Tom ....what a redo! Have few question....1.) Did anyone ever do a new main board concerning the opamp. 2.) Do you have a parts list that you replaced on your redo.
3.) did you end up burning a board?? I have the crown D300A Series II and have not
looked to see if their is any major difference in the D300 or A. Hope you dont mine the
travel back in tine.......thanks allot for your post.......I heard yesterday HBO is buying
the post transcript for a movie......
Finally I found one Amcron (Crown) DC300A too, which I have been looking for a long time.
This one is all original. Marked RCA outputs, Sangamd filtering caps etc.. a little bit rusty transformer.
Well I had to test it without opening up (a bit of cleaning done before that), just to see how it does (knowing it is very risky!).
One channel working like a charm, other one distorted.
Checked bias, which was missing in faulty channel. Soon I realized there was defective biasing transistor MPSA93 (no connection beetween B-E).
Well, I didnt find exact same type so I took 2N5401 which seems to be "close enough". This transistor has to be high voltage type.
Bias problem was solved and it is spot on (according to service manual).
After that, I just fine tuned DC offset and put speakers + input source. Wow, everything was working! Filtering caps seems to be just fine, it plays many seconds after mains switched off.
Sound quality wise, I cant find nothing to blame. Lows, mids and higs no problems, no harshness at all (except with Alpair 10.3's which has harsh treble anyway). And I have dozens and dozens of different kinds of amplifiers.
Very good bass control I have to say. Amazingly good work back then when this amp was designed! Well done. This amp does exactly what it was supposed to do without any flaws I can say, provided it is in fully working condition. No hissing, no hum at all.
Surely I will change all the electrolytics, zeners etc.. just in case.
This one is all original. Marked RCA outputs, Sangamd filtering caps etc.. a little bit rusty transformer.
Well I had to test it without opening up (a bit of cleaning done before that), just to see how it does (knowing it is very risky!).
One channel working like a charm, other one distorted.
Checked bias, which was missing in faulty channel. Soon I realized there was defective biasing transistor MPSA93 (no connection beetween B-E).
Well, I didnt find exact same type so I took 2N5401 which seems to be "close enough". This transistor has to be high voltage type.
Bias problem was solved and it is spot on (according to service manual).
After that, I just fine tuned DC offset and put speakers + input source. Wow, everything was working! Filtering caps seems to be just fine, it plays many seconds after mains switched off.
Sound quality wise, I cant find nothing to blame. Lows, mids and higs no problems, no harshness at all (except with Alpair 10.3's which has harsh treble anyway). And I have dozens and dozens of different kinds of amplifiers.
Very good bass control I have to say. Amazingly good work back then when this amp was designed! Well done. This amp does exactly what it was supposed to do without any flaws I can say, provided it is in fully working condition. No hissing, no hum at all.
Surely I will change all the electrolytics, zeners etc.. just in case.
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I've been looking for an old D300A [big black knobs] for ages and ages, but they very rarely come up for sale here in Australia, and if they do, they are either totally wrecked, bashed, smashed, and/or blown to bits and just don't work.
One's on eBay are always questionable, as sellers never tell you what's wrong with them, knowing only too well they're totally broken, and always sell them "as is" & "no returns". They're the ones that are generally really best avoided, unless you're a tech.
Even these are never cheap, and the cost of repairs are always very expensive here.
I guess I'll never have one again because of that.
I just can't afford it. Especially when you add the cost of repairs, which can then total the purchase & repair price upwards of $600-$700!
I do have an old [non front panel version] of a D150 though. It's one of two.
The other one went DC and took out both my JBL Decade 36 10" drivers.
You're either lucky, or you not, and because there's not many here in Australia, the ones that do pop up, are best avoided.
But if there is a good one, then because of their rarity here, are just too dear....
One's on eBay are always questionable, as sellers never tell you what's wrong with them, knowing only too well they're totally broken, and always sell them "as is" & "no returns". They're the ones that are generally really best avoided, unless you're a tech.
Even these are never cheap, and the cost of repairs are always very expensive here.
I guess I'll never have one again because of that.
I just can't afford it. Especially when you add the cost of repairs, which can then total the purchase & repair price upwards of $600-$700!
I do have an old [non front panel version] of a D150 though. It's one of two.
The other one went DC and took out both my JBL Decade 36 10" drivers.
