RM22 Power issue

[crazyeddie]

After finding out my RM22 has run away bias on one of the tubes they were replaced and the amp given a clean bill of health. In my original post about the amp shutting down I though after checking the bias settings that the tubes were the culprit.

My amp tech replaced the tubes and found a higher than normal plate voltage of 390 volts. Adjusting the bias to a lower setting of around 11mv set the amp in a decent range that wouldn't fry my tubes after a week of use. I was worried if the amp would be weak and sterile but my tech assured me the amp sounded great.

Got the amp home and he wasn't kidding, the amp sounded fantastic. But after an hour of playing the amp shut down like it did when I first got it. I have a video posted on YouTube showing what the amp is doing. Skip to 1:55 to see it shutting down: http://youtu.be/FNJdVpcB9io

Thoughts anyone??

 

[crazyeddie]

Looks like I Stumped the Trunk on this one.....

 

[Jaded Faith]

I'm super busy trying to catch up right now, but I did look into this for you over breakfast today. Unfortunately the RM22 schematic I have does not include the power supply or audio boards.

Looking at page one, I saw nothing that could really contribute to this. My hunch is the power supply page would render some clues if I could get it. If the tranny is like the ones on the larger amps, there is a thermal reset inside them. If your amp has something wrong with the B+ running too high I could see it running hot and showing your symptoms to the tee.

Did the tech truly check readings in the PS?

 

[crazyeddie]

My tech said he opened up the chassis and went through the amp with a scope and discovered the abnormally high plate voltage. As a matter of fact he was concerned about crossover distortion and when checked with the scope he said the crossover was minimal. He didn't actually say he checked voltages from the PT but after finding the irregular plate voltage you would think he would have checked the PT. I'll be taking the amp back to him in a couple of days and will be sure to ask.....

 

[crazyeddie]

This is a strange coincidence. While moving the amp a small piece of paper fell out of the back of the amp. It reads "RM22-PT Built -In Thermal Break."

 

[Jaded Faith]

I think we may have the problem isolated. If he is reading abnormally high voltages, something is failing to create that state. The bad part needs to be identified and replaced. I'll bet the transformer is going into thermal shut down, which looks exactly like your video example. If that sticker got hot enough to fall out of the amp, that's probably a tell-tale clue as well. If possible try to get the amp to the point where it's shutting down and feel the tranny. I'll bet it's too hot to touch.

 

[crazyeddie]

And so the saga continues??

I have good news and bad news (well, kind of?) Good news - I'll be getting my amp back this week. Bad news, my amp tech couldn't repeat the failure my amp was experiencing. Not exactly good news, but I had the same problem on two different occasions and I sure as hell don't want it to rear it's ugly head again. So let's look at the history of the problems my amp has had since I purchased it from ZZsounds in March. Incidentally I didn't know I was buying damaged goods, so to speak. The website stated the amp was an open box unit but was new and carried a full warranty. I bet the unit was purchased and returned because of the problem it is currently having. I can't prove this but just in case anybody decides to make an "open box" purchase on ZZsounds, buyer beware?

Anyway I had the amp a few days before I fired it up. After about an hour of play the amp shut down. I thought the tubes could be bad and sent it out for repair just in case. One of the tubes had runaway bias and were replaced. The plate voltage was high, around 380 volts, but with a lower bias setting all seemed to be good. I got the amp back and it was doing great for about an hour. Then it shut down again. Arrrrggggg!!! So I got out the iPhone and I took a video of the amp shutting down (a link is posted earlier in this thread.) The amp wasn't even being driven and was only in stand-by mode and it kept shutting down every 90 seconds or so. I decided to let it cool down assuming it was a thermal related issue. I re-checked the bias about an hour later and it was in spec. I called my amp tech. He was puzzled. I took it back for service again. The day he got it he had it hooked up to a load and had a signal on the input for about 6 hours and it didn't shut down. He tried that for several days. Still no shut down problem.

And here we are. I'll be getting my amp back and I will put it through it's paces and will see what happens. I think a real world test will have the amp shutting down again. And if it does what the hell do I do then? :-( To be continued?.

 

[crazyeddie]

I haven't had a chance to fire up the amp since I got it back. My mother just passed recently so family matters have been my focus. When things settle down I'm gonna give the RM22 a serious workout....