Eico ST-70 vintage tube integrated amp restoration/upgrade

FTGV

The inexpensive Panasonic ECQ-E series are respectable for coupling purposes aswell.

michaeljhsda2

Fongolio wrote: »

I have my SDA's connected to the 8 ohm taps but the SDA's are rated 6 ohms. Would connecting them to the 4 ohm tap make much of a difference.

I use the 4 ohm taps on my Yaqin MC-10L integrated to run my SDA 1C's. Couldn't tell any SQ difference between the 8 ohm and the 4 ohm taps but I would try it on the 4 ohm taps to hear how it sounds.

michaeljhsda2

I'm curious about that jumper wire on the back...any idea why it's there? Could that be introducing hum?

[Deleted User]

I find that most people who do well thought out modifications will find one capacitor approach and supplier and tend to stick with what works for them. I know Tom Tutay uses the IC "lemon drops" with great success. I have used the Panasonic film types bypassed with .1mF in reVox mods that sound good. Any of those three will be fine. I do recommend cleaning the leads before installing, I believe it makes for a better soldering job. Use any metal polish, such as Flitz to shine up the surface and wipe off any tarnish completely. The "orange drops" are favorites of the guys at Ampex.
You might want to consider improving any of the loading and circuit path resistors while the patient is "on the table".
Regards, Ken

mhardy6647

FTGV wrote: »

The inexpensive Panasonic ECQ-E series are respectable for coupling purposes aswell.

Yes, I think I used those in an Allied 333 (Pioneer SX-34) that I recapped for a colleague at work a while back; space was at a premium under the deck of that critter; the Panasonics I used were remarkably small.

Kenneth Swauger wrote: »

I find that most people who do well thought out modifications will find one capacitor approach and supplier and tend to stick with what works for them. I know Tom Tutay uses the IC "lemon drops" with great success. I have used the Panasonic film types bypassed with .1mF in reVox mods that sound good. Any of those three will be fine. I do recommend cleaning the leads before installing, I believe it makes for a better soldering job. Use any metal polish, such as Flitz to shine up the surface and wipe off any tarnish completely. The "orange drops" are favorites of the guys at Ampex.
You might want to consider improving any of the loading and circuit path resistors while the patient is "on the table".
Regards, Ken

Indeed, the old carbon comp resistors may have drifted considerably since new; good advice there from the Master :-)

Fongolio

michaeljhsda2 wrote: »

I'm curious about that jumper wire on the back...any idea why it's there? Could that be introducing hum?

Good question but there is an answer. On the back of the unit the board on the left is for connecting speaker 1 (left channel) the board on the right top is for connecting the speaker 2 (right) channel. The board below it is the speaker 2 impedance tap. Now from the manual:

"To complete the connection of the Channel 2 speaker, a jumper wire must be connected between the third terminal from the right on the upper right board TB2, and the appropriate impedance tap on the lower right board, TB3, as show in the diagram."

I find this to be a very goofy way to do this and will hopefully remedy this durring my modding process. To make it even worse the positive side of the speaker is connected to common ground which causes a phasing problem but a phase switch is supplied which I operate in the reverse position to get normal sound. As great sounding as this unit is, it had some serious design flaws that hopefully will be fixed.

Fongolio

I just ordered a bunch of parts from justradios.com which is an outfit that specializes in the hard to find caps and resistors for old and antique tube gear. Because of the high voltage values caps for these units tend to be rare and expenisive. However, the place I named has very reasonable prices. I'm taking a chance on an unknown site but I'm not gambling much either. The parts are guarenteed so we'll see. There's a few parts they don't have that I still need but I can find them local or order from part express or digikey. The new tubes have been shipped so by next week I should be firing up the soldering iron.

In getting my parts list together I noticed the big power supply caps are the cardboard wrapped axial types and just seem so low quality. One of them looked like it had puked it's electrolyte so I'm guessing this could be a possible cause of at least some of the hum. Looking forward now to rolling up the sleeves and getting busy with this.

