| Author | Message | ||
     John Van Norman (johnnyv) Username: johnnyv Registered: 11-2009 |
Greetings, all! I'm new to the forum but have been through this great site end to end! I'll try to keep this short in the hopes someone can send me in the right direction. I'm a former professional musician (back in the 70's and 80's) turned pro audio engineer (live events). About 12 years ago I "inherited" a MM 212HD-150 (chassis #2475-150 built in January of 1980 from markings on the circuit board) from an old friend who had passed. The amp is in fairly good condition, but as I recall, was somewhat of a road-warrior back in the day. Not playing much anymore and on the road a great deal doing sound the amp went into storage in my garage. I think I may have pulled it out and turned it on 3 or 4 times in all those years. I've recently started to return to the playing I've missed so much. I pulled the amp out for a party jam recently and it seemed to work "okay". Very noisy pots, some hum, etc. I brought it home and dismantled it for a through cleaning. Lo' and behold, I found a few modifications! There were a couple of jacks (one on the front, one in the back I hadn't paid any attention to), a pot on the back and three "probe ports" (the kind you stick meter probes in for measurements, two red, one black) mounted on the back panel. Next to those are written the bias spec's in sharpie. Interesting. More on these soon. I went ahead and did a through spray down on the pots and a quick inspection did not reveal any visibly damaged components. I fired it up and noticed a 60Hz hum (even with all pots turned down--this hum's never changes volume). Turning things up and down reveled some rather nasty noises--at higher volumes it turns into a nasty "RIZZ" and the tone controls actually effect the tone of the noise on both channels. I found this website and the research was on! I've gleaned much from this forum. As far as the mods---it appears the additional 1/4 jack on the front panel is an effects loop. It's a stereo jack wired, in part, to the Master pot and channel 2 section of the circuit board. The one in the back appears to be a mono line out wired to channel one. The interesting combo is the bias adjustment mod. They removed the trim pot from the drive board and wired it to a pot mounted on the back panel. The 3 ports I referred to are also wired to the drive board. I guess the idea being to do bias adjustment without removing the chassis from the cabinet--convenient and it appears to work for adjusting both high and low bias. The tubes have been replaced at some point--there are Phillips ECG jc 6L6GC's installed. Through my research I decided to recap and re op-amp the unit, a project I just completed last night. I replaced all the electrolytic's (apparently originals) but not the yellow metallized polyester ones--but my noisy amp is still noisy--though perhaps not as much and with some slightly different tonal characteristics. The low volume 60Hz is still there--and so is the heavy RIZZZZZZ at higher volume (no, NOT, normal amp noise) on both channels--though at mid level volumes it seems pretty quiet. The BRIGHT/NORMAL switches effect the tone of the noise, as well as do the tone controls. I've adjusted the bias--though notice, through monitoring, the readings start out very low and slowly creep up as the amp heats up over the course of an hour or so--I assume this is normal and I did the adjustments after the amp had heated up. It does still float around a bit after that. And one other thing I've noticed. When switching from high or low mode to "stand-by" that 60Hz hum gets much loader and then fades away. Since I'm thinking the hum's not normal I don't think that is either and could be a clue? Disappointing to say the least! My question is, where should I go from here? Any suggestions? Have schematics, meters and pretty good with the iron. Im hoping to bring this amp back up to greatness myself--after all we all need a hobby! LOL! Thanks for your time! JohnnyV | ||
| Bill Traylor (bozzy369) Username: bozzy369 Registered: 02-2008 |
well Johnny v,sounds like you got alot going on here ,first sounds like some modding was maybe done ,so that can lead to tons of things ,but what i'm suspecting is a prob most musicman amps have ,they are very sinsitive to rf freqencys ,so its not really 60 cycle electric hum you are battleing really but probally every radio,tv, florescent lights,and other amps possibly ,it got bad enough that musicman put out a service note on things to help fix the prob,those service notes are on this site.try that i think it will help ,Bill | ||
| John Van Norman (johnnyv) Username: johnnyv Registered: 11-2009 |
Hi, Bill! Thanks for responding! Got to thinking about that frequency so I grabbed my handy-dandy-handheld RTA (I mentioned I was a live event audio engineer and deal with hums, loops, and RF crap all the time). The low level hum is actually 3 different harmonics. 60Hz, a little more of 120Hz and a little less 250Hz. Literally nothing in between. Also, I didn't mention we live WAY out in the Southern California High Desert so close by TV, radio and such isn't a factor. We do all that via sat. I'll have to pull the chassis again and check for the caps mentioned in the Service Bulletin. I do recall, when changing the caps, many of those caps already there. I didn't check for the resistors talked about in a different bulletin that talked about the use of Phillips EGC tubes. Could any of the stuff I mentioned be caused by those old tubes? Thanks, again! Johnny | ||
| John Van Norman (johnnyv) Username: johnnyv Registered: 11-2009 |
Pulled the chassis again--apparently, whoever has been in here before read the service bulletins--all RF precautions have already been installed-- BTW--did I post this in the wrong area? Should it also be in Urgent Help section? JohnnyV | ||
| John Van Norman (johnnyv) Username: johnnyv Registered: 11-2009 |
Hey, Bill--you were sure right about one thing! The mods were part of the problem! I yanked the one that was wired into channel 2 on the circuit board, restored the original circuit path and was rewarded with BOTH channels cleaning up! However. . .I still have the low-level hum when everything is turn down. Any suggestions on that would be welcome! JohnnyV | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Hi John, How are the filter caps and or bias caps? Has anyone suggested that yet? With all volumes down you are still hearing slight hum? Is the grey plate area on the power tubes glowing cherry red at all? Not the filaments, but the grey plate area. If the bias is too hot, you will draw excessive current and the plates will tuen red, and you might hear some hum, but my first thoughts are the filter caps need replacing (Usually every 10-20 years). They dry out and hum and sound bad plus can short out the power transformer if they get to the point where they start to short out, usually the fuses will blow hopefully first. This of course is worst case sinerio  Ed at goforthsound@yahoo.com |