October 02, 2013 - 04:07 pm|
|i just found one in a dumpster. all 3 fuse caps are missing and upon opening it up i can see why every electrolytic cap is either blown, about to blow, or got so hot the plastic covering has melted!!! they look like those nasty Seimens things i find in old german radios. !! they ALWAYS are ng. i'm first going to re cap it then see if i've got any ng transistors as a result of bad caps!!! AH, but this won't be as much fun as totally re working my Kenwood 1100u!!!!! N1ACK Russ |
April 17, 2013 - 07:55 pm|
|Hi Max, That's a good point about replacing the electrolytics. In my reply, I presumed that he would replace all of them or test all circuits, to assure that they were good. I found that most of the small electrolytics were bad. (Larger ones tend to be more reliable.) You are quite right, that a bad electrolytic could, for example, throw bias off enough to greatly increase distortion. Like you, I enjoy the sound of the 3070. It might not have the power of some big modern amps but hey, like I said above, at low frequencies where you need lots of power, it can kick the ass of any of those EL84 amps you see going for big-bucks on eBay :) |
April 17, 2013 - 06:33 pm|
|Haven't looked here for a while - If Marco B. didn't replace all of the electrolytic capacitors, this would explain why he wasn't happy with the sound, they will all be well past their best before date by now. I've been using my Cortina 3070 more or less daily for the last year now, and still not bored with it in the least. Could use a little more power occasionally, but generally speaking it's more than adequate for whatever I throw at it. |
September 03, 2012 - 07:39 am|
|Hi Marco, Thank you for your kind words. [By the way, I'm the author of this article. Dave edits the "Dave's Lab" section of Tronola.] I understand your wanting to preserve the original amp. It's often hard to decide between preservation and improvement. Sorry that you weren't impressed with the stock sound of the amp. It's a pity that Eico only needed to change a few details, to make it a beauty. |
You already mentioned the line stage noise, which is easily fixed by replacing one transistor per channel. The other possibility that comes to mind, which could audibly degrade the amp, is the tone control issue addressed in the section, "Tweaking the Tone Controls" on page-2. I should also mention that the low frequency peaking, addressed in the section, "Low Frequency Stability Problem" could cause audible problems, depending on conditions. After finishing the work which I did on the amp, I must say that it sounded magnificent. But I totally respect your decision to keep the amp stock. Thank you for your comments.
September 03, 2012 - 06:39 am|
I'm doing an overhaul of a mint condition 3070 that happened to be found in Milano. It's a 220V set, contrary to what is stamped on the back. The only substitution so far has been the two R308 VAS emitter resistors - I always tend to call them "cathode" resistors... - they were supposed to be 330 ohms but reading 400.
Power transistors - as all the rest - are original, idle current is around 50 mA.
Very extended frequency response, but on the other hand I must say I'm not particularly impressed on how it sounds.
The preamp transistors are rather noisy, but I'm going to leave them in place, to preserve its original condition.
Thanks Dave for your help !
July 24, 2012 - 12:29 pm|
|Hi Walt, Thank you for the kind message. I believe that you are right about the case being on backwards, in the picture. I will make corrections to the text. I might say though, that I have decided to keep my unit as in the picture. The top dips down in the middle along the edge of the original front of the cover. I think it will look better with the flanges forward.|
Looking through several Eico catalogs from the sixties, I wasn't able to confirm that they used a W suffix to indicate factory wired. However, if yours is labeled with that, it seems likely. On the ST-70, you could identify a factory wired unit by a computer-printed label, which was applied to the topside of the internal chassis.