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Old 7th January 2010, 04:50 PM   #1
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Default Rhodes Janus 1 Rectifier

Hi everyone. I'm Tony, a newbie here from Ohio, USA. I'm new at working with electronics, but have done numerous repair jobs with good results. I am in the beginning stages of restoring a Rhodes Suitcase Piano with a Janus 1 100W amplifier. I have procured most of the parts I need to recap and re-resistor the boards, along with some other components, but am having trouble identifying 4 diodes on the rectifier bridge. I'm recapping the entire piano since it is around 30 years old, built in 1980, and none of the components has ever been replaced. It has crackling noise in both channels. My justification for replacing most everything is that the parts don't cost that much and the amplifier is 30 years old. I've searched through all the related forums here and haven't found anything to help. I did extensive Googling searches for the diodes using the numbers on the diodes and the numbers on the schematic, but haven't been able to find any information on them. I guess I'm hoping that some of the experts on here can look at the pictures of the diodes and the schematic and help me identify these components and their specifications and suggest some replacements for them. The only identifying numbers I have for them are "A15A", which is printed on the diodes themselves, and the number "072934" which is printed on the schematic and is some identifier of the bridge itself?? That's all I know about them. Any help greatly appreciated. If this project goes well, I'm going to recap my Marantz 2330B next, which needs all the electrolytics replaced, at least. I want some more experience under my belt first before I tackle the Marantz. Again, thanks a lot for any help. Here are the pictures. If any other information is needed, I will try to answer as best as I can.
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Old 7th January 2010, 05:50 PM   #2
Mooly is offline Mooly  United Kingdom
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Why replace them ? they look like glass passivated types, however any suitable diode will be fine. As those regulators are not on a heatsink the current will be low so a 1N4004 etc will be fine.
I would leave them be
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Old 7th January 2010, 05:56 PM   #3
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If it's crackling on both channels you need to scope the rails (-/+15). It's unlikely both power amps would be faulty the same way... does it crackle with the inputs isolated which you can do by unsoldering one end of R22 (and the same on the othe channel).
You have to narrow the fault down.
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Old 7th January 2010, 05:57 PM   #4
Mooly is offline Mooly  United Kingdom
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And that picture of the regulators... has that 7815 melted
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Old 7th January 2010, 06:29 PM   #5
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I'm guessing your under moderation for your first half dozen posts or so

If that reg is duff... it looks it... then it may well be just that and nothing else. If you replace it check the voltage going to the regs... and how hot are they... if they sizzle it's too hot lol which indicates excessive current draw from whatever they supply.
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Old 7th January 2010, 07:09 PM   #6
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And that picture of the regulators... has that 7815 melted
Thanks for all the input and help. No. No melting. It's just my lousy photo skills. I think the flash reflected off the regulator.

I don't have a scope. 30 year old rectifiers are ok? I found 1 bad resistor on one of the channel boards. A 1 watter. But, that's it. I don't know how much testing and isolating I am able to do. I have found I can replace EVERY single component on all the boards, and the 2 large can filter capacitors, for around $150 getting the parts from Mouser. The keyboard is worth around $3000. They are in pretty high demand for the purists who value the Rhodes "sound'. That's another thing. I don't want to change the "tone" of the instrument. That's what makes it so valuable to others and myself. I'm not selling it, I plan to keep it. I got a good deal on it for $200. It's in really good shape and I may have to just put another $150 or so in work on the action mechanism. So, maybe $500 or so in total. If I were to go out and buy one all fixed up I would have to shell out some serious mullah.

As far as the diodes: I'm more concerned with buying replacements, if I do, with as good as or better specs than what is in there now. But, I don't know what the specs are on the existing ones. I don't know the recovery, amp rating, voltage rating, etc.

I can try to do some isolation work to find the real problem. I don't know how good I would be at it. But willing to learn. I know that's the correct way to do it. Is there any way to do it without having high dollar instruments like a scope?

Thanks for all the advise.
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Old 7th January 2010, 07:29 PM   #7
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Originally Posted by Mooly View Post
I'm guessing your under moderation for your first half dozen posts or so

If that reg is duff... it looks it... then it may well be just that and nothing else. If you replace it check the voltage going to the regs... and how hot are they... if they sizzle it's too hot lol which indicates excessive current draw from whatever they supply.
Hey Mooly.

I honestly thought that it was my camera that made the regulator look like it did in the picture. After reading your post, I went out to the garage and got my magnified headset and came back in to look at it and it DOES look kinda burnt. I never noticed that before. I don't think my eyes are what they once were. I've got a couple new regulators saved in my cart at Mouser because I was going to do the whole board anyway. The only isolation work I did do was to unhook the speakers (2 each channel) one at a time from each channel and it crackled either way, even with no speakers attached to either channel. Thank you BIG TIME for directing my attention to that regulator!! Here's another closeup picture of the regulators.
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Old 8th January 2010, 07:02 AM   #8
Mooly is offline Mooly  United Kingdom
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Let us know if the 7815 cures it. 30 year old diodes should be fine but if you want to change them use 1N4004 or 4007 etc. Just used as old fashioned rectifiers so soft recovery isn't important. They won't have any effect on sound quality here.
PIV... this explains it quicker than I can,
Peak inverse voltage - Wikipedia, the free encyclopedia

current rating of diodes... the 7815/7915 are 1 amp regs (when fitted on suitable heatsing) and with no heatsink the current will be far less, probably less than 200 ma per reg so the 1N4004/7 are perfect.
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Old 8th January 2010, 07:27 AM   #9
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I have never seen a 78xx get that hot before to do that

Cheers!
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Old 8th January 2010, 05:47 PM   #10
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I have never seen a 78xx get that hot before to do that

Cheers!

Thanks for the advise. Is there anything I need to be concerned with then? Is it just a problem with the regulator itself or did/could something else have caused it to melt? The electrolytics will definitely all be replaced. They're 30 years old. Could there be another problem upstream? I will replace the regulator and see what happens. Actually, I'm going to replace both of them. We'll start there and then move on if necessary. I don't know how good 30 year old carbon composition resistors are? I might change all those. But it's true, is it not, that to keep the "tone" and "character" of this piano, that I need to replace the carbon comps with carbon comps? That's my plan anyway. I know they're not the best, and I know electrolytics aren't the best either, but it seems to me when dealing with vintage equipment that part of the value of the equipment, maybe all, is the unique sound that the equipment produces from the old technology components. Would there be agreement on that? If I'm ordering from Mouser, which is what I have to do, then I might a well just buy 4 replacement diodes and be done with it. I can't get anything around my home because there are no electronics stores that sell components left. They all closed up. Well, there is Radio Shack. Heck with them. I'll just do that. That way the whole board is updated.

I'll post back with results and any "new" problems encountered. Sure hope there is none.

Thanks again for the help.

Tony
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