Fongolio

I was just reading in the Yahoo Eico group that the plating on the Eico ST-70's chassis contains cadmium which is toxic if ingested. A respirator is recommended while sanding or polishing. I was wondering why my sinus's were so sore for several days after I cleaned and polished. I thought it was from the Bluemagic polish which has a distict ammonia odor to it. Well, it could still be from that but I'm not too excited about the idea of having ingested some toxic heavy metals. Anyway, be cautious around these old pieces. Some are suggesting the painted parts could very likely contain lead as well. Just a heads up for any future restorers of these old Eicos.

Tour2ma

Sorry, but I laughed out loud at the above... Kinda got this Name of the Rose black finger/ black tongue image... Sorry, my problem to deal with.

The ammonia odor is the likely cause of your nasal irritation.

Anyway, back to my thread lurking... Very interesting so far... Carry on...

geoff727

Fongolio wrote: »

...I'm not too excited about the idea of having ingested some toxic heavy metals. Anyway, be cautious around these old pieces.

That does it! I'm never working on speakers ever again!

Polkersince85

I'd be more worried about that fully charged capacitor just lurking and waiting........

Fongolio

Ok ok I get it. I'm being paranoid. Anyways, thanks for the laughs you guys!! I'll call you when I go into the hospital for cadmiumestosis and need to have my brain and my junk removed!! THEN I'll need a laugh.

On a different note, I've still got the Eico in the main system. Even with the hum it sounds so sweet I don't want to remove it until I really have to.

Oh and before I forget, I've found all the caps and resistors I need except the two big metal cans that sit on top of the board with the tubes and transformers. The one is a dual value cap and the other is a quad style. The one is as 40/20 microfarad @450v the other is 40/20/20/20 @500v. Any ideas on where to find such beasts or how to change them to a configuration that will work with available parts? I figure since I'm eventually replacing all other caps and resistors these should go too. Thoughts?

Tour2ma

The Yahoo group won't share? If not, try AudioKarma forums.

mhardy6647

Unless either www.tubesandmore.com or www.radiodaze.com has the quad cap, you'll have to make do with a two or three section cap (if you can find one "close enough") and add the fourth electrolytic under the deck.

Fongolio

Ok, all the parts except the two new tubes and the quad and triple caps are in my possesion. I'm pretty busy all week but I will likely start replacing caps and initiating the mod this weekend. I've been using the unit every day just for normal listening and have not gotten quite used to the constant faint hum. I will be glad when I can get rid of that permanently. First to go will be the big cardboard cylinder caps. I am still undecided if I will replace ALL the resistors or just the ones that measure out of spec. I have resistors to replace them all but I am kind of an "if it ain't broke don't fix it" kind of person.

By the way the Thorens table I mentioned above is fully operational now. I've got a new belt, lubed the main bearing and motor bearings, polished the platter, obtained two Thorens headshells, cleaned the whole unit, polished the dustcover, added a home made missing base for the table and attached some air cushion feet from a wrecked Rotel turntable I had lying around for parts. I tested the speed using a strobe and it is just a hair off but I can't hear the difference so I'm happy with it like that. I attached and aligned my Audio Technica AT20SLa cart which is very high compliance and a good match for the Thorens light tone arm and auditioned with the Eico. Beautiful! Magical at times and this is before the Eico overhaul. The Thorens is so quiet I'm amazed. Now I know why these tables are so collectible, they are exceedingly quiet for how old they are. I am extremely pleased with these two aquisitions and the Eico has a long way to go still. I'll try to post pictures as the new caps and mods go in.

Tour2ma

Nice progress... I'd completely missed the TD-160 mention in OP. Is it a MK I? MK II?

Huge Thorens fan here... Here's a link to both models' operating manuals (bottom of page)...

http://www.theanalogdept.com/thorens_td_160_dept_.htm

Fongolio

Tour2ma wrote: »

Nice progress... I'd completely missed the TD-160 mention in OP. Is it a MK I? MK II?