You're either lucky, or you not, and because there's not many here in Australia, the ones that do pop up, are best avoided.
But if there is a good one, then because of their rarity here, are just too dear....
I had to comment I’ve read this thread a few times and always enjoyed it.
Oh so many years back I picked up a D150a then overtime I think I ended up with maybe four of them.
Did the basic rebuild on all and they’ve all been good amplifiers a little forward but not terrible.
A few years ago I ended up with a D300a that would go DC occasionally, I tried to get it stable some days it was some days it wasn’t so I got tired of messing with it and gave it to a friend I tried to get it stable some days it was some days it wasn’t so I got tired of messing with it and gave it to a friend.
The same friend loves the DC 300 the early version and I got to admit it doesn’t sound bad I don’t hear anything of the breaking glass signature.
Well I ended up with a black and silver TC 300 a recently, treated the same buddy that amplifier for DC m300a in Matt silver.
Gave it a tuneup fresh caps check the offset tweak that and it seems to be a pretty decent sounding amplifier not harsh a little forward but not harsh.
So I’ve enjoyed the DC300A it definitely has a good low end about it very powerful
sounding.
For me I only use may be as much as 10 W of power, and this monstrosity has been kind of pleasurable. I got to admit I love the looks.
Anyhow that’s about it I read this thread a number of times and I thought finally I’m just gonna comment great work Tom I really enjoyed it
Oh so many years back I picked up a D150a then overtime I think I ended up with maybe four of them.
Did the basic rebuild on all and they’ve all been good amplifiers a little forward but not terrible.
A few years ago I ended up with a D300a that would go DC occasionally, I tried to get it stable some days it was some days it wasn’t so I got tired of messing with it and gave it to a friend I tried to get it stable some days it was some days it wasn’t so I got tired of messing with it and gave it to a friend.
The same friend loves the DC 300 the early version and I got to admit it doesn’t sound bad I don’t hear anything of the breaking glass signature.
Well I ended up with a black and silver TC 300 a recently, treated the same buddy that amplifier for DC m300a in Matt silver.
Gave it a tuneup fresh caps check the offset tweak that and it seems to be a pretty decent sounding amplifier not harsh a little forward but not harsh.
So I’ve enjoyed the DC300A it definitely has a good low end about it very powerful
sounding.
For me I only use may be as much as 10 W of power, and this monstrosity has been kind of pleasurable. I got to admit I love the looks.
Anyhow that’s about it I read this thread a number of times and I thought finally I’m just gonna comment great work Tom I really enjoyed it
RJones.....Dude, how does this happen ...."I ended up with a D300a that would go DC
occasionally" How does it go DC occasionally...did you not go through it,.or did?...I have
the 300A Series II - three of them I got a deal on getting ready to go through.....Before I
use any amp...got to use one of the following. Anyway just what went on with the 300A...
are you still on speaking terms.......Who has custody of the 300A and do you have
visitation rights and do you have the 300A every other weekend....and every other
Christmas?? You cant leave everyone hanging....Is going DC occasionally, like terrible
two's......????
https://www.ebay.com/sch/i.html?_fr...r+Protection+Board+Module+AC+12V-16V&_sacat=0
UPC1237 30A High Power Stereo Speaker Protection Board Module AC 12V-16V
occasionally" How does it go DC occasionally...did you not go through it,.or did?...I have
the 300A Series II - three of them I got a deal on getting ready to go through.....Before I
use any amp...got to use one of the following. Anyway just what went on with the 300A...
are you still on speaking terms.......Who has custody of the 300A and do you have
visitation rights and do you have the 300A every other weekend....and every other
Christmas?? You cant leave everyone hanging....Is going DC occasionally, like terrible
two's......????
https://www.ebay.com/sch/i.html?_fr...r+Protection+Board+Module+AC+12V-16V&_sacat=0
UPC1237 30A High Power Stereo Speaker Protection Board Module AC 12V-16V
My issue.
Had a 300a given to me years ago.
That would randomly go dc.
I mean 50v dc.
No smoke no flames no nothing. It would just go DC. Then like it happened it would stop just sitting there running with nothing plugged in it would do it.
never really figured that out but could never trust it.
Turn it off turn it back on it might be just fine the next time around, I checked every transistor I couldn’t find a problem finally I put it on the shelf and a buddy of mine wanted it so I let him have it full disclosure he knew what it was doing. So it was up to him to work on it or use it for parts.
