yaqin mc-100b - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st February 2009, 04:39 PM   #1
jaks is offline jaks  United Kingdom
diyAudio Member
 
Join Date: Dec 2008
Default yaqin mc-100b

Hello.
This is my first go at this and I am hoping for some advice, but it will have to be in layman's terms.
I feel confident enough to check/ change components but have no idea where to start. My only ability in fault diagnosis is visual inspection and using a meter to check resistors.

This is the story so far.

After lots of reading on the pros and cons of chinese valve amps, I purchased a yaqin mc-100b off ebay. It was not new, so I hoped if there was a problem with voltage these would have been addressed already. for this amp was going to "blow me away", according to the seller.
Hooked it up and sure enough it did blow me away , but could of done with more bass. Leftfield sounded crap according to the wife . A week or so of intensive listening passed and all had been fine then turning “valvy” on one evening I thought I saw a spark from one of the kt88 tubes (v2), so I looked closer at the tube, with that a flash of light came from the board right near the v2 socket. I took the valve out and had a look, part of the board was black and a couple of soldered connections had vapourised. Took the back off and could see no signs of charred components. The burn was near the bias resistors (I think thats what they are so I checked them with a meter and the readings were all the same). I came to the conclusion that a rouge drop of solder had lodged between the two soldered points and shorted . The reason i came to this conclusion. 1 the flash of light was slow almost like a magnesium flare been lit. 2 the right hand channel in the same place as the burn showed that the two soldered point don’t connect to each other. I repaired the damage and risked turning on, the main fuse had blown but it came with a spare attached to the fuse cover, neat I thought.
Turned on eureka! All came to life and sounded great . I was over the moon, my wife called me a smart **** and the kids, well i won’t repeat.
Bias setting were now on my mind, I checked each one, they were all over the place and on ckecking v2 there was no reading at all. The burn had caused this I thought. This was not to be for when I swapped v1 tube for vs2 there was now a reading at v2 test point. On reading other threads on this forum I decided to set the bias at 0.45v not what the manual stated which is between 0.55v-0.6v at 220v. Because of the one dodgy valve I could only accurately set three of the valves but trimmed v2 with v1 valve and left it at that. Again all was fine.
Decided to replace all the stock Chinese valves with Electro Harmonix, the sound difference was astonishing. Leftfield blew us all away even the wife. The bass now... well my eldest son wants to take the amp to his local drum and bass venue and show them what a proper system sounds like. Again over the moon. Almost forgot to mention biasing the new valves, could only get down to a minimum of 0.52v and v2 seemed to glow a little brighter than the rest even though the reading was the same as the others. ( I have been checking readings every couple of days and no further adjustment has been necessary ).
The other evening whilst listening at very low volume there was a loud crackle from the speakers, my wife said she saw fireworks from the valves. The following day I changed the fuse and turned on , the valves started to glow then after about ten seconds v2 valve emmitted a vivid white light the speakers crackled and the fuse went . Before removing the back for another look I put the stock Chinese tubes back in and fired up again, this time all was fine. A couple more days of listening whilst thinking there must be something wrong with v2 when v1 decided to give a display on stat up.
I would be grateful for some constructive advice on whether I should be able to muddle through this or if there is someone in Cornwall England I could take the amp for repair.
  Reply With Quote
Old 1st February 2009, 07:51 PM   #2
Hi_Q is offline Hi_Q  England
diyAudio Member
 