Huge Thorens fan here... Here's a link to both models' operating manuals (bottom of page)...

http://www.theanalogdept.com/thorens_td_160_dept_.htm

Thanks. It was through that website I was able obtain a huge amount of info on this unit. It's the MK I I'm assuming. Looks just like the pics I've seen anyway. I was going to start a seperate thread for the table with before and afters but I dove right in and forgot about the befores. I'm going to change the tonearm wiring eventually and maybe even the tonearm. Right now though, I am very pleased with how it sounds and looks. It looked like a hopeless cause when I aquired it. Now it's my main table

Fongolio

So, mods on the Eico are progressing smoothly, I've got most of the 'lytic caps changed over including the power supply caps. I had some ceramics in there that were nearly microphonic and those have all been replaced with "orange drop" poly's. But I still have the hum problem. Very low level but audible on the main power transformer and through the speakers on any input. Once music is playing I can't hear it. I've tried pulling out one tube at a time but it's always there. I've done hum adjust, bias and balance but the hum is there. Could this be a semi bad transformer? The sound is improving with the upgrades. There is more air around instruments and the soundstage is just spectacular including an amazing SDA effect on almost anything vinyl,cd, or music server I play on it. It's just this humming thing driving me nuts. Oh and the background hiss is way down now at full volume nothing playing. Any suggestions would be greatly appreciated. Thanks. Pics up soon.

ben62670

reported

Tour2ma

Ground loop? Have you tried a different house circuit?

Fongolio

I made an error in my post above. I have NOT yet changed the two big "can" caps and I think one of these is for power supply. Perhaps this is where my hum problem originates from? Tour2ma, I've tried several different house circuits all with same result.

mhardy6647

The first section of one of the two cans is the primary filter for the P/S; yes, it will usually be the culprit.

The other problem with kit amps (or was yours factory? I've already forgotten) is that they're as good - or bad - as whoever built them. The hum could be one, or 20 poorly dressed solder joints; or it could be poor dressing/twisting/location of the AC filament voltage (I believe/imagine all the tubes have AC on their filaments?)... or it could just be intrinsic to the design (these were low-cost kits/amps of their day, don't forget).

Fongolio

I'm leaning to a little of all of the above. It IS a kit amp and the soldering and wire routing in some places is iffy (not that I'm totally gifted in that area either) and any that are suspect I'm cleaning up. I will feel better when I replace the can caps. There's a lot of old resistors on the unit too and I will replace them all eventually. The hum is actually considerably down from where it was when I started and the amp is sounding better all the time, I just won't be happy until all the hum in the speakers is gone. Many online owners of this amp have reported near silent operation. Anyway, I'm still having fun with it and that's what really counts here. Fun with my nerdy hobby!!

Fongolio

Udate: I had hit a road block with this project in that I had done 90% of the mods but there was a serious channel imbalance that I was unable to resolve so I just shelved it for a few weeks while I took care of some new career stuff. Well today I was in the right frame of mind to take another crack at finding the problem and indeed I did!! Turns out one of the new orange drop caps wasn't making a good connection. I had been warned previous in this thread to dress the ends carefully and now I see why. I fixed that and then decided to search for any other problems. I found another. A 1megaohm resistor was soldered to the wrong post at one end on one of the input channels. This would totally explain the huge volume difference from one channel to the other. I re-soldered it to the proper terminal and checked for any other errors I may have made along the way. Finding none I powered the unit up on the bench with my Polk R20's and an old Technics table. EUREKA!!! Everything working properly and sounding great. Even the anoying hum that has been persistent since I've owned this unit is now dramatically reduced. Balance between channels seems dead on and everything works exactly how it should. So next was to put it on the SDA's and see how it sounds. I put Paul Simon's Graceland on the the Pro-ject table with the Benz cartridge and PS Audio step up transformer and it sounds fantastic!! Clean clear and with gobs of bass and just a tiny hint of hum that is audible only with my ear at the speakers. I still need to replace the big can multi-caps so even this last trace may disappear when that happens. There is a couple last mods to do with the power amp section and the can caps so I will delay those until I have the new cans. For now it's just sit back and enjoy the improved sound of this unit so far. I'm a very happy modder today.

