| Author | Message | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
last night I was running late for my gig and arrived after the rest of the band was already set up and impatiently waiting for me so we could start; I was really under the gun to get my shit together and start the first set. normally, I am very methodical in my set up proceedure but last night I rapidly got my amp into the stand, flipped on into standby and grabbed my 335 out of its case. I plugged in and flipped it out of standby and heard that sound every electrician who has ever pulled or spliced a wire dreads hearing on startup--the dreaded "whooomp!" of a dead short. I initially thought fuse and checked. no love: fuse was fine. I then shoved the amp off the front line and set up a POS I keep on hand for just such emergencies. anyway, I didn't get my amp taken apart until this afternoon and began looking for tube problems as my starting point for trouble-shooting...it was really easy to spot in standby; NO HEATERS. at that point (while simultaneously praying it wasn't the transformer...that "whoomp" could spell tragedy). all the voltages checked out except the heater circuit. it was dead short to ground...now I haven't yet removed the start and finish of the circuit from the pins on the 12AX7 because I wanted to ask you good folk before I went any further. do you guys think, from my initial analysis of a dead short power transformer is correct or is this a problem that you have encountered and the fix is otherwise than shucking out the bread for a new xfmr (if I can find one...does Mojo still have 100-65 MM transformers?). c'mon, please tell me I'm wrong; this is one time I would love to be full of foul but false crap. as always, thanks in advance for any help you good folk can provide; ed | ||
| Reed Ohrbom (reedmoh) Username: reedmoh Registered: 06-2006 |
Ed; I'm curious what you've determined/discovered about your "whoooomp"? If you haven't figured it out yet, let us know. Reed | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I'm betting on filament melted in tube and layed over. That would be more likely, too me anyway, that a low voltage winding going to ground. It IS possible however. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Hi Ed, you may have said, but did you take out all tubes and set a meter to AC and check the heater pins/sockets? You may (hopefully) have just a shorted tube, but does the power come on at all (pilot light)? If your transformer's filaments winding is dead and the high voltages are there, the cheapest way to get around this would to get a small filament transformer that has ample current to supply the fils. Unless you really want to keep it stock. Those power tranny's are pricey! You could do this til you get a new tranny if the winding is gone. I don't like to drill holes in a good mint amp, so you could do a cheaptrick and mount it with ATV silicone as a temp solution, sounds cheezey, but works, haha, or tie wrap the tranny in place, till you spring for the new tranny. Sorry, just my early morning ideas going through my groggy head :^) | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
I wish it had been the tube, mike. In fact, the splitter may be fried now but I have since removed the heater circuit from its first stop on its tube travels (the PI) and can get no voltage (expecting 6.3 vdc) reading from the wires themselves... one bright note is that I don't think the circuit grounded inside the xfmr because I don't have either wire coming off the secondary windings with an indication of being short to ground...and now that I've looked a little closer on the schematic, one half of the circuit goes to ground on the first grounding point available in its travels throught the tubes--pin 9 of the PI tube. unfortunately, what my investigations have led me to conclude is that a short somewhere (perhaps within the 12AX7) led to a short being made with the secondary winding of the power xfmr. since I can subsequently get no voltage from that source I have to think the transformer is shot. My major problem is that while I am certain of how I arrived at my conclusion, I am unsure of whether I left something out of my assessment of the situation...like I said one time before, I'm really good with large scale electrical matters (as long as it is the size of an oil refinery or hydro project I'm pretty sharp at figuring these things out) where I am unsure of myself is in the area of audio electronics. I don't know if I left any factor out of my calculations that would mitigate my conclusion of a bad xfmr. did I leave anything out? my next question is equally obvious if my analysis of the situation is correct: does anyone know where I can find a transformer? specifically a 1-65 Music Man power-type xfmr. you know, it is sort of ironic that after running my amp with a relatively "hot" bias of 24 mA per output tube (a set of chinese EL34s) I lowered that factor once I installed my NOS 6CA7s down to 22 mA. I dropped the bias along with giving my sockets a "tune up" of new caps and resistors about three months ago. the amp was running as sweet as I've ever heard her run...and now this hassle (its not as if I don't have other amp projects I'd rather be investing my free time into). oh well, as my DI told me in 1967: "deal with it, solberg." I'd sure like to get my baby up and running sweetly again (partially because I want to begin experimenting with the concept of "voiceing" my preamp with different opamps (or, maybe all of the same type?) in each IC position. anyway, is there an obvious flaw in my troubleshooting /reasoning that is immediately obvious to any of you folk on the board? thanks to you guys for considering my problems, you all usually have very useful ideas or solutions to inject into repairing my amp. ed (the other one) | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Ed, I see that you posted the fils. dead short to ground, but where the tubes all removed when taking the readings? I hope that it is jsut a bad tube, but its possible the winding is gone... :^/ | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