I mentioned having that problem to a buddy and told him it had been a few years ago and the amplifier was long gone, his response was see below.
********************* his reply *************
silver dipped mica caps leaking around the lva transistor on the main board is what I found that does this
that transistor is the one that is towards the center of the board lower by the IC its like a 151 oR a206
Apparently silver dip Mika’s, the silver can migrate causing all kinds of fun problems.
If yours is going DC I definitely wouldn’t hook it to a speaker you want to keep.
Also I was planning on adding the fuse as crown recommends just to be on the safe side, i’ve seen speaker relays welded together after a failure.
Had a 300a given to me years ago.
That would randomly go dc.
I mean 50v dc.
No smoke no flames no nothing. It would just go DC. Then like it happened it would stop just sitting there running with nothing plugged in it would do it.
never really figured that out but could never trust it.
Turn it off turn it back on it might be just fine the next time around, I checked every transistor I couldn’t find a problem finally I put it on the shelf and a buddy of mine wanted it so I let him have it full disclosure he knew what it was doing. So it was up to him to work on it or use it for parts.
I mentioned having that problem to a buddy and told him it had been a few years ago and the amplifier was long gone, his response was see below.
********************* his reply *************
silver dipped mica caps leaking around the lva transistor on the main board is what I found that does this
that transistor is the one that is towards the center of the board lower by the IC its like a 151 oR a206
Apparently silver dip Mika’s, the silver can migrate causing all kinds of fun problems.
If yours is going DC I definitely wouldn’t hook it to a speaker you want to keep.
Also I was planning on adding the fuse as crown recommends just to be on the safe side, i’ve seen speaker relays welded together after a failure.
Attachments
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Hi all,
I've already opened a thread but unfortunetely I could not get any answers.
Let's see if someone here can help me
I'm refurbishing my CROWN DC300A and I need to replace the output and driver transistors.
In my crown I have the following (as in the attached picture Crown_transistors.jpg):
- Outputs : RCA 62085
- Drivers : RCA 3626
I've been searching around the forum and I've seen people using MJ21194 for the outputs.
However for the drivers, I did not come to a conclusion which ones would be the best.
I know some people used MJ15024 for the drivers, but while the amp will work, distortion and oscillation will be the end result.
Can someone help me?
And since I'm doing this overhaul what other critical components would you recommend to replace?
Thanks in advance
I've already opened a thread but unfortunetely I could not get any answers.
Let's see if someone here can help me
I'm refurbishing my CROWN DC300A and I need to replace the output and driver transistors.
In my crown I have the following (as in the attached picture Crown_transistors.jpg):
- Outputs : RCA 62085
- Drivers : RCA 3626
I've been searching around the forum and I've seen people using MJ21194 for the outputs.
However for the drivers, I did not come to a conclusion which ones would be the best.
I know some people used MJ15024 for the drivers, but while the amp will work, distortion and oscillation will be the end result.
Can someone help me?
And since I'm doing this overhaul what other critical components would you recommend to replace?
Thanks in advance
Attachments
DC300A is designed to produce DC on output. If you have a diesel motor and want to control rpm by hydraulic demand, it is just the device to move the throttle actuator. If you have speakers, this is the wrong thing. As posters previously noted, if the silver migration doesn't get you, oxidation of the zero pot will. they will mysteriously & suddenly produce large DC voltage on output.
This was the amp to have about 1973 for big stage shows. By 1980 the Peavey Cs800 was out, also with 5 pairs output transistors. I was cruising those on ebay this week and those are still going for $2-400. Worth it IMHO, great schematics on line, semiconductor cross table, everything. Worth repairing.CS800 will only go DC out if you have toasted an output transistor. I just upgraded a CS800s with new AC mains caps. As bands have sluffed CS800 rev A,B,C, X, off for class D trash because of the 70 lb weight, your local repair shop might have one they are using as a doorstop. Make him an offer. X & S suffix have a DC detect & disconnect relay internally. A,B,C have a DC detect crowbar triac.
My speakers cost $600, and now that the burglar got them, the next ones might cost $1200. No DC producing amps for me. I don't have an electro-control diesel engine to use one.
