One more Eico ST70 rebuild thread

[oldman55]

Picked this up for fair deal and was going to rebuild. Sound (somewhat) in both channels and tubes tested good. Cosmetics OK but need a couple of new knobs.
When I took off bottom cover, I got concerned with the transformers as the wires had black goo on them. Before I spend any time and resources on this, is there an easy way to check the transformers and does this indicated that they are damaged?
Thanks
oldman

 

[gadget73]

pull all of the tubes, power it through a dim bulb tester with a low wattage lamp. Or use an ammeter if you have one. You should see no light, or a few MA of current draw. With all the tubes out, it may still have a pilot lamp and you'll have a bias supply but there should be extremely little load. If the bulb lights or you see any real amount of current draw, something is wrong.

Visually that looks OK to me. If it had a blob of tar running out of the transformer, usually with a blob on the bottom panel and a horrible stink about it, then you might have trouble. If its got a good fuse of the proper value, thats also a very good sign.

 

[oldman55]

Thanks. Will order parts If they check out.

Hoped that there might be resistance and voltage values for these.

 

[nerdorama]

You certainly can use an ohmeter to check for open windings and for approximate balance on the primary of the output transformers and secondary of the power transformer in case there are shorted turns.

Look here at the schematic. Click on it to enlarge. The approximate resistance readings for the transformer windings are listed on the it.
http://tronola.com/html/st-70_hotrod.html

John

 

[crispycircuit]

xmers from that era were dipped in wax to keep them from buzzing. In use they heat up and emit the wax. Usually see dripping on the bottom plate. Completely normal. To check for shorts. Put in a 1amp fuse and see if it blows. I'd remove all tubes first. Have fun!

 

[primosounds]

Picked this up for fair deal and was going to rebuild. Sound (somewhat) in both channels and tubes tested good. Cosmetics OK but need a couple of new knobs.
When I took off bottom cover, I got concerned with the transformers as the wires had black goo on them. Before I spend any time and resources on this, is there an easy way to check the transformers and does this indicated that they are damaged?
Thanks
oldman
View attachment 870688 View attachment 870687 View attachment 870685 View attachment 870684

Actually, that is fairly normal for the EICO units. The power trans runs hot and most of them have like a seepage of black stuff that migrates down the wire over a long period of time. But restoring any old amp requires a check up on the transformers which should be done first. If you have a variac and a solid state replacement for the tube rectifier it is easy to do with a DMM digital multi meter. Simply plug in the unit to the variac and without the tubes and just ss rectifier. Put the variac on 10 and measure the voltage at pin 8 of the rectifier tube socket. It should be around 50vdc. If you don't have any voltage you need to check the fuse. I was going to list more checks but if you have the owner's assembly manual they have a very good run down on troubleshooting if no power.
Using a variac allows you to run checks at low voltages which is safer. If you have the 50vdc at pin 8 of the rectifier you can check the dcv at the output tubes sockets. Check the voltage on the plate and G2 of all the power tubes sockets. This will let you know if your output trans are good. There should be the same voltage as on pin 8 of the rectifier. If everything checks out, you are 99% sure the transformers are good.

 

[oldman55]

Edited: further reading cleared up question.

Is there a reasonably priced sub for the 40/20/500v?

 

[primosounds]

I did mine with separate axial caps which will fit under the chassis. Make sure you stay with 40uf for the first cap on the rectifier. After that you can add some capacitance. Also you might want to add the inline diodes before the 5ar4 pins 4 and 6 which cuts down the load on the 5ar4 and helps it from flash overs. Especially important for the currently made ones which are not as stout the originals.

 

[oldman55]

I did mine with separate axial caps which will fit under the chassis. Make sure you stay with 40uf for the first cap on the rectifier. After that you can add some capacitance. Also you might want to add the inline diodes before the 5ar4 pins 4 and 6 which cuts down the load on the 5ar4 and helps it from flash overs. Especially important for the currently made ones which are not as stout the originals.

1n4007s?

 

[primosounds]

Yes, that is good enough, but i only had some FREDs high speed rectifiers which is what i used.

 

[oldman55]

I am an novice and my knowledge of circuits is limited at best.
In reading another article on a rebuild, I saw where a 50/50/500v can was used to replace the 40/20/500v original filter (much more economical). Seems like this is pretty far off but apparently ok?

 

[RWood]

Thats what I did on mine, using a JJ can which is fairly low price.
I also got their 40-20-20 or maybe it was 20-20-20 but never used it as the second can in the Eico was fine. The first two positions got handled with the 50-50.

 

[oldman55]

Thanks, will try it.

 

[Dandy]

With a tube rectifier, you have to check that the first capacitor does not exceed maximum input capacity. But 50uF after an 5AR4 is fine. 60uF is the maximum.

 

[blhagstrom]

A nice unit showed up here. To-do... restore

 

[oldman55]

Waiting on the large cap order to come in and ordering the film caps and resistors today.

Should I make mods while at it?

Looking forward to getting started but not having to look at it. May put a paper bag over the faceplate.
A redesigned faceplate/knobs on the market would be nice.

 

[gadget73]

I've always heard that the sound overcomes the cosmetics on these. I don't think they're horrible looking but there were definitely better looking pieces of gear.

 

[oldman55]

Thoughts on the mods vs stock rebuild?

 

[Dandy]

I'm inclined to say restore to stock. It's always good to hear something as originally intended, i.e. with new components where needed but using the original circuit. You can also be sure you get it working correctly, and if there are problems someone will be able to help you locate the original assembly manual. There is much more potential help out there for a stock restoration.

 

[primosounds]

Thoughts on the mods vs stock rebuild?

Well, the eico 70 is running the 7591 tubes at near max ratings which means longevity and reliability is a concern. For currently made tubes this factor is even worse. If you are considering a long term relationship with the amp it would be best to operate the amp a bit more conservatively. Also the amount of gain for the long tailed pair driver/splitter is really high and noise issues can be a problem. Steven Lafferty and Dave Gillespie have addressed some of these issues in depth in Steve's Tronola web site. When running as it should the ST70 is an impressive amp.