Yaqin mc10l "popping" - diyAudio
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Old 4th August 2008, 04:46 PM   #1
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Unhappy Yaqin mc10l "popping"

Hello,

After a particularly long listening session of about 10 hours
my Yanqin MC10L (version 2 ) generated some rather loud pops & crackles via the speakers & blew its onboard fuse yesterday

I replaced the fuse with the spare and then checked all 4 bias were where I left them -at 0.35V, which they were

There are now still a couple of pop & crackles when the amp is powering up and some more on shut down.

I've altered the bias to 0.32V on all 4

Now it doesn't seem to be playing up as much as when the fuse blew, but I've only had 4 test runs of 5 minutes since the incident yesterday

- I've had this amp for about 2 weeks. It's been working just brilliantly(switched on for about 3 hours every day) untill this little hiccup

I'd really be most gratefull for any assistance !



James
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Old 4th August 2008, 05:26 PM   #2
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Seems like there are a few reliability problems with the Yaqins, but you can deal with most of them.

Some questioned the use of F2.5A (Flink, Fast-acting fuse) in the mains circuit, and I would too. Try a T2A like the DIYers here

Yaqin mc10L

No doubt the EL34s fitted at the factory are not the best quality types. And if they are running output valves as hard as they do the 300B model, it won't be long before they'll need renewing. A ten hour session could have fatigued the screens in such a setup - the crackling could be screen - cathode shorts... does the amp have an HT fuse? I would fit one quickly if not!

I like to use these inline fuseholders, if I'm in a tight spot

http://www.rapidonline.com/Electrica...AC-white/80051

and these, if I have time and space

http://www.rapidonline.com/Electrica...seholder/71463

If it blows HT fuses (try T500mA), consider stocking up on some EL34s before anything more serious gets blasted when the EL34s really blow up.

The EH300B is good & reliable, and if you want to ride first class, get some Svetlanas.
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Old 5th August 2008, 04:07 PM   #3
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Many thanks - yes thinking about it, I ran the amp for more like 14hours.


I don't thing it does have a HT Fuse/s. I came across this schematic that Les posted ....

http://www.diyaudio.com/forums/attac...amp=1194907581


I note that he's running 6CA7's, which after a couple hours "researching" / lost in an internet valve vortex, I was begining to consider too.

Unfortunately, I thought that these were exactly what I was after : http://cgi.ebay.com/ws/eBayISAPI.dll...MEWA:IT&ih=006

-the hunt begins again.. !
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Old 5th August 2008, 04:42 PM   #4
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- The issue is on both channels.

Excuse my ignorance - Does that point to the HT circuit or could multiple valves be failing ?
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Old 5th August 2008, 05:36 PM   #5
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The Power supply is common to both channels, and since power supply noise rejection in SE circuits is effectively zero, you will hear breakdown events from both speakers.

Schematic Useful, and highlights two immediate design errors:

- The 440V supply is uses pairs of series connected 250V electrolytics, with their voltage balancing resistors not provided. The design thus expects big-cap leakage currents to balance within 30% under all conditions, which will not happen for long, even if it held for the amp when new. leakage gets really out of hand after the amp has not been used for a few weeks, so I expect MC10 owners returning from a good holiday may be in for some cracking moments!

Fix this easily with 2W balancing resistors properly accounting for leakage. 150K by default of a calculation. Check resistors are 500V rated.
This job is only for High Voltage trained persons! This amp shows no signs of any HT bleed resistors either, so the dangerous voltage will remain on the amp after switch-off! Danger!
This fault will also be corrected by balance resistors.

The next fault is even stupider, inasmuch as even fewer extra parts would have been needed to do it right. It's the screen resistors - none fitted! at 440/470V found in this amp you need 1K 5W screen resistors on an EL34 - even in UL circuits, as here.
Fixing this also involves handling the dangerous bits of the amp - so be trained or get a trained tech/DIYer to do the fix!

Leaving these resistors out really puts the screens at risk on any EL34, and the more so on cheap valves. sudden failures, or sudden loss of quality will result when the screens get overstressed one day.

There is usually a guitar amp tech nearby, wherever you are, so if you are not HV trained, track 'em down.

Both these fixes are standard fit in Marshalls of various types, so a guitar amp tech is ideal.

Lots of scope to improve the sound on these, while you are at it. Probably worth figuring out a mod pack for these, since everyone wants an amp at their prices!
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Old 5th August 2008, 05:43 PM   #6
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The noises you hear may not be incipient output valve failures, or capacitor breakdown.. you may just have cheap 6N1s that have become noisy.

So if you are looking for something to try without voltage hazards, try one of the Russian ebay vendors of 6N1 that other DIYers here have recommended.

I'd still have the design errors fixed soon though, and would avoid leaving it powered & unattended.
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