Leaky/Intermittent Tantalum Capacitors

[Old Sparky]

I've recently been repairing a couple of Radiomobile 108SR Radio/8 Track Players.  In one the cause of no radio reception was an o/c 100 ohm resistor in the +ve supply to the tuner.  Unusual, but easily put right.  However the second 108 was a bit more of a challenge as the radio would play OK for anything from 30 seconds or so to 2 or 3 minutes.  This I eventually traced to two coupling capacitors feeding the radio audio into the preamp circuit.  The caps in question were 100nf tantalum bead types.  Not having any 100n. caps physically small enough to fit in the spaces on the rather crowded preamp PCB, I fitted a couple of miniature 1uf aluminium electrolytics.  These worked fine, so, as a precaution I also replaced 4 otther tantalum bead caps, each 2.5uf, with  2.2uf Aluminium electrolytics.  Getting these into the available spaces was a bit of a struggle, and getting the PCB back in positon wasn't easy because the electrolytics are taller than the bead caps they replaced, but the 108 should now perform relliably.  For those who don't know this model, the PCBS are mounted component side down atop the metal framework of the stereo, which is why component replacement is a bit fiddly.  (Thanks, Voxson !!. The Italian Company who designed & built the 106/108 series and the previous 103/104 models)

[Murphyv310]

Hi Sparky.
I'd think the only reason for using Tants in this application was down purely to size. Some tants are pretty awful and unreliable, they have been known to go bang in power supplies too.
So long as you've managed to get everything assembled OK then that's another one off the bench, well done

[Old Sparky]

You're probably right, Trevor. In the early 70s, Blaupunkt made a good claim for the virtues of Tantalum Bead Capacitors, and printed data for them in their Car Audio Service Manuals, in which they were used in quantity. As I noted the Radiomobile 108SR, etc., which were made by Voxson in Italy used them, but , IIRC they weren't much used in sets made in the UK, at least not in the makes with which I was familiar, such as Radiomobile, Motorola, PYE, & Elpico. Neither, AFAIK, did Philips use them, at least not in their UK made Car Radios

[Murphyv310]

Hi.
Yes, the use of them in British designed and manufactured car audio and even other consumer items was unusual. I did see them in Grundig, Saba and other makes, generally far more unreliable than aluminium electrolytic caps. I've some unused Tants in a drawer, probably 40 years old and every one is duff!

[Pat Pending]

As a trainee TV serviceman in the seventies the "blue resin tants" were on the company hit list for replacement as a matter of course in the rental sets, along with ITT CVC5 switch suppressors and the Decca boost ht filter caps amongst others....

[ppppenguin]

Good quality tants, used correctly, can give a lot of low leakage capacitance in a small space. However many were not either good quality or used correctly and are an utter liability. Notably if they are used as decouplers where there is a significant amount of current available they can be very nasty. I've had one, fitted across the main reservoir in some professional kit, actually catch fire. Had this happened in service (in an unattended area) rather than on my bench the results could have been truly horrible.

[Old Sparky]

The tants. in the 108SR to which I referred were used as coupling caps. between the (AM)Radio detector o/p and AF Preamps., and also between the tape preamps. and AF stages, so no high voltages or currents involved, but leaky coupling caps. obviously upset the bias on following transistor stages. Just another example of something not expected to last and be in use over 40 years after it was made, I guess!

[Refugee]

I have seen one across the smoothing capacitor in a CCTV camera control unit that was used to look at the innards of an atomic power station.
There was a big patch of soot on the inside of the cabinet and no bits floating around inside apart from the remains of the leads still in the PCB.

[pamphonica]

Notably if they are used as decouplers where there is a significant amount of current available they can be very nasty. I've had one, fitted across the main reservoir in some professional kit,  actually catch fire. Had this happened in service (in an unattended area)  rather than on my bench the results could have been truly horrible.

I have had a significant number of tants catch fire in Tektronix test equipment of the 1970s era, especially the tiny 500 series rackmounted modules.  These tants first glow brightly then emit smoke then go phut!  Easy to fault find subsequently why the voltage rail was zero, though!  If they have already self-immolated, the easiest way to find them is a low-range ohm-meter like the ToneOhm etc.  It's so simple to slide the test prods along a power trace and hear the tone frequency peak as you pass the exact faulty tant.

I have always replaced them with modern 105 degree decent quality electrolytics - much better nowadays as power rail decouplers/smoothers.

Happy days...

Jeremy

[AlanBeckett]

It's a bit odd really. I used them for years, even as Timing Capacitors, without any problems, although they were always High Quality ones within their Rating.

Alan