About the drivers. Drivers need to be faster than output transistors. In the PV-1.3k Peavey used MJ15020-21 as drivers. Ft 20 mhz. They don't make those anymore. They do make plastic MJL1302-3281. 30 mhz.
This was the amp to have about 1973 for big stage shows. By 1980 the Peavey Cs800 was out, also with 5 pairs output transistors. I was cruising those on ebay this week and those are still going for $2-400. Worth it IMHO, great schematics on line, semiconductor cross table, everything. Worth repairing.CS800 will only go DC out if you have toasted an output transistor. I just upgraded a CS800s with new AC mains caps. As bands have sluffed CS800 rev A,B,C, X, off for class D trash because of the 70 lb weight, your local repair shop might have one they are using as a doorstop. Make him an offer. X & S suffix have a DC detect & disconnect relay internally. A,B,C have a DC detect crowbar triac.
My speakers cost $600, and now that the burglar got them, the next ones might cost $1200. No DC producing amps for me. I don't have an electro-control diesel engine to use one.
About the drivers. Drivers need to be faster than output transistors. In the PV-1.3k Peavey used MJ15020-21 as drivers. Ft 20 mhz. They don't make those anymore. They do make plastic MJL1302-3281. 30 mhz.
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Hi TomWaits, currently I try to repair my Crown DC300A which has 100Hz and harmonics of it as low noise in channel 1. Channel 2 almost quiet.Or do what I did: Tear it all down and build up from scratch?
Good times.
Reading your posts I have seen that you replaced the carbon composite resistors with metal film ones on the new pc board. Since carbon composite resistors are more suitable for radio frequencies because of almost no inductance the replacement with metal film ones may create oscillations of a hi amplication circuit.
So my question is whether your DC300A with the new resistors work fine without rf or any oscillations?
Perhaps measured with rectangle input signal and oscilloscope at the speaker output.
Since I already opened my amp I think of replacing all of the resistors with metal film types.
Thanks in advance for any input.
Hi radiogaga,
I serviced these for years for touring companies, recording studios and home enthusiasts.
I would question your intent to service it on your own. It is a job for an experienced audio technician. For noise, you're just guessing and don't know. There are things I would check, and enhancements that make a real difference, you will not find these on the internet. I'm sure you're very good at your job where you have expertise. Well, same thing for proper audio service and that involves specialised test equipment and knowledge of part characteristics.
Right now you are combating age and the requirement to replace components no longer available with ones having different characteristics. Oscillation is always a risk, and you need an oscilloscope for that. You also need to understand the causes and how to approach each cause to solve or avoid these issues. Plus, you will have components that have deteriorated, and I am not talking about "recapping". Of course that may help other issues, but you must use the correct parts. My point is that this is not a simple failure of a part on a new-ish piece of gear.
My advice? If you want to listen to and enjoy this amplifier, get it into the hands of a skilled audio technician. We are not all created equal unfortunately.
I serviced these for years for touring companies, recording studios and home enthusiasts.
I would question your intent to service it on your own. It is a job for an experienced audio technician. For noise, you're just guessing and don't know. There are things I would check, and enhancements that make a real difference, you will not find these on the internet. I'm sure you're very good at your job where you have expertise. Well, same thing for proper audio service and that involves specialised test equipment and knowledge of part characteristics.
Right now you are combating age and the requirement to replace components no longer available with ones having different characteristics. Oscillation is always a risk, and you need an oscilloscope for that. You also need to understand the causes and how to approach each cause to solve or avoid these issues. Plus, you will have components that have deteriorated, and I am not talking about "recapping". Of course that may help other issues, but you must use the correct parts. My point is that this is not a simple failure of a part on a new-ish piece of gear.
My advice? If you want to listen to and enjoy this amplifier, get it into the hands of a skilled audio technician. We are not all created equal unfortunately.
Good reply....does that apply to the 300A series II or the PS400...I have a 300A II I was going to pull from storage......and saw a PS400 cheap...
thanks Anatech.............
Hi anatech,
I understand your hints. But be sure I AM an experienced audio tech and an EE engineer as well. Further I have measurement instruments which many hobbyist don't have. Therefore I can do all necessary measurments. My intent is to make the amp stable for future use. Carbon resistors do change characteristics over time and/or get noisy. Of course I can not estimate whether the circuit of the DC300A will oscillate when changing to other restistors. It just a guess. Therefore I try to ask somebody like TomWaits for his experience since he did all the replacement.
