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Old 26th Aug 2015, 7:43 pm   #1
'LIVEWIRE?'
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Exclamation 1980s Frequency Counter stopped working(PSU Fault)

Maybe I should have said 'How to miss the blindingly obvious cause of a fault! I've owned this 'Play Kits' Italian made Frequency Counter for over 20 years, and never had a problem with it until today, when I switched it on and it was totally dead. The PSU in these is a simple full wave rectifier, followed by an LM309 To-3 regulator to give +5v stabilised supply. I checked that there was 10-010vAC at the Transformer Secondary, and at the anodes of the 2 x 1n4004 rectifier diodes, then found that there was only<1v at the Cathodes. First thing, snip out the 4700uf main smoother. Voltage at diode cathodes now ca. 8v. Tack on a new 4700uf, which I'd checked for capacitance and ESR . DC voltage back down to <1v. By then I'd disconnected the lead to the LM309 I/P. It turns out that the two 1N4004s are faulty, with odd readings on the Diode test facility of my DMM. A couple of new 4004 or similar diodes will solve the problem. (Should have changed them sooner. My excuse is that I don't recall when I last had to replace 1N400X diodes in any piece of equipment. They very rarely fail in my experience!)
NB I think this is the right section for this thread as the Freq. Counter must be at least 25 years old by now!( I bought it second hand & ready made)
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Old 4th Sep 2015, 11:06 am   #2
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

It turns out that one of the 1n4004 diodes was o/c. Having no 400x diodes to hand, I replaced both with a pair of 1300PIV(actually VRRM) similar diodes from my spares box, fitted a new 4700uf capacitor, and the genny is up and running again. Whilst repairing this instrument I was reminded of how technology moves on. The display on this unit consists of 6 x 7 segment Red LED displays, and there are over 20 14 pin DIL I.C.s, all mounted in sockets in the circuit, which id built on a fiberglass PCB, plus a pre-scaler on a seperate small PCB, and another board with the mains on/off, Khz/Mhz, and prescaler switches - good old fashioned push-button types. Not a microproccessor in sight! That having been said, it isn't the easiest of things on which to work. The circuit boards are bolted either side of an aluminium plate, and held in place via brass spacers that drop out when the screws are undone, but then it was originally supplied as a kit for home assembly. Does anyone else remember, or still have one of these counters?

Last edited by 'LIVEWIRE?'; 4th Sep 2015 at 11:15 am. Reason: To add further information about the instrument
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Old 4th Sep 2015, 11:35 am   #3
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

Good result.Possibly 1N4007's will work OK as well.
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Old 4th Sep 2015, 12:13 pm   #4
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

I'd be tempted to fit a suitable MOV and low-value fuse on the mains input to pamper those elderly ICs, and to use something like 1N5402 in place of the original diodes. Maybe I'm just naturally cautious, but as hobbyists we can afford to be generous with ratings as we don't have OEM accountants breathing down our necks. 1N4004 seems a high voltage rating for a 10-0-10V secondary, perhaps their demise was current/heat-, rather than voltage-related.
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Old 10th Sep 2015, 10:30 am   #5
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

Now I should know what a MOV is after nearly 45 years in the radio repair trade, but I've forgotten. Enlighten me, please! There is a3A fuse in the plug, and the Counter is already fitted with a 500mA Fuse internally. This has been there from new, maybe 25 years or more. The diodes I used were BY133s because I happened to have a pair handy, and had run out of 1N 400x devices. However I'v just looked at the original parts list, which specifies 1N4004 or eqivalent for the rectifiers. For the past13 years my workshop has been in a Cellar, which, like most such places, can get damp, so that may have contributed to the failure. It certainly hasn't done the black enamel paintwork of the instrument case any good! As a matter of interest, some of the ICs used are; SN7400,7410,7413. 7490, & 7493, which gives you some idea of the age of the thing. These days I guess one single LSI Chip could do all that this instrument does, and more!!
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Old 10th Sep 2015, 8:34 pm   #6
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

Hi LW, Metal Oxide Varistor, a symmetrical device with a highly non-linear resistance vs. voltage characteristic- ISTR the resistance is inversely proportional to something like the thirtieth power of the applied voltage, so inconsequential at the rated voltage and dipping abruptly to dissipate spikes. I fit them wherever possible (and it usually is, they're physically a bit like a ceramic capacitor) to semiconductor kit in particular. All too often, I hear of folk trying to source 30-40+ year old semiconductors (especially long-obsolescent ICs for superceded purposes) and finding them either unobtainium or sit-down-sir expensive, so I think it's worth protecting them wherever possible. In theory, supply decoupling should help quench supply spikes, but in practice older capacitors may posses significant impedance, particularly super-annuated electrolytics with raised ESR. MOV construction gives low self-inductance and they have nano-second response times. Of course, getting spiked is only one of many ways that semi-conductors expire, but doing something to guard against it is cheap and straightforward.

A caveat is that, under prolonged overload (say lost neutral in a three-phase system), they can destructively overheat and even melt/explosively disintegrate if current isn't limited so it's important to have a low-value fuse before them in the supply- and the fuse will assist in normal operation by presenting a tad more source impedance for effective spike suppression. The usual suspect suppliers stock them in various working voltage and energy (physical size!) ratings, I use Philips or Siemens (Epcos) 275VAC types.

I've a couple of Nixie counters (HP and Marconi) tucked away and they're awash with 7490, 74141 etc. Yes, they've got primary-side MOVs fitted and NO, no-one's having them for a Nixie clock!
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Old 15th Sep 2015, 9:31 am   #7
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Default Re: 1980s Frequency Counter stopped working(PSU Fault)

Thanks for the explanation, turretslug. I really should have known, as stated, what a MOV is. Essentially the same thing as a Voltage Dependent Resistor (VDR)no doubt. I'm not sure whether the counter includes a MOV. Both it, and it's circuit diagram, are in my workshop, almost a mile from here, so I can't check. If none is present, perhaps I'll fit one,.
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