Wrong from new (again)

[Studio263]

Here's one of those things that turn up from time to time, an assembly error that only gets spotted when the equipment gets broken up after a long and (presumably) trouble-free life.

In this case its a Sony turntable from the early 1980s, which has a little control circuit and a preamplifier for the magnetic cartridge mounted on the underside. As you can see, C56 (the output coupling capacitor for the amplifier IC) has been inserted backwards. Looking at the circuit, the legend on the PCB is correct and the one in the other channel is the right way round. Needless to say the capacitor still measures perfectly.

Polarity - overrated?

[Nickthedentist]

Not typical of Sony though, surely?

Have you measured the voltage developed across it? I wonder whether it would fail if you put it in the right way round after all these years.

Did you spot it in the absence of a fault? I'm very impressed if you did (even though it's right on the edge of the panel).

N.

[Studio263]

Hello Nick,

One end is roughly half way up a 12V supply, the other is at 0V.

I'm currently transplanting the whole RIAA circuit into something else (a lab instrument which we are designing - it measures the Young's modulus of metallic samples under dynamic conditions) and since the components are of high quality and are still in good condition I'm re-using them all. Modern alternatives, especially the film capacitors in the feedback loop, are not easy to obtain in the same grade in prototype quantities.

Picture attached - a quality RIAA stage that fits in a matchbox (with a PP3 battery taped to the side!).

[Nickthedentist]

Now that's real "upcycling", Tim! And I suppose the Sony can still be used with a normal hifi amp.

Mr Morita would have been proud of you (probably).

Nick.

[Philips210]

Hi

I remember a quite decent Aiwa micro Hi Fi system had distortion on the left channel from new. Its owner returned it to the supplier but the service department couldn't find the problem and it was out of guarantee. When I had a look at it and the circuit diagram, it was evident that a resistor was never inserted. Normal operation was restored after fitting the correct resistor.
I used to come across a few build errors in TVs. I remember a couple from Thorn in particular.

Regards
Symon.

[Mooly]

I remember the 1N4148's fitted in place of ZTK33B's on the original Toshiba FST's back in the 1980's. Tuner drift... what tuner drift.

[Graham G3ZVT]

The first time I encountered this sort of thing it was a Thorn 850 TV that crossed my bench for refurb, not repair. One of the dropper terminal lugs had never been soldered.

[yesnaby]

I had a pair of PC loudspeakers from Freecycle where the wires for one speaker were never connected inside the housing, they were just tied in a knot. The previous owner was blissfully unaware of this!

[Boater Sam]

My Seymour radio had lots of wax on the bottom under the mains transformer, it all worked but obviously ran hot.
The 5v windings for the rectifier, 1 Amp, and the rest, 6 Amp, had been transposed from new.
Sam.

[crackle]

I wonder if the plastic sleeve was fitted the wrong way round or if it has slipped round.
It may have been fitted based on the long and short wires.

Mike

[McMurdo]

I'd have thought it would be on a bandolier-fed auto insertion machine. Possible the bandolier was made up with the capacitor backwards.

[Graham G3ZVT]

Years ago my sister-in-law bought a new freestanding cooker, right from the start the ovens overheated whenever she used them.
She got an engineer out but he said he could find no fault.

A second engineer was equally baffled.

On one occasion she used the top and bottom ovens together, it wasn't perfect but they worked reasonably well. That was a clue.

I had a look at it myself and found the problem.

(I bet some of you are ahead of me here).

It wasn't an electrical problem.

When I traced the capillary tube from the bottom oven thermostat, its phial was in the top oven's pocket and vice versa, the tubes were all neatly routed and the clamp screws sealed with paint in the factory.

[Herald1360]

That's a neat exposition of the difference between an engineer and a technician.

The people called out to look at the cooker weren't engineers!

[AC/HL]

At one time I worked in a TV factory (Bairds). You get those sorts of fault on the production line. They couldn't possibly happen in service, so you have to look for the implausible. Sometimes they were obvious, wrong valve or picture upside down, but they can be head scratchers.

[dominicbeesley]

Originally Posted by McMurdo

I'd have thought it would be on a bandolier-fed auto insertion machine. Possible the bandolier was made up with the capacitor backwards.

Or the capacitor is the right way round but the marking sleeve is on backwards?

[boxdoctor]

Originally Posted by AC/HL

At one time I worked in a TV factory (Bairds). You get those sorts of fault on the production line. They couldn't possibly happen in service, so you have to look for the implausible. Sometimes they were obvious, wrong valve or picture upside down, but they can be head scratchers.

A friend and I went to the (probably) same factory at Leeds in an old mill, on a dealer's engineers tour in the late sixties.
We watched with admiration the skill and speed of the people on the assembly line, until we came to a lady soldering parts in the line/convergence part of the P.C.B.s She deftly soldered at one side of the component in the (tinned) board, and then quickly almost flicked the iron to the other side, resulting in a sheen of solder that looked as if a perfectly good joint had been made. It hadn't though, as the tag on the component could not have possibly been hot enough in that brief moment to have taken a flow of solder, or even been tinned. All the connections she did were done thusly.
My friend commented quietly "A few more call-outs in the making."
I often wonder how many of those joints failed in service. Tony

[Craig Sawyers]

Back to backwards capacitors. I had exactly that on a Tektronix 7000-series plugin (forget which one now) that had not surprisingly bulged and leaked. But that plugin has clearly been operating just fine on someone's bench probably for a decade or more.

I also had a backward one in my Quad 33 preamp (C401). That rendered it open circuit. This was on the bootstrapped tone control input transistor, which follows the volume control. Fed by a 68n capacitor C400, that circuit has some subtlety. It is arranged at a second-order rumble filter, cutting at 15Hz with a second order response. If C401 is open, the frequency of cut does not change so much, but it changes to a first order filter.

Craig

[Jon_G4MDC]

Racal RA117 40Mc/s filter. The silver plated wire from the coil neatly wrapped around the silver plated trimmer tags ready for soldering. Actual soldering - overlooked.

That worked for years until a few went high resistance at the joint.