Just found the reason for the little hum. Pressing the IC uA739 on its socket reduced it to almost zero. Crappy socket I guess or bad solder joint under the pc board.
Lol!
Okay. I'm looking at 5 19" racks of HP / Agilent / Keysight test gear plus other makes. Audio has been my profession for 45 + years and I have rebuilt many various amplifiers. I also have a test and measurement background and went to university. Not your average tech at all.
So to answer your questions. No, using metal film resistors in the DC300A will not cause any instability at all. Solder the uA739A directly, sockets are never good for all kinds of reasons which you no doubt are aware of.
Look at the various characteristics of all the components to figure out what types are best. Be aware modern metal film resistors can have breakdown voltages as low as 50 VDC! This is also a class B amplifier, so lots of HF components where it switches outputs on and off (comparatively speaking). For certain make sure the voltage doubler capacitors are new, very good quality types. Because they are axial and those are being discontinued, you might want to put a spare set aside for later.
If you're concerned, run a Bode plot once you're done. Then add a capacitance across the dummy load to see if it is still stable. As an EE, you know the drill.
-Chris
Okay. I'm looking at 5 19" racks of HP / Agilent / Keysight test gear plus other makes. Audio has been my profession for 45 + years and I have rebuilt many various amplifiers. I also have a test and measurement background and went to university. Not your average tech at all.
So to answer your questions. No, using metal film resistors in the DC300A will not cause any instability at all. Solder the uA739A directly, sockets are never good for all kinds of reasons which you no doubt are aware of.
Look at the various characteristics of all the components to figure out what types are best. Be aware modern metal film resistors can have breakdown voltages as low as 50 VDC! This is also a class B amplifier, so lots of HF components where it switches outputs on and off (comparatively speaking). For certain make sure the voltage doubler capacitors are new, very good quality types. Because they are axial and those are being discontinued, you might want to put a spare set aside for later.
If you're concerned, run a Bode plot once you're done. Then add a capacitance across the dummy load to see if it is still stable. As an EE, you know the drill.
-Chris
Hi Chris,
great that you have this experience and lots of test gear. I feel that you judge without knowing me. I am not an average tech.
I started working with vacuum tube audio, vacuum tube TVs and all kind of silicon audio gear. My background is indeed radio/TV tech befor I went to university for the EE and computer science. Finished I worked until retirement for Hewlett Packard. So I have some of their measurement instruments and a lot of other makes including Sound Technology. I know ST is old and not for spectrum meas but still worth for f-response and general THD and IM and Noise. And I said that not all hobby people have this many meas instruments. But some have of course, especially when being professionals. Anyway, thank you for the valid tech inputs.
I am very leery of the Crown DC-300A as-is. I have the option to acquire a busted one relatively cheaply, and if I actually pull the trigger, I would use it as a jumping-off point for a totally different design without the inherent quirks and gotchas. That means totally different circuit boards, maybe different output devices, too, using the amp basically as a power supply and heat sink assembly.
I've had a fried DC300A in my piles o'crap for years not knowing what to do with it. Stock with that damn uA739 opamp is not an option, waste of time to rebuild it that way. I once thought about getting a White Oak P/L 400 board for it but never got around to it. Both amps are Class B with minimal heatsinking and output are stages very similar. Both amps have their fair share of problems.
Craig
Craig
I also have a monster of a Biamp Labs amplifier lurking around the house (~300W/channel). It has a much nicer design than the Crown all around, with complementary differential input stage and with a lot more protective metal around the power supply and heat sinks.. I originally nabbed it as a consignment item from a musical instrument store, and fixed it by replacing some power resistors that had overheated. I might consider re-tooling it for lateral mosfet outputs, as I have a fair number of TO-3 parts lounging about looking for gainful employment. This might reduce the output power somewhat, but there still should be plenty to go around...
It turns out the amp I have is a Biamp Systems 2400, with 310W/channel. I was also able to download the schematic and operating manual. I will approach the beast with some respect, but will try to find a way of getting rid of those power resistors if I can, as they've always been a sore point in the design.
It turns out the amp I have is a Biamp Systems 2400, with 310W/channel. I was also able to download the schematic and operating manual. I will approach the beast with some respect, but will try to find a way of getting rid of those power resistors if I can, as they've always been a sore point in the design.
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