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Another Eico ST 70 thread

NJ -- To answer your questions :

1. 125 PF is ideal. The change was to accommodate using a readily available component value. The difference is not critical.

2. Yes, and important to make sure it's added.

3. Yes.

4. 50 ohms is ideal (two 100 ohm resistors in parallel), but 47 ohms is fine. The change accommodates use of a single component. The difference is not critical.

5. An optional change to provide AC balance. The original values without the pot work well if no AC balance is desired.

6. Yes, and important to do. 4 ohm tap is no longer grounded.

These changes are only for the power amp section, but still allow this section to be completely compatible with the original input section. Therefore, the drive connections into the power amp section remain unchanged.

The modifications allow the Common terminal of the OPT to be grounded (versus the 4 ohm tap as originally grounded), while still maintaining the original feedback factor (18 db). But they also greatly enhance the HF stability of the unit. As a result, where as the original design was only conditionally stable, the modified design displays absolute stability. This can be clearly shown with the excellent and greatly improved 10 kHz square waves the modified power amplifier section will now pass, with the frequency response of this section easily meeting +0/-.5 db to 70 kHz due to the superb output transformers employed.

I published these modifications many years ago on the Yahoo Eico Forum. BuzzK implemented them in his recent ST-70 efforts, and was very pleased with the results. You have now picked them up from there.

There are also simple power supply upgrades that can help as well, but they can be far, far surpassed with the installation of EFB(tm) in this section. I am currently nearly rounding third for home in that effort, and the results are very promising indeed.

For the benefit of those trying to follow along, you might update this thread to glue all of this information together now, so that others can understand and have the benefits that these modifications provide if they wish.

I hope this helps!

Dave
 
As Dave as so excellently summed up as he usually does. I previously PMed him asking about some Mods on the ST 70. I asked him a while back about the Tronola article. He said while it is a good update, he would go about it differently. When I asked what that might be. He pointed me towards BuzzKs build (a beautiful piece which he deleted the tone controls, updated power supply, and added a CCS. ) Dave had assisted Jon in the feedback to accommodate changes do to tone control section being removed. (off the top of my head I don't know the thread, but if you haven't read it and are interested, ask and i'll find it for you). So anyway, Jon, was generous enough, to post schematics of his project . So being the quasi tube nube that I am, just to get a handle on the amp section change (I wanted to retain the tone section). So I sent Dave questions about the changes and unable to attach schematics to PM, they became post 19. I hope my rambling post fills in the blanks. If not please feel free to ask.
 
I have a favor to ask. If anyone have done a rebuild on one of these, I'd like an underside photo. The one I have, I'm not in love with the component positioning and wiring routing and I was looking for a neat example to follow. Thanks in advance.

PS to BlackEmm
While I haven't dug in yet (I have pretty much everything now). My intention is to use Jon's power supply circuit, and power amp circuit with the tone controls in (less the CCS which I understand didn't seem to improve the sound) Oh and Dave's EFB module when it becomes available. If I can find them quick, i'll add them to this post. OK amp section too large
 

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Excuse me Dave, At the risk of exposing my Arse once more. What is the benefit of the AC balance and how would you go about setting it?
 
Oh I already was corrected on the electrolytic issue, for some reason I thought if a cap had a curved line on one side (in the schematic) it was polarized. Seeing it physically and on the parts list cleared it up as well as somebody in an earlier thread pointing it out. Thanks
 
The benefit of an AC balance control is to provide a more complete circuit adjustment to accommodate unbalanced output tubes. The DC balance control allows you to adjust the quiescent current of each output tube for a match when no signal is present. This adjustment primarily helps to maximize output transformer performance, but also helps to maximize tube life as well. However, this is only a static adjustment, because it is made under static conditions.

Output tubes that are balanced under static conditions is no guarantee that they will remain balanced when excited with a drive signal. In fact, more often than not, they won't be. The AC balance control then allows adjustment of the drive signals presented to the output stage, so that the output tubes can be balanced for equal output under dynamic conditions.

With proper adjustment of both DC and AC balance controls, both output tube and output transformer performance can be maximized with otherwise unmatched output tubes.

The best way to set an AC balance control is with a Distortion Test Set or Distortion Analyzer. But setting the adjustment this way also requires proper load resistors, a good signal generator (if using an analyzer only), and ideally, a scope. There are ways to adjust the control without an analyzer or generator, using only a DVM and load resistors, which I have detailed in a number of threads. You can likely find that with a search when or if (if you install the control) the time comes.

