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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Dec 2014, 7:36 pm   #1
David G4EBT
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Default Speedy Fault Tracing

We all have our own way of going about tracing faults, and I wouldn't presume to place my approach above that of others. There is the obvious approach that I think most of us take, in looking for loose connections, checking voltages as compared to the circuit, checking the value of resistors to see if they've drifted high in value, substituting suspect valves and so forth. On valve radios, where higher voltages are concerned, I do a lot of testing without the set connected to the mains - continuity of coils, transformers, switches, valve heaters etc, checking if resistors are within acceptable tolerances and replacing any that aren't, replacing certain components that I know I'll be replacing come what may, such as 'that cap' (the grid coupling cap in the audio stage) and the audio stage cathode by-pass cap - usually 25uF 25V.

But aside from this, for speedy faulty finding - especially on a dead set - for decades now, (since the mid 1950s to be more precise), I've been in the habit of using the 'signal tracer/injector approach' which has saved me countless hours of fruitless effort. By applying the probe of a signal tracer (just a simple little amplifier) to the slider of the volume control you can check to see if there are tunable signals at that point. If so, the frequency changer and IF stages are working, if only after a fashion. If not, you can work your way to the front end of the set, seeking out signals with the probe at various points to pinpoint the fault. (A scrap PC amp/speaker is fine as a tracer, or one of those LM386 Mini Audio Amplifier Modules which abound on e-bay for £3.00 or less post free).

If there are signals at the slider of the volume control you can then use a signal injector to squirt a signal into the AF stages at the slider, and if you hear no tone from the speaker, you've narrowed the fault down to the AF Stages and can look further into that. EG, if you apply the signal tracer to the primary of the output transformer and can hear signals, but not at the secondary, the OPT is suspect. If you do hear signals there but not at the speaker connections, if there's an earphone socket that mutes the speaker, bridge that out. Still nothing from the speaker, apply the signal tracer to the connections to the speaker. If signals appear, the speaker is faulty. If no signals, suspect one or both of the output pair of transistors and their associated components.

Without a signal injector/tracer and a methodical approach, it's easy to start making speculative guesses that 'maybe this or that transistor is duff, so let's yank it out and test it'. 'No - not that, maybe the detector diode then?' 'No not that...and so on it goes.

Same approach applies to any set - transistors or valves.

It's hard to beat the Velleman 'K7000' combined Signal Injector/Tracer kit:

https://www.esr.co.uk/velleman/products/frame_kits.htm

An hour spent making that will save countless hours wandering all over sets trying to discover where the fault lies.

To give a little example, I was recently asked to look at a Bush TR82C MK2 (the one with AF117 transistors in the RF/IF stages). Rather than homing in on the transistors due to their notorious reputation, I injected a signal to the V/C slider which showed the AF stage was working fine. I then checked with the signal tracer at the volume control and heard nothing. I moved the probe to the base of TR3 - the final IF and heard tunable signals, which showed that the front end of the set and the IF stage was working fine. I moved the probe to the secondary of the third IFT, (pin 6) and still got signals. Hence, the AF117 transistors were behaving themselves, (against the odds perhaps!)

Tried the tracer at pin 5 of IFT3 - no signals. The OA90 detector diode is hidden in the IFT3 can and was suspect. I connected a new diode across pins 5 & 6 of IFT3 and the set came to life. No need to strip the IFT out to remove the duff diode and fit the new one inside the can, which is a fiddly job - just left the duff diode to R.I.P inside it's 'coffin' and soldered the replacement diode externally across pins 5 & 6 of the IFT. All in all, about 20 minutes work.

Without a signal injector/tracer, I'd most likely have gone off on a frolic making an erroneous assumption that the AF117s had fallen victim to the 'tin whisker syndrome'.

I've attached a pic of two signal injectors I made many years ago, which I've given to other forum members recently. The upper one was made in a Woolworth's icing sugar syringe, using two OC44s about 1959 which at the time cost me ten shilling each (£10 each at today's prices when adjusted for inflation!). The lower one used a couple of 10p BC108s, built into a felt tip pen case. They've certainly earned their keep.

All in all, I'd rank a signal tracer/injector second only to a multi-meter in usefulness.

Just my thoughts - others may come at this from a different angle, which is fine by me.

