| Posted by Steve L. | February 23, 2012 - 01:48 pm |
| Hi Max, Bear in mind that when you replace the drivers, their betas should be matched. Well, the paralleling story is understandable enough. While the typically low feedback in tube amps lets you get away with it, I'm kind of uncomfortable with it there, too. Anyway, nothing to be embarrassed about. All part of the learning process. I had a trying experience with an amp, when I was in high school, too. I was building a SWTPC amp kit called the L'il Tiger, for a friend. It had a circuit somewhat like the one in the 3070. http://www.swtpc.com/mholley/PopularElectronics/Dec1967/PE_Dec1967.htm Due to various problems, we had gone through two sets of output transistors. Finally, we got it all working, and turned it off. I had the cover and a screwdriver in my hand and was about to put it on, when the screwdriver slipped out of my hand and fell into the amp. I thought I saw a little spark, from the charge left in the power supply cap. Breathlessly tried the amp again--sure enough it was dead! A defeat snatched from the jaws of victory! To this day, I'm very nervous, holding anything metal above a solid state power amp! :) |
| Posted by Max H | February 23, 2012 - 12:52 pm |
| I plan to replace all electrolytics in both tuner and amp. Like you I'll get a full set of transistors for both channels, just to be on the safe side. It's a shame the little ones on the driver board will no longer say RCA on them, but at least it should be functional! To be honest, the reason the amp has sat so long, is I am a bit embarassed by how it got damaged in the first place. When I was still in high school, I was frequency sweep testing a speaker, and decided that more power was needed. Why not put my recently re-capped EICO in mono, and parallel the speaker outputs? It worked very briefly, then fuses blew, and goodbye EICO. I knew enough about electronics even then to know it was a bad idea, but I thought hey, you can parallel a Dynaco ST-70, this thing should be able to handle it too! I realized there was some risk involved, but assumed that worst case I'd blow the fuses. New fuses did nothing, and now the voltage measurements were all off. At the time I was more interested in tube amplifiers, so the EICO was put into storage. The tuner stayed part of my system for years after, but eventually it stopped working properly, and was replaced by a Pioneer. I'll worry about the tuner once the amplifier is working. I have experienced what you are talking about with small electolytic coupling capacitors. The worst device I have ever worked on for this problem, is a Phase Linear 4000 preamplifier. It must have 100 1uF capacitors in it, and nearly all of them on the verge of failure. I replaced them one at a time, and gave up on it for the time being. Some day it will be a rebuild project too, but for now I am appreciating the more minimalist EICO. |
| Posted by Steve L. | February 23, 2012 - 08:20 am |
| Hi Max, Yeah, what would we do without the Net? Ironic that the pinnacle of modern technology is such a boon to supporting vintage audio. On the channel with the good output transistors, one would suspect the drivers, of course. In the tuner, the German sourcing is reminiscent of the vacuum tube HFT9X series, in which the RF tuner was a cast-aluminum module, made in Germany. The one time I ventured inside, I found an amazing a 3D sculpture of components. I tip-toed back the way I came... :) I have found that the larger Eico electrolytic caps are pretty reliable, though filter caps might need to be reformed. Sadly, those little 2uF caps (and similar) are a different story. Those are among the few components that I replace en masse. Ceramics should be fine. |
| Posted by Max H | February 23, 2012 - 01:56 am |
| Dug out the EICO. On one channel, both output transistors are shorted. Other side blows the fuse, but the output transistors test okay. Going to order parts before I continue to troubleshoot, but thankfully the design is simple enough there's only so many things which could be wrong. Took the cover off the tuner. Electrolytic filter caps have a date code of 1967. Tuner is interesting - the RF and IF sections are made in Germany, and with only the multiplex board and power supply made by EICO. Tons of capacitors to replace, mostly axial 2uF electrolytics on the multiplex board. Glad you got the transistors from Surplus Sales - isn't the internet great? |
| Posted by Steve L. | February 22, 2012 - 07:26 pm |
| Hi Max, That is a great find! Thank you so much for posting about Surplus Sales of Nebraska. I just ordered ten units for myself. Have dealt with them in the past but didn't look there searching for the transistors. Google didn't find them at the time, either. It's neat that your great-uncle bought your amp and tuner new. Glad that you're going to save it. By the way, there is a Summary of Modifications on page-2, in case you didn't see it and want to make sure you have all the parts (for the things you choose to do). And yeah, those vacuum tube amps don't get any awards for efficiency. But at least in the winter, when the heat is on, you can be consoled by the fact that 100% of the energy goes into heating the house. One might carp that electric heat is more expensive than fuel-based heat but at least the energy doesn't go to waste. (Of course, summer is a different story!) |
| Posted by Max H | February 22, 2012 - 04:50 pm |
| Just ordered 10 units of real RCA 40312 from surplus sales for $1.50 each, plus shipping... their min. order is now $10. I should be able to get a pair out of ten units, hopefully same date code. |