ah, more input! you're right, I didn't try the ac settings...I'll give it a shot and get back to you guys later today. also, ed, your idea for a stop gap measure sounds promising. I was about to just find and order a tranny (I want that amp back, BADLY). I'll try the ideas you guys suggested and get back to you...again, thanx ed | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Ok, I finally re-read where you checked the voltages, and the high voltages are there, great! Be sure you set the meter to ac for the filaments not DC as you posted. In DC you will get nothing, in ac you will see about 4.5 vac in low power and about 6.3 vac. I hope for you that changes matters, But be sure to remove all tubes first. Oh you should be able to get the power tranny from Mojo Musical supply. I really hope this helps... Ed G the other Ed :^) | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
An interesting point I just realized. When in low power the filaments are about 4.5 vac, lower filament voltage is supposed to raise gain in preamp tubes. The effect partly what Van Halen got when using the Variac set at about 90-96vac. The powerubes get a brown(out)effect, causing the powertubes to breakup sooner and warm up a bit, especially with 6CA7's that he used in those Marshalls in place of EL-34's, It also raised the gain on the 12ax7, though the plate voltages where also dropped, lower headroom, more distortion, browner sound. What a tone! I do like the sound of the HD-65/130 in low power better for overdrive Solo's, most MM amp sounds great that way! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Just as an added little idea, if the tranfsormer IS out on the filament side but the rest is ok, why not add a filament transformer? THey're cheap and easily available and would allow you to use the original transformer yet. Just a thought. | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
that sounds like a pretty decent idea guys. the voltage is so weird in that particular circuit yesterday I had 4.2vac on low power and thought: "my problems are over! the elves came in and fixed it last night...." alas, no such luck. half an hour later I was getting 93 volts ac from the same heater wire. I've never thought in terms of a filament transformer, do you think it would be much trouble? sounds like the way to go...my power xfmr is the 1-65 (not the 100-65; most of the houses that have the unit are 100-65s) it may be a little difficult to replace that early model? something funny happened last night. I made a huge mistake of assuming the power was off and the caps discharged (I'd fired it up and discharged so many times yesterday that I just plain forgot where I was and I neglected to look at the switch positions)... so, anyway I was trying to trace this wire on the rectifier board and, while holding on to my cast iron bench top, stuck my finger in to the rectifier area to move some wires. man, did I get lit up...I think I got the whole B+ rocket of 728 vdc up my left arm. man did that hurt like a bastard (not as bad as some of the high voltage, high amps/current shocks I've receive in 23 or 24 years as an electrician....but bad enough to hurt and scare hell out of me). after it stopped hurting and the stars faded away from smacking my head on some conduit in a rack I started kicking my ass but good for violation of elementary saftey precautions....perhaps that gentle reminder has smartened me up and will help me not take some things like safety rules and high voltage for granted. I wasn't laughing last night until long after the tingle wemt away... take care guys, thanks for the help... a little more information on the filament xfmr would be appreciated, have a great day ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
oh, and now you know why they call us electricians: "sparky" e | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
If the rest of the transformer windings are fine and unaffected by the heater winding's problems, you could try isolating those transformer leads and simply substitute a stand-alone heater transformer. I would first make sure that your heater leads aren't shorted to some other secondary winding that could effect the DC supplies. Also check for shorts from the heating winding leads to the chassis or the core of the transformer. If the heater leads are reasonably isolated from everything else then a replacement filament transformer would be a viable fix without having to resort to something much more involved and harder to accomplish. That 93VAC reading is strange, but if it is isolated from everything else it might be OK. If it was truly 93VAC and any tubes were in place they would be toast ("fusible heater"). Did any of the tubes get zorched? Steve | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
