X cans problem

[louismark]

Hi all i need some help/ideas on what could be wrong with my x cans. It will work for about 5 minutes from cold, but the it will give a distorted pulsating sound, even when the volume is turned down all the way the sound is still there, could this be a blown valve or cap? Here are some pics of it.   

[operater]

hi,
is both (left and right) channels same? If sound is different try to switch valve.

[louismark]

I had forgot all about this lol, i did try and swap the valves but it was the same, what should i be looking for? can i test voltages to see if anything is a miss? Thanks

[PinkFloyd]

How long have the valves been in service? If more than two years I would replace them because they will definitely be out of spec, as to the distortion it's most definitely down to some capacitors that have gone high ESR...... the main culprits will be on the top board, especially the two 100uF ones that sit directly next to the regulators. Best thing to do is replace all of the caps on the top board and this will cure your problem. If the valves have been in the amp for years, change them too as they will be well out of specification by now.

All the best,

Mike.

[PinkFloyd]

To confirm that it's the caps remove the valves and have a listen without them..... you should hear music but only high frequencies, if you still get distortion it's most definitely one or more capacitors that have gone high ESR.

Mike.

[louismark]

Thanks PinkFloyd will try that later.

[PinkFloyd]

Try that and let me know how you get on mate.

Best regards,

Mike.

[louismark]

Hi Just tried that and yes the sound is distorted and there is a strange hum in the background?

[louismark]

Just to add when i unplugged the power the sound jumped in volume and wasn't distorted just for about 2 seconds, not sure if that gives any clues.

[PinkFloyd]

It's definitely the capacitors, replace all of them on the top board and you'll be good to go. You can replace the 470uF caps with 1000uF ones or better still get yourself 10 x 35V / 1500uF Panasonic FR ( 12.5mm Dia. x 30mm high) these are good 105c temperature rated capacitors and they are a perfect fit. Don't go for the Panasonic FC or FM as they are 35mm tall and one of them will foul the top of the enclosure. I would recommend Samwha VA series 35V / 100uF for the two caps next to the regulators..... they are 130C rated and I have never had problems with them, they are good solid / reliable capacitors purpose built for withstanding heat. The 85C 100uF caps that MF use next to the regulators ALWAYS go high ESR after a few years and are pretty much built in obsolescence..... absolutely awful that they should use such inappropriate caps.

C13 will be fine, it's a metalized polyester (470nF) so no need to replace that.

The 16V / 10uF caps on the bottom board can also be problematic so I would (if it were me) recap the whole amp whilst I was at it..... in for a penny, in for a pound! Your amp looks absolutely 100% clean, no scorch marks or signs of being left on 24/7/365 (which some people very stupidly do) so I would strongly suggest that you replace all of the electrolytic caps and treat the amp to a nice new pair of valves....... you won't believe your ears when you hear the major improvement in sound quality!

All of the 10uF caps on the bottom board are 16V electrolytics but you are better to fit NON polar 10uF caps to C103 / C203. C103 and C203 are input coupling caps and if you have no DC offset on your source you can actually dispense with them entirely and simply jumper over the pads with a wire link (an offcut from a capacitor leg for example)...... non polar caps fitted in C103 / C203 improve the sound quality and if you are certain that your source has no DC on its output then wire links across the pads are even better.

I don't want to go into too much detail because that tends to confuse some people...... essentially, your amp is old and a recap is essential..... no point finding the one or two "culprits" that have gone high ESR and replacing them...... the amp deserves a good service...... you wouldn't replace "one" spark plug on your car, you'd fit a new set. Trust me, it's well worth doing.

Hope that has been helpful to you Louismark.

Best regards,

Mike.

[PinkFloyd]

Mar 28, 2016 17:21:50 GMT louismark said:

Hi Just tried that and yes the sound is distorted and there is a strange hum in the background?

Capacitors, capacitors, capacitors...... all day long I assume that you have already ruled out the source as the problem or even the interconnects..... if so, my advice above still stands.