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About cronie

  1. DC300a biasing

    Good news Amp fixed!For those still following this thread.After a medical problem I got back at the amp. I was barking up the wrong tree,well sort of.Being there was buzzing in both channels I thought it must be a bad part common to both channels . After replacing the opamp power supply components a resistor, a capacitor a couple of diodes and a zener diode to no avail I started looking elsewhere.I started fiddling with the input and offset pots when the buzzing stopped. Whatever part was bad failed completely. Now I had crystal clear music from the left channel but occilating dc voltage on the right. I'm not sure which part fixed the amp because I used the shotgun approach. I replaced the neg predriver,the pos predriver,the biasing transistor on the power board and three small transistors.I rebiased as instructed in the crown service manual and reset the input and offset pots.Now I have no hum or buzz and neglible dc at outputs. I think the bad part might have been Q201 but just guessing.I think it was feeding back to the good left channel.
  2. channel down

    I just bought two ua739s off ebay and they both work fine.The part joust mentioned is also listed.But its good to know youre off and running again.Cronie
  3. DC300a biasing

    Well,I finally got back at the amp-Twenty transistors out,twenty transistors cleaned and checked and twenty transistors put back in with new silicon pads.What a time I had getting all the thermal paste cleaned out of the holes.This did not fix the amp but I can rule out bad mica gaskets. My next assault at fixing the amp will be new zener diodes and caps near the opamp. Cronie
  4. DC300a biasing

    I bought this amp off ebay. The guy who had it had it serviced to get the hum out. New isolators were put in with thermal paste.It didn't fix it so they gave up and decided to sell it as is on ebay.I bought it very cheaply knowing about the hum and decided to take on the challenge of fixing it. so far it is kicking my There is quite a lot of thermal paste around the transistors.could the thermal paste itself become conductive? I think I read somewhere it might. I also changed the opamp to no avail.
  5. DC300a biasing

    First,thanks for the reply.I also found out myself by looking at the dc300 type 2 schematic.Right now im pulling my hair out trying to eliminate 1/4 volt 120 cycle buzz equal in both channels. So far ive changed the bridge rectifier,the "beer can" caps with known good ones. Also changed the caps and diodes near the 42ac feed to the main board. Now im fiddling with the bias.I think Ive eliminated all the "easy" solutions-loose screws,broken ground straps,loose wires etc..Its funny-gains off it hums,gains in middle-no hum and gains full hums again. I can adjust where the hum appears and disappears using the input pot but cant get rid of it totally. But if the input pot is set according to spec it hums quite loudly. Any ideas?
  6. DC300a biasing

    Im a bit confused.Im trying to set the bias on my dc300a according to the service manual.It calls for .310 to .345 mv across resistors 132,232,139,239 on the heatsink board.You do this by adjusting {changing] the resistor at the plugin sockets on the main board. Are those mvs Ac or dc? The manual is a bit confusing. Also do you think the amp gets adequate cooling without those heavy heatsink boards with normal home use? Thanks,Cronie
  7. dc300a guru needed

    First , I have a hewlett- Packard 412A analog Vacuum tube volt ohmmeter which i love and is extremely accurate. I pulled an output transistor from the good channel and it read like the crown manual said it should read. I pulled all eight trans from the bad side and six read identical to the known good transistor. One of the others read .4 eb and it should have read "large" like the others.The last transistor read 10ce and should have read "large".like the others. Im not sure what scale I had it on but It was very sensitive to the touch. For comparative purposes the scale shouldn't matter. Each transistor was measured twice each leg-each polarity. These 2 transistors may be ok but i would rather have eight that read the same. Thanks for listening-Im still new to transistors and am trying to learn as i go. I have new isolators ordered. Cronie
  8. dc300a guru needed

    carlos or joust--is the2n3773 a drop in replacement for th rca 62085 which i have now or do I need to to replace all eight with the same number. Two of the rca 62085 transistor read a little different out of circuit and I would like to replace them. The 2n3773 is readily available. Thanks Cronie
  9. dc300a guru needed