excellent point, steve. I'm not sure that voltage anything more than a transient. there may be a connection between the filament windings and the other secondaries, or an intermittent short that occurred. no tubes were visibly fried in this adventure. though I did have a set of NOS 6CA7 Syl and an Amperex 12AX7--I'm gonna be really reluctant to try those tubes again (just in case), once I DO get my new 100-65. oh yeah, I found bulletin #4 about xfmr substitutions. it may be a few days yet before the unit arrives--while not adverse to buying a needed part I'll be damned if I'll pay for overnight shipping! I think I've read elsewhere that the color codeing on the 100-65 is different; if so, I'm sure I'll be eagerly asking a thousand more questions. thanx gentlemen, ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
a question just occurred to me as I was thinking over the implications of installing a new transformer. each transformer will output some value voltage as a percent of the average of 725 vdc (my present transformer generates a B+ of 728 vdc). given that the actuaL voltages WILL vary from the average/norm, it will affect every device within the system of its operation, no? given that fact, I deduce that the change in transformer will probably shape my amp's voice differently (maybe not a lot, but different and conceievably not a change for the better if I get a cold running xfrmr in a system that likes it a little bit hot). are my deductions correct? and if so, will the changes drastically affect the voice of my amp? a man wonders about these things as he impatiently awaits the arrival of his new xfmr. I really like the way my "baby twin" sounds now (well it IS silent now, but you know what I meant) and don't view the possibility of a changed voice with anything resembling enthusiasm. and then again, maybe I'm just over-thinking the thing and worrying for nothing. upward and onward: ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
okay guys, here's the latest installment of the series called "ed's transformer change." I was very stoked last friday afternoon when the UPS guy showed up with my new 100-65-120 power xfmr... or at least I was for the first ten minutes or so. after I freed it from its packing materials I noted an ominous series of numbers stamped on the transformer's bell: 100-65-250. the freaking stock clerk sent me a damned export transformer...wait, it gets better. this being the labor day weekend and the last weekend for people wanting to go camping, gambling, etc. most business establishments are rolled up tight until tuesday morning. damnit, I don't want to wait until tuesday to explain the problem cause now I can't see the xfmr exchange happenining in less than 3-4 days and I will have to wait for the weekend for installation. thus, one more gig using my damned POS backup (my good fender stuff is with my brother and son: a thousand miles away in washington state). at least I have begun to figure out good settings on the backup...the weekend of the 19th I sounded like crap because I didn't know the amp. but damnit, I want my baby twin back! ASAP. ed s | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
HI Ed, sorry to hear of the common BS that seems to happen when idiots like that are too big a hurry to go on their 3 day away. I would complain to the dept manager and demand a discount or reimburse you, your shipping costs, since "you could not gig because of their fault" they should be more than happy to take care of you, if not I would turn them into better business. (Those Bastuds!) | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Is there a way, I wonder if the export can be wired for USA wiring? Maybe Steve or someone else here might know, before you have to send it back and go through all that, maybe there's a way to wire it up so you could. Some of the Fender transformers had multi taps that could be wired up in alternate ways so you could use it here or there so to speak. Some of the MM amps I have seen in German Ebay listings have a voltage select swtich on the back like some Fender export models. Maybe, maybe not, just a thought... Maybe the proper wiring diagram is listed and it can just be hard wired to US specs. I hope anyway for you! | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
According to the schematics, the 100-65 domestic transformer has taps for 125VAC (US) and 100VAC (Japan). Similarly, the Export 100-65 transformer has taps for 250VAC and 220VAC. Unless you have a 120-to-240Vac step-up transformer (or wish to tap into your 240Vac dryer power outlet) you won't be able to do anything meaningful with that Export transformer. That's a bummer... holidays always take a week to 10 days out of normal reality it seems. Steve | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