Hi_Q's Avatar
 
Join Date: Oct 2007
Hi Jaks, sorry to hear about your 100b, this is supposed to be a very well made amplifier. Looking at your thread I am left with the belief that you have had an unfortunate episode with the KT88's.
If the bias voltage had disappeared, I would have expected significant hum and the large plate (Anode) inside the KT88 would have glowed a very bright Red or even Orange. The fact that the original spark appeared inside the KT88 makes me think that it had an internal short.
Although these valves should be good for 800V, I wonder if the arcing was due to full HT, around 500V, being present instantaneously before the valves warmed up. You mentioned burning around the bias resistors? Are these marked with the colours Brown,Black,Black (10 Ohms) or Brown,Black,Red (1k Ohms)? This will ascertain whether it is the cathode monitor resistor or the Grid bias feed resistor. It would be easy to suspect the coupling capacitor feeding V2 but then again, I would have expected a cherry red glowing anode. It concerns me that most valve arcing problems seem centred around V2 and a poor joint on the output transformer here might cause large spikes well in excess of 800V to appear on the valve anode. Likewise check that your speaker cables are in good condition as an intermittent open circuit here would not be liked by the valves. Also can you confirm that the amp was set to UL mode or was it set to Triode mode? Don't ever operate this switch by the way when the amp is in operation as this could also produce very large harmful spikes to the valves.
I still have my money on faulty valves at the moment as you can imagine, it is not easy to remote fault find your amp but only to give a few pointers. Personally I would try to set the valves to 55mA and run them just slightly cooler but that is my preference.
You can run the amp with V2 removed and measure for correct negative grid volts on pin 5 and check it is steady. Remember that from the top side, pin 5 will be the 5th pin anticlockwise from the spigot key way. If you are not sure about doing this yourself then DON'T as there will be 500V lurking on other socket pins!
Maybe someone on the Forum can give you any info about the present day quality of KT88's as I do not use them in my amp and have never had to order any.
Hope you can get it sorted OK
Les
  Reply With Quote
Old 2nd February 2009, 09:55 PM   #3
jaks is offline jaks  United Kingdom
diyAudio Member
 
Join Date: Dec 2008
Hi Les,

Thank you very much for help.
I have taken the back off and checked the resistors with a meter, they all give the same reading that is printed under each one on the board. Ihave followed the circuit to where the original burn happened. X marks the point of the burn where the two resistors are adjacent to each other on the board . ... v2 socket terminal 4.....3w 180ohm resistor ...X....3w 10 ohm v2 socket terminal 8. Looking closely again at the burn, there is possibly a bad solder between the 180 ohm resistor and the wire leading to the switch that alters from ultra linear to triode mode. If this is a bad connection, would this cause the problems I am having?
I have checked the speaker cables and connections and they are ok. I have not checked the voltage on pin 5 for I have no fuses left , will have to order more.
Thanks again and I hope this message makes some kind of sense. All the best John..
  Reply With Quote
Old 3rd February 2009, 12:20 PM   #4
Hi_Q is offline Hi_Q  England
diyAudio Member
 
Hi_Q's Avatar
 
Join Date: Oct 2007
OK John, Yes it is worth checking out anything that could have caused your problem and a dry joint on the Output Transformer primary side is not a good thing to have. It may have been the cause of the original crackle .
Make sure there are no carbon deposits left on the board that may cause tracking later and yes, those two resistors should never touch LOL!

Hope you marked the possible dodgy valves just in case but I find my mind in conflict here as I want to think it was a bad valve yet it seems to happen in V2 position all the time and this after running quite happy for a time. So check all soldering around V2, look for rouge wire hairs, any solder blobs, dry joints etc.

I do not have a 100B (it's on my Xmas wish list but Santa does not call here much more ) but I have a circuit diagram to drool over. I have the lesser MC10L which has given faultless service for over 3 years and now owes me nothing, but you have to ditch the Chinese valves, which you appear to have done. All the best of luck John, don't be tempted to increase fuse values, the amps do not need it, mine is still running on its original one after all this time.
It's a pity I am in Watford as I could have otherwise popped over to help.
Cheers
Les
P.S. Always allow a good cool down time in between switching amp on and off. Failure to do this causes large surges (a characteristic large thump from the mains transformer) because the control bias voltage takes a little longer to establish. The valve heating time provides this necessary time delay.
  Reply With Quote
Old 3rd February 2009, 09:44 PM   #5
jaks is offline jaks  United Kingdom
diyAudio Member
 
