Measuring and testing equipment

(and some thoughts on testing tubes)



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Brand and type: All are Philips instruments. The oscilloscope at the left is type PM3230. In the middle the AC-millivolt meter GM6012. At the right the RF signal generator PM5320. I also have the DC-millivoltmeter GM6020 which looks similar to the AC model, but has much more tubes.

Produced: The Netherlands, around 1960.

Do you need this equipment to repair a radio? Some say you can't do without, others say they will never use it. In my opnion, it takes common sense to start with. With a screwdriver, a (digital) multimeter and a soldering iron, one can solve 95% of radio problems. Provided, you have some knowledge of the matter, that is.
For example, it's easy to recognize a burned resistor and to replace it , if you can find out its value. Step 2 is to find why that occured, before plugging it in again: even more important!

But, it is possible you run into an incrossable border with your DMM. For the more intricate cases, a scope can do wonders for you. The same goes for the RF generator or the AC millivolt-meter. If the local oscillator of a radio doesn't work, provide the signal with the generator and check if it works then. It also provides modulated antenna signals for all bands, including FM. The scope can inform you if this local oscillator is at work. Or maybe there's some nasty oscillation in your audio output which cannot be heard while being high frequency (or very low, also possible). More important, the oscilloscope gives the possibility to determine and measure AC voltage with high frequencies. Up to 400 Hz the DMM reliably can be used. But in a radio, frequencies below 400 Hz are not relevant, so the scope quickly becomes indispensable.
The DC-millivoltmeter has its advantage in a very high input impedance, and it easily determines very small DC value changes.

Some thoughts on tube testing

Click on the picture to enlarge

Do you need a tube tester in this hobby? I would say no. The best test still is to plug in a known good replacement tube.
But, as you probably don't have spares for all the tubes in your equipment, a tube tester can do a reliable check for you. This particular device, the British AVO type CT160, checks for internal (partial) shortcuts, first with the cold tube, then the same checks on the warmed up tube-under-test. It also determines gassy tubes, so with a bad vacuum. The final test is a check of dynamic properties: conductance. Never rely on testers which check for heater continuity only, and in some cases for (cold) shorts circuits as an extra feature. The outcome is fully worthless, you would get the same result from a cheap multimeter connected to the filament pins on the tubes base.

Adjust the settings and run the test. The device gives the result on a meter with green/red coloured scale, or as a percentage from 0 to 100.

Now what if the tube fails in the "shortcuts" test? A real shortcut gives a zero-reading, which almost never occurs. Any values better than 100 kilo-ohm (so higher ones!) measured between electrodes don't have to be disastrous for the tubes performance, but consider that (partially) shortcuts can affect the readings of the actual performance test.The same can be said about a worser performance than 100% in the conductivity test. It is very well possible that a tube that tests as 30% , or even less, still operates satisfactory in your radio.

But be careful: a weak tube also can damage other components in your equipment. The reverse effect more often occurs: rotten electrolytic filter caps easily can blow out the rectifier tube. In fact the mains fuse in a radio should prevent this to happen, or the tranformers thermal fuse should cut off the current. Unfortunately, this only works in some radios.....
The same for coupling caps between stages, which can cause the current through the tube to increase to an impermissible value.

For instance, a weak audio output tube doesn't necessarily result in bad audio, if you listen to MW at humble audio level. Who said you have to listen at full output level? (which only results in heavy distortion, even with good tubes!). Also RF and IF tubes are sometimes not very critical in their demands. It is possible that your tube works great in radio A, and messes up the sound in radio B. For HiFi gear it's different, some people even can hear the tubes brand from the sound they produce.

So, I never dispose of a "bad" tube, unless it has full internal shorts and is a modern type. Those can be dangereous for any radio, so they end up in the basket. All others, with some performance, I keep. As long as there's a filament, there's life.
Especially the real big and ruggedly built oldies, I consider them pieces of industrial art, and it's a shame to put them in the basket pitiless, even if the filament is dead. Did you know there are people collecting dead tubes?

So, what's the use of a tube tester then if a bad tube works anyway in your radio? The tester simply compares the tubes properties to given data, in a dynamic testing procedure. If it appears to be OK, then it is OK. When out of specs, interpretate the outcome into your specific situation. Or simply try it it as is. Don't blaim me if anything goes wrong, OK?

Cabinet: All equipment is well built from top-class components in a very solid way, to high workshop or laboratory standards. The Philips instruments are a bit more sophisticated.

Tubes: Of course, the scope is the modern guy having also some transistors aboard.

Obtained: The scope came from a public sale at the local university some 15 years ago. I paid way too much because at home I found the picture tube broken, literally with a crack so no more vacuum. Caused by an idiot messing around with the wires attached to it. Years later I found the same new tube by sheer chance in a dump shop for another 100 guilders, so in the end I paid 200 guilders for it.

Some time ago I could buy US Tech scopes (I think it was Tech) for only a few guilders each. They looked real huge and impressive but were much to big to handle. That was the main problem of the seller also, he had to carry them home again. Poor guy.

The AC voltmeter comes from a junk market for 15 guilders. The signal generator from a fellow collector, he wanted 125 guilders for it. After changing a tube I have a reliable instrument now for 150 guilders. Its DC antipole came from a flea market, where the seller told me it was a transmitter and a power supply in one case. I had my own ideas about that, and I paid him half his price, a 25 guilders.

Condition: All complete and in good working order, though I can't track the right input plugs for the AC meter.