RM50 Combo just blew!

I just took a chance on a RM50 combo from GC online. When I got it, I noticed that it had EL34 tubes (I think stock are 6L6s), the stock Celestion speaker had been swapped for an Eminence Maverick and there is a toggle switch on the back of the chassis near the transformers that is labeled "Hi" and "Low". I assume it is a modded pentode/triode switch but have been afraid to try it until after I have a tech look at it. I have played through the amp a couple of times for about an hour and it sounded great. Tonight, I fired it up on standby for about 20 minutes, then began playing. After less than 5 minutes I heard a pop and the power light went out and the amp died. I could smell a faint burning type of smell coming from the small vent located between the power cord input and the footswitck jack, which is also in close proximity to the modded switch, but I am not sure if the blow out is related to the mod.

Any ideas what might have happened?

No power light, no fan means your power circuit is dead... Hopefully just one or both fuses. But could have more damage... causes? Anything from a short circuit to power line problem to a power transformer failure... I am sure Rob from JF can guide you to find the problem.

I have seen you post questions about this amp several times and the honest answer is one of two things based on your questions and reservations:

1: You should get this to someone qualified to look at it and figure out what's really going on and what the switch is doing.

or

2: Return it while you can and get an amp with less shady origins.

I'm being very honest with you and I fix more of these amps than anyone out there. Do one of the above. There's nothing more that can be done by posting questions on a forum. You need an experienced set of eyes to get inside to go over it or wash your hands of it.

I also forgot to mention that you have blown fuses and this doesn't happen for no reason. Something is wrong and pulling too much current. Replacing fuses without repairing the problem only opens you up to more expensive repairs when more than a fuse blows. All the more reason to get it to a tech.

Jaded Faith said:

Return it

Click to expand...

^ This. It's quite possible you'll spend more time and money chasing a gremlin than it's worth.

Yeah, use that 30 day return @ GC, just tell em it blew on you. It's no questions asked tho, so you don't have to tell em anything really. And, just find another one. Shouldn't be too terribly hard to find these days.

Get Rid of it! Buy One from one of these cat's or Jaded

I've actually been keeping an eye on ebay, craigslist and other internet resources, as well as regularly checking all my local music shops. Unfortunately, I am not seeing RM50 Combos coming up that often. I learned subsequent to buying this one that Rob has one for sale, but even if I returned my amp and got my $379 back, I am sure that I would be well short of what I would need to scoop up that beauty. Believe me, if I had the money, I would do it in a heartbeat! I am going to give this amp at least one shot. I may be able to get GC to cover the service call, as long as it is not too outrageous, because they know that I can still return it and they would just have have it serviced anyway (by the the same guy I use-Jeff at JFK Electronics) in order to sell it.

I dropped it off at Jeff's shop today. Thanks to all of your input and research on this forum, I was able to give him some suggestions for areas to investigate; e.g., he does not have much experience servicing the MTS line and was not familiar with the arcing issue. He seems pretty confident that he can get the amp working properly. He is just about the only tech still servicing amps in my area. He has the local warranty service contract locked up for all the major amp manufacturers, as well as Guitar Center. When I mentioned the amp was designed by Bruce, he said, "Oh, great I know Bruce and can call him for advice if needed". He also said that the modded "Hi/Lo" switch appeared to be wired to a transformer.

BTW, I mentioned in another post that the stock Celestion had been swapped by the preious owner for an Eminence Maverick with a built in power attenuation dial. The dial was frozen up, so I emailed Eminence for advice on how to free it up and they sent me a brand new speaker at no charge! I won't forget that great customer service the next time I am in the market for a speaker! They said the plastic they used in the dials on the initial production runs was discovered to be prone to absorbing moisture, thus swelling and freezing up. They have since switched to a new material.

Jeff is a good tech and he and I were in contact about this in case he needs help.

Wow, that is so great that you guys all know each other! If I lived closer to NJ, I would have brought it to you, but Jeff really seems to know his stuff as well. It is just that he has not had very many MTS amps cross his bench. Thanks for offering to help and for all f the advice so far. The MTS forum is truly one of the greatest communities on the web!

If it doesnt work out let me know, I will be selling my combo soon. keeping the head as I have cabs. I could match your price. Shameless plug I know.

Still no word from the shop, but thanks. GC will not take the amp back now, so I am really hoping it can be fixed!

I got a text from Jeff at the shop yesterday that read, "You're amp is done and it works great!" He did not mention what caused the problem, but I will ask when I pick it up. I am also curious to find out more about the power reduction mod that was wired up to a transformer. Unfortunately, it is going to cost me $200! His work hours don't line up well with mine and it is a bit of a trip out to his shop, so I might not be able to get over to pick it up right away. I have now put a total of about $600 into this amp, which is more than I was looking to spend, but at least I got two of the more expensive stock modules, plus the Eminence speaker with the power reduction dial. It is supposed to take a 50 watt amp down to 6 watts, so that should be cool!

Glad to hear that. FYI, I had to send my RT2/50 back to Randall when I first got it due to one channel having very low output. In that case, it was a disconnected wire. My RM50 needed a new power amp board and output transformer, which I repaired myself. Why did I go through the trouble and cost? Because I love the way the amp sounds and what you can do with it.

The tech said that one of the power tubes failed, leading to an arcing issue with one of the high voltage traces on the board. He was able to fix it and it works great. I assume the board looks pretty knarly now in the area of the short, but out of site, out of mind!

This is work I did on my amp:
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair

arc'ing just about destroyed my first RM50, Randall sent me a new board with some area cut out and insulated around them resistors and also a wire wrap mod underneath.

Yeah, I think Jeff mentioned that he had to cut out portions of my board as well!