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rrcrain

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About rrcrain

  • Birthday 02/20/1956

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  1. I have an issue with two 10 drivers out of an AR90. The obvious problem is a voice coil lead became disconnected from one terminal on both drivers (same speaker). It can be reconnected, but it is a bit short.
     

    The potentially severe issue is the DC resistance. They never measure over 3.5 ohm. After manually moving the cone, I've seen it drop to zero, OK, .5 ohm to be precise. The voice coil is not rubbing. I checked for a short to the frame and that checked fine.

    What the heck is going on and what are my options?
     

  2. Well, it's been a while, but one of my two amps died. My first thought, because I saw something glow red how in the case when I turned the unit on, was the unit suffered a cascade failure. AKA not cost effective to repair.

    Fall is here, my depression over the event has abated a bit and I decided to open the unit to see how bad the damage was before trashing. No burn marks, no scorched wires or melted or loose connections to be found. Checking further, both 5 amp fuses were blown in the right channel, the channel that failed.

    I decided to check the TO-3 transistors first, and found two that are a dead short between B-C-E. Each was part of a pair, it's matched transistor was good, er, acceptable given it's age.

    Checking further, I noticed whatever repair facility replaced 2 of the 4 NPN transistors in the channel, and used the wrong part number.
     

    The original part numbers are now obsolete (MJ15024 and MJ15025) but two substitutions are available. The better choices for the TO-3 NPN and PNP transistors is MJ21193G and MJ21196G. Mouses strongly recommended replacing all eight if going with these options. They have a slightly higher gain than the originals, supposedly better linearity and a high SOA.

     

    If this fixes the amp, this plus the new electrolytic caps should be a minor upgrade in performance and perhaps extend this old Heathkit AA-1800's life a few more decades.

     

