 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
Anybody have any thoughts on why the right channel of my amp has an annoying crackle? It's only in the right channel, is present regardless of whether a pre-amp is connected, and is not effected when the amps volume pot is turned down. It comes and goes, and is not present at a steady level. Any ideas guys? Your thoughts would be appreciated!
Follow Ups:
Hey-Hey!!!,
I have a 275 behind me that did what you are describing. Sound form one channel, a bit like driping water, a bit of a sizzling sond to it. Tried swithing all the tubes to no avail. I found a bad cathode bypass cap on the 12AX7, something like a 3volt 100 uF 'lytic. Try measuring the resistance of these caps, as the voltage of an ohm meter is comparable to the actual woring voltage you might see the screwy behaviour. My HP multimeter would not settle down to a range on the bad one like it did with the good one. The 12 uF 250 V 'lytic coupling caps from the cathodes of the 12AT7 to the grid of the power tubes also failed on the other 275 I have. Suspect all 'lytics is the short story.
Short experiment, un-solder the bypass cap from the bad channel's 12AX7 cathode, less gain is all that will happen, and if it's bad you'll have your answer.
BTW, the 12AT7's section to section match is a bit critical for it controlls the actual grid voltage of the power tubes, if you have slightly mismatched power tubes and 12AT7's you can measure and match for better performance. just measure votlage at the power tube's cathodes after you measure the DCR of the cathode winding to determine current, Mc spec is .6 v
regards,
Douglas
Thanks for the suggestions. I'll check it out tonight. The units been working well for a long time and I have no reason to suspect a huge problem. Lytics are always suspect - right. Thanks again.
I did replace those cathode bypass cap's for my 240, which turned to be leakey over the years. The improvement to sound quality was great. But the sound as described by the author is crackle/pop instead of sizzling so the situation might be slightly different.I am not sure whether RB is from Hong Kong and I heard that a local guy bought a MC275 having the same symptom but the diagnosis was bad transformer with short turns, no refund, of course.
Describing a sound without a few shared experiences to establish a baseline is difficult at best, impossible usually. Like, what is this 'Air' everybody speaks of?
Anyway, the cap could be further along its failure or failing slightly differently than mine did. Or it could be something else.
That circuit is far from easy to grasp from its schematic. I wish him(assumption here) luck with its repair.
regards,
Douglas
RB,Probably best to try an isolate the section(s) with the poor contacts.
First: With amp plugged in and speakers attached, no inputs---try slightly giggling each tube to reproduce the noise. If you find one tube replicates the problem---turn the amp off and check the pins of the tube (bent or corroded) and the tightness of the female sockets.
If no tubes will duplicate the problem. Flip the amp over and take off the plate to expose the guts. KEEP THIS IN MIND AT ALL TIMES: HUNDREDS OF VDC! Enough to whack you.
Get ONE cheap all-wooden chopstick. Turn the amp on, speakers connnected. Very low volume (if it has controls). Now, systematically probe the resistors, caps, and all soldered jumpers from the speakers' side of the amp to the input. Include all input jacks and their connections. KEEP THE YOUR LEFT HAND OFF THE CHASSIS (if you are right-handed). You do NOT want to "test the path of least resistance."
Especially, look for poorly soldered or loose ground points.
Be careful, but most of the time this method isolates the culprit(s). There can be more than one poor solder point or bad component, so check the whole enchilada.
Good hunting.
make sure it isn't a used one, soaked full of electrolytes( soy sauce ) which would render it a conductor. Just get a NEW one and keep it dry. Also don't use a pencil, the graphite is a beautiful conductor and will remind youof it if you try something stupid,like using it in place of dry chopstick...
regards,
Douglas
Thanks for the thoughts. That's the first place I went, but to no avail. Guess I'll have to dig a little deeper.Rod
Well, it was a try... If you've ruled out connectors, control knobs, STANDBY switches, and such---it sounds like a bad cap or resistor. One of those with a small internal crack or where the lead has loosened off the body.Sometimes, in old guitar amps which haven't been used for awhile, the resistors and mica caps absorb water from the air. This can muck up the continuity. Just a WAG....
The "chopstick" method is effective. However, the MC275 is capable of > 90W RMS one channel driven. If you find something big by poking around you might send a massive pulse to your speaker. If your speakers are really robust or have protection circuits probably no problems. If unsure, I'd be inclined to connect a dummy load in place of the speaker. Then connect a 50-100 ohm resistor in series with the speaker and parallel this across the dummy load. If you do find something massive, the dummy load takes the punishemnt but you'll still be able to hear it thru the speaker although reduced in volume by the series resistor. It also reduces the startle factor. Radio Shack has everything you need if you choose to go this route.Whatever you do, be sure that there is some load across the amp to prevent potential damage to the OPT.
At tubes or the volume control. Of course, cannot eliminate the possibility of electrical faults. From an electrical point of view, any intermittent faults are quite hard to trace.
The possibilties are nearly endless, but here are the most likely, with the most likely first, least/last: 1: Bad tube-- could be any. 2. Leaky coupling capacitor 3. A carbon composition resistor (most likely a voltage dropper that sources a front end tube) has deteriorated and is intermittently opening and closing, electrically speaking. I've seen carbon comps that looked fine, maybe a bit darker brown than normal, that crumbled upon being touched. 4. Loose tube socket-- again, intermittent contact. These WAGs are based on the original 275, which I owned and loved and thus stared at for hours and hours. I have never seen inside a reissue.
I had the stereo/mono rocker switch on one of my 2100's(ss) do just that. Had similar with the center contact of the rca jack on another. Used De-Oxit many here recommend to clean with great sucess.
nt
www.caig.com
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