Dave
 
That amp has a huge number of resistors and replacing them is a daunting task. How are you going about doing it? Methodically testing them all one by one and replacing the out of spec ones? Selecting the most important ones, testing, and replacing if needed?

I have a box of resistors that will eventually go into my ST70, if I can ever tear myself away from it to work on it some more.

And...any hints or tricks to deal with soldering the lugs that have 5 wires attached? It seems like a difficult soldering task. Just apply heat and wait till they all heat up enough to melt solder? Or go left to right? And I see you have a soldering station. Do you make any modifications to temperature to handle situations like this? (I recently Got Religion and purchased a Hakko soldering station)
 
I am definitely not the expert. I ordered basically a complete set of resistors from the service manual with the addition of part values from the Tronola article and the other st 70 article plus elrctrolytics, couping caps and a few bits. My intention for now is to change all resistors in amp, preamp, phono, and power supply sections and stay away from controls for now. As far as the desoldering station mine might be newer to me than yours to you. I'm still on the learning curve (I am psyched about how it works though.
That would be a lot if plunger and solderwick. I'm definitely open to guidance. I think you're safer experimenting point to point than PC boards. Oh had tragic accident, broke a piece of tab of mini nine pin socket last night. Totally made my night when I was able to extract broken socket contact and replace with one from a socket I had laying around. Saved me from replacing whole socket with some short tightly packed wiring.I just plan on going slow and triple checking schematic and values, do both channels together so the steps and drawings will be fresh in my mind.As far as 5 wire lugs, just take your time planning how everything will fit any shape in carefully. Start with lug as clean as possible. Once your happily with how it fits. Apply heat form one side and solder from other. Make sure you get a good flow and nothing moves before it sets. (I find cleaning tip frequently a must for best heat transfer). Those are all the pearls I have for you. I'm open to correction, criticism, and suggestions
 
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I have a question. Does anybody have recommendation for wire to replace existing chassis wiring. Its sort of ugly in places and melted by some terminations. Also a good source place. I was hoping to get some hi temp solid in as near to original colors for the rework? (I don't know 15-20' each should be more than enough)???
 
nj pheonix said:
Hey Shoc, Do you have a soldering station or a desoldering station? I think I misunderstood.
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It's a soldering station, Hakko FX888D, which I recently bought after reading some posts in the DIY forum.

Now all I have to do is get the most out of it.

I saw one YouTube video which recommends 350-400 C, 660 - 750 F, for almost all work. But the station can go higher or lower; I'm tempted to try a higher temperature setting for difficult stuff like those intimidating 5-lead connections to a lug. But higher T has some drawbacks; the tip oxidizes faster, and flux burns off faster, for example. The experts could set me straight on that.

As for the resistors, yeah, it looks like it just comes down to measuring and replacing all of them. No quick and dirty shortcuts. Trouble is, there's just one more recording I want to listen to before opening the amp back up.
 
Ok I stand corrected. I think I have the same soldering station as you. Also only on my bench a couple of weeks. I kind of go by feel. I would start out at lower temps for PC work (which I haven't done with this yet) point to point more forgiving. I just like to see a nice flow. I suspect some of the more experienced techs can chime in. I'm all ears too!
 
nj pheonix said:
Easy question......Pin 1 on the 7591 is open? No connection to inside of tube Yes?
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I looked at the schematic, and there's no pin 1 shown. I looked at the tube data sheet, and there's no pin 1. Finally, I looked at the tube itself, and there are only 7 pins, and a hole where pin 1 would be. So it looks like there's no pin 1 at all.

How are you doing on the resistor replacement/modifications? Me, I've gotten as far as about half my Haydn symphonies...maybe after that I'll start replacing out of spec resistors.
 
I never thought to look at the tube itself (duh). Through a couple of other sources I came to same conclusion. Thanks. I have most of the power supply rebuilt. I only can spend a couple of hours a night plus because I'm trying to do BuzzK's power supply and amp, and the Tronola phono stage mod. Its not piece for piece. That and my lack of experience is making me double and triple check and sometimes I have to rethink routing. So , in a nutshell slow! I'm not unhappy with what I've got done so far. I'm happy no one is waiting on me to finish.
 
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