| Author | Message | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
My amp, which has a full cap job, new tubes, new speakers and Ed's modified Paul C mod had been performing flawlessly for sometime now. At practice it was fine, I power it up 3 days later and it has a 60 hz hum, even at zero pre amp volume and master volume. The volume was also very low and distorted. I cant fine any bad connections or joints. The hum is there even with all the tubes out. as soon as you switch out of standby, it pops and hums, in hi power and low. The voltages in hi power mode are screwy The + / - 46 vdc going to the preamp is fine at +/-48 v however, the 725 v is only 680v and the 340 is only 300. Any ideas ??? Thanks | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
I just checked the voltages again all tubes pulled. They seem fine, 730 vdc and 347vdc but there is still as buzz / hum even at zero volume and no tubes. | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
OK...A little more info for ya. I have reflowed all the joints on the rectifier board and the driver board. With no tubes in (no power or inverter tubes) and volume and master at zero it still hums. I'm thinking I may have a bad c44 or c45 100MF filter cap OR a bad rectifier. I have scoped the High Voltage supply and there is a 80v p2p 60hz ripple, ita a nasty looking sucker. Here are some more measurements / observations. Again, no tubes, power on high and volumes at zero. 725 v point H, measures 728v with the large ripple.( 80v p2p) 360 v point G, measures 346 volts Clean...no ripple(I do wonder why this is low) 340 v point F, measures 346 volts CLEAN...no ripple + / - 46 volts points A and C measure +/- 48 v, CLEAN no ripple. So the only place I have the bad ripple is on the 725 supply after C44 and C45 should be filtering it out....unless D9 and D10 are faulty and C44/C45 cant fully filter. | ||
| Andrew Wilson (awilson40) Username: awilson40 Registered: 05-2006 |
Well...It looks like the whole problem was a bad 100uf 450v cap. I just did a cap job on it last year using Illinois Capacitors for these 2. If it cooks another one of these caps I'm gonna try to wedge in some Spragues. | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
andrew, your voltage fluctuations remind me a bit of something that is occurring in my amp (210 sixty-five from 1975, with driver/PI 12ax7). I had voltages that would fluctuate (no, they went lower with no upswing whatsoever) when the power tubes were installed. my B+ would drop into the neighborhood you described but my "G" voltage of 360vdc WAS correct and it was my "F" voltage that was out of whack and also at 360vdc rather than the called for 340vdc. it is the low voltages that are REALLY squirrelly on my amp. all of my positive (+) low voltages are missing but my negative voltages are all present and measure as nominal sums. the removal of power tubes makes all voltages return to nominal values with the exception of the positive low voltages which are still MIA. I have replaced my entire bias section (caps, resistors and diodes) and there is no change. all of these problems began after replacing my power transformer (model 1-65 replaced as per service bulletin No. 4 to a 100-65 along with the necessary changes to the on-stndby-on power switch and the relevant wiring changes this entailed). Since then I have gone through my work many, MANY times looking for a flaw or fault I may have caused.... I wish it was that simple. I have also rebuilt my rectifier board completely and the driver board as well; to no avail unfortunately. I have recently lerarned from another musicman owner of the old type amp like mine with the PI tube that he has had a similar problems since he also replaced his power xfmr (actually, he did his xfmr a few months before mine went on the fritz and required replacement.) one of my first reactions when the new transformer went tits-up, after an analysis much like your own, was to replace an apparently faulty 100 uF cap in my filter section.... I also replaced the caps on the driver board at that time as well because the fault had thrown these way out of spec as well..... another datum: all electrolytic caps in my amp were less than a year old. are you beginning to see a thread of suspicious "coincidence" here? myself, I tend to discount the "coincidence" theory when I begin to see patterns emerging from the disparate bits of data I've gathered. I don't wish to alarm you but I am very interested in seeing whether your amp develops any more breakdowns similar to those suffered by Josh and I. so far, the problems have been very resistant to analysis and resolution. Stay alert, bro. ed s. |
Tue, 03/25/2014 - 09:30
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