| Author | Message | ||
     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. ideas, anyone? thanx ed s | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I assume that you have tried a second set of tubes? Also, I assume that your idle current is not off the charts? I haven't been around here much-what amp are we talking about? THat much voltage drop could be driver problem with a cap going to ground or just a crap tube. Although I would think the driver would go poof first. If it only does it with the power tubes in, I'd start there. | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
I agree, if the massive current drain only happens when output tubes are installed then that can indicate bad tubes, incorrect biasing, bias curcuit problem or a bad driver/P.I. circuit. It is also possible that your biasing was set on the old transformer at a time when it was already on its way out (unless the current problem is the reason why the power transformer failed in the first place! It sounds like something on the order of a short circuit or close to it. No popping fuses or red plates? I guess you wouldn't want to leave it on long enough to find out! Steve | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
thank you, steve and mike, for your responses. I really can use a little help with this head scratcher. I was looking around the site yesterday but never did scroll down to this topic (I had forgotten I posted here and concentrated on threads in the "transformer" and other topics higher up on the page). I think steve is indeed right about the source of the fault; it is probably the fault that burned up the secondary winding for the heaters in august; that is if it isn't new damage caused by the original short circuit. when getting ready to wrap it up for the night I ran across a couple of "strange" voltage readings on my DMM. First, my amp's rectifier board is NOT wired as shown in the schematic for 2275/2475 or the representation of the rectifier board found in Service Bulletin #4. the high voltage terminations are found in the series A thru C and the low voltages are in the terminals E thru I. this is a factory wiring job and I really don't know how others' 210 sixty-five amps are wired but mine has always been different from the factory provided information; and it has functioned flawlessly for over thirty years before that saturday in august. anyhow just before leaving my bench I tested points C and B (correspond to points F and G in the schematic/bulletin) and found anomalous voltages....not terribly far out at 354 vdc each, but the fact that they were identical troubled me because there should have been a 20 volt difference in the two. this new datum made me go a little further down the chain to see what else was messed up. I next went to the output of the driver board and checked the voltages output from the PI. here I found something REALLY wrong: both outputs to the pin 5 grid leak resistors were -20 vdc. now, I really have never measured here before but shouldn't one of those numbers be positive to acheive phase "inversion"? I guess at this point I should start looking closely at my rectifier board for the source of this difference--I'm positive this out-of-whack signal voltage is the reason my limiter lights up like a christmas tree with the power tubes installed. if necessary I have a new set of diodes for the thing and, though I tested the filter caps out of circuit with my analog meter I didn't look at either the driver or rectifier boards' electrolytics...another avenue of approach. mike, I have been using two sets of tubes for my testing (and I have a virgin set of JJs for when this problem is resolved; I'm understandably nervous about reinstalling those NOS sylvania tubes though they are still okay I think) so I knew the probability of the fault being in one of the tubes was pretty low. and steve, you are quite right about my not wanting to leave the power on for any longer than necessary--I use clip-probes, get my numbers and shut down fast. losing one transformer is enough to make me real gun-shy about leaving that circuit operating for any more than about 30-seconds. any further input would be most welcome gentlemen (and anyone else who has an idea). thank you for your responses, ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
gentlemen, I found a couple hours to put into my benched 210 sixty-five and came up with another indication of where the problem may lie (the "bread crumbs" are headed back toward the rectifier stage). I followed up on my intention to test my electrolytic caps in the driver and rectifier stages. testing the caps out of circuit with my analog meter I determinied that the rectifier caps were in working order (not surprising in lieu of their newness). such was not the case on the driver board. here, at the junction of R58/R59 and the cap making the ground connection, I found that (polarized e-cap, 150 uF, 50 vdc) C38 was gone--the analog needle pegged with leads reversed to either infinity or zero upon contact with the probe. here the confusing information presented by the schematic (2275-65/2475-65) and bulletin #4 drawing of the rectifier conflicting with the actual wiring crops up. the point "B" connection of cap and resistors appears as a low voltage (signal) junction in the schematic/bulletin but on my board, the actual point B is high voltage dc (360 or 340 vdc depending on whether the actual point represents "F" or "G" of the schematic/bulletin). On my board, too, the connection is low voltage but the source seems to originate somewhere among the points "H" or "G"...point "B" would input 367 vdc here. I am assuming this is where the inverted signal leaves the driver stage to enter the respective tube via pin 6 (tied with the 1.5K resistor to pin 5 and it's connection with the screen grid--?