Raindog Industrial Arts

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  1. Yeah...Now you got an "old graybeard" Vet all over me. I thought it might be moot to discuss the functionality of such an old product but you prove otherwise. I'll be back soon with some measurements. Cheers, Shawn.
  2. The DC300A VI limiter circuit also once known as SPACE is not transparent to the function of this amplifier when it is "not in use". The VI limit circuitry is an integral portion of this amplifier's output bias, signal output and feedback path. If you disable the VI portion of the schematic, the DC300A can not be biased and it will not function properly. When D108/208 are at fault, the entire amplifier's performance suffers. The VI limit control circuit is directly tied to the bias of the output transistors. Also, the negative feedback loop is feed back to the DC300 OpAmp right through the middle of the VI limit circuitry. I think the VI limit portion of this design is being treated like a separate entity when in fact it is embedded deep in the heart of this amplifier's design and function. It is not a separate switch that turns on and off yet it is a feature embedded in the core of the DC300A. I think I will have to provide some performance data based on a known working DC300A with D108/208 substitutions so you folks can really see the effect the VI limit design has on the performance of this amplifier as a whole. I'll set it up and see if I can get some data posted on the net. I feel bad for stirring up *beep*. That is not my intent here at all. I should get the DC300 website running properly and post the data there, so the Crown forums are not full of 1975 technology debates. I'm working on it... Cheers, Shawn.
  3. Hi, I think there may be some confusion here; D108 and D208 are in the BIAS SERVO which in turn becomes part of the VI limiter when the VI limiter is in effect. Faulty D108/208's will affect the bias of the output stage of a DC300A. A poor bias setting will certainly generate distortion even though these diodes are not in the feed back loop or in the signal path. If the output of the DC300A is shorted or driven into a very difficult load for a sustained period, it is very possible the VI limiter circuitry will force too much current through these diodes. The peak operating current of a 1N270 is 325mA @ 25 degrees Celsius! The Central Semiconductor part CDSH270 is indeed a better part for this application as it has a wider operation temperature and the forward surge current is higher. The Central part also has a continuous forward current capability much higher than the “old school” 1N270. If you talk to any true professional service tech that worked on Crown equipment in the 70's and 80's, they will most certainly site these two diodes as one of the known problems with DC300’s. I hope that helps. Cheers, Shawn.
  4. Firstly, and this is important, I think it is very possible at some point in my life I have indeed soldered naked! Red Green is a Canadian Icon! Nice to hear his message is universal. The most difficult part for me to find was indeed the 25uF bipolar radial lead capacitors. I have since found a higher quality version of this cap is made by Nichicon in their ES electrolytic product line. It is near impossible to find a 25uF cap any longer as they are now 22uF. You can certainly purchase Nichicon from many distributors in the US. The 25uF feedback caps are indeed critical as they are found inside the feedback loop of the DC300A. The tolerance was not specified in the BOM and as far as I can tell the devices do not reveal this either. 22uF is the best we can do. Was your DC300A already in working order or did you buy a damaged unit from eBay? Good to see you hanging out with an old friend again. When you were bouncing racks of amps around my Dad was bouncing me on his knee! Cheers, Shawn.
  5. I don't see any 22uF capacitors in my DC300A or on the schematics I have. There are 0.0022uF caps and there 25uF non-polar caps and there are .22uF but I do not see these "22uF's". Perhaps you have a different version of the amp than I? Maybe you are calling your large power supply caps 22MFD. That’s it, you Americans and your old nomenclature? Just kidding… we still have some of that here in Canada. I think it more universal to call them 22,000uF. I mention these caps further down in my post. One of the electrolytic capacitors at the top of the driver PCB has been known to cause problems. It is C5, 10uF radial lead electrolytic. There is an Axial 10uF beside it and you should replace that too. It is C4 but you should keep it at 10uF and raising the voltage of both of these caps is good as Bill says above, 160V to 200V is great. Predominantly the old models had problems with C5. These caps are filter caps for the Op Amp power supply and you can raise the value in capacitance and voltage for this one. The service bulletin from Crown stated that the caps were faulty from a supplier but it may have been specified at too low a voltage rating. If you have the amplifier open, the driver board off and you are servicing it, then for the love of XXX replace the ceramic disk capacitors with MKP, type caps. 200V rating will work nice. There are various ones like C7, C102, C202, C121, C221...that may be all of them, I think. If she's an oldie, you will have a heck of a time adjusting input and output bias with those old dirty potentiometers on the board. Replace them if you can and or clean them well. I have a friend in the UK selling industrial temperature uA739DC Op Amps in the ceramic package. They are not cheap but heck they are NOS inventory and have never been used. Drop me an email and I will hook you up with him. The power supply capacitors hanging off the back of the chassis could easily be replaced by calling Apex JR. and Steve will hook you up with some big caps for the mains. He has all kinds of stock. Google Apex JR. and when you hit his site look for the Hitachi caps as he has posted pics of them. Great Guy, Great Service! The DC300A is an audio classic! It is an iconic semiconductor amplifier. Perhaps the greatest quasi complimentary design ever made, at least it is the most famous for certain. Go buy one and restore it now before they cost $2000.00 broken on eBay. Preserve our Audio history! After a few key points of restoration, you may have a chance to hear what the original designers heard? Not the shrill tweeter squeaking amp we all heard while listening. What'd ya expect for an old bugger of an amplifier? It needs new parts to sound new again. I love my DC300A and I'll be cooking up number 2 this year and I can't wait. I hope to have the website looking like a real web site in a month or two and it should be a little interactive for visitors. I need a little more time. Cheers, Shawn.
