Bad tube (I think) took out my MC-100B. 180 ohm 3W resistor fried
After 10 years of flawless operation, and even a couple years of driving a set of Tung Sol KT-120s, my Yaqin MC-100B has let out some magic smoke. I turned the amp on, hit play on my R2R, and walked into the kitchen to make some eggs. Music started playing, I got a few items out of the fridge, and then I heard loud static from the living room, for just about 2 seconds. The only other time I've ever heard something like this is when, a few years ago, a well-used JJ tube made quite a nice little light show while I was listening to the amp. I shut it down immediately then and no damage was done.
But this time, before I could get back to the living room the noise was over. The music slowly died away, as if the power was switched off. I touched the tubes, and V1, V3, and V4 were warm to the touch. V2 was stone cold. Not a good sign.
So I unplugged it, removed it from the rack, pulled out all the tubes, flipped it over, and removed the bottom. There's a nice char mark under one of the 180 ohm 3W resistors on the right side of the V2 socket (Well, right side if the unit is upside down and the volume controls/front inputs are near your body)
Pic:
Here's a close-up of the offending resistor:
So, two questions. 1) What likely happened here? 2) If I replace that 180 ohm resistor, pop in a new main fuse, and replace the KT88 that was in V2, will that likely fix it? I don't see any other parts burned on the board, but are there any others that could be damaged by that resistor burning?
Thanks!
Charles.
After 10 years of flawless operation, and even a couple years of driving a set of Tung Sol KT-120s, my Yaqin MC-100B has let out some magic smoke. I turned the amp on, hit play on my R2R, and walked into the kitchen to make some eggs. Music started playing, I got a few items out of the fridge, and then I heard loud static from the living room, for just about 2 seconds. The only other time I've ever heard something like this is when, a few years ago, a well-used JJ tube made quite a nice little light show while I was listening to the amp. I shut it down immediately then and no damage was done.
But this time, before I could get back to the living room the noise was over. The music slowly died away, as if the power was switched off. I touched the tubes, and V1, V3, and V4 were warm to the touch. V2 was stone cold. Not a good sign.
So I unplugged it, removed it from the rack, pulled out all the tubes, flipped it over, and removed the bottom. There's a nice char mark under one of the 180 ohm 3W resistors on the right side of the V2 socket (Well, right side if the unit is upside down and the volume controls/front inputs are near your body)
Pic:
Here's a close-up of the offending resistor:
So, two questions. 1) What likely happened here? 2) If I replace that 180 ohm resistor, pop in a new main fuse, and replace the KT88 that was in V2, will that likely fix it? I don't see any other parts burned on the board, but are there any others that could be damaged by that resistor burning?
Thanks!
Charles.
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Hi,
The heater wiring above the burned resistor looks a little charred and would benefit from being replaced.
Aleks
The heater wiring above the burned resistor looks a little charred and would benefit from being replaced.
Aleks
Also have a very close look at this 10 ohms (supposedly) resistor lefthand next to the blue .22µ capacitor. This might be the cathode resistor and also has seen excessive current. So my guess is the related KT88 tube might have become gassy, or it's coupling capacitor might be leaky, or the bias arrangement might be faulty...
Best regards!
Best regards!
The wires for pin 3 and 4 are not touching. That's a 100k ohm resistor, and it appears to be fried too. Is that 1/2 watt or 1 watt, do you think? I did have a couple of gassy tubes in this batch (I bought a large batch of Teslovak KT88 tubes and weeded out the gassy/bad ones. Apparently I missed one!).
Charles.
Charles.
Hi Charles,
by it's rings' colours I would have bet on it's value of 10 ohms. If it indeed has been a 100k one (brown-black-yellow), then it most probably is an anode resistor, and there might be also an issue with the PI or last amplifier stage before the finals.
Best regards!
by it's rings' colours I would have bet on it's value of 10 ohms. If it indeed has been a 100k one (brown-black-yellow), then it most probably is an anode resistor, and there might be also an issue with the PI or last amplifier stage before the finals.
Best regards!
I'm looking for some easy upgrades to my MC-100B. I've got it apart on the bench and am waiting for some resistors to arrive, so i can replace some burned out parts due to a gassy tube. While I've got it apart, I'd like to upgrade the amp.
I've found this post on AK: Yaqin MC-100B - not as powerful as advertised, but still a good value. Part 3, final. | Audiokarma Home Audio Stereo Discussion Forums
but never did get my questions answered. I don't really understand circuits, but I can replace parts pretty easily. (I can solder well and have all the necessary tools). I'm basically looking for a step-by-step, replace this part with that part. Not any major circuit redesigns.
From that post on AK, sounds like I can jumper R15 and replace R6. But is that really all it takes? It seems like I'm missing something.
