Quad FM4 Tuner

[SteveCG]

This tuner was turned on recently after a few months of no use. After about 1/2 hour everything suddenly went off, with no particular 'sounds' before it went. The T63 ma fuse was open-circuit. It was replaced, however an AVO on its AC current range and a variac were used to bring the voltage up slowly. The unit came up fine - all functions working - and drawing 25 ma (240 Volt mains). So it was left on (without the meter or the variac in circuit this time) and after about 1/2 hour, off it went again - blown fuse again.

Clearly something is warming up with a longish time constant and taking the unit out. I noticed that there is a Rifa anti-click capacitor-resistor across the switched mains. Visually it looks alright, but I wonder whether Quad's tight fusing rating means that the fuse blew before a runaway leak resulted in the capacitor body cracking apart - as one see with other Rifa failures?

Any opinions please.

Mains transformer?

[SteveCG]

Possibly - I checked its temperature and it was cool to start with but was somewhat warmer (but not hot) by the time the fuse failed. The PSU used a regulator device mounted on a small heatsink - now that was too warm to touch.

I'll have to set a test rig up to monitor the PSU voltages (pre and post regulator) as well as the input AC current to the unit. The tuner is all mounted on a dual sided PCB so it is not straight-forward to intercept the PSU lines to monitor the currents directly.

I had originally wondered whether the fuse failing was simply due to old age as there was no visible break - evidently not. I don't suppose there are enough of these tuners around for anybody to have detected a 'stock fault' situation arising from ageing alone.

[percival007]

I did one of these fairly recently and found quite a few of the small light blue electrolytics had leaked.

They were all across supplies dotted around the PCB.

I didn't have a circuit so can't be more specific. I seem to remember they were either 10uF or 22uF (?).
Looking at the PCB from the top with the front panel towards you there was definately one, front left that had leaked the worst.

Sorry for being a bit vague and hope what i wrote helps,

James.

[percival007]

The attached Photo may help.

As I say, that front left one was worst for me, exact same fault as yours.

James.

[SteveCG]

James,

Thanks for the tip, I'll have a really good look - when I first looked (a quick look) they all seemed visually alright (compared to others I've seen) - no bulging or leaked electrolyte.

I note in your photo that the Rifa capacitor I mentioned (just next to the mains transformer) has been replaced with a capacitor and a resistor in series. Interesting.

[KeithsTV]

I had exactly the same problem with my FM4 changing the mains transformer before finding the Rifa cap was the cause.

Keith

[Michael Maurice]

Replace the cap, if that doesn't cure it then change TX

[SteveCG]

Thanks to James's and Keith's suggestions I am 99.99% sure I've found the problem. I looked closely using a jewellers eye-piece and bright sunlight and discovered by suitably holding the tuner in the light, that the Rifa capacitor has a tell-tale crack in one side. It was difficult to see as it is the side facing the power sockets - and the other sides and top are in pristine condition.

I'm going to replace it with a X rated capacitor with a 220 Ohm, 1 watt resistor in series - the wattage for the maximum working voltage rating, not the power.

All the electrolytics look fine - no bulging or any sign of electrolyte leakage.

[SteveCG]

A postscript: I removed the Rifa capacitor (actually a snubber) and it was indeed cracked only on one side. A replacement X2 class 47nF capacitor was fitted in series with a 100 Ohm 1/2 watt carbon composition resistor (my old RS catalogue gave the maximum working voltage of 500 volts for this type of resistor - desirable for mains voltage circuitry). I noticed in the latest CPC catalogue that the Rifa devices are still made and sold - I wonder if these newer ones are any better?

The tuner now has worked without a problem for a couple of hours. 'Problem fixed' I reckon.

[SteveCG]

A postscript to the postscript.

The FM4 tuner is still working. On my suggestion the associated Quad control unit - from the same era as the FM4 - was checked a couple of days ago. Here the Rifa snubber (same type as in the tuner) was found also to be ruptured. Rifa capacitors/snubbers really are an Achilles heel.

[fetteler]

Indeed they are! I've just replaced one in a friend's 33, like you I used an X2 cap and resistor in series..
As to weather or not the modern Rifas are any better than their older versions, well I don't know but it's a good question. Maybe someone can shed some light for us?

Cheers,
Steve.

[Radio Wrangler]

We won't find out for ten or twenty years even if someone is brave enough to try them!

David

[benisjamin]

Originally Posted by SteveCG

A postscript: I removed the Rifa capacitor (actually a snubber) and it was indeed cracked only on one side. A replacement X2 class 47nF capacitor was fitted in series with a 100 Ohm 1/2 watt carbon composition resistor

I know this is an extremely old thread but I have the same issue with an FM4 tuner, visually checking the RIFA capacitor directly in front of the main transformer, I can confirm it has cracked/blown. Can anyone recommend where and what make replacement components to buy to upgrade/replace this blown capacitor ?

Thanks

['LIVEWIRE?']

I always replace RIFA Capacitors in any equipment, using, as Steve CG has said, a modern 47nf X2 cap. I don't always fit a series resistor, although the 100 ohm 0.5watt type suggested by Steve isn't a bad idea. AFAIK though, those RIFA devices were simply Capacitors with no(added) series resistance.

[Nickthedentist]

You can find similar capacitors in scrapped household electrical goods if you like fiddling with old stuff you find dumped on the pavement or in skips etc. Just avoid any that are cracking!

Or eBay, but make sure spacing between pins is correct for your Quad: http://www.ebay.co.uk/sch/i.html?_od...nf+x2&_sacat=0

As the others say, look for the marking "X2" on any replacement, with a capacitance of 0.047uF (or 47nF, same thing!).

N.

[SteveCG]

It is a bit ago now but I am pretty sure that the RIFA was a snubber (that is a capacitor with a series resistor) and not just a capacitor. The vales of both R and C were on the side of the RIFA.

[nutteronthebus]

Hi I know it's a bit late but I have the cct diagram in PDF form but it is to large to post here PM me if you want it


Dave