--). this signal would necessarily be low voltage (witness the voltage on the e-cap) and would suffer great damage by being connected to point "B" on my amp. so, given the contradiction in plans and reality, which assumption is correct? I reason that it MUST be a low voltage connection but I don't know if this is so (what blew the cap? perhaps the low voltage with a significant overcurrent? I'm confused as hell at this point as to what voltages I should anticipate in a properly functioning amplifier). I don't even know if my question was intelligible or not. can one of you help unscrew my brain as to what to expect at this end of the driver board...I am becoming ever more certain that the fault originates somewhere on the rectifier board (the incongruent voltages at points "B" and "C") but I would really like to have a complete analysis of my driver stage before I simply replace the blown cap and move on to the rectification.... help? ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
here's another update, gentlemen. it wasn't just cap c38 which had failed, but the whole chain of caps along that branched line (c36 and c37) as well. the film/polypropylene 600 vdc .047 uF caps on both sides also failed as well as the electrolytic cap. I am wondering if this scenario is likely, perhaps one of you can tell me as much or whether I'm barking up the wrong logic-tree. say, one of these three caps failed at some point in the recent past. is it possible that the failure of one cap caused a radical shift in the bias voltage which then further compounded the problem by increasing the current toward overload and taking the rest of the capacitors in that chain out as well. finally, already dealing with large currents as a normal state of affairs, the effect on the heater circuit was increased current to the point where the secondary winding (6.3 vac) burned out on my power xfmr. is this possibly the scenario that fried my power xfmr and remains the source of the overcurrent within my amp? after replacing one transformer, I am understandably nervous but if this line of reasoning is the case, can I then just replace the parts, rebias and be back in business? ed s | ||
| Roy Simison (froggy) Username: froggy Registered: 09-2006 |
Edward I am not really to good at this tech stuff but it seems to me the heater windings are a totally seperate circuit and should not be affected by power supply problems. I had to replace a transformer with bad heater windings even with heater leads disconected from the amp the tranny got hot because of the internal short the rest of the voltages were correct. I am just guessing here but if you check and replace all the bad caps and replace the zener diodes d 5,6 and 17 which serve as regulators you would be a long way twards solving your problems Diodes are so inexpensive I would go ahead and change all of them for safety, or at least check them all.Just the ramblings of a novice---good luck and hope you are back in buisness soon. Roy | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
hi roy, and thanx for your input. I mentioned the 6.3 vac heater circuit mainly because it was the weak link in the system that "gave" when the overcurrent problem developed and took out those secondary windings in my power xfmr. I've since followed the overcurrent around the driver and rectifier boards and have come to roughly the same conclusions regarding a rePAIR strategy that you suggested. I am prepared to lose any number of caps and/or resistors but only one power transformer is included in this years "maintenance budget" :o)...so, slowly and carefully are my bywords for this repair. I have some very strange voltages running through my rectifier stage but the damage was wrought in my driver stage: three caps and one resistor. I am opposed in principle to using the "just throwing parts at a problem until it disappears" strategy, but I'm afraid it has come down to just that--I've purchased twice as many materials as needed to repair the damage. as far as a causitive explanation of the problem, I've reached the "clutching at straws" stage of figuring this thing out. I'm only waiting on some locally unobtainable parts from mouser before I can proceed much further. (and oh yeah, most of us here are novices at "obsolete" audio electronics but lots of us are accepting the challenge to educate ourselves in