I have to admit that mojotone responds to customer needs in a timely fashion--they called me at 7:01 pst this morning, so I guess they read their e-mail. I spoke with Jesse (he has something to do with shipping) and he promised to have the xfmer out to me today. I am truly gratified to hear that and I'm glad I didn't bad-mouth the company for a mistake that any of us could have made. one thing does puzzle me though: Jesse said they didn't carry the export xfmr.... but I know for a fact that it was offered on special for $50 last week 'cause I almost bought one by mistake because of the price...it was only by checking the part number and realizing that the input voltage was indicated in the part number that I backed away from buying the 100-65-250. so, needless to say, I was fairly upset when I saw the model number on the unit. the unit is also a bit bigger and heavier--I'd guess more windings because of the different input voltages. I've read that the domestic xfmr weighs c. 7-lbs while the one I have here weighs 8.2-lbs according to the packing slip. so a wise word to fellow parts shoppers: check the specs, check the specs and then check the specs (that goes double for shipping clerks!). I'll update you guys further when there is something positive happening at this end. ed | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
Well guys, my new transformer arrived late yester-afternoon and it IS the correct unit this time. The folks at Mojo Musical Supply have been more than helpful beyond that understandable, though seriously inconveniencing, initial screw up. Maybe shipping the wrong transformer out in the first place has made them nicer people, but I think that is a doubtful proposition (at best)?.their sincerity shows in the way they conduct business over the phone and the internet. Does anyone know if Mojo is still getting their transformers made/rewound by Heyborer? I had a vintage ?twin? xfmr re-wound about a year ago and was really happy with the resulting sound of the OP xfmr (my brother informs me that it still sounds as sweet as the day I loaned him the amp). I have read elsewhere on this site that Heyborer was doing their work after blue-printing Mojo?s last MusicMan transformers? I guess I was wondering if they were still doing the work. Last night I managed to find some time to disconnect and dismount the old transformer? some time later I MADE the time to mount the new unit but that was as far as I got last night. To show a small thing of value about this site, a perusal of prior posting concerning the 100-65-120 transformer saved me a lot of head scratching time when I discovered some anomalous differences between service bulletin 4 and the actual unit from Mojo. One thing that really concerned me was the ?missing? black/yellow wire from the harness. A thread by ?JR? resolved that question as well as my confusion over the new switch wiring. I don?t know how many times I?ve said it but I will reiterate: this web site is a godsend for anyone desiring to understand and work on these wonderful amplifiers. ed (the other one) | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
well, guys. here it is 4 AM on the west coast (it is now saturday morning my watch has informed me though it does feel more like really late friday night) and I've had some promising sucesses with my new transformer. I'm taking it very slow because I can only devote a few hours to it (on the days I've managed to find any time at all) and my pace has helped me avoid an error that might or might not screwed me up...the amp probably wouldn't have worked at all, actually. I misinterpreted the drawing of the voltage divider us folk replacing the 1-65 xfmr are required to install to help with shifting the "on-off" function to the back of the amp and using the "off" poles on the new DTDP for the new "standby" function. the illustration was backward from the text describing the process and I had skipped from a cursory read into building the voltage divider as per the schematic. in the schematic they had represented the switch "schematically" and I made the faulty assumption that up/down on the picture corresponded to the actual switch...I caught it when I was reviewing my "finished" work prior to my first power-up. by the time I had it rebuilt and rewired it had become 3:00 AM and I only had time to establish that all of the required voltages were present and, after installing the tubes (a fresh set), that the heaters actually lit up. viola! nothing to it (he said with the sly grin as if he had sold oceanfront arizona property to the nice old couple from Des Moines). I'll do some more ambitious testing in the morning (after I've slept a couple of hours), but so far so good. however, I still haven't ascertained why the heater secondary winding shorted out in the first place--I don't like a mystery that could reemerge to bite me in the ass a month from now. things happen for reasons and sloughing the winding failure off to "old age" is just too pat a non-explanation to satisfy me. so, I will proceed cautiously and systematically as we test out the new component. later, guys ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
need a little help gents. I've got the 100-65 xfmr installed and every thing is fine with the tubes out: B+ of 741 vdc and every other reading is within acceptable parameters. then when I put the tubes in every voltage drops by half (even the half power drops another half). with all this errant extra current floating through the power stage no wonder the original power transformer burned out one of the secondaries! which brings me to the heart of my problem: anyone got an idea what might be wrong here? I've gone through the amp's power stage with and without tubes and can find no cause for such a fault but obviously something is wrong--anyone got an idea what might have failed within the power circuit? (I did remove the diodes per ed g--but I don't think this caused the problem. the timing is wrong). ideas, anyone? thanx ed s |
Tue, 03/25/2014 - 09:29
#1