| Author | Message | ||
     Seth Lorinczi (slor) Username: slor Registered: 05-2011 |
I'm about to dig into my bandmate's early-model (with 12AX7) 2475-130 combo amp. All in all it's a really solid and good-sounding unit, but lately channel 2 sounds about 30% quieter than channel 1. I'm guessing there's an issue with an op amp in channel 2, but if anyone has an EDUCATED guess I'd love to hear it. This also brings up the issue of general upkeep: Besides any old electrolytics that might be hanging around in the amp, are there any components I would expect to swap out during routine maintenance? Many thanks, in advance! | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Check the bypass line on the Fetzer valves. Don't forget to use ball bearings. -Gordon Liddy | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
And here I thought MM's used roller bearings! Seriously, a general checkup should include cleaning and re-tensioning the sockets, all the eletrolytics changed, including the ones on bias supply and driver boards, maybe removing and cleaning the op amps sockets, and then scoping the inny and outy to check for any breakup along the way. I like to replace the big carbon comps on the sockets with some 5 watt cement resistors or 5 watt flameproof but that's me. Then clean and spray a little caig corrosion killer on the input sockets and see what she sounds like. You might have less output on channel two just from a corroded jack if he doesn't use it much. Mike. | ||
| Seth Lorinczi (slor) Username: slor Registered: 05-2011 |
Interesting....thanks for the tips. I cracked open the chassis today and it appears that at least one plastic cap was swapped out at some point (I was in a hurry and didn't mark down which one, but it was the only modern Mallory in the house). Other than that, there are old electrolytics on the bias board, so I'm assuming the filter caps are similar vintage. I'll change them all out. As for the volume drop, the amp has been in regular use, so I don't think it's jack corrosion (though I'll burnish the jack for good measure). From my limited experience, seems like a bad transistor would fail completely or become noisy, not pass less signal. But I can't think of a more likely place to look, so that's my plan for now. Again, if anyone has a brighter idea I'd love to hear it! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I'd probably look at the gain structure around the ic's in the preamp and see if a cap has taken a crap and gone to ground. IC3 and 4 are for the second channel so I'd look around there and if nothing else, replace every freaking cap around there. MM didn't exactly use top shelf caps and they ARE getting old. Also look at the diodes protecting the input of the ic's and the .1 caps off of ic 3 into 4. | ||
| Seth Lorinczi (slor) Username: slor Registered: 05-2011 |
Awesome, thanks so much for the targeted advice. I'll let you know what I find! | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
C'mon Mike, nowadays it's all ball bearings! And now moving away from movie references you have to be the age of Mike "210" or yours truly (or older) to appreicate, into amp upkeep.... Just my 2 cents: What Mike said, plus maybe a little IC swapping to pinpoint any IC issues. If you swap IC-1 with IC-3 and Channel 2 (reverb/trem channel) is now louder than Channel 1, IC-3 should be replaced. If the swap does nothing, put the two IC's back to where they were and swap IC-2 and IC-4. Again, if you get more volume out of Channel 2 now, IC-4 is defective somehow. If neither swap yields any audible difference, your IC's are probably fine and Mike's threory should definitely be checked out. Don't forget you have two 'spare' IC's to test with in these amps: IC-5 and IC-6 used in the reverb section. Cheers, Harry S. Truman (charge this post to the Underhills' account) | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Lars, roller bearings are IN! (maybe in my bicycle). Sometimes, like Lars says, you just have to start moving crap around as your diagnosis progresses. I have a problem right now with my gigging amp that I have to get down and find and that is some distortion coming along SOMEWHERE in the chain. Scoped it the other day and really didn't find anything that jumped out and screamed "fix me". Didn't have time to do an in depth but now I have to-it pisses me off and I'm not happy with the clean sound(sound great distorted-duh!). Going to have to track it through but it may be something as simple as a preamp valve not liking where it's at. You just have to keep looking. Good tip Lars about having the extra ic's to swap-who the hell uses 'verb anyway? (ONLY KIDDING!) Mike. |