this area--this is great fun for me, btw.) it is in the latter context that knowing the WHY of this fault that drives me. (perhaps I am conditioned by my former profession as the senior electrician for several industrial electrial construction outfits: big stuff like oil refineries, power generation facilities and such. my former positions and disposition would never have allowed me to turn in results thus obtained--and still keep my job. it would have been totally unacceptable to my former employers to RISK a reoccurance of a problem thus "fixed.") the expense risked here is small enough to justify my use of the scatter-gun approach but my curiosity still gnaws at me something fierce. along with replacing the parts on the driver board, I've decided to take your advice on the rectifier stage. though my diodes have tested out as good, I am going to replace them all. hopefully these measures will solve this problem--even though I might spend the next year trying to understand the WHY, at least I'll have my performance amp back (the POS I'm using these days is pretty hard to swallow after being tonally-spoiled by my vintage stuff). again, thanx for your input, roy. it has helped me come to the decision to just go for the expedient solution. I'll let everyone know the results of my proposed solution to the problem [if THAT doesn't work, I'll proceed directly to the "tearing-out-my-hair-by-the-roots" stage of the process. wish me luck, :o)] ed s | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
roy, "you da man, dude!" your suggestion to also check out the diodes used as regulators on the bias board (D5 and D6) would not have occurred to me to check for weeks yet! (well, maybe not THAT long ...but you saved me some time). I think your note above had a misprint or your amp is substantially different because D7 (paired with D8 on the other tube socket) is the surge protector between plate and ground on the 6ca7's. as it was, of the four diodes on the rectifier board, three were two-way conductors [I replaced the fourth just for "balanced composition" in the "big picture":o)]. have you any tips for getting the big preamp PCB in a position to solder...I see seven screws and obviously need to disconnect the tremolo/reverb pedal circuit from the back side of the amp. any other helpful hints for easily accessing this work area? anywho, thanx again for your helpful suggestions. I am putting them to good use. worked on the amp into the wee hours this morning and got a lot accomplished. I need only mount the poly .047uF caps (in transit) and acquire and install the pair of 1N4745 diodes and I'm ready to begin some very cautious testing. my feelings about cranking in the test voltages are a mixture of excitement and trepidation (as I said: losing one transformer this year was quite enough, thank you!). thanx for all your help folks; ain't this board a great site for exchanging technical information and help! the informal, yet well mannered, exchanges by the users are a great source of information, interesting "gossip" and even humor sometimes. it has that "other" musician's web site beat all to hell when it comes to the relaxed atmosphere without cliques or politics, bad-mouthing, and just plain libelous/slanderous behavior exhibited elsewhere in the internet-ether. thanx to steve for creating and maintaining this place.... ed s I'll send in another progress report once I have finished installing the new parts and have some test results... wish me luck, guys e s | ||
| Roy Simison (froggy) Username: froggy Registered: 09-2006 |
Maybe I grabbed the wrong schematic but it looks like D17 I refered to serves a a regulator for control grid curent.A lot of guesswork here like I said I am a novice but as I understand it Zener diodes are regulators in that they do not bleed off the circuit till breakdown voltage is reached just something I thought might affect the output.I wish you best of luck and will love to hear what really caused your amp to go south Keep us informed all this is valuable information. Roy | ||
| Edward Solberg (edward_solberg) Username: edward_solberg Registered: 05-2006 |
roy, I think we must be viewing a different schematic. I've been using 2275-65 (paired with solid state 2475-65) for all the years I've been puttering around with my 210 sixty-five. my amp (and the schematic) has only 12 diodes (D1 thru D12). when I saw the "D17" in your note I just assumed it was a typo. so, the question remains: "what schematic are you using?" The sheet you want to look at is the one using the tube phase inverter, perhaps that is the difference? I hope to fire the amp up sometime today when I get a little more time and can visually inspect all of my previous work (including another review of my work from the xfmr installation). thanks for your help, pal. ed s | ||
| Roy Simison (froggy) Username: froggy Registered: 09-2006 |
Sorry Ed I guess I was looking at the wrong schematic but the basics should be the same for regulation. http://www.ernieball.com/mmonline/techin fo/old_amps/2100-65.pdf Good luck looking forward to a sucess story. Roy |