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dc300a guru needed


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#1 cronie

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Posted 09 February 2013 - 01:47 PM

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

#2 joust

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Posted 11 February 2013 - 09:07 AM

First of all you need to access the state of your output devices: remove all the o/p devices and measure for continuity. It is possible that some devices may have shorted out, especially if it was loaded. Second, measure output emitter resistors, those low resistance ones.
Thirdly, now you can verify drivers, pre-drivers, bias, current limitter circuit, etc.
Check low resistance resistors: they're the most likely to have taken the brunt of the accident.
Make a note of the defective output transistors: If one side appears bad, it is likely that the same driver side may have crapped.
Replace components from the input to the output section, refaining from installing output devices yet.
If the driver section is operational, such as no DC at the output, and an appropriate bias voltage is detectable on the drivers, then you can install the newer output transistors.
If the isolators are of the 'mica' variety (looks metallic), replace them for either the 'plastic' or tissue (SIlpad) version. The micas, with time, leak and can generate low level hum or other electrical noises. Lastly, adjust input and output offsets.

Hope this helps!

Alain

#3 karlos the dc300a

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Posted 15 February 2013 - 02:16 PM

Good advice from mr joust!

I would say the transistor you grounded with the front panel will be dead for sure.

If you are really lucky you didnt kill the output transistors.

Dont be too hard on yourself, i have worked on hundreds of dc300a's and done a similar stunt myself, those little round heatsink can get very close to the front panel very easily! we are all human and all make mistakes.

Check the two corner transistors and the one thats almost between the two, no good me reeling off part numbers they could be any of dozens fitted over the years, check the resistors 82 and 820 resistors near lower edge of mainboard.

Obviously look for smoke damage!

good luck

#4 cronie

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Posted 22 February 2013 - 07:13 AM

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.

#5 joust

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Posted 22 February 2013 - 10:04 AM

What current symptoms do you have?
Is it blowing fuses or do you have DC at the output?
If you need the service manual, let me know an address to send it to.

Alain


#6 cronie

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Posted 22 February 2013 - 02:29 PM

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

#7 joust

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Posted 22 February 2013 - 03:06 PM

To help preserve the outputs from damage, remove them! If you still have DC, then it may be easier to troubleshoot. I believe, even without the output transistors, you should have normal signal on the output rail, at least, if the section is not damaged. You could also troubleshoot by removing stages at a time from both sides. Start off with the outputs, then the drivers, etc. Without the outputs, the amplifier should behave normally except it can't drive current! Just a thought.

Alain

#8 cronie

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Posted 01 March 2013 - 07:10 AM

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

#9 karlos the dc300a

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Posted 06 March 2013 - 01:32 AM

i dont think you will find info about the ss7304.

The 2n4929 and 2n3637 were used in later models and i would say they will be fine.

#10 joust

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Posted 06 March 2013 - 12:06 PM

From the Crown Replacement Transistor Guide, the SS7304 is subbed with : D2923-7! And it is still available!! It is a replacement for 2N3637.

Alain

#11 karlos the dc300a

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Posted 06 March 2013 - 03:07 PM

dont waste ordering crown transistors for this repair, 2n3637 were fitted to later dc300a mkII they will be fine for your amp.

From crown parts list: D 2923-7 SERF XSISTOR, 2N3637 175V PNP  B)

#12 joust

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Posted 08 March 2013 - 08:39 AM

My dear Karlos, you have privilege information! ;)

#13 cronie

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Posted 11 March 2013 - 08:44 AM

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

#14 joust

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Posted 11 March 2013 - 10:05 AM

(Still holding my breath, cronie!)

Since you have a -60V at the output it appears that the positive side is locked up and that the -60V is not being pulled up! Check for open resistors also.

Alain

#15 cronie

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Posted 15 March 2013 - 05:11 PM

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