    I just had all ten transistors out of the ch2 board.Plus two transistors out of the good side to compare to. They are the mica ones with the tab. I wiil change the isolators. Channel one would also have distortion on it on startup but would go away after about two minutes. A year later the same happened to ch 2 but would not go away. So far I have swapped the large beer can power cap and retightened the screws to see if it helped- same symptoms. Then I changed the 4 nonpolar timing caps and two other white vertical caps on the main board. Its funny when i was doing the ohm tests on the transistors i was wondering if the dow compound was conducting. It was smeared on pretty heavy. There is one more large yellow cap on the main board is this the hv cap? I will replace it also.
  10. dc300a guru needed

    just replaced the neg and pos drivers rca3626 with 2n5804s. The rcas were almost totaly shorted emitter to collector. No more -60 at output. Original problem still exists. I have crackling distortion at ch 2 speaker terminal - shows up as about 1 half to one volt dc. I wiggled everthing and tapped on everthing to no avail. Carlos, you may remember my original problem from a post years ago. any ideas ? cronie the distortion problem is what I was trying to fix when i was changing caps - my first post. Also, what is the replacement for the rca 61061 predriver and the rca62085 output transistor? Thanks again Cronie
  11. dc300a guru needed

    Well,Igot a 2n4929 from american microsemiconductor thru amazon. I soldered it in but before I powered up I clipped 5 wires connecting the main board to the ch 2 board. The common brown,two feedback wires and the yellow/white striped and purple/white striped.I think this totally separates the boards. I powered up and still -60 at the speaker terminal. I also have 9 volts dc coming from the yellow/white wire from the main board. I will replace the rca 61061 next and then start pulling output transistors. So much for the "easy" fix. Something that joust knew all along LOL. I will look at the neg rail first and maybe bid on a broken dc300a on the bay right now for a parts stash.I think this repair is a little over my head but I will give it the old college try. Thanks for listening Cronie
  12. dc300a guru needed

    Id like to start by replacing the ss7304 just to see what happens. Information about this transistor seems to have vanished off the face of the earth. I called crown and they told me to call ae techron who still repairs the dc300a for information about a substitute. I will do this as a last resort but for now my research is telling me a 2n4929 or 2n3637. Any ideas? also, has anyone ever dealt with UT source holding company limited for parts? The site didnt seem to work very well. Thanks Cronie
  13. dc300a guru needed

    Alain,No blown fuse but continuous 63 volts at output. Have service manual from crown website. was afraid to measure current- maybe further damage? Update- got the magnifing glass out and brite light and found a hole burnt thru the case of the ss7304 transistor. Now the parts hunt is on and maybe ill get lucky yet! or not! The heatsink got hot at the time of the accident- I think the current limiter protection might have kicked in. Any thoughts will be appreciated. Cronie
  14. dc300a guru needed

    First,thanks for the replies joust and karlos. I was hoping to get lucky and fix the amp "easy" using the "sniper" approach.Well its not working. I checked every part of the good channel against the bad channel and they read the same. I will probably have to use jousts textbook approach which was so nicely outlined to me. Tedious but surefire. karlos-the resistors checked fine and yes im sick to my stomach over my mistake. I bought another dc300a off "the bay" and it works fine. Now I have time to properly work on the amp. And in case youre wondering the positive predriver is a rca 61061 and the negative predriver is a ss7304-the one that touched the front panel. The 2 drivers are rca3626 and the 8 output transistors are rca 62085. It may be awhile but I will post my findings. thanks cronie.
  15. Well, I accidentally blew out channel 2 on my dc300a.I had just finished changing a couple of caps on the main board and was trying it out when the faceplate accidentally touched the heatsink on the corner transistor on the main board. A loud spark and now 60volts dc at the output. Goodbye speaker and goodbye channel 2. Where do I start? Did I avalanche the whole side? The diodes on the ch2 board read ok. As do the resistors. Should I start with the predriver transistors? I have traced signals thru guitar tube amps sucessfully but transistor amps seem too confusing to me. Should I send the amp out to be fixed? Any help will be appreciated. Thanks, Cronie