Hope these ramblings may help a few others on the forum.
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Old 15th Dec 2014, 7:54 pm   #2
G6Tanuki
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Default Re: Speedy Fault Tracing

It's always interesting to compare techniques: your approach is to work backwards, injecting a signal into the audio stages then working forwards until the signal stops.

My preferred approach has been to connect a signal/antenna into the front-end and follow it down the chain until it stops getting-louder.

Both are equally valid: my approach will take as long to find an open-circuit loudspeaker coil as yours will to find an open-circuit antenna-coil primary.

Things get 'interesting' when there exists multiple paths through a piece of equipment: a simple example being an old analog 405-line TV with video- and audio-chains separate. It gets even more fun when trying to chase-down a fault on something like a "Wadley Loop" receiver where the various locally-generated oscillators need to be 'perking' and the loop locking before you have any hope of receiving anything.

I must admit, my two favourite fault finding tools are a neurologist's rubber-faced hammer [as used to whack you on the knee to see if your patellar reflex is OK: I use it for 'percussive investigations' of valves, earth-points, switches etc] and a hairdryer [used to heat things to bring on component-drift or failure].

I'd add in a Variac [so you can wind the supply-voltage down to see which oscillator has barely-enough gain and drops-out, or wind it up to get borderline parts to finally release the last of their magic-smoke].
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Old 15th Dec 2014, 8:45 pm   #3
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Default Re: Speedy Fault Tracing

When I was a field service engineer (tellies) all I had was a multimeter and the usual array of hand tools and up to 20 odd call outs per day, first job was to give the set a good thump on the top, that quite often restored normal service, that was then followed by some valve wiggling, tuner tapping and a tap around with an alignment tool (plastic knitting needle) to pinpoint any offending connection(s) If you didn't nail the fault in 10 minutes then it was going to be a nasty so into the workshop it went.

When I was an apprentice I built a signal tracer/injector etc, must admit it ended up on the shelf gathering dust, together with most other PW gizmo's I built.

The best servicing tool you have is knowledge, that's the skill, you've either got it or you ain't, if you ain't then go get some, that's my tip of the day.

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Old 15th Dec 2014, 10:28 pm   #4
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Default Re: Speedy Fault Tracing

Received wisdom has it that the most efficient way to trace faults (in any linear system, not just electronics), is to start at the middle, and see which 'side' is faulty. This side is again divided in half, the faulty half determined, and so on.
Logical in theory, but not always pracical!
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Old 15th Dec 2014, 11:06 pm   #5
paulsherwin
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Default Re: Speedy Fault Tracing

Signal injector / tracers were standard beginners' projects in the 60s and 70s and are still worth making today. They mostly consist of two transistors which operate as a multivibrator (inject mode) or two transistor amplifier (trace mode). There are lots of examples around if you google.
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Old 16th Dec 2014, 2:10 am   #6
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Default Re: Speedy Fault Tracing

dseymo's advice to start in the middle is the successive approximation technique. Works in fault-finding, works in alnalogue to digital converters.

The optimum ploy is not to pick the middle on a circuit diagram - say as measured by ruler, but to pick the point with equal probabilities of the fault being before or after it, then you do the same trick on the faulty 'half' to get it into 'quarters'


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Old 16th Dec 2014, 9:10 am   #7
kellys_eye
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Default Re: Speedy Fault Tracing

I recall making signal injectors for my marine radio training at college in the 70's. Even then I had a penchant for 'miniaturisation' and enjoyed cramming the circuit into the smallest felt-tipped pen housing I could find!

Certainly, the tool is invaluable for fault tracing - as is the humble diode-probe - another 'device' that got the miniaturisation method. Matching felt-tipped pens were the order of the day (blue for tracing, red for injecting).

Happy days! Thanks for reviving the memory David. I'm off to make some new versions!
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Old 16th Dec 2014, 10:29 am   #8
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Default Re: Speedy Fault Tracing

Aside from checking voltages, I agree that some sort of audio signal and something to trace it with will [usually] save hours of work. And as Paul said, a few components, even salvaged from an old board, are all you need to make both an injector and an AF/RF tracer. Great little wet Sunday afternoon projects that everyone can make.
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Old 16th Dec 2014, 7:13 pm   #9
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Default Re: Speedy Fault Tracing

Never bothered much with signal injectors/tracers apart from an RF signal generator but I can certainly appreciate their usefulness. When I first started my career in radio I was instructed to build a diode probe which was used for tuning up and finding faults in transmitters. I think I still have it somewhere, built inside an old biro case of course. This is where you definitely need an analogue meter.
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Old 16th Dec 2014, 8:10 pm   #10
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Default Re: Speedy Fault Tracing

I built my signal injector/tracer when I was 16 years of age ......... 50 years ago, just as I left school. Last time I used it was this morning to fault find on a neighbour's hifi.

