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Yaqin MS-300C Integrated Amp Keeps Shutting Down

Hi, I've been stumped for a while with my Yaqin amp shutting down. Most times when I turn the unit on, I have no output until I touch one probe of my multimeter in test opening, then it comes on. Many times however, I can't even get to adjust the bias for 24 hours as it appears to go into protective mode. After the 24 hour wait, the protection circuit goes off and I can bias the 300B tubes to .8 Volts per the manual. However, after a day or two, the process repeats itself: I have to touch the probe to the test point to start it, or it shuts down for 24 hours. I bought it used, and was told there were few hours on the stock tubes. Could it be the 300B tubes are just not holding their bias? I've used the 4 ohm and 8 ohm taps with several speakers with the same results.
Any help would be greatly appreciated.
hillbilly559
hillbilly559 OP
33 posts
 
When it works, the plates don't over heat or get red hot. It sounded good when working. It's just half the time when I turn it on, and after the green light comes on, there is no sound. When I try to bias it, it will not register on my multimeter, meaning the protection circuit kicked in. I then have to turn it off for 24 hours for the protection circuit to reset itself. Sometimes I can bias the next day, but half the time it enters the protection mode.
I just ordered a pair of Genalex 300B tubes, and will see if those make a difference. 
Would the 5Z3P  274B  or 5U4GB rectifier tube have anything to do with my problem?
imhififan's avatar
imhififan
2,228 posts
 
Would the 5Z3P 274B or 5U4GB rectifier tube have anything to do with my problem?
I think the rectifier tube is dying, my suggestion is replace it first, If after replacing the rectifier tube bring back the amp to life, next step is replace C112, C113 from 330uf to 68uF, it will give the rectifier tube a longer life. You can also increase C122 from 100uF to 220uF for longer start-up time to reduce tube stress.

https://china-hifi-audio.ru/_si/0/37232385.jpg
hillbilly559 OP
33 posts
 
Thanks imhifiman,
I found the original  rectifier tube from my Woo WA6 and swapped it in. It started up today without me having to bias it, and the amp didn't go into protective mode. I'm hoping that was the problem. I should be getting the 300B tubes this week, so I'll see if they work without any problems. Hopefully, I won't have to perform any internal changes, but I appreciate the info in case I do.
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