mhardy6647

Cool! Congratulations!
I am officially jealous, now ;-)

txcoastal1

Sounds like Sunday sunshine even if there is none...enjoy

Ron

Fongolio

Riding the wave of confidence I was on after fixing the imbalance problem I proceded to complete all the mods except for changing the big can caps and a seperate loudess control mod (makes the loudness switch work properly). The unit sounds totally sweet with a just a trace of hum left. The main power transformer emitted a huge amount of hum not to the signal but just sitting there. I found a very good fix. I pulled the top off exposing the the paper and windings, the bottom is already open, and use varathane on a brush and recoated everything. I did this top and bottom. I then let it dry several hours until it was good and tacky. I put the top cover on while it was still wet as I wanted it to seal it shut and eliminate all vibration. After about six hours everything was dry to the touch. I even coated the outside of the stacked iron. While waiting for the transformer to dry I did the last of the power mods. The very last one involves adding a couple of AC pots that allows for adjustment of AC distortion. The problem was how to mount these pots so that they could be connected below but adjusted above. The pots are square but the holes I driled are round so the solution was shoe goo. I had drilled the holes and mounted the pots without connection several weeks ago allowing them to dry totally. When I swap the can caps out I'll take pics of everything so you can see how this was done. The last thing I need to do is adjust the pots for minimum AC distortion. I've generated a 1 khz wave in Audacity and will feed it into the unit at 0.5 volts and using the same points as used for bias adjust set the pots to 0 volts. Not having proper test equipment this is the suggested way to trim the AC pots. Anyway even without having this done the amp sounds fantastic and the noisy transformer is barely perceptable now from the listening chair. I can tell there is an increase in power and at the same time a decrease in noise. The bass has loads of slam and the soundstage is wide and deep. Even without the can caps replaced this unit now sounds nearly as good as many tube amps I've listened to in dealer showrooms. Now off to do the AC distortion trim. Thanks to all for your encouragement and tips along the way.

Kelvin

mhardy6647

heh, now you're completely qualified to post in either of the recent "low powered amps" threads...

;-)

Fongolio

I finally got around to playing a cd from my Rotel tube modded cdp to the nearly fully modded Eico and I am just blown away at how great these two pieces work together. I was listening to the Junkhouse Strays cd (circa 1993 Canadian band) and I got seriously freaked out hearing stuff behind me. There's a part on one of the songs where it sounds like someone rattling jars and also whispers that are barely perceptable and they really sound like they are coming from behind my listening chair. Very freaky but takes the prize for best SDA effect so far. This comb is coming very very close the quality sound I get with vinyl. Using the computer based music server and outputting through my dac sounds very good as well.

On a slightly different note, I've ordered jj replacements for the big can multi caps and will swap them out as soon as they arrive. I'm just loving this Eico piece!!

Fongolio

I did the loudness switch mod today as well. The rumour is that way back in 1961 or so when this amp was being designed the sales and marketing end of Eico wanted a loudness switch to appeal to consumers who listen at low levels. The engineer/designers hated the idea and said no way, it ruins the pure sound (does this debate sound familiar?). The sales and marketing team apparently won and upper management ordered a loudness switch be designed in. The engineer/designers apparently designed the switch purposly to work so poorly that no one would want to use it and I must say it was really just a glorified mute switch on my unit. The problem was the design was so bad it also affected the sound of the amp even when not used. The mod I performed today fixes the design error and makes it functional again but more importantly gives the amp better bottom end even when not in use. It really does sound much better in either position. So now the only mods left are to replace the hideously poor speaker connection terminals on the back and all of the input and output rca connections. Not a mod but still an upgrade is the replacement of the can multi caps which are on their way to me as I write this. It's just amazing how good this 50 year old beast sounds now. Tune time!