Never assume anything!
 

Just been working on an R1155 that I rescued from the skip a few years back. I had to re-wire it completely as the insulation had turned to mush and I also replaced most of the caps & resistors.

Recently it got very quiet on the highest frequency band and inspection showed that the local oscillator was very weak and stopped working completely on part of the band.

After scratching my head for a while as all the voltages & the bottle were fine, I was not relishing getting at the inductor which is very well buried in these sets.

A bit more poking about revealed a faulty PS capacitor which I had used to replace the padding cap. with. As I probed the leads of it, the osc. output varied.

It just goes to show that one can not assume that the nice relatively new and still shiny components must be blameless!

Re: Never assume anything!
 

I’ve had this a couple of times before. I can understand why the goods in team at an electronics company I worked for actually QA’ed all the incoming components. I’ve taken to doing the same. Has caught a few duff ones, particularly cheap resistors.

Re: Never assume anything!
 

Regular problem at goods in QA. I experienced regular failure of some critical components on military equipment build. On receipt testing, we ultimately found in excess of 30% failure. The supplier lost his supplier status for over a year.

Colin

Re: Never assume anything!
 

I worked at a place where they contracted out populating PCBs.
Someone put all the caps in the wrong way round.
The test department started making crackling noises.

Re: Never assume anything!
 

A place I was at contracted out everything they could. The firm that did cable assemblies got live and earth crossed on a mains input connector in a batch of cables for a prototype in R&D.

Someone just assembled the first proto and plugged it in. No checks.

Ye gods was it hard to get it through the skulls of some labbies that you don't just plug new things in without thinking.

Larry Niven created the phrase "Think of it as evolution in action"

David

Re: Never assume anything!
 

Even some big companies can have problems, Behringer has problems in their larger iNuke amps with a faulty batch of diodes which would cause the amp to fail, and this wasn't some no name Chinese company but International Rectifier.

Re: Never assume anything!
 

Was it one of those small axial capacitors with the clear plastic outer through which you can see the wound-up foils inside? Usually just marked with a 'short-form' value/voltage-rating code and no manufacturer's name?

I don't know who actually makes them but I wish they'd stop: I've had plenty of these fail in recent years. Some are useless-from-new: I had a bandolier of 330pF ones where both the wires were connected to the same foil !

Re: Never assume anything!
 

I can remember working at a small place where we were making uo transformer boxes.
The boss was helping. After being interrupted he rotated the diagram some how.
Several units got made with the power switch across the input from the mains and the primary of the transformer.
The first one to get plugged in emptied all of the sand out of the 13 amp fuse and vaporized one of the end caps and half of the other one.

Re: Never assume anything!
 

I remember my father doing something similar in the mid 1980s. He'd imported a load of hardware from Taiwan which was 110v so he hired a guy (whos other part time job was a commercial pilot?!?) to put new transformers in everything and stuff an adapter on the plug. He goes "Hey John, the fuse keeps going". My father in his wisdom says "just put a bigger one in it and see if that's ok". So he couldn't find one and stuck a soldering iron tip in the fuse holder...

Woosh! Turns out PVC smoke can fill a room pretty quick!

Re: Never assume anything!
 

Yes it was. I don't know the manufacturer, but never had a problem with them before. I used to think that silver mica types were practically blameless, but have had to replace one some while back. The interesting thing about this was that it measured correctly on the bridge, but I think there must have been a bit of variable leakage which was dependent upon the applied pressure.

Re: Never assume anything!
 

"Suflex" polystyrene capacitors.

Re: Never assume anything!
 

With the 'put a bigger fuse in' theory I also while still a trainee saw a fail. Another but supposedly more experienced trainee built a power supply. Connected and switched on but the glass fuse popped. So he tried the next size up with the same result. The next size up again didn't so much pop as explode leaving only the end caps and vaporised glass. To his credit he did stop at that point and ask me what I thought might be the problem. I pointed out that connecting the secondary to the mains and the primary to the rectifier / regulator probably wasn't wise. I'm guessing it produced a nice HT supply for a fraction of a second......

Re: Never assume anything!
 

Polystyrene capacitors have been the cause of many awkward and intermittent faults over many years. They tend to have very thin wire leads, for a good reason - to reduce the heat from soldering travelling along the wire and softening the polystyrene, causing poor contact with the foils. Once damaged in this way, they represent a fault waiting to happen, possibly years later. Use a heat shunt when soldering, or better still, use an entirely different type of capacitor.

In an earlier thread on this topic, Suflex claimed that their polystyrene capacitors did not exhibit this fault mode. Unfortunately, other makes are available... I hate the things, and suspect them immediately.

Re: Never assume anything!
 

Ditto.

Al.

Re: Never assume anything!
 

I've just wasted half a day fruitlessly fault-finding on a piece of SMT equipment, which, it turned-out, I'd repaired properly first time and the test rig had developed a fault...despite having tested 4 other similar instruments the previous day.

Re: Never assume anything!
 

I am working on a cheep and nasty Bush /Chinese record player ,the motor didn't run I wrongly assumed that the motor switch needed to be closed to run the motor in fact it needs to be open ???,due to there being a control cct Mick.

Re: Never assume anything!
 

Thanks for explaining why Suflex polystyrene capacitors have such thin leads. I had always wondered why.

Re: Never assume anything!
 

They may well have been some early Suflex ones, they were terrible for going/already -from-new, being s/c!
I have hundreds here, I always check them before I fit them.

David.

Re: Never assume anything!
 

Took all the valves out of a radio chassis to turn it and replace a few caps. It was working OK.
Replaced the 5 valves, the EZ80 had been replaced with an EZ81 and the output was a used Russian EL84 equivalent. Assumed (ha!) that the tall valve was the output valve, nearest the output transformer wiring.
Strange, its not working now. Oh, the rectifier is glowing bright, oh, now its sparking over inside. Duff rectifier.
Fast off, pull it out, oh heck its a pentode, the output valve !!!
Yes, I'd put them in the wrong sockets.
Even stranger, with the same valves in the right holes, it works just as well if not better than before!
Tuff beggars these Russian valves.

Re: Never assume anything!
 

It's a real puzzle when a piece of kit from a reputable manufacturer turns out to have been wrongly wired half a century ago.

I had a recent example with a Bruel & Kjaer dual condenser microphone power supply with versatile switching to route either or both microphones to the two outputs.

The audio worked fine, but the two little neon lamps showing the active input(s) made no sense. I assumed it was just a design oddity. It was only when I got hold of a second identical unit that DID make sense, with the neon adjacent to the active input(s) lighting up, that I realised my first unit was actually defective. Couldn't now live with it - had to rectify the problem!

Hampered by not having the circuit diagram (in the hen's teeth category), I removed the relevant reed relays - they checked out fine. Transistors - all good.

Finally, having as a last resort traced out the very tidy cable-form connections, I noticed a minor wiring colour difference between the two units. Sure enough, the defective unit had two wires swapped over in the cable-form. Reversing the two finally put the indicators right. It spoiled the immaculate look of the cable-form though - clear evidence that it had been wired wrongly all those years ago.

A satisfying puzzle to solve!

Martin