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Old 18th Dec 2014, 11:39 pm   #11
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Default Re: Speedy Fault Tracing

I built my signal injector out of the module found inside a musical christmas card. An AA battery fitted to the coin cell terminals and a 1000V 0.1uf isolating capacitor and it's still playing 'we wish you a merry christmas' 20 years on.
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Old 19th Dec 2014, 2:36 am   #12
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Default Re: Speedy Fault Tracing

Brilliant idea, much better than a plain buzz and topical again every year.
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Old 20th Dec 2014, 10:18 pm   #13
Phil G4SPZ
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Default Re: Speedy Fault Tracing

I've enjoyed reading about the different approaches to fault-finding. I confess that I don't generally use a signal injector/tracer (other than a signal generator for alignment following repair) as I rely on the sort of 'cold checks' and 'replace on sight' actions that David describes. It's most unusual for a set to remain dead following these basic checks, but if it does, I try an external loudspeaker followed by voltage checks using the specified meter. In my experience, it's pretty rare for a radio to remain faulty if all the DC conditions are correct.

The techniques that I try to recommend to newcomers include mentally separating the circuit into stages and sub-stages, and pinpointing the faulty stage by a process of elimination. The danger I think is that some people replace all the wax capacitors on sight - and maybe several other components - before applying power, which denies them the ability to look for the initial fault symptoms when powered up, and also introduces the potential for inadvertent wiring mistakes or damage to adjacent components, which can make subsequent fault-finding much more difficult. Yes, waxies are unreliable, but only one or two actually need to be replaced for safety reasons before powering up.

The other thing I tend to do is check that the set still works after each component replacement. Sometimes this is easier said than done and I might stretch a point sometimes these days, but there's nothing worse than a set which initially worked (after a fashion) then refused to work after you've replaced half-a-dozen components. Where do you start looking?
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Old 20th Dec 2014, 11:04 pm   #14
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Default Re: Speedy Fault Tracing

When I first started fiddling in the 60's, my main tool was an old pair of ex-BBC high impedance brown bakelite headphones, with the metal band (which were incredibly uncomfortable!) with a crocodile clip on one lead and a waxey of some value on the other. This would produce a deafening click when testing live anodes! In hindsight this probably wasn't the safest approach, but hey I was young

Now it's a 1950's Advance genny and an 80's TQ scope - or even just a 70's Philips portable + cap plumbed into various points of the gubbins!
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Old 21st Dec 2014, 1:59 am   #15
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Default Re: Speedy Fault Tracing

A battery operated portable amplifier built into a small plastic box is useful at work for plugging into Krone 237A strip connectors to quickly check for the presence of audio signals when fault finding. In a previous job I had a microphone socket mounted on mine to check radio fist mics for mic audio and PTT/hangup functions and also to test control room operator headsets. Very useful for finding intermittent leads. This unit was commonly known as a "listening box" in the trade.
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Old 22nd Dec 2014, 10:00 am   #16
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Default Re: Speedy Fault Tracing

For speedy fault finding, a metered power supply can be very useful.

A low voltage supply is very useful for most portable radios and the like (essential for many sets where the batteries don't stay in place with the back removed - Philips Popmaster anyone?). You can learn a lot from the supply current, zero current indicates power connection problems, several milliamps which vary slightly between AM and FM looks promising and if it changes with tuning, the front end is probably OK (AGC effects), and if it varies with the volume turned to full, there's probably a problem with the speaker connection.

A metered isolated Variac is useful for mains powered stuff, even if it is not live-chassis. Again, zero current tells its own story, as does watching the current as the valves warm-up. For transistor stuff, for example HiFi amps which have suffered blown output stages, ramping up the mains slowly whilst monitoring the supply current can reveal remaining faults without destroying the parts you've already replaced.

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