than with anything else. It might be bulky (and make some low-level noise) but it's a nice idea..

[N i g e l]

.... We do need to see more initiatives like the US "right to repair" to avoid this ever-growing mountain of e-waste,

The EU & UK are both on it.

https://www.which.co.uk/news/article/ne ... W160h9DWLL

I replaced a door catch & heater on my washing machine recently and was shocked at how reasonable the parts were.

[ajay_m]

There's a pile of folks on ebay selling similar products e.g https://www.ebay.co.uk/itm/226041926772 ... Sws7Rl8B6F
and also on Amazon.
A cheap 405nm laser pointer will cure it instantly - note it does get quite hot when curing, so exercise caution around anything particularly heat sensitive, but chips and components can withstand soldering heat which is far higher, I'd guess this maybe can reach 80 degrees when curing but a small drop obviously doesn't really contribute a lot of heat.
I just bought a set of three laser pointers like these ages ago
but any cheap 405nm pointer should work fine.

https://www.ebay.co.uk/itm/305335562409 ... BMsOiOm85j

[Folderol]

About those inverters and cold-cathode tubes, I have the same problem in some used 17'' ELO touchscreen monitors I bought on Ebay several years ago. There are kits for putting LED backlights in, or were, maybe no longer, if the monitors became too rare for the maker of the LED kits to make them. Replacing inverters and tubes together was always intended as a fix, so LED's can make a lower maintenance and higher brightness alternative. I balked, mainly because I hate messing with displays, there's always some nasty bit of dust or airborne fibre that wants to get in there the first chance it gets. I'm actually considering making my own lightbox with matt polyester film as a screen for a tiny projector! This has the advantage of huge dynamic range, and a I could make an arbitrary screen size more easily than with anything else. It might be bulky (and make some low-level noise) but it's a nice idea..

I've successfully used LED strips to replace fluorescent strips + inverter on a number of occasions. There is usually a 12V supply available, and on one occasion I strung a load of LEDs in series due to the size limit, and ran it from about 40V with a transistor series current controller. This was on an obsolete industrial control panel. They were a bit surprised that it had changed from white to red (lowest voltage LEDs) but happy to have the machine usable again.

[Lostgallifreyan]

I've successfully used LED strips to replace fluorescent strips + inverter on a number of occasions. There is usually a 12V supply available, and on one occasion I strung a load of LEDs in series due to the size limit, and ran it from about 40V with a transistor series current controller. This was on an obsolete industrial control panel. They were a bit surprised that it had changed from white to red (lowest voltage LEDs) but happy to have the machine usable again.

The series-chain idea is nice. There are kits from the US that make a very nice job of this, but the cost is ludicrous. Over £120 shipping, and by the time VAT gets paid, I'd do better to buy a new med-grade monitor! The projector-in-a-box notion looks better by the hour, I'm good at thermal management. I never used the touchscreen much so I could live without that. I might try the LED strip method myself when I've got nothing left to lose, because if it works I'd have three monitors with plenty of life in them.

[ajay_m]

Well, Digikey are certainly awesomely efficient. Ordered on Saturday and I assume they didn't even process the order till Monday but the goods arrived via UPS today, all the way from the US. Except unfortunately somehow my two tweeters had turned into one of the correct tweeter and two other Peerless 1 inch tweeters (6 ohm instead of 4 ohm).
But a quick session on their chat screen and another tweeter is on its way and I get to keep the other two as they are apparently not worth returning, so I'm not too bothered. In fact, usefully this let me compare the intended tweeter with the other ones shipped. As I expected, the unit I intended to use is very efficient and I set the back HF switch to -2dB and - to be honest - if there had been a -4dB option that might have been good. I can easily enough EQ the top end down a bit if I want, anyway, and certainly it's a quality speaker. [it's a bit tricky finding replacements as the ferrite magnet units are a bit too low in sensitivity and you need 4 ohms and 60W+ power handling which restricts choice].

Interestingly the 6 ohm units, while obviously the wrong impedance and also less sensitive in the first place, while supposedly also 1 inch dome tweeters, expose more of the dome and surround and the waveguide is less deep, so I think they probably have a broader dispersion and with several dB of EQ on the top end they certainly sound nice - but alas, there's no 4 ohm equivalent of these, or I might have been tempted to swap, so I'll stick with the original choice.

