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.
July 24, 2012 - 11:14 am|
|I built an EICO 3070 back when I was 13 or 14. I sold it years later. I have no idea if its still in use by anyone, and if my young attempts at soldering skills held up.|
As a stroll down memory lane, I purchased a 3070W. I assume that the "W" suffix means it was a factory assembled unit (correct?). BTW, I am in need of a replacement knob, if anyone has one lying around.
BTW, isn't the pic you have of the rear, showing the case on backward? Should not the end with the bent-edge go toward the rear, filling in most of that open gap?
Much thanks for a great article.
March 26, 2012 - 07:55 am|
|Hi Max, Congratulations on restoring the 3200, along with your 3070! Sounds like you have done a thorough job. On the loudness control, you could simply remove C1 and C2, to eliminate the HF boost. Though the Fletcher-Munson curves have been revised somewhat in modern times, the HF boost at low levels has remained more-or-less similar. Perhaps you mean simply that loudness switches have come to be just a bass-boost option in modern usage. In any case, I guess it is a matter of personal preference. http://en.wikipedia.org/wiki/Robinson%E2%80%93Dadson_curves |
March 26, 2012 - 01:16 am|
|Tuner now perfect. After some use, AFC was in and out - traced to a cold solder. Thank goodness too, really didn't want to have to open RF stage.|
Found a box of #49 lamps. Stereo indicator works properly now. Took apart tuning meter and glued scale back in place with contact cement. It was loose and causing needle to bind.
Very happy with the 3200 and 3700. Next time I order from Mouser will get some low noise transistors for line stage of 3700 as per your mod, it would be nice to kill the hiss completely.
Another useful mod, would be to make the loudness control boost treble less, it appears to follow an outdated curve which has excessive treble boost compared to modern loudness controls, and it limits its usefulness.