I was also considering replacing all 6 of the .22uf blue box capacitors in the amp with .47uf, as I've heard that helps generate a little more bass.
Thoughts?
Charles.
I've found this post on AK: Yaqin MC-100B - not as powerful as advertised, but still a good value. Part 3, final. | Audiokarma Home Audio Stereo Discussion Forums
but never did get my questions answered. I don't really understand circuits, but I can replace parts pretty easily. (I can solder well and have all the necessary tools). I'm basically looking for a step-by-step, replace this part with that part. Not any major circuit redesigns.
From that post on AK, sounds like I can jumper R15 and replace R6. But is that really all it takes? It seems like I'm missing something.
I was also considering replacing all 6 of the .22uf blue box capacitors in the amp with .47uf, as I've heard that helps generate a little more bass.
Thoughts?
Charles.
I would start by measuring the amp to see if it meets its specifications.
The place to start with all Chinese amplifiers is with the output transformers. You can disconnect the wires going to the speaker posts as well as the center tap to B+ connection on the primary, then use a 6.3V transformer to measure the step-down ratio of the output transformer. Replacing the OT's will be expensive, but it's a big part of the cost savings on your Chinese amp.
As far as the electrical design goes, this is typical of a company that is copying a vintage design and not making some very necessary changes. This amp is way too sensitive on either setting. Get rid of that gain switch and at least go to a 12AT7 front end (will require some resistor changes).
The 68K/3W R9 is a serious performance bottleneck in the amp. What I would do is measure the voltage across the 68K/3W resistor (should be around 5mA), then buy a constant current source kit and set it for 5mA. Instead of just connecting it in place of the 68K resistor, connect the output to the cathodes of that stage and the input to the -70V rail for your bias supply. Next remove R8 completely and replace C5 with a 100 to 500 Ohm resistor (a wire jumper is probably OK too). The CCS will make perfect AC balance in that long tail pair, and using the negative rail that's already in your amp will allow you to dump that coupling cap and its associated issues.
With those changes made, you can experiment with increasing the value of R6/R7 (you now have lots of plate voltage available, yay!), and at the same time decreasing the value of R12 and R13. In fact, you might just try swapping them for kicks.
Another mod that I would suggest would be to remove the triode/ul switch and just leave the amp wired in UL. With that mod in place, remove R12 and R13, the jumper pin 1 to pin 3 and pin 4 and pin 6 on the 6SN7 socket and give it a listen/measure.
The place to start with all Chinese amplifiers is with the output transformers. You can disconnect the wires going to the speaker posts as well as the center tap to B+ connection on the primary, then use a 6.3V transformer to measure the step-down ratio of the output transformer. Replacing the OT's will be expensive, but it's a big part of the cost savings on your Chinese amp.
As far as the electrical design goes, this is typical of a company that is copying a vintage design and not making some very necessary changes. This amp is way too sensitive on either setting. Get rid of that gain switch and at least go to a 12AT7 front end (will require some resistor changes).
The 68K/3W R9 is a serious performance bottleneck in the amp. What I would do is measure the voltage across the 68K/3W resistor (should be around 5mA), then buy a constant current source kit and set it for 5mA. Instead of just connecting it in place of the 68K resistor, connect the output to the cathodes of that stage and the input to the -70V rail for your bias supply. Next remove R8 completely and replace C5 with a 100 to 500 Ohm resistor (a wire jumper is probably OK too). The CCS will make perfect AC balance in that long tail pair, and using the negative rail that's already in your amp will allow you to dump that coupling cap and its associated issues.
With those changes made, you can experiment with increasing the value of R6/R7 (you now have lots of plate voltage available, yay!), and at the same time decreasing the value of R12 and R13. In fact, you might just try swapping them for kicks.
Another mod that I would suggest would be to remove the triode/ul switch and just leave the amp wired in UL. With that mod in place, remove R12 and R13, the jumper pin 1 to pin 3 and pin 4 and pin 6 on the 6SN7 socket and give it a listen/measure.
Also, the deeply discolored 100k resistor is brown, black, yellow, gold on the other channel. It's marked 100K on the PC board right beneath the resistor.
What's PI? What other problems should I look for?
Charles.
What's PI? What other problems should I look for?
Charles.
Replaced all of the 3W 10 ohm and 3W 180 ohm resistors, repaired/replaced the burned wiring, upgraded the four .22uf caps in the signal path to .47uf caps, replaced the burned out blue LED with a red LED, and plugged a new tube in. Powered right up, and I adjusted the bias voltage to .55v for each tube. Hooked up some test speakers and she sung! Completely fixed, I'm really happy! BUT, I'm going to take it back apart and do a little re-wiring. I want the front panel input to bypass the volume control. Looks like a really simple change 🙂
Charles.
Charles.
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