Eico ST 70

Any advice for treating the chassis.
After my cleaning, the bright shinny steel chassis is slowly oxidizing, showing a light brown tint.
-Brad
 
Metal polish generally includes a corrosion inhibitor, commonly an aliphatic or synthetic wax.

How did you polish the chassis?
 
Simichrome contains diethanolamide as an anticorrosion coating, but the MSDS doesn't specify much. I don't know how well it works.

You could try polishing it again and then coating with a synthetic car wax to protect the surface since it seems to be prone to corrosion.

Based on the reviews, I was going to try using one of the car resin glazes (BlueMagic TR-3) to seal the surface on my next chassis project. I do not, however, yet have any direct knowledge of how well it works.
 
Simichrome contains diethanolamide as an anticorrosion coating, but the MSDS doesn't specify much. I don't know how well it works.

You could try polishing it again and then coating with a synthetic car wax to protect the surface since it seems to be prone to corrosion.

Based on the reviews, I was going to try using one of the car resin glazes (BlueMagic TR-3) to seal the surface on my next chassis project. I do not, however, yet have any direct knowledge of how well it works.


I did as you suggested with a second round of polishing and a couple rounds of car wax.
Here is a couple shots after I have started to reassembly.


_DSC0172sm.jpg
_DSC0175sm.jpg

Still undecided about what modifications to the original circuit I'm going to do. For sure ditching the center channel, not sure about the tone controls.
 
Great work xbart. I took the cover off my modded ST40 to check an OPT number for another thread here, and was reminded of how pitted my chassis is, and also how I had to clean the sockets and replace those carbon comps to get mine quiet. But the chassis is built like a tank, e.g. that bottom plate with its 13 securing screws.

I'll be interested to see how you go with those tone controls and associated PECs. (Mine are marked Sprague 29751.) If you exclude them, you lose part of the goofy charm of the multiple controls, but probably gain sound quality. Another route is also have an input direct from from Tape 1 and Tape 2 inputs to the driver tube grids, for a power only option.
 
Great work xbart. I took the cover off my modded ST40 to check an OPT number for another thread here, and was reminded of how pitted my chassis is, and also how I had to clean the sockets and replace those carbon comps to get mine quiet. But the chassis is built like a tank, e.g. that bottom plate with its 13 securing screws.

I'll be interested to see how you go with those tone controls and associated PECs. (Mine are marked Sprague 29751.) If you exclude them, you lose part of the goofy charm of the multiple controls, but probably gain sound quality. Another route is also have an input direct from from Tape 1 and Tape 2 inputs to the driver tube grids, for a power only option.

My path might be directed by my last obstacle, the volume, balance, bass and treble potentiometers are all really bad. I spent a good amount of time working on them, but don't think they will be usable.
I didn't see much in the way of new for replacements for the bass and treble.
 
Those ganged / concentric controls are virtually impossible to replace, especially since they must match with the knobs.
The treble control is the Achilles heel as it also has the power switch AND is located in the lower corner - easy to get knocked into.
Maybe you could get some from a donor unit, or from someone who's ditched them entirely for something different. Alternatively, you could be the next one to try something entirely different!

Btw, I always love your pictures, and the chassis looks fantastic. Here are a couple of the one I just finished -
 

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when I did my HF85 pre I was faced with the same pot problem.
No originals available.
So I went to AES for regular dual pots of correct values but needed to use shaft extensions.
So now both chs adj. rather than individualy which is fine by me and everything works fine.
Also shitcanned the loudness sw. Much better now. Dont needed it.
Had to replace knobs with AES new black ones with set screws as the spline shafts went with the old pots.
Very not original but better looking IMO.
judge for yourself.
I rebuilt the tone PECs with discreet parts on perf bds. Much better.
 

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Since it's been a few weeks, I wanted to ask if the new polish with the synthetic wax as a corrosion inhibitor helped to prevent new oxidation or rust?
 
Since it's been a few weeks, I wanted to ask if the new polish with the synthetic wax as a corrosion inhibitor helped to prevent new oxidation or rust?
The Simichrome polish is holding up well, with no signs of oxidation.
With so many of the original pots being bad, I'm going stray from the original build a bit and will be building it as an integrated without any tone control.
 
Now wondering if all the coats of polish are going to have a negative effect on grounding, in particular on the grounding lugs of the power tubes.
Would soldering a ground wire in series from socket to socket create a ground loop?
 
I probably have extra controls, since I normally remove them.
I'm following your lead and ditching them also, Thanks anyway!
I have been busy with life and finally had a change to do a little more work on this.

_DSC0377.jpg
_DSC0378.jpg
_DSC0379.jpg

Still waiting on a few caps to come in and need to do the phono section,
I hope I drew up the new phono section correctly. I have a hard time following the rotary switch routing in the original schematic.
Anyone see any mistakes, please chime in.

Schematic rev.jpg
revised with 3 position selector switch.

ST7006.jpg
Original.
 
I missed this one until now. I know when i finished mine and was exchanging emails with Dave G , he seemed to like the 2080 design than the st70. I'd be lying if i remember exactly why. I think the part setup matches pretty well with the 70 having a few part choice advantages. I'm reaching here on my poor early morning slightly caffinated memory. I think 80 used 12au7 instead of 6sn7 and it used some nearly equivalent 6v tube where the 70 used an 12ax7. I kind of remember the sequence of the volume, tone balance was different. I think that was where the 80 had the advantage. You make want to ask him. I wasn't motivated enough that when he told me that i would go rip it apart and start again. I really like your clean up work,
 
Just a suggestion. The 70 had a wonky input selector set up (not for it s time frame but for modern use) AM/FM , Tape head. If i were as far down the path as you are now i think i'd try to make more useable inputs.
 
Just a suggestion. The 70 had a wonky input selector set up (not for it s time frame but for modern use) AM/FM , Tape head. If i were as far down the path as you are now i think i'd try to make more useable inputs.
I'm going with just 3 inputs (see my revised schematic). Phono, CD & Aux.

I was hoping I could get my reworked schematic of the phono section double checked.
-Brad
 
I missed this one until now. I know when i finished mine and was exchanging emails with Dave G , he seemed to like the 2080 design than the st70. I'd be lying if i remember exactly why. I think the part setup matches pretty well with the 70 having a few part choice advantages. I'm reaching here on my poor early morning slightly caffinated memory. I think 80 used 12au7 instead of 6sn7 and it used some nearly equivalent 6v tube where the 70 used an 12ax7. I kind of remember the sequence of the volume, tone balance was different. I think that was where the 80 had the advantage. You make want to ask him. I wasn't motivated enough that when he told me that i would go rip it apart and start again. I really like your clean up work,
I'm following BuzzK's build of his Eico ST-70 Rebuild Redux, from his write up, it was nice sounding.
 
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