Join Date: Dec 2008
Hi Les,
Again thank you so much for you help and advice, I am hoping you can help with a couple more questions before I replace the cooked valve and brave turning on.
Since reading your last message I have been looking for anything suspicious around the v2 socket. Armed with my 55watt hunting lamp and magnifying glass I have not found any obvious problems, apart from the one possible dry joint that I mentioned earlier , which under the bright light does not appear to be that bad after all. But under close inspection, the 3w 10 ohm resistor (terminal 8) has slight discolouration and a very small bulge around the centre two black identification bands. Do you think this needs replacing even though the meter reading is displaying the correct value and could this be the cause of the tube failure?
Cheers John
P.S I do hope this is the reason for the problems rather than having to assume faulty tubes.
I might have to take up your kind offer of taking a look, even if it means driving a couple of hundred miles or more each way ....... only joking...... but thanks for the offer.
  Reply With Quote
Old 5th February 2009, 01:45 PM   #6
Hi_Q is offline Hi_Q  England
diyAudio Member
 
Hi_Q's Avatar
 
Join Date: Oct 2007
Hi John, the resistor problems depends on the type fitted. Usually they now fit film resistors designed to blow open circuit if overloaded rather than the old wire wound types that would simply light up like a fire bar and burn a big hole in the board. You could be crafty here and substitute the same resistor from one of the valves on the other channel. If the fault travels to the other channel then you know the resistor is not right. Normally I would recommend changing the thing if you are in any doubt but finding a replacement may be tricky, hence the transplant suggestion. My biggest concern is that the intermittent fault is loading down both the mains and output transformers and thankfully the fuse has so far protected these. Intermittent faults like this are normally due to a shorting electrode in one of the valves. You could of course try swapping over valves from channel to channel in the hope of forcing the fault to appear on the other side, this would definitely show up an unreliable valve. This is where a valve test rig would be useful, something on which you can heat up the valve then check for inter-electrode shorts. Getting back to the bulge in the resistor, it could be showing an overload at this point but the film track has just remained intact. If it went open circuit I would expect the valve to cease functioning but then if it was intermittent then perhaps other nasties might occur like very large transients at the output transformer. So I think in the absence of a valve test rig, it might be worth trying to make the fault move over to the other channel by swapping the valves and resistor. Just an idea.
Les
  Reply With Quote
Old 7th February 2009, 08:26 PM   #7
jaks is offline jaks  United Kingdom
diyAudio Member
 
Join Date: Dec 2008
Hi Les,
Thanks for the last post, I have taken on board your suggestions and I would go with your idea of transplanting the suspect resistor to the other channel to see if the problem swapped sides, but the company that I purchased the valves from have agreed to take them back, test them and replace under guarantee if found to be faulty. So as a precaution I have ordered replacment resistors for both channels, both the 3w 180 and the 3w 10. (They must have taken quite a beating in that initial short). I am hoping that after the valves have been checked the report comes back as, one faulty. So at least the resistor change is only for precaution.
I will let you know if this is successful when I have fitted the new parts
All the best John
  Reply With Quote
Old 23rd February 2009, 07:11 PM   #8
jaks is offline jaks  United Kingdom
diyAudio Member
 
Join Date: Dec 2008
Hi Les,
The kt88’s were put on a test rig and v2 was found to be faulty and a replacement sent. Changed both 10 ohm and the 180 ohm resistors for v2 only. Turned on and after a few crackles from the left channel speaker all seemed fine, the amp was on for just over an hour with no problems. The next time it was turned on (several hours later) smoke billowed out next to v1 socket and the fuse blew. The weird thing was after all that smoke there was no visible evidence whatsoever on the board, the resistor values were spot on according to the meter. The only option I had left was to replace the 10 and 180 for the v1.
Since changing the v1 resistors there have been no problems so far ( just over a week ) so I am hoping the troubles are over.
Again thanks for your advice and all the best John
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 09:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2