  3. Well, I DID use Kimber wire throughout the project, and did replace all of the old wire. The Alphacore copper foil inductor in the tweeter section did an excellent job as well. Some say capacitors don't wear in or break in, and I have to admit that I was in that camp myself not that long ago. All I can say is that the sound has altered just a bit as the new parts settle into their new home.
  4. The 4700uf monster leaves you little choice other than to use a new electrolytic. I've read some comments that the new oil filled caps are rather good. These might be an option, but I've absolutely no experience with them. Personally, I interpet "oil filled"as "electrolytic" by another name.
  5. I have to politely disagree on replacing the caps in the woofer section. I used a 100uf and 150uf Sloen cap to get the correct value, and the improvement in the bass response is almost beyond description. I'm not used to feeling the pressure of the peddle drum at low wattage hit me in the chest like it does now.
  6. oops! I miscommunicated here. The issue with running hot was taken care of with the new crossovers. I'm looking to replace the power supply caps as a preventative measure before they suffer a catastrophic failure. The rails in these amps are pos and neg 100 VDC fused at 5 amps each.
  7. You'r comment got me to thinking last night about my amps and speakers. It wasn't until last summer that I had rewired the speakers to enable them to be biamped, or that I had even acquired a second amp for the purpose. The AA-1800 is rated at 250 per channel into 8 ohms, 500 watts peak per channel and a bit more than that into 4 ohms. It's only been in the last few years that my old amp started running on the hot side and I had assumed that it was the electronics showing its age. Apparently, I was right, but it wasn't the electronic components that I had imagined. Since electrolytics do dry out with age, anyone know of an excellent cource for large electrolytic caps in the 13,000 uf range? I suspect that I should start looking for neww ones to drop in my amps.
  8. I suspect the real issue is my ears and what I've become used to hearing. I'm not yet used to precise, well defined bass with a distinct report to it when its in the recording. As an added note. I'm using a pair of Heathkit AA-1800 amps to drive the AR90s in a horizontal biamp arrangement. As an unscientific observation, both amps used to become rather warm at moderate listening levels, around 30 to 40 watt peaks. With the new crossovers, they are almost stone cold at the same listening level.
  9. Amazingly, the local shop was able to locate a supplier that has a stock of new upper midrange drivers on hand. Unfortunately, the first replacement that arrived was bad. Based upon the dome geometry, it appeared that the voice coil wasn't in the manet groove. The second replacement was a perfect sonic match to the 20 year old driver; well as perfect as my ears can determine. I'll see if I can hunt up the old inductors and get some reliable DC resistance measurements on them and get the measurements posted later this week. I thought you were going to tell me the bass would become a bit over pronounced, not a dropoff in bass response. I do NOT want any more bass out of these puppies! If it's a bad recording, the bass can be unforgiving. If its a well done recording thats been well mixed, its georgeous.
  10. > Personally, I'm of the opinion that in this >case, "don't fix it if it ain't broke." Soundminded, your advice is valid for anything that works, including speakers. In the case of my speakers, age had taken its toll. Once the new crossovers were installed in both speakers, it became extremely apparent that the upper midrange driver on one cabinet was dead. I first thought was that I had made an error in wiring one crossover and I spent a few evenings tracing my wiring and taking resistance measurements on both assemblies. Once I was convinced both were correct and exactly the same, I swapped the upper mid drivers, and the once dead circuit produced sound. Since I lack a sig gen, I was stuck with having to compare the drivers and used a DMM set to the diode setting; enough voltage to produce a click and nothing more. One driver produced a click, one didn't. I did take the driver to the local shop the next day and confirmed its death. My best guess is that the driver had been dead for several years and had been damaged in one of my moves. Simply put, the voice coil had been forced out of the magnet groove and was resting on top of the magnet face. Amazingly, the dead driver was producing just about as much sound as the good driver with the original crossover assembly. I checked the original capacitors and dicovered that they had drifted from their original value by over 20% and had a high leakage current. Your comment on the bass section though especially caught my interest. I did replace the original inductor with a solen 15 gauge inductor, which if memory serves me correctly is one wire gauge larger than the original. What if any negative change would you expect to hear, and how would you recommend counteracting this change?
  11. Brian, my apologies for not having answered your question sooner. The only parts in the speakers I recycled were the two resisters. New caps, new inductors and wire. The inductors were all 15 gauge copper wire except for the tweeter where I used a Goertz 15 gauge copper foil inductor. Cost wise, the total project was over $200. I paid $200 up front to the local shop to order the parts and over 2 months after the fact, I have yet to be handed a bill for the remainder. Best guess is the total project was done for around $300. I've mentioned this issue to one of the co owners and he wasn't concerned about it. Not included in this cost was the cost of one new upper mid driver. I discovered one was dead, and had been dead for some time. It just wasn't obvious until the new crossovers were installed. BTW, a Heathkit IT 2250 was used to check the new and old caps.
  12. I suppose this could be viewed as a positive or a negative effect of the crossover rebuild. The new crossover revealed that one of my upper midrange drivers appeared to be dead. My first thought was that I had made some sort of wiring error and spent quite a bit of time going over and over their wiring. After I was convinced the crossovers were correct, I decided to order and install a new driver, and original parts are indeed available. Once installed, the new driver had a startling effect, working as designed. Since the old driver is unrepairable, I carefully unassembled it to determine what had happened. What I discovered should NOT be an issue with other speakers (hopefully). The dome had two creases in it directly opposite of each other, probably from a moving accident, that had deformed the voice coil to the point it had popped out of the magnet groove. This defect was not audible with the old crossover since the driver in the other speaker was all but totally dead. Having the upper midrange drivers actually working makes these old AR's sparkle.
  13. I purchased the AR90s new and know exactly how they have been treated; Used but never abused. I didn't realize that the bass crossover was "leaking" high frequencies until after I had rewired the speakers for bi amping. I hooked up one section at a time to proof out my wiring job and heard Shania Twain's voice coming out of the bass section, abet very softly. Simply put, the old caps leak rather badly from age. Also, they flat out will not hold a charge. My bet is that most of the existing AR90s have worn out caps in them by now simply due to age.
  14. The effect was a bit of both. The detail is vastly improved as well as the over all clairty. I wasn't aware that I could not clearly hear the lyrics to some music until I after the upgrade and I was hearing it clearly for the first time. Increased detail, openness and clarity were the subtle changes. The major changes were due to simply to having decent caps in place that now prevented information from going to the wrong section; such as female voices coming out of the bass drivers. Given the effect of replacing the lower mid and bass caps, I do recommend replacing the caps on the lower crossover as well. I did have to use a 200 and 150uf cap in paralell to get the 350uf for the bass section, plenty of room in the cabinet. Tighter bass and precise midrange will be what you get. I've driven these speakers bi-amped with the old crossover and was very pleased with the sound; Brighter and a detailed sound stage. I'll be bringing my bass amp home tomorrow night (power spike took it out) and will see what the speakers sound like bi-amped with the updated crossover. Total cost for the upgrade with new caps, inductors and wire was around $300
  15. The crossover rebuild, the task is done. All capacitors were replaced with Solen Poly caps, even the 350uf in the bass section. I did have to use a 150 and 200uf cap to get the proper value in this circuit. All of the inductors were also replaced with Solen 15 gauge except for the .1uh coil in the tweeter section where I used an AlphaCore copper foil inductor. All wire was replaced with Kimber TCX 15 gauge copper with Teflon insulation and the speakers are bi-wired. Several of the replacement cap and coil values were close but not exact replacement values. Given the old parts were rated plus/minus 10% and the replacement values were no more than 4% off and rated plus/minus 5%, it was felt by Solen and myself that this would have minor if any impact in performance. Also, I did remove the attenuation switch assembly. It's not missed! The changes I'll try to describe are due as much, or moreto the condition of the old capacitors as it is to the quality of the new components. The most dramatic change is the upper mid range driver now being a major contributor to the sound stage. Previously, it was nearly silent with the lower midrange driver performing 80 percent of the work. Female voices are now primarily reproduced by the upper midrange, male voices are mixed equally between the upper and mid. Female voices, such as Shania Twain no longer eminates from the bass section. Overall definition and soundstage has dramatically improved with quite a bit of high frequency detail now reproduced that had been missing. Overall, this was a project well worth the effort and price paid.
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