Go with your gut

[agardiner]

Hi all,

So; a simple reminder when repairing vintage radios, to go with your gut. I am sure many people here are very experienced, but sometimes find themselves questioning their own diagnosis or ability. Sometimes you also end up fitting a 'new' part which is defective as per another thread.

The most important equipment for diagnosing faults in vintage equipment is: Experience, Knowledge, Multimeter, Oscilloscope - in that order. I, like some others, have a raft of fancy equipment at my disposal, including semiconductor analysers, LCR analysers, ESR meters, valve testers etc etc. Yet sometimes these can mislead you.

I was repairing a Bush TR130 over the weekend - had been in regular service before breaking down with severe distortion. First suspect was the AF117's, but these were innocent. 'Gut feeling' said the output transistors were faulty, but testing them showed they were innocent. Further tests with all my fancy equipment suggested maybe the driver transformer, but substitution proved fruitless. Quiescent current was fine. After hours of troubleshooting, I replaced the output transistors anyway, and low and behold the problem was fixed. The moral - just because the part tests OK on a fancy analyser, does not mean it will work in its intended application. Sometimes you can have too much equipment!

Adrian.

[toshiba tony]

Strange post Adrian, I spent a total of fifty years at Sony and LG and have seen some illogical faults. I mainly ignored flow charts but always 100% used the manual. Had the no power\not plugged in charts but some were quite good. Over the years you tend to get a feel, how things have changed over the last ten years. I retired three years ago due to dementia. Maybe it came at the right time! Glad to be out of it now, take care. Tony Walker

[HamishBoxer]

Indeed Adrian, there are test conditions and working conditions, different worlds at times.

[paulsherwin]

Transistor faults are strange things, and are often not picked up by standard testing methods. You can't beat substitution - the substituting part doesn't need to be precisely correct to expose a fault.

Ge output transistors seem to be becoming increasingly troublesome.

[agardiner]

They do indeed. I have found several times when I buy new old stock, that the analyser will often state that the collector and emitter are reversed. Once put into service they work fine and indeed if retested, test fine too.

[paulsherwin]

Component analysers are known for not handling Ge transistors very well, presumably because they are over optimised for more modern Si types.

[G6Tanuki]

I generally use a mix of intuition, cold logic, random stabs-in-the-dark, and sometimes a small rubber-faced mallet!

After a while you get to develop a feel for the sort of fault-symptoms different components can cause which helps you to take shortcuts rather than working through a formal fault-finding chart; in my experience the worst faults are the intermittent ones - which never show up when you've got the thing stripped out of its case and the meters/scopes/logic-analysers connected, but which re-appears a couple of days after you've put the thing back in its case and returned it to the client.

More than once I've had a not-reproducible-in-the-lab 'fault' which turned out to be either some external influence at the customer's site causing the problem, or was user-error.

[Tractionist]

Easy to say after the event I know - but did you offer it some freezer spray or heat? The 'acid test' I suppose is whether a device works as intended [reliably] irrespective of the provenance of certain components. I have the odd receiver which various 'testers' have confirmed to contain defective valves [low emission etc. etc.] - nevertheless these sets work fine i.e. as well as they do when NOS replacement valves are fitted in the same locations.

Of all potential faults - I hate distortion! (Worst still - intermittent distortion!) That said, checking external influences [routers/LED's etc.] and a scope can make things a little clearer.

Nonetheless - you make a very fair point ... i.e. "don't take anything for granted": all fault finding is ultimately a process of logical elimination.

[Radio tirana]

Hi Adrian
I have a scope, several multimeters, a signal generator and component testers. Often they are not the first thing I reach for when faced with a faulty set. I find that a really good visual inspection under strong light, a sniff for burnt or overheating components and a prod with an insulated tool just as enlightening. Isolating sections of a circuit and finding faults by elimination and substitution are powerful tools. I used to jump to the conclusion that the most expensive or hard to get parts would be at fault but often it turns out to be more mundane bits switches, valve holders, tag strips, fuse holders etc. the wealth of knowledge on this forum is a resource I find absolutely invaluable.