  6. Replace D108 & D208 germanium diodes as they are the achilles heel, always leading to distortion and coloration when they go bad, which does not take much effort. I'm working on substituting some modern day equivalents as the original spec'd ones are hard to get and the circuit is still weak after you replace them with new ones. The old part number is 1N270. I already used 1N771 but has even less current capability. 1N34A is in my amp right now and it is a "modern" part but it has not been on the scope yet. It does work fine, for now. *beep* bad spot in a design for a light weight germanium diode. Cheers, Shawn.
  7. Finally I had a chance to put this thing on a scope & distortion meter. The left channel was hard to bias and gave a final result of 1.8%THD. The right channel came more realistic at under 0.05% THD. I have a small tweak to make to the left and it should be fine. Looking back in hindsight, I put in a few hours on this DC300A project and I now regret that I did not match the transistors, especially the pre and driver stages. I'm certain I would have achieved much better results. All in all, it sounds fantastic and I am very pleased with the sound. Just thought I give an update. Cheers, Shawn.
  8. Thanks. That's what I thought. The uA739 OpAmp had a '75 date code. Cheers, Shawn.
  9. Can anyone advise what year my DC300A came off the production line based on the serial number? Cheers, Shawn.
  10. Thanks again! I gleaned all of the data posted on the Crown site and I have been fortunate that a member at DIY Audio graciously sent me all of his service bulletins for the DC300. eBay is good and I have had my eye on it for a while. Perhaps some blokes dropping in on this forum may have some stuff too? Cheers, Shawn.
  11. Thanks. I've had it cranked up for a day now and it sounds wicked. Any insight how I can get my hands on some old documentation? I'd love to incorporate some cool old graphics into to the website when I rebuild it in detail. Regards, Shawn.
  12. Hi folks, I found an old DC300A abandoned in the basement of a downtown bar while I was there doing some electrical wiring. The owner gave it to me and I took it home and listened to it on and off for a year or so. In the end it was just too awful to listen to anymore, so I ignored it for another year or two until I was reading the DIY Audio Site and felt inspired to breathe some new life into an old classic. I started the restoration just this past September and I'm nearing completion. I'm going to ramble on here so if its not your cup of tea just go check out the pictures of it HERE. After stripping it down I scanned the driver board and created a new PCB image using PhotoShop. I did the same for the output board but decided to use the original output PCBs as the copper is as thick as a dime on them. I etched the PCBs at home; tin plated them and drilled them out. I used 1% metal film resistors and 5% silver mica caps where ever I could. The output transistors have been switched to MJ21194's and I believe Crown is using these today on some current designs. The power supply caps were 17,000 uF and have been replaced with 80V 47,000uF Hitachi's from APEX JR surplus. I updated the output transistor compensation scheme to handle the faster On Semi Transistors. I modded the amp to use 100k Alps potentiometers. I stripped the transformer down and coated the iron with a fresh coat of coil winding varnish and I cleaned and painted the bells. The aluminum chassis got cleaned up and I had some heat shields made up to protect the output transistors and save people from electrical shock. I stripped the front down and inserted a temporary piece of brass just to see how it looks. I replaced the plastic uA739 Op Amp with an industrial temp ceramic version I received from a friend in the UK. Basically it is a complete replica using all new components. It sounds fantastic and the bass is not too tight but more deep and full. The high end is no longer shrill sounding. Overall it sounds fantastic. It was a labour of love and I did it because nobody else would. I have many more pictures and some technical documentation that is not obtainable from the Crown web Site. However, I do not have any old brochures or manufacturer documentation from Crown. I would love to include this type of material on my website as I rebuild it into an awesome place to go and learn all things DC300A related. If you have such brochures and Crown documentaion regarding the DC300, I would love to hear from you. I think I can be emailed through the Crown forum here? All the best from a budding hard core amp builder! Cheers, Shawn.