Also I am running the repaired unit with the 6 ohm tweeter temporarily on top of the cabinet and this gives me a sacrificial unit in case anything goes wrong, but it's racked up several hours since the repair so I think its tweeter-eating days are over (at least for now). Just need to get the other tweeter and print off some 3d printed mounting plates and we're done.

[Folderol]

I would recommend putting in-line 22mm 1.5A fuses in series with the tweeters. This would be technically less than the peak current in the units, but way more than the average, so everything should be quite happy.

[ajay_m]

Yes, that is sage advice and had I had my wits about me at the start of the whole affair, I would have been out a fuse or two instead.
Ah well. Just printing the first mounting plate on the smaller 3d printer now. Took a couple of hours to design in freecad and I will probably print a thin flexible gasket in TPU to fit underneath once I'm sure everything fits ok.

[ef37a]

Regarding the economics of repairs? Not all bad news if the public can be educated and the repair shop has a good turnover of jobs.

If a tech' is putting in a 35 hr week (hah!) then quite a lot of the work will be relatively simple jobs that give a quick cash turnaround. Then there are 'stock' faults* that can be done in a trice but earn a good wedge, these 'subsidise' the jobs that take hours of work and maybe days of thought but even those very tricky repairs can be economic in the long run? Next time you will know!

Fundamental to the business model IMHO is a non-returnable charge and moreover a significant one. £25 to £30 at least. This helps cover overheads and helps cash flow. The charge also dissuades time wasters and puts off those that think you work for £5 an hour. The charge also helps prevent the workshop clogging up with uncollected repairs. If the punter is already 30quid down he is more likely to come back and pay. VITAL to this matter is a clear understanding of consumer law with regard to uncollected equipment. You MUST make sure the customer is in no doubt what you will do with his amp in three months time if he does not come back for it. The law is surely a bit different from my understanding of it 30 years ago?

Yes we urgently need legislation. Service manuals should be freely available but that means some better control of people like B....you know who I mean!
Hard to fix parts such as SMT SMPSUs should be made available as 'service replacements'. The White Goods industry got these concessions decades ago but not for brown goods.

*A data base needs building where a make and model number can be plugged in and know issues displayed. I recall that many of the major electronics companies had a (quarterly?) news letter most of which dealt with known faults and mods. Co's like RBMurphy, Philips and Ferguson come to mind.

Dave.

[forumuser931182]

I was in the market for some new small monitors recently. Found a pair that I thought would be good.. class AB amp because i really can’t listen to class D amps ( maybe just cheaper ones ) but when i read user reviews a number of people complained that when the speaker went into standby ( auto when no signal for 5 minutes ) it was almost impossible to override back to normal operation!!!
I ended buying an as new secondhand pair of ESI monitors instead because I knew they had a straight forward ( old fashioned ) design that i could repair if required. I already had a pair of similar but larger ESI monitors that i obtained for free ( faulty ) and just had to replace the LM3886 ic’s and main power caps. I also contacted ESI and they readily supplied me with a schematic.
This is the same reason i bought a Dualit toaster… it was the only model that still sold replacement elements.
I think this type of behaviour is now called “old man yelling at clouds”

[James Perrett]

I've been working on the Otari Mtr-90 over the last few days. My lad was amazed to see that they had included extender cards to make access to the boards easy and test points on the boards for 'scope probes. The idea that people would design gear with servicing in mind was totally new to him.

[Folderol]

Yes. Design has been going backwards for years. The peak in servicablility came in the late 1960s - particularly TVs, and Thorn/Ferguson were a service engineer's wet dream!

[ef37a]

Yes. Design has been going backwards for years. The peak in servicablility came in the late 1960s - particularly TVs, and Thorn/Ferguson were a service engineer's wet dream!

Ha! I don't know about in your 10/20 Will but here in darkest Npton dealers cooperated with each other. They tended to specialize on a few brands. I worked, second job for a private firm and we did Ferguson, RBM, and Grundig TV which many folks did NOT like (but then WTF uses THYRISTORS as horizontal scanning devices!) Dealers would therefore pass on brands they were not familiar with and we even helped each other out if we were short of a valve or other component.
Was not all sweetness and light! We knew who the "cowboys" and rip off merchants were. "You took it to HIM?!"

This was all at a time of course when dealers could charge a decent price for gear. A 26" telly cost as much as a pretty good second hand car. The coming of the rental giants, Magnet and bloody Amstrad put paid to electronic servicing.

Dave.