cronie

dc300a guru needed

23 posts in this topic

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

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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

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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

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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.

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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

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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

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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

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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

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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.

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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

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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)

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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

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(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

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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

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Hey cronie, I've come once on a 'similar' problem, where the output components isolators were made of mica, and with time, started to be conductive! Solution: replace all micas with silicon or plastic isolators. The mica isolator has a small tab at one of its ends, looks greyish and metallic.

Do you see the described noise at the output of the main IC?

How about the hi-volt biasing circuit? There's a HV cap that is usually very problematic...

Alain

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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.

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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

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Crown has used the 3773 in the past. It seems like a good replacement for the outputs. Perhaps Carlos can concur.

When you say that two transistors read a little funny, what is the difference?

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To be honest im not sure about the 62085.

2n3773 are usually ok, the trouble these days are finding decent original genuine transistors, there are so many fakes about.

use a reputable supplier farnell rs comps etc etc

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Crown has used the 3773 in the past. It seems like a good replacement for the outputs. Perhaps Carlos can concur.

When you say that two transistors read a little funny, what is the difference?

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

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Hi Cronie,

I am currently going through a similar repair process and was interested when I found this thread. I am not sure what is blown in mine yet but I cannot seem to source "genuine" 2n5804 transistors. There is only one seller showing on fleabay and he has unbranded parts. I may not need them yet but I am trying to find a source/alternative for all transistors in the amp. Could you please tell me where you sourced your 2n5804 parts?

I am planning to go with 2N3773 parts for the outputs should I need any replacing. Still can't find an equivalent for the pos. pre-driver RCA61061 though. Any thoughts? For the neg. pre-driver I will use 2n3637, available from Farnell.

Thanks!

Vinny

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