Kind Regards RT

[Dukebox]

Originally Posted by agardiner

Hi all,

So; a simple reminder when repairing vintage radios, to go with your gut. I am sure many people here are very experienced, but sometimes find themselves questioning their own diagnosis or ability. Sometimes you also end up fitting a 'new' part which is defective as per another thread.

The most important equipment for diagnosing faults in vintage equipment is: Experience, Knowledge, Multimeter, Oscilloscope - in that order. I, like some others, have a raft of fancy equipment at my disposal, including semiconductor analysers, LCR analysers, ESR meters, valve testers etc etc. Yet sometimes these can mislead you.

I was repairing a Bush TR130 over the weekend - had been in regular service before breaking down with severe distortion. First suspect was the AF117's, but these were innocent. 'Gut feeling' said the output transistors were faulty, but testing them showed they were innocent. Further tests with all my fancy equipment suggested maybe the driver transformer, but substitution proved fruitless. Quiescent current was fine. After hours of troubleshooting, I replaced the output transistors anyway, and low and behold the problem was fixed. The moral - just because the part tests OK on a fancy analyser, does not mean it will work in its intended application. Sometimes you can have too much equipment!

Adrian.

Personally speaking Ade, I think it’s because you are “the King of the Thetford Castle” when it comes to this sort of stuff!
Sorry to “gate crash” in on this thread but having had many radios repaired/restored by your good self I feel the need to express kindest thanks and to say to anyone reading this that if you need a radio repaired/restored/realigned give this fine chap a call. Good old fashioned service and impeccable work.
Laurie 👊🏻

[agardiner]

Kind words, thank you. However, there are many talented engineers within these forums, keeping the wonderful sets of yesteryear alive!

[duncanlowe]

Yes, it's one of those maxims, never trust your instruments. I mean do, but don't. It's something I've tried to pass on to younger Engineers. Just like in this case, believe them with a dose of scepticism. If what they are telling you doesn't add up, don't stick to 'it must be true 'cos my meter / scope / whatever says so'. Aliasing on digital scopes is a great example of this, what you are seeing isn't neccesarily whats going on.

I had another, where a system was saying that the airflow measurement sensor was out of range under certain conditions. The computer said it was a sensor problem, but replacement sensors made no difference.So the service department insisted it must be a wiring or other electrical fault, and made us spend hours replacing wiring and stuff. Everything was always OK as we could test it 'on the bench' but as soon as it was run up to full operation, the same thing tripped the diagnostic. I got fed up, and told the service department that maybe the flow sensor was actually telling the truth that there was a real problem with the airflow. They did a proper investigation, and a clip wasn't on properly which meant that under low pressure conditions (where we could test) the air feed pipe was pressing against it's mating point enough to seal and the airflow was within spec. But as soon as the pressure / flow was turned up, it pushed away and there was a massive air leak. The sensor was right. The diagnostic was misleading. The fault was a badly fitted part.

On the other hand, trust your instruments. Had someone who designed a simple relay based circuit to avoid applying a high current supply to a circuit with a short circuit fault where we had seen some short circuits in prototypes (because of a cable insulation rub through). They tested it on a part built unit that didn't include the area where the known issue could occur, but the simple circuit detected a short and refused to apply power. They tested for resistance with a simple DMM, which confirmed the short, which was logically unlikely given that they were essentially testing a single wire that ran from one end of the part unit to the other, and was connected to nothing else apparently. So the test circuit AND the DMM reported a short, but they refused to believe it, it was 'impossible'. But of course said wire had been trapped under a component as it was bolted down and a compression short was present. My boss muttered 'I think we have a problem with some of our Engineers' as we walked away from that one.

[majoconz]

Like any good detective work, the man who is asking the question of the suspect already knows the correct answer , he is just checking if the suspect comes up with the same answer.

Testing a suspect component is much the same really.